Dimitrios Sampatakos Thesis 2014 by Dimitris Sampatakos

May 2014

MSc in Architecture, Urbanism and Building Sciences Building Technology, International Facade Master

Development of three dimensional PV structures as shading devices for a Decentralized Facade Unit of the Future. Report

Mentors: Arie Bergsma Craig Martin Wijnand van Manen

Dimitrios Sampatakos S.N.: 4251687

TU DELFT FACULTY OF ARCHITECTURE DEPARTMENT OF BUILDING TECHNOLOGY FACADE MASTER

GRADUATION REPORT

TITLE OF GRADUATION PROJECT Development of three dimensional PV structures as shading devices for a Decentralized Facade Unit of the Future.

PERSONAL INFORMATION Name: Dimitrios Sampatakos Student Number: 4251687 Address: Nieuwelaan 144 Postal Code: 2611 SB Place of residence: DELFT Telephone number: (0031) 629469277 Email address: dsampas@gmail.com D.Sampatakos@student.tudelft.nl STUDIO Track: Building Technology, International Facade Master First mentor: Arie Bergsma Second mentor: Craig Martin Third Mentor: Wijnand van Manen

Table of Contents 1.1 Introduction 1.2 Problem Statement 1.3 Aim of the Research/Final goal 1.4 Research Question(s) 1.5 Methodology 1.6 Summary Thesis Structure 1.7 Time Planning 1.8 Introduction to first Part Research Topics 2_Decentralized Facade Concepts and Solar cell integration

13-18

2.1 Next Facade and Decentralized Facade Concepts 2.2 Climate Box integration options 2.3 Solar cells as part of a decentralized Facade system 2.4 Potential of incorporating solar cells in Facades for maximum output 3_Solar Energy and Sun angles

18-26

3.1 Sun and the Solar spectrum 3.2 Solar Irradiation (The Netherlands) 3.3 The orientation of FLAT solar panels 3.4 Sun Tracking as a way to increase performance 4_Solar Cells

26-52

Introduction The Photovoltaic Phainomenon Crystalline Solar Cells 4.1 Monocrystalline cells (Production Processes) 4.2 Polycrystalline cells (Production Processes) Thin film Solar cells 4.3 Inorganic Thin Film cells 4.3.1 Layering layout and production 4.3.2 Amorphous Silicon solar cells 4.3.3 Micro/Nanocrystalline and Micromorphous solar cells 4.3.4 Cadmium telluride (CdTe) solar cells 4.3.5 Copper indium disulphide (CIS) solar cells 4.4 Organic Thin film cells 4.4.1 Dye Synthesized solar cells (DSSC) 4.4.2 Polymer solar cells 4.5 4.6 4.7 4.8 4.9

Advantages of thin film solar cells over crystalline cells Organic solar cells compared to crystalline and other thin film cells Solar cells and transparency Encapsulation medium and back face Flexible and/or curved solar cell structures

4.10 The problem of partial shading in solar sells 4.11 Custom, non rectangular solar panel shapes in facades today

5_PV System Peripherals

53-62

5.1 Grid Connection 5.1.1 Fully stand-alone systems 5.1.2 Hybrid stand-alone systems 5.1.3 Grid connected systems 5.1.4 Systems selectively combining the two types 5.2 Peripherals. Devices 5.2.1 Cables 5.2.2 Controllers 5.2.3 Inverters 5.2.4 Storage Means 6_Polymers as PV Substrates on Facades

63-66

6.1 Lightweight and Transparent Polymers 6.2 Photovoltaics on Polymers/Membranes

7_Methodology (four phases) towards final Design Proposals

67-70

7.1 Four Phases Diagram 8_Phase 1 Analysis of possible light control structures Light Control/Shading and PV Integration characteristics

71-110

Introduction. Facade Integrated PV cells 8.1 PV cells and light control/Solar shading 8.2 External roller blinds 8.3 Roller based folding structures 8.4 Horizontal blinds/louvers 8.5 Vertical louvers 8.6 Sliding flat panels 8.7 Sliding 3d panels and Rigid origami 8.8 Linear Foldable structures and parallel Origami folds 8.9 Linear Deployable Origami Shapes 8.10 Flexible Shading_Bending, twisting 8.11 Reference Project_Soft House by Keneddy Violich Architects 9_Phase 2 Shape research in terms of Surface Area Potential Surface Comparison through parametric tools 9.1 Solar cell efficiency and 3d shapes 9.2 Shape optimization in different scales Research References 9.3 MIT 3d solar cells project

111-138

9.4 Other projects and research related to 3d shaped solar modules 9.5 Phase 2 shape Generation 9.5.1 Parametric definition of surface area Potential 9.5.2 Origami shape characteristics and Models Testing 9.5.3 Preliminary shadow study. Shadow patterns 9.5.4 Results and Evaluation of Phase 2 Shape Generation (graphs/tables) 9.5.5 Comments on Phase 2 and additional aspects 10_Phase 3 Solar study of Phase 2 Shape Results. Solar Exposure, Partial Shading percentage Comparison

134-135 139-166

10.1 Solar Analysis Process Solar Analysis Graphs/Tables 10.2 Solar Analysis (Ecotect) results 10.2.1 Results Table “Sorted by Energy gain”:Criterion 1 (per footprint area on facade) 10.2.2 Results Table “Sorted by Criterion 2” (per PV surface area) 10.2.3 Results Table “Sorted by Criterion 3” (combined)

153

11_Phase 4 PV-Laboratory TUDelft Shape Testing. Real Thin film module Performance evaluation

167-192

11.1 Importance of Phase 4 as a next step after the Solar Analysis 11.2 Creating the testing models 11.3 Testing Procedure and shapes 11.4 Models Lab Testing Results 11.4.1 Results per Inclination (Graphs/Tables) 11.4.2 Assessment of Results per Inclination 11.4.3 Ecotect results compared to lab results 11.4.4 Results per shape (Graphs/Tables) 11.4.5 Assessment of Results per Shape 12_Integration of the results into the Next Active Facade Analysis and presentation of different design proposals Solutions Categories 12.1 Category A (proposals) Outwards Convex curved Surface 12.1.1 Solution 1 12.1.2 Solution 2 12.1.3 Solution 3 12.2 Solution 1,2 Material Selection 12.3 Solution 3 and Category B,C Material Selection 12.4 Category B (Proposal) Vertical Corrugated Folding Surface 12.5 Category C (Proposal) Origami (Miura Ori) Structure 12.6 Comparing and evaluating the developed Solutions/Proposals

179

193-236

13_Options for the complete PV system based on the final designs (Possible use of the gained Energy)

237-246

13.1 Energy Produced and consumed (loads) 13.2 Needed Capacity of Storage means for Stand-alone Option 13.3 Assessment of the calculated Stand-alone Option 13.4 Options in order to use a partly autonomous system 13.5 The possible concept

243

14_Summary of Results of the 4 Phases and Designs

247

15_Conclusion, Recommendations and further Potential

249

1.1 Introduction

It is officially agreed within Europe that Net-Zero-Energy buildings (net zero-energy balance on an annual level) are a goal for the near future. For achieving this, new or refurbished buildings need to both reduce their energy demand significantly by increasing their efficiency as much as possible and simultaneously gain the remaining needed energy efficiently with the local use of renewable sources. Solar energy constitutes the most efficient source with the highest potential for local energy production in buildings. Relevant technologies have reached a high level of maturity through a wide range of existing applications that have already proved themselves in the market. Building integrated solar energy solutions are a relatively new approach to applying these technologies to buildings, one of the most popular ones being Building (or Facade) integrated PV cells. The idea of replacing building elements with energy producing devices that also have an additional function (as cladding, shading device or other parts of a building, thus partly reducing the cost) has produced some very interesting ideas and results in the built environment. The potential of achieving grid parity and replacing the existing energy sources with more sustainable ones has been the driving force for the development of these systems. The need for local use of produced energy and integrated solutions as a result of governmental (financial) incentives and subsidies (or lack of them in some cases for grid connected systems) as well as the potential of using bigger areas especially in high buildings with large facade surfaces compared to roof/footprint for maximum output, all create a fertile ground for Facade Integrated solar devices. At the same time the concept of decentralized facade integrated (building) services and climate control has recently become popular and shows an important potential especially for modern office buildings. Including most vital climate installations in the building envelope gives additional possibilities for efficient refurbishment of buildings, as it becomes less dependent on the specific characteristics of each structure/building or existing installations. Imagining a future where the building envelope will be to an increasing extent responsible for the reduction of the energy demand as well as for the energy production (in an active way) of a building, one can conclude which characteristics will be of major importance for an efficient complete system. Easy and relatively inexpensive or -at least- cost effective refurbishment of the existing building stock will depend on the integration of systems that are efficient (in terms of TOTAL energy produced or saved) and cheap/easy to produce, fabricate, transport, install and operate. A much more frequent need for facade refurbishment and upgrades of the components (solar cells, insulation etc) with better ones more times within the lifetime of a building will also play an important rule in the development of the new active facade modules/structures regardless of whether they are designed for new buildings or for refurbishment. Lightweight, inexpensive, transparent and flexible structures and materials and the ability to use them as the basis/ substrate for PV cells has opened new perspectives for engineers and architects who develop active solar devices for future facades with the aforementioned characteristics. Production processes and materials that reduce the PV cell price and enable three dimensional shapes forming lightweight forms instead of flat rigid panels give the possibility of solar and structural shape optimization as well as integration on buildings of any shape or facade type. They also become an important tool for architects envisioning an adaptive and bio inspired built environment. This Thesis is based on the assumption and vision that solar cell technology will become more and more accessible and cheap for people to use, shape, and experiment with not only due to the low price of the cells but also due to the characteristics of the substrates that can be a wide range of materials with cells printed on them. High performance cells, which will be the energy producers within efficient solar plants, will probably remain the one of two directions on which scientists will base their efforts for further development. The second direction which is already rapidly growing is the lower efficiency, low cost and more integrated approach mentioned above. In this case cost of the active solar material will become less important and the goal will be to increase the materialâ&#x20AC;&#x2122;s performance for a given footprint, through shaping it in three dimensions and thereby fulfilling the ever growing demands for better energy balance of buildings. Considering building integration 3d shaping of lightweight facade integrated pv modules has the potential of creating low cost efficient building skins formed specifically for each given location and aesthetically more interesting than flat panels attached on facades. 1

1.2 Problem Statement

The aforementioned conditions related to the expected need for zero energy buildings set new standards and impose new design and integration strategies as well as thorough research into local use of renewable sources. New technologies in the fields of PV-cells and substrate materials provide great potential for applications in buildings in the future due to their special properties. At the same time modular decentralised facade systems placing the climate control units within the facade have the potential of more frequent and easier facade refurbishment, but also that of better operation close to active solar devices. Active solar device as part of the NEXT FACADE The development of an integrated active solar device for the Next facade concept in cooperation with Alcoa Architectuursystemen forms the first basic brief for the following research. The characteristics of the device must follow the idea of an adaptive (for different building types), modular element. More specifically the use of new emerging flexible PV cell types/technologies will be researched and the possibilities of using them on lightweight structures and substrate materials within facades. Their integration and cooperation with the climate control devices of the â&#x20AC;&#x153;Next facadeâ&#x20AC;? places an extra challenge but can also add extra value to it. Problem Statement. Relation to standard structures and structural problem of lightweight structures. Solar cells used on buildings are until now usually thought of as rigid standard sized rectangular panels installed on rooftops. Other, more integrated or customized applications are often considered too expensive by engineers and clients, or less efficient. On the other hand architects and clients are also often negatively biased against active solar devices due to their standard shape and dark color. Within this research and final design development the possibilities, advantages and alternatives provided by current and emerging solar cell production technologies and materials will be analysed, as well as the potential of providing an efficient alternative to flat solar devices on facades. Apart from the basic questions, which -compared to standard solar panels- any non standard solar cell structure needs to find answers to (cost, production etc), lightweight structures and thin flexible designs also place extra challenges and difficulties. There are still extremely limited examples of new solar cell technologies integrated on lightweight 3d shaped (or free-shape) structures on facades. Most building related projects making use of the aforementioned properties are exhibition structures, military or third world structures space applications and many small scale applications. Challenges that need to produce efficient solutions are the inherent problems of thin, lightweight structures on building facades, namely rigidity against wind pressure and resistance against the elements in general. The different ways of dealing with these problems will be analysed, as well as possible kinetic and adaptive characteristics. Performance related Problem Statement The Netherlands as well as other countries have a relatively high percentage of cloudy days with a total of about 60% of diffuse light compared to only 40% of direct light conditions (sunny conditions). The question arises as to what extent it makes sense to use expensive active solar structures but also how the performance in these conditions can be improved with new solar cell technology and through specific design decisions (shape optimization) compared to a flat panel with standard silicon PV cells, that theoretically have a higher efficiency than new still less mature PV technologies. The properties of new pv cells and substrate materials enable more kinetic and adaptive structures which on the other hand need to deal with the wind loads and structural demands. At the same time they enable free three dimensional shapes that can have a big advantage when optimized according to specific solar conditions. Shape optimization in different scales (facade/building scale, or module scale) is the basis on which compact, lightweight PV structures can in the future operate more efficiently compared to standard rigid and flat solutions.

1.3 Aim of the Research/Final goal

The goal of this research is to investigate how the special properties (such as excellent diffuse light performance, flexibility, and even printability on different substrates etc) and great price reduction potential of new generation PV cells can lead to highly efficient three dimensional alternatives of flat and rigid solar modules as well as the way these can be integrated and operate within a decentralised facade unit in the future. Three dimensional energy producing structure proposals in the form of light control/shading structures will be the result of this process, which will constitute the multifunctional active device of the “ACTIVE NEXT FACADE” concept in cooperation with Alcoa. The designs researched and the finally developed solutions should highlight and make use of the special properties of the aforementioned materials and technologies and exhibit advantages over standard flat modules especially in terms of diffuse light and partial shading performance, weight, design freedom and adaptability. Especially for countries with low direct light percentages within a year this can lead to a considerable solution in the future. Finally the thin, lightweight shape of the basic material should lead to a structure that retains these characteristics but is also able to withstand the forces and elements on a facade and exhibit a good performance in all conditions. As a total the NEXT FACADE should maintain and highlight with the integration of such a system its main inherent advantages, namely the good potential of integration in a variety of building types and locations and (through the use of an efficient innovative PV-light control system) a high efficiency in terms of climate control. In terms of Performance the research and design aims to find the simple,light and low cost alternative to other “performance increasing” strategies namely precise tracking, and concentrated photovoltaics. Three dimensional shape and design should be therefore optimized in order to increase the performance compared to flat panels for a given surface area, partly through better exposure angles and mainly through an increase of the total exposed surface for the same “facade footprint”. The Graduation Research Part will therefore have the following main focus points: - Solar cell technologies and Facade Integrated PV solutions. - Light control/Shading structures. - Shape optimization and three dimensional PV structures. - Decentralized Facade solutions and integration of the active device within the Next Facade Module.

1.4 Research Question/s

How should a lightweight, thin and flexible photovoltaic system be shaped three dimensionally and integrated in a decentralized facade module in order to make use of the properties of emerging PV cell technologies and lightweight transparent substrate materials/polymers and exhibit performance advantages compared to flat systems? How can these characteristics lead to advantages compared to conventional flat solar panels in terms of performance, adaptability to different conditions and added value for Building Facades? How should the total PV system be designed and what -energy consuming- devices can benefit from the energy production of the structure? Sub-questions could be related to the aforementioned possible advantages of flexible structures and the challenges or inherent problems they set. These are: How can such a structure overcome its inherent problems of rigidity and resistance against the elements without sacrificing its advantages and structural simplicity and low weight? How can PV technology and more specifically emerging Thin film Photovoltaics benefit from (or become the basis for) parametric methods and shape optimization processes in different scales?

1.5 Methodology A. The first part Research aims at collecting data from literature study and existing projects about Solar Energy, existing and emerging PV technologies and relevant Materials (either as Active photovoltaic material or substrates) as well as decentralized facade systems. B. For the second part of the Graduation Thesis a methodology consisting of four (4) basic Phases is developed in order to reach the final design proposals. 1. The first Phase includes literature study and research on existing projects related to light control/shading systems and relevant mechanisms. 2. The second Phase includes research and literature study on papers and projects dealing with shape optimization and surface maximization for active solar surfaces. Parametric tools and genetic algorithm software are used to produce a large number of surface area efficient 3d shapes. These are compared and evaluated creating relevant graphs and tables. Models (origami,prismatic,corrugated) are constructed to observe partial shading patterns under artificial light. 3. In the third Phase, a Solar Analysis of 56 of the produced 3d shapes is performed using specific software. Results are collected evaluated and compared creating tables and graphs. 4. The fourth Phase includes PV Laboratory Testing of different shapes in order to compare with Solar Analysis results and evaluate performance potential of specific 3 dimensional PV structures/shapes. Results are extracted from specific PV testing software, transformed to Graphs and Tables for Comparison and Evaluation. C. In the last Part the best performing shapes are translated into real design proposals (as energy producing light control devices) for integration into the NEXT FACADE Concept by Alcoa Architektuursystemen. Possible operation methods and mechanisms are developed and modeled. Material Selection Software is used to select best performing Polymer materials for the designed structures. Finally proposals for the operation of the complete PV based system are made and calculations in order to define the possible use of the gained Energy by the PV cells.

1.6 Summary Thesis Structure The first part of the current Thesis consists of a research on topics related to Solar Energy, PV technologies and Materials used as part of active solar structures (from the active materials to substrates) as well as their integration in the form of custom shapes. The analysis of these topics reveals the potential of new and emerging PV technologies, which combine very lightweight and flexible characteristics, with a very good performance in partial shading and diffuse light conditions, but most importantly a very high price reduction potential due to specific fabrication methods and materials. In a next step a (four phase) Methodology is developed in order to observe how the process of shaping a three dimensional PV structure in order to increase its surface area per facade space can lead to performance advantages compared to conventional flat and rigid PV structures. This part consists of a first phase, where possible light control/shading systems are compared according to specific requirements related to shading efficiency but more importantly to PV integration potential and PV surface increase potential. The second phase is a shape research in terms of Surface Area Potential and a comparison between 3d shapes using parametric tools. Thereby the ability of different shapes with various folding or curvature patterns to increase their surface for predefined total dimensions is evaluated. The results lead to structures which can provide much more active PV area for a given facade area. The third phase is a solar analysis of 56 of the three dimensional shapes tested and compared in phase 2. Thereby the real average solar radiation reaching the surface of various prismatic or corrugated 3d patterns is calculated and graphically presented. Incident Solar Energy values and partial shading percentages within every shape clearly illustrate which 3d shapes have the highest potential for making maximum use of solar radiation. The relation between surface area increase through denser folding or curving patterns and total received solar energy constitutes the most important finding of this phase. In a fourth phase Laboratory tests are performed at the PV-LAB of the Faculty of Electrical Engineering TUDelft. Different 3d shape versions are tested using flexible Thin Film solar panels. The fact that solar cells performance (output) does not decrease linearly proportional to partial shading and total incident solar energy changes makes real Lab Tests an important verification of the solar analysis results. Finally the best performing shapes are used as the basis to create design proposals in the form of a light control/shading device. The PV optimized 3d surface is thereby developed as a facade structure with possible ways of operation and integration into the NEXT FACADE Concept by Alcoa Architectuursystemen. Additionally ways of using the produced energy and creating a complete efficient PV based system are proposed.

1.7 Time Planning January weeks First Research phase P2 Presentation Additional Research based on feedback Research on mounting structures, assembly disassembly. Interviews with companies/specialists.! research on decentralized facades and storage Phase 1. ! Analysis of light control/ shading devices Shape research and parametric shape optimization Reference Projects and relevant Research Study! P3 Presentation Phase 2.! Shape Generation (parametric tools) Surface Comparison. Phase 3.! Solar Study Phase 4.! Model construction! PV Laboratory Testing Development of ! Solutions based on ! 4 Phases. Drawings/! 3d Models P4 Presentation Final adjustments of! proposed Solutions and! of complete system proposal Visualization, 3d Rendering of Designs Final Presentation

February

March

April

May

June

July

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Basic Development Units

Topic Definition Preliminary research

Phase 1

Literature Study. Study of existing structures, projects and conducted research

Phase 2 Phase 3

Main Development Part

Phase 4 Solution Proposals Implementation

Final adjustments Visualization

1.8_ Introduction to first Part Research topics

Renewable energy Industrialization, prosperity and the ever rising population numbers lead to a growing demand for energy worldwide. Previously mainly oil and coal covered the energy needs of most countries. However fossil fuels are finite resources (predictions vary about future reserves) and also irreversibly harm the environment. Nuclear energy was presented as a very powerful solution, but due to its risks and the -at least two- major relevant accidents (Chernobyl/1986 and Fucusima/2011) its use is strongly declining (for instance the German Government has decided to fully stop nuclear energy production by the year 2022). In addition to that, in times of climate change, a change of thinking has been considered not only desired, but unavoidable. Electricity and heat generation from renewable sources is becoming increasingly important. Sun and solar energy The renewable energy discussion nowadays has evolved to the point that once the terms alternative or renewable energy are mentioned, the first thought is directed to the sun, certainly also for historical and psychological reasons dating back to the very origins of the mankind. The sun is the center of our solar system, and it is a huge nuclear reactor since in its interior energy is produced through the fusion of hydrogen into helium at temperatures of 15 million degrees. What makes the sun energy so attractive is, in addition to its vital role for all creatures on earth as their existence is fueled by light from the sun, the further fact that the sun reliably rises every morning and its radiation is thus according to human appreciation repeatedly and endlessly available at no costs, i.e. fully renewable and sustainable. The primary energy performance of vertical solar radiation on a square meter is 1,36 kW (solar constant, i.e. the amount/ density of solar radiation received at the top of the Earthâ&#x20AC;&#x2122;s atmosphere) and is composed (by total energy) of about 50% infrared light (> 750 nm), 40% visible light (380 to 780 nm), and 10% ultraviolet light (<400 nm). At ground level this decreases to about 1120â&#x20AC;&#x201C;1000 watts/m2, and by energy fractions to 44% visible light, 3% ultraviolet (with the Sun at the zenith, but less at other angles), and the remainder infrared. Thus, sunlightâ&#x20AC;&#x2122;s composition at ground level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of visible light and 32 watts of UV radiation. According to estimates, the sun provides each day the equivalent of the daily global energy demand of eight years calculated in view of the present consumption. Use of this energy dates back to over 3.000 years ago when in every religion the Sun was personalized in various Gods, whereas in areas of high solar irradiation, like Egypt for instance, the most important god was that representing the sun (God Ra). Initially the sun energy was mostly combined with the use of mirrors or concentrating lenses of a very simple but still very effective technique linked often to fire, the almost unique energy form at that time. In the Netherlands the total solar radiation on a horizontal plane is approximately 1000 kWh/m2*year. As a result of the relatively large number of cloudy days, the irradiation consists approximately of up to 50 to 60% of diffuse light (indirect light scattered by the atmosphere) and only 50% to 40% of direct light. Both direct and diffuse light is extremely useful when efficiently harvested with appropriate technology.

Electricity (photovoltaic) and heat (thermal) energy from solar energy To start using the solar radiations thermal energy , no complicated invention has been necessary as the effect can be experienced by simple sun exposure or through space arrangements and positions that ensure more exposure to the sun. For electricity generation however the humanity was obliged to wait until 1839, when the photoelectric principle, which forms the basis of all types of solar cells, was discovered by a 19-year-old French physicist Alexandre Edmond Becquerel (1820-1891). From that moment on solar energy was in the center of interest of many engineers and inventors. Although initially heat production was mostly envisaged, when Jan Czochralski (1885 - 1953), a scientist from West Prussia developed - in 1916, while working at AEG in Berlin- a method to grow crystalline silicon (material used until today) and in 1921 Albert Einstein (1879 - 1955) received the Nobel Prize for his explanation of the photoelectric effect in 1905, development of photovoltaic energy production gained steadily ground. Development of solar cells was boosted by the space industry for being a key factor for the energy production in space vehicles and satellites, but this remained for several years in the sphere of high technology (high tech solutions). It was in 1994 when the First World Conference on Photovoltaics took place in Hawaii.

Photovoltaics (PV) and buildings ďżź It was not earlier than in the last 20-25 years, that photovoltaic solar energy was widely used first in open field utilities and later more and more on buildings, even though first attempts to practically add PV cells to buildings were already made in 1959, when the Solar House in Lexington, Massachusetts, represented the result of a solar house design competition of the MIT Faculty of Architecture. Once all the data had been collected during three heating seasons, MIT sold the house to a private owner. Buildings offer an enormously large, eco-friendly potential for use of solar energy, becoming even more attractive when stimulated by specifically addressed governmental incentives for following this direction. Building Integrated Photovoltaic Systems (BIPV) In a first phase PVs were â&#x20AC;&#x153;simplyâ&#x20AC;? added to buildings, usually on their roofs. They were not particularly designed for this new use, but were mainly based on existing modules fixed on planned or existing convenient building surfaces, mainly pitched roofs, flat roofs and opaque flat parts of facades. These systems are now called Building Added PV Systems (BAPV). In distinction to that, components and systems that also serve conventional functions in the building envelope and simultaneously convert solar radiation into electricity represent multifunctional building-integrated photovoltaics (BIPV). A photovoltaic system is truly integrated into a building when it is integrated both architecturally and as a part of the construction of the building. The PV systems embedded in buildings play a double role by producing green and clean energy as a benefit for the occupants of the building and the environment, and by simultaneously contributing in saving costs of construction materials and energy. They are ideal building elements with multiple roles and benefits by participating in functions, like insulation and sealing applications, shading, highlighting aesthetic and architectural design. We can assume that they will prevail more and more, especially as they also amortize their initial costs over the course of their utilization period.

Facade Integrated Photovoltaic Systems (FIPV) As the use of BIPV grew during the recent years, more specific sub-areas were defined, the most common of which are the ones dealing with integrating PVs into facades. Since space in the cities becomes increasingly rare and expensive and the roofs represent only a small part of the outside area of a building, (especially when dealing with high office buildings with a relatively small footprint) it became more and more of a challenge for the architect to implement the new renewable energy production requirements on the facade through a combination of the most appropriate techniques in terms of integration and performance with an interesting and appealing design. It appears that the (at the moment still not so common) abbreviation FIPV (Facade Integrated Photovoltaics), will be more and more of a standard term in architecture of the future, given that Facades will probably contribute to a growing extent to the energy production of buildings. Lightweight (and/or flexible) building and facade structures The origins of the use of flexible lightweight materials for human built structures can be traced back to thousands of years ago, when the first shelters made of animal skins supported by wooden sticks were built. In a later stage loosely woven fabric tents associated with nomadic people were considered one of the first and most successfully implemented examples of fabric structures. The tensile tent idea with the advantages of lightweight (easy to transport and deploy) and efficient sun protection spread throughout the world and became an unsurpassed finding even until today, used in different forms and layouts especially in specific sectors where low weight, ephemeral character or transportability is the ultimate goal, namely in military structures,ephemeral sports or exhibition (pavilions etc) facilities and of course in large span structures. The work and experiments of Frei Otto on tensile membrane structures, as well as Buckminster Fullerâ&#x20AC;&#x2122;s structures in the beginning of the last century were a strong influence that again brought flexible tensile membrane structures into the vocabulary of the contemporary architect. Recent development of materials and especially polymers has created an even higher potential with more durable, transparent and climatically better performing structures. The inherent problem of wind resistance due to limited material rigidity, especially for movable structures has been dealt with in a variety of ways. Solutions for stabilizing the systems have been developed apart from the conventional principle of tensile surfaces with compression elements, like for example inflated or vacuum systems or even non tensile systems where plastics are formed into specific shapes (through corrugation or folding) to withstand forces. The advantages of these lightweight structures go beyond the gains deriving from their low weight in terms of cost, simplicity of construction/installation and even reach a point where they become very efficient insulating skins or part of the climate control installations (for example by controlling the light or preheating air) of a building. In this sense they have the potential of significantly contributing to the climate control within a facade. Lightweight materials in the form of polymer sheets or membranes instead of glass (for rigid solutions) or fabric (for flexible solutions) will form the basis/substrate for the PV integration developed within the Graduation Project.

2_Decentralized Facade Concepts and Solar cell integration

2.1 Next Facade and Decentralized Facade Concepts During the last years especially in western Europe, several concepts of decentralized facade units have been developed, a few of which have already been implemented in buildings. The building envelope and services together cost approximately 50% of the entire construction, which as a percentage can theoretically grow if more high tech systems are used in the future and the load bearing structures become cheaper. In a decentralized facade the main climate installations and services are integrated in the facade and operate in a decentral level in favor of a specific part of the building, instead of using central installations that control the entire building. The idea of combining the building envelope with the building services (climate control installations etc) is not new, but the integration of these modules in office buildings in the future has a high potential due to several reasons related to the foreseeable situation in the building industry and new climate control or energy production technologies. First of all, the fact that the construction of new buildings in Europe shows strongly decreasing trends compared to refurbishment of the existing buildings has to be taken into account. Strategies for efficiently and easily renewing our buildings have to be developed, which are also closely related to easy integration of standardized facade modules in different kinds of buildings with different installation arrangements/layouts. The flexibility (and possibility) to attach better performing facade units in different types of existing - office- buildings is a big advantage in this sense. Size of the installations and piping also plays an important rule and decentralized systems integrated in the facade or floors enable bigger floor heights or more floors for the same total height. Another aspect mentioned before is the increasing speed of development of new technologies in climate control devices and energy producing devices. This development combined with the probability of a decrease in prices for these devices will definitely make the frequency of facade replacement, refurbishment or renewal (either as a complete unit or as specific climate control parts) rise dramatically. Not only have the times past, when a facade would practically have the same life cycle as the building itself (and never be renewed or replaced), but even the renewal every few decades will be completely reconsidered. Already ideas about facade leasing or extremely frequent refurbishment/renewal of office building facades have emerged. Decentralized facades with integrated services, being independent from existing installations layout and requiring less modifications and labor, will have a relative advantage in this case.

fig. 2.1 (source: ALCOA Architectuursystemen)

Possible disadvantages of these systems are until now mainly related to increased maintenance cost and low flexibility in design solutions for architects, since they are relatively limited to specific predefined products. High complexity of the units, with devices coming from different companies is one more aspect to consider, since communication and maintenance responsibility can in some cases be a problem. The idea of the NEXT facade can be compared to a system comprising different devices which can be controlled by the user through a computer. It will also be able to adapt to the needs of the inner space in case of an unoccupied room and ensure low consumption. The complete unit is customizable to fit the needs for every specific building and can be placed horizontally or vertically. The advantage provided by this project apart from the common advantages of decentralized systems, is that NEXT would be responsible for the operation of the entire system, also taking up the responsibility for consulting and contracting with architects and clients. High customization possibilities provide more freedom to the architect and energy producing devices would further improve efficiency (calculated at approximately 20% better than centralized systems). Energy2 Facade by Sch端co The E2 Facade is another example of a decentralized facade incorporating operable windows, sun shading, decentralized ventilation and PV modules. PV modules were in a first stage used on the opaque/intransparent part of the facade and later integrated as semi transparent thin film sheets within the glass windows. Decentral ventilation units are integrated on the external part of the slabs (fig 2.3), thus enabling higher story heights and an absence of shaft and service areas. The system was presented at the BAU 2009 exhibition in Munich.

fig. 2.3, 2.4 E2 Facade cassete and Section (source: Schueco)

fig. 2.5 E2 Facade external view of thin film PV glass panels (source: Schueco)

2 Degree System Schüco

Two years after the Energy2 Facade Schüco develops an-

other decentralized multi-functional unit called 2 Degree Concept Facade. The name is used to express an optimistic vision of decreasing the global temperature (global warming threat) by two degrees. The design follows a layered approach where adjustable/movable layers of different properties and functions comprise the units surface. These are either panels with high insulation properties, sunshading systems or thin film Photovoltaic panels. Smart Box by Cepezed Architects, Energy Research Centre of the Netherlands (ECN), TNO Bouw & Ondergrond Cepezed participated in the development of a project called “Zonwel”, which uses a “smartbox” unit at the junction between the floor slabs and the facade, similar to what the Energy2 facade by Schüco proposes. This leaves more space free between the floors and additionally a suspended ceiling can be avoided. The use of ducts is avoided. The idea is to place all the climate control units exactly between the inner and outer space and make use of both to improve efficiency of the decentralized installations. The Smartbox includes, apart from decentral ventilators, also a heat pump and a heat exchanger that uses the temperature difference between inside and outside. The system can be combined with floor integrated systems. (The Future Envelope 2: Architecture, Climate, Skin)

fig. 2.6,2.7 2 Degree Facade in BAU 2009 exhibition (source: http://www.fat-lab.de/archive/portfolio/bau-2009)

fig. 2.8 SmartBox climate devices. Ventilators, Water pump, Heat exchanger (source: http://www.archello.com/en/project/smartbox)

fig. 2.9 SmartBox facade integration in section. Use of slab width (source: http://www.archello.com/en/project/smartbox)

TEmotion developed by Wicona The TEmotion facade is another concept including an opening element, decentral ventilation unit, heating and light control through adaptable sun shading. It can optionally include active use of solar energy through votovoltaics on lamellas. Wicona promises over 40% energy savings compared to conventional facades and installations. The installations are placed only vertically and design flexibility is therefore slightly limited. Integrated facade for “Capricon house” by Schossing and Gatermann In 2008 the construction of the Capricorn house in Düsseldorf (Germany) was finished, with a goal to become a low energy building with an extremely low operating cost and a primary energy requirement 20% below the German energy saving regulations. Trox was responsible for the decentralized ventilation unit used in the facade of the building. In this system the outside air first passes through an independently operating mechanical volume flow controller, then reaches a heat recovery unit and is heated by the extracted air through a heat exchanger. The air conditioning installations/devices are in this case all fitted in 20cm wide sill units, comprising the opaque (red colored) part of the façade. fig. 2.10,2.11 TEMotion facade view and installation box (source http://www.wicona-int.com/en/Product/Facade/TEmotion-Intelligent-facade-concept/)

fig. 2.12, 2.13 Trox ventilation Unit used in Capricorn House facade (source http://www.trox.cz/xpool/download/en/technical_documents/air_water_systems/projects/pi_fsl_11_en_2_capricorn.pdf )

fig. 2.14 Capricorn House Duesseldorf

2.2 Climate Box integration options The aforementioned decentralized facade examples provide different solutions for placing the climate control installations. In some cases the box containing the devices is placed vertically or horizontally on the facade plane (as a sill for example) either with or without the flexibility of choosing which part of the facade module will be transparent/opaque (fig 2.15). In other solutions the box or cassette is placed on the edge of the floor slabs using only the width of the slab (fig. 2.16). This solution gives more freedom for architects to design the facade and in some cases also provides more transparency. Solutions that combine the climate installations with active energy generating systems (either PV or Solar Thermal collectors, or just air preheating) have an additional advantage and potential of combining them with a heat pump or heat exchanger. Even though the option of placing all the installations within the floor width has the aforementioned advantages and a good potential for a very transparent façade, when combining most of the devices that can create a complete system with energy generation and climate control (especially including heat pump for example) the needed space often exceeds these dimensions. Apart from this, a solution that uses mostly the floor part and an extension of it can be considered less modular and “retrofit ready” as a refurbishment solution of different types of buildings (or office buildings more specifically). In the products/designs researched not the optimal layout for a new building will be chosen, but the one that combines performance (and cost efficiency) with a modular character and flexibility to easily fit into different buildings (and replace their existing façade).

fig. 2.15 Vertical Integration parallel to facade plane (source http://www.troxuk.co.uk/uk/products)

fig. 2.16 Horizontal Integration as part of the existing slab (source http://www.troxuk.co.uk/uk/products)

fig 2.17 The Next Facade integration provides the flexibility of placing the climate box either vertically or horizontally. Presented solutions include a vertical unit within the grid of the facade windows. (source: 3d model by author)

2.3 Solar cells as part of a decentralized Facade system Providing electricity to the building by renewable sources is an important step towards more efficient systems that can in combination with a reduction of consumptions lead to the goal of energy neutrality. In case of decentralized systems, which incorporate different climate control devices (that use energy to operate) on facade level, an additional potential exists, since the produced energy can be used locally within the same module. Thereby an energetically independent (as long as the produced energy is enough to run the climate devices) external layer could be created, incorporating practically both the climate control units and the active solar devices that provide the needed energy for the facade and depending on the output also for the rest of the building. Even for 100% grid connected systems where the produced energy is entirely fed to electrical grid, the fact that energy producing devices (solar cells) are part of the same independent module that controls the inner climate is important for the efficiency of the system, the simplicity of the structure (connections etc.) and as a result for the refurbishment/retrofitting potential of the facade, which is an important characteristic. 2.4 Potential of incorporating solar cells in Facades for maximum output. Taking into account the EU guidelines for zero energy buildings in the near future, harvesting of â&#x20AC;&#x153;cleanâ&#x20AC;? renewable energy will become a major factor in achieving the best possible energy balance. Even though in most cases until today roofs have been used for the installation of solar panels as added devices, building integrated systems that replace building elements (like cladding material, shading surfaces etc.) have emerged, many of which deal with facade integration. The fact that most large scale, tall office buildings have a very small footprint compared to their facade surface, makes facade integration more appealing and efficient (compared to only flat roof installation) despite the fact that optimal angles of the active panels is more difficult to achieve. From the above it can be concluded that the most efficient use of the available facade surface area of a building could become one of the first goals of either new or retrofitted energy producing facade systems/modules. The fact that emerging PV technologies show a potential of significant price reduction by cheap fabrication methods and substrate materials, in combination with the need for maximum energy output for a given facade surface (in order to comply with the predefined energy neutrality demands), will probably lead to a new way of approaching facade integration of PV cells. Solar cell ouput per available facade surface will in such a case become increasingly important compared to the output per cost ratio that until now has been the ultimate efficiency indicator. The question that arises, as a result of the above assumption -based on the two tendencies of strong PV cell price decrease and the ever growing demand for a better energy balance of future buildings- is which structures,shapes or solutions could make use of the new PV technologies (and those that seem to prevail in the future) in order to achieve the maximum output for a given facade. Within this Thesis a first analysis of the existing technologies (and context, climate) will be attempted and used as a basis for the development of a solution for maximizing the energy output within a facade using the PV technology with the highest potential.

3_Solar Energy and Sun Angles

3.1 The Sun and the Solar spectrum

The electromagnetic spectrum of the sun radiation has its greatest intensity in the range of visible light (sunlight). The degree of absorption of the solar radiation by the atmosphere depends on the wavelength. The radiation reaching the earth’s surface depends substantially from the current weather and the sun’s position. Only almost half of the solar radiation power reaches the Earth on a clear day. The non-visible radiation near-infrared radiation is absorbed by about a quarter in the atmosphere, mostly by water molecules. The visible light range of 380-780 nm carries the most energy of the sun’s total irradiance spectrum reaching the earth surface. fig. 3.1 Solar spectrum graph. Definition of different wavelengths and light intensity (source http://isites.harvard.edu/fs/docs/icb. topic831443.files/WK14-Fundamentals.pdf)

The sun path The combination of the sun’s and the earth’s movement in space results in having every moment and for every position on earth (depending on the latitude, longitude and orientation) a changing angle of the sun rays relative to a line perpendicular to the earth’s surface (this is called the zenith or altitude angle (α) relating to the different solar heights and the sun’s position relative to the north-south axis, the azimuthal angle (θ).) Thus every day has its own solar orbit with the sun being at exactly 12 o’clock solar time (corresponding to a particular standard time depending on the longitudinal of each time zone) at its highest point. Detailed calculations can be based on respective formulas, like for instance one of defining the number of daylight hours (N) for a given latitude (φ) of a specific place on earth:

Where δ is the angle between the rays of the sun and the equator plane and can be calculated by the following formula where (n) is the n-th day of the year (e.g. 1 of February: n=32)

Specific software applications available enable the creation of sunpath graphs for a given region at the specified period. The software tool developed by the University of Oregon (source http://solardat.uoregon.edu/SunChartProgram.php) has been used to define the sunpath of different regions. For Delft (coordinates 52.00oN; 4.34oE) a chart is created giving solar elevation and solar time date for a period from June to December. fig. 3.2 Graph showing Solar Elevation (Altitude) and Azimuth in Delft for the period June 21-Dec 21.

(source: Author. Created with sorftware from source http://solardat.uoregon.edu/ SunChartProgram.php)

From the chart it is possible to extract in addition to the solar elevation angle α (y-axis) change, an estimation of the time for sunrise and sunset (for instance on October 21st: sunrise is around 7:30h and sunset at around 17:30h). Further tools allow more precise results. For instance Susdesign (source http://www.susdesign.com/sunangle/index.php) creates a more accurate result table for the above data (Delft). Nowadays sunpath calculating and simulating tools are integrated within (or cooperating with) BIM or 3d design software and enable efficient solar optimization during the design phase. fig. 3.3 Table showing detailed Data for a given location (Delft) and time (21st October, 15.00 pm).

(source: Author. Created with sorftware from source http://www.susdesign.com/ sunangle/index.php)

3.2 Solar irradiation (The Netherlands) The term “solar irradiation” or “solar radiation” is the amount of radiated solar energy per unit area in a given period. Yearly solar Irradiation is measured in kWh/m2*year. An overview of the direct normal (β=0) i.e. flat horizontal surface) solar irradiation for Europe and the Mediterranean area shows that southern Mediterranean countries reach values of approximately 2000 kWh/m2*year or more in some cases, while central Europe and northern countries only achieve about 1000 kWh/m2*year or less in the northern parts of the United Kingdom. The Netherlands have a yearly amount of solar irradiation close to 1000 kWh/m2*year. The statistical sun hours (average sun hours per year) in the Netherlands show a slight advantage for western seaside regions in terms of sun hours per year reaching approximately 1600 sun hours, while eastern regions only have about 1400 to 1500 hours in average. Specific year sun hours (in this case the example of year 2000) give more insight to regions with more or less solar irradiation as well as fluctuation within the months of a whole year. On the basis of the above sun hours and the sun relevant intensity the annual solar irradiation for the Netherlands can be seen in figures 3.4, 3.5.

fig. 3.4 Direct Normal Irradiation Annual (source http://solargis.info/ doc/71)

(Graphs source: http://www.knmi.nl/klimatologie/achtergrondinformatie/Zonnestraling_in_Nederland.pdf: and http://www.solar-drenthe.nl/informatie/zonne-energie-in-nederland/) fig. 3.5, 3.6 Sunhours in the Netherlands

fig. 3.6

fig. 3.7 Percentage of solar radiation per Month in the Netherlands compared to total Annual Radiation.

The total solar radiation on a horizontal plane in the Netherlands is, as mentioned before approximately 1000 kWh/ m2*year. Data shows that the coastal region receives about 10 percent more radiation than the eastern Netherlands. For example, the annual solar radiation on the horizontal plane in the city of De Bilt is 973 kWh/m2*year. On the basis of that the amount of solar energy received by the Netherlands each year is about 40 times as large as its total energy consumption. As a result of the fact that the weather is very often cloudy in the Netherlands, the irradiation consists approximately of up to 50 to 60% of diffuse light (indirect light scattered by the atmosphere) coming from all directions (this fact being important for the following design decisions in the present thesis). Time fluctuations of the radiation Due to variations in the yearly insolation yearly accumulated solar irradiation varies from year to year. Over a period of 10 years during the “best solar year” in De Bilt, the irradiance on the horizontal plane was about 15 percent higher, while during the “worst solar year” it was 15 percent lower than the long-term average of 973 kWh/ m2*year. The corresponding differences for the optimal orientation (south orientation at 180°, 36° inclination, see below as to these angles) are 18 percent above and 18 percent lower than the long term average of 1123 kWh/ m2) (source: Meteo Standard version 4). Furthermore, there are apparently differences between the irradiation values per month, whereas June is the best month on average for generating solar energy.

fig 3.8 Annual insolation on the horizontal plane (in kWh/m2*year) courves in the Netherlands (data of November 2012)

3.3 The orientation of FLAT solar panels The orientation of a solar collector panel with respect to the sun rays is apparently of paramount importance. In order to maximize the available solar radiation it is necessary to give the solar collector system a suitable orientation. This includes both the orientation in relation to the horizon (normally mentioned as an angle γ referring to the deviation from the south (whereas γ= 0 means oriented at 180o i.e. exactly to the south) and the inclination of the collector with in relation to the horizontal plane (angle β). It is apparent that the collectors should be directed in such a way to the sun so as to provide the best energy yield. Optimal positioning is given for each location on earth either by general charts or through calculation software, which gives more detailed results.

fig 3.9 Annual insolation percentage for varying Panel tilt and panel orientation. Point (Panel) at 45o Southwest and 30o tilt performing within 95% of maximum possible.

Graphs (example fig 3.9 from www.hollandsolar.nl) are able to show the PV panel’s energy performance as a percentage of the maximum possible yearly yield for a fixed panel at optimal orientation (that is in the Netherlands at an orientation to the south under angle of 36° with the horizontal plan) for various orientations and tilt angles. For instance a façade mounted collector (β=90o) facing south has an accumulated annual output lying at 70% of the optimum). It can be observed already that even quite substantial deviations from this orientation can still provide an acceptable yield. For instance at a slope between 15° and 57°, the yield is still within more than 95 % of those of the optimal slope of 36°. Of course the optimum 100% is calculated for a non moving mounted solar panel and not for the overall maximum possible performance including tracking and movable panels. In that case an efficient moving system would achieve higher output than the 100% of 36o. As a result of the above data derived from PV specific calculation tools/software it is possible for architects and engineers to conclude about the possible and best performing positions and mounting angles for PV panels within a given building structure at a specific location and date. (flat roof, slope roof, facades etc)

fig 3.10 Insolation percentage for varying Panel positions on building and orientation. (Source: http://www.vrtechniek.nl/elektrotechniek/Zonnepanelen.html)

Optimum Tilt of Solar Panels by Month for Den Haag Figures in degrees from vertical

(source :http://www.vrtechniek.nl/elektrotechniek/Zonnepanelen.html). Example of optimal tilt for The Hague (from www.solarelectricityhandbook.com).

fig 3.11 Optimal Panel Angles for den Haag (Source: Created by Author using software tool by www.solarelectricityhandbook.com)

3.4 Sun Tracking as a way to increase performance As the sun’s position changes during the day and also the daily sunpath itself within the year, the idea of directing the surface of any kind of collector in such a way towards the sun, so that the sun rays hit the PV surfaces at the optimum angle (mostly 90°) inspired people to create systems that would enable this motion, which is generally referred to as “tracking”. Following the sun’s movement is as old as the idea of using the sun’s energy. This is apparent from many techniques and examples used for concentrating sunrays, which is a basic and simple method to heat a focal point and create fire. Precise tracking of the sun is achieved through systems with single or dual axis tracking (structures that rotate around one or two axis). There are many sophisticated tracking systems (some of them called “Heliostats”) so as to control the position of the collectors based on the signals from relevant ilumination sensors and relevant software, but also passive solutions that use smart materials, bimetals, hydraulic pressure or other means in order to actuate and control the desired motion. The effect of tracking on the solar panel performance assuming that the system achieves the desired movement, largely depends on the type of tracking, the type of solar cells used and the location (climate and sunpath). Locations with high percentages of direct light conditions (sunny days) generally benefit more from precise tracking. Conventional silicon based solar cells also depend more on precise insolation angles while new technology thin film and especially organic solar cells are far less sensitive and also perform well in diffuse light conditions.

The energy yield of fixed (at 30o from horizontal) and one-axis (around the horizontal axis only) PV systems for each month are compared. The maximum energy yield occurs in July for both lines. The gain of such a tracking method over a fixed panel is 5.87%.

fig 3.12 Performance Graph example of horizontal axis (only) tracking compared to fixed panel at 30o tilt south Orientation.

The same comparison to a fixed module is made using one-axis tracking around the vertical axis only. Again the maximum energy yield occurs in July. The gain of such tracking over the fixed one is 20.12%.

Studies, calculations and experiments give more information on the question of what the exact gains are, that are possible with one axis horizontal, one axis vertical and two axis tracking. Examples of studies performed in places with high solar loads (Jordan in this case study) show the additional efficiency of a silicon based solar panel for the three different types of solar tracking.

fig 3.13 Performance Graph example of vertical axis (only) tracking compared to fixed panel at 30o tilt south orientation

The third test compares a fixed solar panel at 30o with a two-axis tracking system that is able to more precisely follow the sunpath. The gain of such tracking over the fixed one is 30.82%.

Dutch climate building integrated tracking evaluation. Within the thesis of S. Broersma (TU Delft, June 2008) a comparison was made with analytical calculations about the effect of tracking of PV collectors in general and when arranged on the façade of a building (in connection with sun shading blades) in a Location in the Netherlands. Under chapter 7.2 on page 85 a table depicts the difference in total annual energy gain (kWh/year) for the city of De Bilt and for four scenarios: a) PVs positioned directly on the façade (100% vertical surface angle α=90°) b) Fixed PVs at the optimal angle for Netherlands of 36° c) 1-axis tracking around the horizontal axis d) 2-axis tracking, namely around the horizontal axis and around the vertical axis for +/- 30°

fig 3.14 Performance Graph example of dual axis tracking compared to fixed panel at 30o tilt south orientation.

Even though practical experiments and theoretical calculations often show a high performance improvement potential with tracking for free standing or roof installed solar panels, facade installed or integrated solutions and systems in locations with smaller amounts of direct sunlight throughout the year, exhibit significantly smaller gains. Facade mounted systems, even those used on exactly south facing facades, are limited to much shorter range (much less than 180o azimuth) of vertical axis tracking compared to free standing systems. Apart from loosing a large range of possible tracking angles due to the building integration, these systems also suffer from self shading of the panels when rotating, which reduces the surface that can be covered by solar cells. Horizontal Axis tracking on the other hand in less influenced by the facade integration since the angle range of the sun altitude for example in the Netherlands is only between 0o and 62o (which is the maximum altitude value). However it also suffers from the self shading problem if the PV area is not reduced in order to have all panels entirely facing the sun. Two axis tracking on the other hand is extremely rare for facade structures and practically only used in concentrated PV projects that need precise tracking. Again the azimuth tracking range is strongly reduced, leading also to an overall performance gain reduction. The added cost of any of the single axis tracking solutions is further raised by two axis solutions which also demand more space on the facade and need to solve even bigger self shading threats. 25

According to these results for PV cells arranged on a façade only a 1-axis tracking pattern could make sense, if any, since the energy gains for the much more complicated 2-axis tracking are minimal (this result will be considered in the later phase of the present thesis). Further calculations show that the restricted angle a façade element can track with a vetrical rotation axis (as mentioned before theoretically 180o but in practice much less due to self shading and space restrictions in facades) and vertically (theoretically 90o but only about 45o needed according to Dutch altitude through the year) combined with architectural and space related restrictions, makes tracking a very complicated feature if it is not combined with a high performance PV or STC device that is very sensitive to precise angles, and in a location with large amounts of direct sunlight. Research on tracking systems on facades and relevant existing research papers to a large extend oriented the development of this thesis towards less complex non tracking solutions and to systems that can improve the performance of a flat panel through shape.

fig 3.15, 3.16 Tables Performance Tables comparing fixed, horizontal axis and dual axis tracking on facade. Source: TUDelft Thesis Repository, Graduation Thesis 2008 S.Broersma â&#x20AC;&#x153;De PV-wirefree zonweringâ&#x20AC;?

Tracking type

Horizontal axis tracking

Vertical axis tracking

Tilted axis tracking

Dual axis tracking

Tracking Layout

Tracking Characteristics Tracks Sun Altitude changes within day and during seasons

Advantages -Relatively simple/cheap (simple side supports) -Can be manually adjusted for only seasonal tracking -Good for south facades in facade integration

Disadvantages -Does not efficiently track daily -Azimuth (east-west). -Prone to partial shading in low Latitudes (high sun) in facade integration. -Makes less sense in high Latitudes (low sun). -Does not efficiently track Sun Altitude change -Prone to partial shading in low Latitudes (high sun) in facade integration. -Needs constant automatic tracking during the day (running cost)

Tracks Sun Azimuth changes within day and during seasons.

-Relatively simple/cheap (top-bottom supports) -Good for south or east/ west facades

Tracks Sun Azimuth and Altitude changes within day and during seasons.

-Good (similar) tracking characteristics for much lower cost compared to dual axis.

-Difficult to integrate in Facades. -Needs constant automatic tracking during the day (running cost)

Tracks Sun Azimuth and Altitude changes within day and during seasons.

-Very high performance potential especially with concentrated PV. -Limited space requirements per output if combined with concentrated

-Complicated structure -Expensive. -Difficult for facade Inte gration. -Cost only justified for concentrated PV and high direct light values.

Either central hinge connection or 4 side control

fig 3.17 Table Comparing tracking systems (source: Author)

4_Solar Cells

Introduction The term “PV cells” is often used for describing solar cells, as an abbreviation of the word “photovoltaic” referring to the underlying phenomenon of converting light (“φως” -phos in greek) into electricity, the measurement unit of which is the volt (V). The idea of creating energy in a process that is friendly for the environment, silent, without moving parts and with a minimum demand for maintenance and running costs has fascinated people for many years and has lead to a fast development of the sector through research and testing by scientists and a huge interest by the industry and governments for supporting further development and applications in order to be able to compete with other more environmentally harmful and non renewable energy sources in the near future. Solar cells converting solar radiation (both direct and diffused) into electricity have for many years been mainly based on silicon. Recently new materials have been used and are of interest for the potential they provide for lower cost production, flexibility and (semi-)transparency. With their exposure to sunlight solar cells absorb most of the visible spectrum beginning from less than 400nm and up to near infra-red up to 1100nm generating thereby direct current (DC) which is either used directly or converted to alternating current (AC) using inverters. It can also be stored in batteries for use during the night or during cloudy periods when the PV systems cannot produce enough electricity. Inverters are shut down automatically upon loss of grid supply for safety reasons. Grid connected systems supply electricity for use in the building and when demand from building loads is higher than maximum PV supply, all the PV output is used (grid consumption and utility bill reduced). When there is low building demand, electricity is exported to the grid. The efficiency of solar cells, apart from their type (material and structure) and shape, also depends on the incident angle of the sun rays and the module’s temperature, with increasing temperatures leading to a slight or strong decrease depending on the solar cell type. Common PVs available on the market (or developed enough to be brought to the market) are mono-crystalline silicon, polycrystalline silicon, thin film and third generation organic PVs cells. Within this research the focus point is mainly on the materials and cell types that enable free custom shapes, flexibility and semi-transparency. In conventional standard applications solar cells are arranged in bigger DC electrical units called modules or panels produced in standard or custom sizes, mainly with a rectangular shape. Groups of modules are referred to as arrays. In order to perform better without additional light reflection losses or temperatures they are meant for outdoor use and need to withstand the elements. Therefore solar cells need to be additionally encapsulated in a transparent sheeting material.

The Photovoltaic Phenomenon Conversion of sunlight to electricity takes place in a solar cell based on silicon in case of conventional (first generation) cells, or other compound materials in case of thin film cells and on polymers in case of organic third generation PV cells. When light (photons) strikes the cell, a portion is absorbed within the semiconductor and knocks electrons loose, enabling electrical charge to flow freely within the material. The PV cells have built in electric field that acts to force electrons to flow in a certain direction. This field is created by “doping” (controlled introduction of impurities) within pure silicon (when silicon based) which has 4 outer shell electrons, with elements like Phosphorus which has 5 outer shell electrons (or Boron) to create N-type (negative) and P-type (positive) zones.

fig 4.1 Doping of silicon using Phoshorus and Boron. (source:http://www.evoenergy.co.uk/wp-content/uploads/2010/07/ photovoltaic-cells-doping-semiconductors2.jpg)

By placing metal contacts on top and bottom of the PV cell, the generated current can be used by passing through an external circuit.

fig 4.2 Cross section of conventional solar cell. source: http://solarpanelhisar.com/wp-content/uploads/2013/05/solar_cell.png

4.1 Monocrystalline solar cells (Production processes) ďżź Monocrystalline cells are usually manufactured from a single crystal ingot of high purity most commonly grown by the Czochralski method (crucible drawing process) with diameters usually of around 12.5-15cm but possible to reach even 30cm. This homogeneous single crystal structure enables high potential efficiencies. In this process a cylindrical silicon rod with a length that can reach several meters is simultaneously slowly rotated and pulled out of a silicon melt with a temperature just above its melting point and an additional dopant in it. The rotation produces the crystals cylindrical shape. The produced Ingot is first trimmed in order to become a rectangular longitudinal block with a side length of around 10cm to 15cm. In this procedure material is lost in favor of a shape that enables closer/denser positioning of the cells in a panel, even though in some cases cell shapes remain circular or semi-circular and material can be melted and used again (not the created silicon dust).

fig. 4.3 Czochralski process (source http://cnx.org/content/ m40280/latest/?collection=col10719/latest)

The resulting blocks are sliced/sawn into thin (about 0,2 mm) slices/wafers with a wire saw, during which procedure a significant amount of silicon dust is lost. The dust is afterwards cleaned/removed with chemicals. In a next stage high temperature phosphorus diffusion creates the needed thin n-doping layer to finally acquire the p-n junction characteristics of a solar cell. After that and in order to increase the amount of absorbed light into the cell, (thus higher currents) a thin ANTI-REFLECTIVE Coating, like Silicon Nitride or Titanium Oxide, is applied by vapor deposition. The specific characteristics of monocrystalline solar cells in terms of the wave length which they better absorb (long wave in this case) imposes the thickness of the coating (about 70nm) and causes the dark blue appearance seen on these cells. By leaving out the anti-reflective coating and showing the cells natural gray color, reflection losses increase from 3% to 30%, which is not acceptable for the vast majority of applications. Other reflective coatings can give a range of other colors (custom made), but efficiency is slightly reduced depending on the color.

fig. 4.4 SIlicon Brick slicing (source http://pveducation.org/ node/496)

fig. 4.5 Monocrystalline Final Cell (source http://image.ec21. com/image/cetcsolar/pre_ GC03520316/Monocrystalline_Solar_Cell_3-busbar.jpg)

Another method from making monocrystalline silicon is edge defined film-fed growth (EFG) (fig.4.6) and string ribbon process. In this case wafers are grown in the right thickness from the beginning and slicing (material losses) can be avoided. The silicon ribbon is pulled out of a graphite die placed again in a silicon melt. Carefully adjusting the temperature profile of the graphite die causes the silicon to crystallize with large grains. For the production of the final cell a metallisation grid consisting of thin â&#x20AC;&#x153;fingersâ&#x20AC;? that conduct current to central collectors or bus bars, is screen printed. This metallization pattern on the sunlit face of the cell is a compromise between shadow losses and resistance losses that reduce electrical output. Back contact can be applied to more surfaces. Full surface aluminum coating is printed and processed to help the cell efficiency. When cells are fabricated into glass-glass laminates a shiny back face is visible from inside.

fig. 4.6 Edge defined film-fed growth (source http://pveducation.org/pvcdrom/manufacturing/other-wafering-techniques)

As a final cell monocrystalline silicon cells have a relatively high efficiency of about 12-18% and a dark color, either black or blue and sometimes gray. Their performance is significantly reduced in diffuse light conditions. The part of the spectrum they are more sensitive to lies in the long wavelengths.

fig. 4.7 Monoctystalline Production Processes (source: Detail Photovoltaics)

Monocrystalline

Trimming

Sawing into wafers

Phosphorous diffusion

Screen printing Contacts

Solar Cell

4.2 Polycrystalline solar cells (Production Process) Polycrystalline (or Multicrystalline) silicon cells are compared to monocrystalline easier to produce with simple techniques and therefore usually cheaper than monocrystalline PVs. However compared to the homogeneous and pure material structure of the latter, polycrystalline cells show grain boundaries -frost like structures- that act as material imperfections and reduce the overall performance -which lies between 11.5-15% , by slightly blocking specific carrier flows. The production process of this solar cell type involves casting of the molten silicon into a cuboid form/ingot (fig 4.9). As the silicon solidifies under precisely controlled temperature, large crystals with different directions are formed with grain sizes from a few mm to a few cm. To avoid significant losses a minimum of grain sizes is required. Often bigger cells are used in practice compared to monocrystalline cells in order to further reduce cost. Their color in their final form is usually medium or dark blue with characteristic frost like crystal patterns. The color as for all solar cells is however a result of the anti-reflective coating which is used on top of solar cells in order to reduce reflections and absorb more solar energy. The thickness of this layer depends on the wavelength in which each cell type performs better, which in turn is responsible for the color. Before the coating polycrystalline cells have a gray color (fig 4.10).

fig. 4.8 Polycrystalline Silicon Casting (source http://www.algor.com/news_pub/cust_app/SPI_minicaster/first_casting_L.gif)

fig. 4.9 Polycrystalline Silicon block/cuboid (source http://www.ise.fraunhofer.de/bilder/presseinformationen/2011/11-pi-19-de-siliciumblock)

fig. 4.10 Polyctystalline Final Cell with and without coating stage (source http://www.tf.uni-kiel.de/matwis/amat/semi_en/kap_3/illustr/ poly_si_solar_cell.gif)

Grain boundaries in polycrystalline cells are a result of rotation, parallel movement and dislocation of crystals within the silicon material, which are otherwise identical but not organized in a uniform geometry. The effect of diffuse light and temperature increase on Monocrystalline and Polycrystalline PV cells performance is relatively similiar. In terms of temperature increase there is a slight disadvantage for polycrystalline cells which show a Performance drop of approximately 0,5% per Degree Celcious compared to about 0,4% for Monocrystalline cells. All solar cell types show a performance decrease for higher temperatures however crystalline silicon cells are more sensitive. fig. 4.11 Grain Boundaries responsible for lower performance (source http://pveducation.org/pvcdrom/manufacturing/ multi-crystalline-silicon)

fig. 4.12 Polycrystalline Production Processes (source: Detail Photovoltaics)

Polycrystalline

Cutting Blocks

Sawing into wafers

Phosphorous diffusion

Screen printing Contacts

Solar Cell

4.3 Inorganic Thin film solar cells

Inorganic Thin film cells Inorganic Thin film cells are considered a second generation solar cell technology and have currently a market share of only 15-20% which shows however a growing tendency. They are produced by applying several extremely thin layers (in the range of a few micrometers for the light absorbing layers) of photovoltaic material and their contacts on an inexpensive substrate or superstrate. The connections between the solar cells (made usually of Ni or Al) and all other additional elements needed for the cells operation are part of its production/ fabrication process and therefore the complete module is practically finished simultaneously. This process is also one reason for their cost reduction potential, together with the extremely limited amount of material required compared to other technologies, given that their thickness is about a hundred times smaller that of a conventional crystalline cell. The photoactive semi-conductor material used in thin film cells is either silicon based or of other compound materials. Silicon based cells are made of Amorphous Silicon (a-Si) ,which is the most commonly used technology, Microcrystalline Silicon (μc-Si) and their combination called “Micromorphous” Silicon. Non Silicon thin film solar cells include semi-conductors made of Cadmium telluride (CdTe) and a combination of Copper (Cu), Indium (In), Selenium (Se) or Sulphur (S) or Gallium (Ga) producing a material system called (CIS) or (CIGS). In terms of performance and production cost (CIS) / (CIGS) cells achieve the highest efficiency and show the highest potential for further gains, while Cadmium Telluride (CdTe) has the potential of very low production cost. Deposition of the different layers of thin film cells takes place at temperatures significantly lower than those required for crystalline silicon wafer cells, reaching only 200oC to 600oC, while crystalline cells are manufactured at almost 1500oC. The low energy consumption and related more environmentally friendly process, as well as the capability for high automation and speed in production, thus reducing the cost significantly, are additional advantages that need to be taken into consideration. 33

fig. 4.13 Typical structure of Thin film cell layers in a Cadmium Telluride cell example (source http://www.homepower.com/articles/solar-electricity/ equipment-products/peek-inside-pv)

fig. 4.14 Thin film module example (source http://materia.nl/article/breakthrough-solar-cell-material/breakthrough-solar-cell-material-7/)

Inorganic Thin film cells Silicon based Amorphous Micromorphous Microcrystalline

Compound Semiconductors Cadmium telluride (CdTe) Copper-indium-gallium diselenide (CIGS) Copper-indium-disulphide (CIS)

fig. 4.15 Basic types of inorganic thin film cells (source: Diagram Author, from Detail Photovoltaics)

4.3.1 Layering layout and production. Depending on the semi-conductor material type used and on the specific application different layouts/arrangements, coating procedures and sub/superstrate materials are possible. The back contact for the thin film cell/module is usually a simple metal layer, while the front contact is usually a very transparent low reflection (through special surfacetreatment) conductive Oxide (TCO). The photoactive layers and contacts are deposited/coated either first on the sun facing front glass surface (or other material) with the front contact being the first layer coated, or starting with the back face material.

fig. 4.16 Layering of typical thin film structure (basic layers) (source http://www.biosolar.com/images/biosolar_backsheet_img.jpg)

In order to achieve flexible, lightweight and non glass solutions based on thin film cells the layers must start being first deposited on a backing surface material, which can be a polymer or metal foil in order to achieve the desired characteristics. Technologies for the production/coating on back/front surface, of amorphous (a-Si) as well as microcrystalline (μc-SI) Silicon include PECVD processes (plasma enhanced chemical vapor deposition) (fig.4.17) as a result of the decomposition of silane (SiH4) and hydrogen. Because of the low temperatures used during the process, low-cost substrates can be used, but also metal, plastic films and glass. The relatively low deposition speed of 0.5 to 2 nm/s further limits the demand of energy during the industrial production. The transparent and conductive front contact is here usually a doped metal oxide such as Tin or zinc oxide. Other common coating methods used are simple vapour deposition, Sputtering and Electroplating Baths.

fig. 4.17 Plasma enhanced chemical vapor deposition Process (source http://materia.nl/article/breakthrough-solar-cell-material/breakthrough-solar-cell-material-7/)

4.3.2 Amorphous Silicon Solar Cells A-Si solar cells are as mentioned before the most common type of thin film solar cells. Amorphous, as a term, refers to the irregular arrangement of silicon atoms created after the silicon vapor deposition procedure compared to the crystalline arrangement of conventional silicon solar cells. The reciprocal action between photons and silicon atoms occurs more frequently in amorphous silicon than in crystal silicon. This allows much more light to be absorbed by the material. Thus, an ultra-thin amorphous silicon film of less than 1μm is possible to be used for power generation. On the other hand the large amount of electronic imperfections in the silicon structure does not allow for comparable efficiency, which in this case reaches only about 5-10% (real world efficiency, with lab values reaching almost 15%).

fig. 4.18 Single junction amormpous Silicon cell (source: http://panasonic.net/energy/amorton/en/solar_battery/)

Apart from single junction a-Si cells also multijunction (with 2 or 3 junctions) are produced for additional performance since every layer absorbs different wavelengths. In most cases A-Si cell layers are first coated on the front sun facing transparent glass layer. As a result of the specific wavelengths in which Amorphous silicon cells operate best, their color is red or brown/red. 4.3.3 Micro/Nanocrystalline and Micromorphous Solar Cells Microcrystalline silicon consists of crystalline silicon grains embedded in an amorphous matrix. Compared to amorphous silicon it has reduced light-induced degradation over time and a good absorption coefficient in the red and near infrared light spectrum. This means that due to the absorption coefficient in this range being lower than in the visible range, the needed thickness of the microcrystalline silicon layer is bigger. Efficient light trapping through textured surfaces is need ed in order to achieve good performance from thin layers of this type. The production method of this material involves relatively high deposition temperatures. Due to the inherent disadvantages of microcrystalline silicon it is often combined with a layer of amorphous silicon above it (fig 4.20). This way amorphous silicon absorbs the smaller wavelengths close to the visible spectrum, while the next microcrystalline layer makes use of the red and infrared wavelengths. Thus micromporphous solar cells achieve efficiencies twice as high as amorphous silicon. Their color is usually dark gray to black. 35

fig. 4.19 Multijunction thin film cell with 2 pin junctions for additional performance (source: http://phy.syr.edu/~schiff/Publications/DengHandbookPV03. pdf)

fig. 4.20 Microcrystalline thin film cell with additional amorphous layer (source: http://www.nexpw.com/images/tech_16.jpg)

4.3.4 Cadmium telluride solar cells CdTe as a material is a direct band gap semi-conductor with a high optical absorption coefficient positioned broadly in the visible portion of the light spectrum. Furthermore, the high optical absorption coefficient allows an absorption of almost 99% of the available photons within only a 0.5 μm film. The materials good chemical stability allows solar cells to be fabricated using a wide range of different film coating methods, like close-space sublimation, vapor transport, vapor deposition, sputtering, and electro-deposition. CdTe films are polycrystalline, with small grain sizes of 0.1 to 10 μm. Within the thin film cell the CdTe semi conductor material forms the positive-conducting element, while a thin layer of cadmium sulphide (CdS) plays the role of the negative-conducting materials to form the desired photoreactive p-n junction.

fig. 4.21 CdTe thin film cell layers (source: “Flexible Solar Cells”Pagliaro M., Palmisano G., and Ciriminna R.)

The color of CdTe solar cells is usually a homogeneous dark brown or black. Efficiency is relatively high for a thin film solar cell reaching values of about 6-11% in real world module performance and up to 16,5% in laboratory conditions. 4.3.5 CIS solar cells As with other well performing thin film semi-conductor materials, the material combination of Copper (Cu), Indium (In), Selenium (Se) and in some cases Gallium (Ga) shows a very high optical absorption coefficient which however is positioned in variable parts of the solar spectrum. This enables the use of the material as a very thin foil of 1,5-2,5 μm. The most common layout of a CIS cell includes a the deposition of the photo-active layer directly onto a molybdenum coated glass or steel surface. The most common vacuum-based process is co-evaporation or co-sputter of the three materials onto the substrate and then annealing the resulting film with a selenide vapor to form the final CI(G)S structure. The P-N junction within the film is achieved between the semiconductors CI(G)S and a Zinc oxide (ZnO) layer constituting the negative conductor. Even though the vacuum based (and selenide vapor based) processes lead to efficiencies of about 14% for the final cell, the high temperatures (reaching more than 500o) and cost have resulted in testing of other methods using sulphur. These exhibit lower efficiencies of around 8%.

fig. 4.22 CIGS thin film cell layers (source: “Flexible Solar Cells”Pagliaro M., Palmisano G., and Ciriminna R.)

fig. 4.23 Co-evaporation of CIGS materials (source: http://www.solarnenergy.com/contents_img/777160.jpg)

4.4 Organic Thin film Solar cells

Organic Solar Cells are cells based on organic photo active layers and form the newest “third generation” solar cell type. They were introduced in 1986 but only recently became very popular due to their extremely high potential of very cheap production, roll to roll printing on cheap flexible substrates and even the ability to create photovoltaic paint. Other important advantages are the ability to create semitransparent, colored flexible modules and their surprisingly high diffuse light performance. The main types of organic cells are Dye Sensitized Solar Cells (DSSC), polymer based solar cells and so called small molecule solar cells (SM cells). 4.4.1 Dye Sensitized Solar Cells (DSSC)

The principle of operation of the cell is similar to the action of chlorophyll in the process of photosynthesis. In the Grätzel-cell, the electrons of a very thin dye layer are excited by the incident sunlight and then flow through a semiconductor layer of titanium dioxide in the attached glass conductor layer. The dye itself compensates this charge deficit with electrons from an overlying layer of iodine solution. The titanium oxide is actually a white paint which is often used for adding mass in a cheap way in current toothpastes, and proves even in permanent irradiation to be very durable. fig. 4.24 Dye Sensitized solar cell structure (source: http://www.energyharvestingjournal.com/images/v5/articles/820x615/main4707.jpg)

Dye Sensitized solar cells are also known as Dye Solar Cell (DSC) or under the name of their inventor (1991), the German chemist Michael Grätzel, as Grätzel-cells. They are not considered fully organic solar cells since they retain inorganic elements. These thin film photo-electrochemical cells based on a semi-conductor formed between a photo-sensitized anode and an electrolyte, work with non-toxic titanium dioxide and have an efficiency of between 8% and 12% while being extremely cheap to produce using conventional roll to roll printing techniques. A special feature is their sensitivity to diffuse light: Even under cloudy, diffused light conditions they are able to achieve efficiencies up to 10%13%, higher that all other types of solar cells under such conditions.

fig. 4.25 Dye Sensitized solar cell structure (source: http://technologyvoice.com/sites/default/files/DSSC.jpg)

4.4.2 Polymer solar cells Polymer solar cells are as the name indicates solar cells based on polymers. Structurally, as most organic cells, they include an electron donor and electron acceptor layer material, instead of semiconductor p-n junctions found in inorganic cells deposited between a top and bottom electrode and in between electron blocking layers. Polymer cells are usually processed by spin-coating or ink-jet printing. The range of possible polymer materials is practically unlimited in contrast to the more accurately defined material requirements and selection range found in other technologies. This also enables an adjustment of specific properties as for example the spectrum range under which they operate best.

fig. 4.26 Polymer solar cell (Konarka Power Plastic) (source: http://www.semi.org/en/sites/semi.org/files/docs/Konarka_McCauley_NEF0911.pdf)

In terms of disadvantages compared to crystalline silicon technologies they are less chemically stable/ mature, have a limited lifetime, significantly lower efficiency and suffer from photochemical degradation. fig. 4.13 Polymer cells production (roll to roll) (source: VISIONS â&#x20AC;&#x201D; CTRL+P Australiaâ&#x20AC;&#x2122;s Largest Solar Cells) fig. 4.27

fig. 4.27, 4.28 Layers of polymer cell module (source: http://www.semi.org/en/sites/semi.org/files/docs/Konarka_McCauley_NEF0911.pdf)

4.5 Advantages of thin film solar cells over crystalline cells. The main advantages of thin film technology over conventional first generation solar cells can be categorized in characteristics related to: 1. Performance 2. Cost (production cost, installed module cost and run ning/maintenance cost) 3. Environmental impact, embodied energy 4. Design freedom and Integration potential Performance related advantages of thin film solar cells must be seen within the scope of their application in buildings. It is true that for non integrated applications of solar cells where maximum efficiency is the major priority and less restrictive factors exist, thin film technology is not the first choice. It is only when seen in an integrated system with more restrictions, that thin film solar cells start showing their advantages in performance. In contrast to optimally designed free standing solar plants, other systems/structures are less capable of providing optimal performance conditions. Buildings, everyday products or small scale structures are an example of this use, since they are unable to ensure perfect insolation angles, cooling of the solar cells, and absence of partly shaded modules. In these fields thin film technology (and organic third generation solar cells even more) shows significant advantages since it is much less sensitive to shaded parts and to sun ray angles diverging a lot from the perfect 90o. At the same time they show a much less temperature dependent performance curve, making them perform much better when modules are poorly cooled or ventilated or when the external temperatures are very high. The same advantages are visible in regions with low direct sunlight percentages throughout the year which is usually the case for countries in Central and Northern Europe. Diffuse and low light conditions during cloudy days comprise one of the strong points of thin film (and organic) solar cell technology. Additionally reports indicate that Energy pay back time for thin film solar systems is about half the time for crystalline cell based systems.

Low production, transportation and installation cost are also important advantages as a result of the highly reduced amount of material used and of the low overall weight. As already described complete module fabrication in a practically one step automated process, through deposition of all layers including the photoactive material, connectors, substrate and front coatings, significantly reduces the cost. More gains are a result of less material use, and lower fabrication temperatures. Additionally the need for extreme amounts of silicon and silicon based materials leads to a shortage and price rise or at least to a stabilization of its price compared to the decrease in other applied materials. In terms of the environmental impact especially high performance triple junction thin film solar cells which are too expensive and high tech to be included within the building integration options form a threat since they have toxic semi conductor materials in some cases. The toxic heavy metal cadmium (CdSe, CdS, CdTe) is also a concern when considering thin film technologies. On the other hand CIS cells for example use indium, a metal in relatively short supply. Otherwise the embodied energy of thin film solar cells is lower than that of crystalline cells leading to a much shorter payback time even with lower efficiencies. Supportive structures construction, framing, panel assembly, transport and all other processes before the final installation account for a large energy consumption that can be reduced by thin film technology. The ability to easily produce different shapes and sizes and a smooth color without predefined patterns as well as a more homogeneous transparency effect creates much better integration possibilities. At the same time flexibility, a thin profile and extremely low weight are very useful properties of elements for building and facade integration more specifically. The ability of creating three dimensional optimized shapes and of using different flexible and lightweight materials as a basis can to a large extend outweigh the efficiency disadvantages as absolute values.

4.6 Organic solar cells compared to crystalline and other thin film cells Fully organic photovoltaics (OPV) based on polymers have an efficiency that theoretically does not allow them to compete against other mature crystalline or thin film solutions, especially taking into account their currently short lifetimes. Maximum efficiency of single junction modules is about 14%. However the non consistent light spectrum absorption curves responsible for the low efficiencies of these cells are expected to be significantly improved with tandem (multiple layer) cells absorbing a larger range of wavelengths and reaching efficiencies of around 20%. The main advantage of organic cells, however is not the possible maximum efficiency but their very high potential for cost reduction, their flexibility and their performance under diffuse and limited light conditions even compared to second generation thin film technologies.

fig. 4.29 Polymer solar module (source: http://www.greenoptimistic.com/wp-content/ uploads/2013/05/080206154631-large.jpg)

Cost reduction during the fabrication is linked to the cheap roll to roll production process that enables vast amounts of photovoltaic surfaces to be printed with a low cost and within limited time. This means that due to the maturity of the process itself and the low amount of easily available and cheap material, upscaling and providing increasingly large amounts of organic solar cells within a few years is possible and much less problematic than with other technologies. Furthermore organic cells as one of the thin film cell categories shares many of the aforementioned advantages. Cost has an even higher potential to decrease than inorganic thin film cells while the integration potential remains the same or is even better due to smooth semi transparency and different color options.

fig. 4.30 Inorganic Thin film solar module (source: http://buildaroo.com/wp-content/uploads/2011/01/ f3203b7085611820612.jpg)

It can be concluded that organic solar cells are a technology with the highest potential compared to all other solar cells, but aspects like durability and maximum performance still suffer and need further improvement.

4.7 Solar cells and Transparency The different degrees of transparency that can be achieved with varying PV cell arrangement when used with glassglass laminates give a wide range of possibilities to the architect. Positioning of the opaque crystalline cells with smaller or bigger transparent gaps between them is the technique that is mostly used, since real semi-transparent cells are at the moment not popular as market products and not as efficient. Existing examples are usually thin film solar cells or dye sensitized solar cells with the latter option providing a true homogeneous semi-transparent surface. By the aforementioned arrangements of cells the performance is roughly reduced by the percentage of the transparent area within a panel compared to the theoretically fully covered surface.

In a larger scale semi transparency is possible by using relatively small opaque solar cell stripes or other module shapes and leaving smaller or bigger gaps between them, thus creating a specific pattern that reveals the transparent or translucent substrate material. Performance, cost calculation and quality of the view to the outside (but also aesthetical criteria) will define whether a solution with a large amount of cheaper opaque units placed with a certain distance from each other is preferable compared to a truly semitransparent lower performing expensive module. In the future Organic solar cells may become an answer to this by producing both perfectly smooth semi transparency with high transparency percentages, better performance compared to other equally transparent technologies and the significant price advantage deriving from the simple production methods.

For crystalline cells the space between them that can create a semi transparent system can vary between 1mm and about 30mm. Covering crystalline cells with small perforations (created by laser or by groves on both sides of the cell) is another technique that can create a softer,more balanced transparency effect. Thin film cells provide slightly more options for fine and neutral transparency without easy distinguishable opaque and transparent parts. Longitudinal stripes of varying sizes can remain transparent (through the use of laser machines to remove material) thus creating strip like effects, but also fine checkered motives by removing material in both directions are possible. These techniques make it possible to have practically smooth semi-transparency. Organic polymer based solar cells provide the biggest potential for semitransparent and colored solar surfaces. Designs with a smooth transparency of about 40% or less already show relatively high efficiencies reaching value of 6-7% or more. fig. 4.31 Organic Solar cells. Cells with the highest potential for smooth uniform transparency and color ( source: http://www.belectric.com/ typo3temp/pics/5a30e5b9d2.jpg)

fig. 4.32 Crystalline solar cell transparency through spacing (source: http://www.osps.eu/ information.php?category_id=3&information_ id=32)

fig. 4.33 Crystalline solar cell transparency created by pattern and spacing variations (source: Author)

fig. 4.34 Fine checkered Thin Film transparency pattern (source: http://www.greensolar.hu/ content/why-thin-film)

fig. 4.35 Thin Film solar cell transparency created by spacing of cell strips and material removal in both directions (source: Author)

4.8 Encapsulation medium and back face In order to protect solar cells from damages due to the elements or mechanical damages an encapsulation medium is required. The most common material used is Ethylene Vinyl Acetate (EVA) or Polyvinyl Butyral (PVB). Cells are laminated between the foils in vacuum in a specified temperature that ensures that the material melts and completely covers the solar cells. The surface of the module exposed to the sun needs an additional anti-reflective covering since the aforementioned encapsulation materials are not UV resistant. A low iron toughened highly transmissive glass pane is in this case usually the chosen option when dealing with rigid modules. The material used must also withstand high internal temperatures and therefore stresses caused for example by possible partial shading. The backing surface in standard modules is usually a thin opaque film like Tedlar (PVF- Polyvinyl fluoride), polyethylene terephthalate (PET) or a metal surface, ensuring the electrical insulation and protection of the module. These materials are usually colored white in order to reduce the temperatures and achieve higher efficiency. Alternatively glass backing can be used for transparency between cells (glass-glass laminate).

Usually standard encapsulation is again lamination with Ethylene Vinyl Acetate (EVA). The back of the module can be finished with Tedlar or a metal film. Raw thin film modules of amorphous silicon and Cadmium Telluride (CdTe) are coated onto a superstrate which forms the front glass through which light enters. It is not possible to use tempered glass for these superstrate sheets as the high temperature used for the semiconductor coating would destroy the glass strengthening. If thin film module needs to be structurally strong (e.g. for a faรงade application), it must be laminated with a sheet of toughened safety glass. In the back any kind of glass or other materials can be used. Copper Indium Diselenide (CIS) and amorphous silicon coated on a substrate usually need a front glass, which needs to be low-iron white glass for high transparency.

New module concepts are developed proposing cells without any encapsulation medium in direct contact with the PV-cells. Cells are installed inside a cavity of a double glazing unit in an inert gas atmosphere where electrical interconnection can be achieved by a press contact. In case of thin film solar cells the connections between the cells arranged in series are embedded within the thin layers during the production process. However encapsulation is still required.

fig. 4.36 EVA Encapsuation between glass, TEDLAR backsheet (source: http://img.alibaba.com/img/pb/267/077/499/499077267_507.jpg

fig. 4.37 EVA crystalline cell Encapsuation, backsheet and glass front sheet (http://www.tjskl.org.cn/products-search/pz531bed7-cz5090b39-030mm-0-80mm-thickness-laminated-roll-solar-cell-eva-film-for-pv-modules-encapsulation.html)

4.9 Flexible and/or curved Solar Cell Structures Curved designs In the vast majority of PV applications until now flat elements are used especially when considering ground mounted high performance systems or building related panels. In these cases the cost efficiency and simplicity of the flat design dominates over other design related preferences or over the idea of lightweight and thin structures. However during the last years researches have also focused on curved solutions mainly stimulated by the integration potential in special applications like the car industry, electronic devices or other similar technological fields that do not benefit from flat design and where small curvature radius is needed. In cases of very large radius curvature, designs have until now usually been based on flat units, which through faceting created the needed curve. PV products with a curved shape have emerged and showed a high potential even for building related applications, some of which can already be seen. The majority of examples until today can be seen on roofs that have a large sun facing surface which benefits structurally (or due to design decision) from the curved shape in order to cover large spans or to save material and weight for shorter spans. The low weight demand often leads to thin film solutions, although there are also larger curve radii crystalline cells possible.

fig. 4.38 Roof integrated Curved thin film panels (source: http://img.archiexpo.com/images_ae/photo-g/roof-fixing-systems-pv-installation-60396-1759371.jpg)

fig. 4.39 Car roof integrated Curved solar modules. Car industry stimulating curved design (source: http://www. aboutmyplanet.com/wp-content/gallery/solar-powered-cars/pininfarina-bollore-b0-electric3.jpg)

Crystalline and thin film cells in curved and flexible Designs In the case of crystalline silicon cells curved modules are either created by integrating the crystalline cells in curved glass panes or by curving the finished product, which two options include the processes of hot or cold bending. The possible radius is however limited. Hot bending is a process that due to high temperature is mainly possible with very resistant cells. Cells fabricated using low temperature, as thin film solar cells or organic cells would get irreversibly damaged. Most crystalline cell designs are usually in the form of acrylic panels where slightly curved polycarbonate PC (or other material) instead of hard glass is used for lamination. The reason is mainly to avoid the issues related to glass bending and the high total cost. Minimal cold bending radius for this type of acrylic modules with 10X10cm cell arrays is 350 times the thickness of the strongest acrylic plastic sheet. (building integrated PV-Handbook) . Crystalline wafer cells themselves are not suited for excessive bending as they have brittle material properties/characteristics. The minimum radius fabricated from crystalline PV cells is about 0.9m. Their mechanical strength and bending ability also decreases strongly with a reduction of their thickness. Brittle material characteristics also largely reduce crystalline cells flexibility. As a result crystalline cell applications integrated in curved designs are extremely rare and very expensive compared to flat solutions. Bigger curvatures are usually achieved with faceting (using smaller flat surfaces tangential to the desired curve) and curved designs are mainly low radius plastic/acrylic sheet applications. Due to the deposition process used with thin film solar cells they can not be laminated from the beginning on a curved surface. Practically the use of curved glass is extremely difficult even in a later stage since bending the thin film coated glass pane would damage the cells.

fig. 4.40 Semi flexible (large radius possible only) Crystalline cells (source: http://www.greenenergynorthwales.com)

fig. 4.41 Thin film cells on curved roofs (source: http://www. solar-constructions.com/flexibel%20zonnepaneel.jpg)

fig. 4.42 Crystalline cells integrated mainly in curved polycarbonate sheets (source: http://www.solar-constructions.com/flexibel%20zonnepaneel.jpg)

Besides other advantages of thin film technology over crystalline silicon technologies one important characteristic is that the cells can be bent down to very small radii and are permanently flexible. Minimum radius for flexible currently sold thin film modules is approximately about 3-5 cm in the best/lowest cases. Folding or creating sharp angles is not possible since it will destroy the inner structure of the cells and probably the contacts. However solutions with longitudinal or transversal parts not covered with solar cells but only with flexible materials can produce sharper angles. In this case electrical connections have to be carefully designed in order to withstand the bending movement. The film layers previously described in the structure analysis of thin film solar cells are able to constitute a flexible ensemble. To create flexible, lightweight modules these layers must always be first coated on a backing surface material, which can be a polymer or metal foil in order to achieve the desired characteristic of enabling flexible deformations. Glass can of course not be used for the front protective layer or the backing material if flexibility must be retained and since flexible glass is still an emerging high-tech and expensive solution.

fig. 4.43 Organic thin film in low radius curvatures. Bus station San Fransisco (source: http://duelingfuels.com/uncategorized/ san-francisco-premieres-solar-powered-wi-fi-bus-stops.php)

In a next step flexibility can even be retained within the final structure of the product if flexible substrates are used. Examples that will be researched are mainly polymer membranes and thin flexible polymer sheets but the same technology also enables textile/fabric or other flexible and lightweight materials as a basis. The non brittle properties of thin film solar cells make them mechanically extremely strong compared to crystalline cells, enabling even walking on them when installed on flat roofs or theoretically even on the ground.

fig. 4.44 Thin film as flexible units on boats (possible for sail or deck). Advantage in mechanical properties, shape adaptability and weight over rigid solutions. (source: http://img.nauticexpo. com/images_ne/photo-g/solar-panels-boats-23244-6581243.jpg)

4.10 The problem of partial shading of solar sells. Solar cell connections Facade integrated PV solutions demand due to limited space, often non flat shapes and possible adjacent objects a precisely designed module arrangement and connection layout between them. Performance, depending on the solar cell type and efficiency of the connection plan, can be slightly influenced by partial shading or extremely decreased. In many cases an inappropriate design can also cause partial overheating, and damage. Unequal insolation or partial shading and their performance impact is therefore the most limiting factor when designing non flat/standard solar cell structures, given that the cost could be reduced by cheap manufacturing processes and inexpensive supporting materials. Solar cells within a normally sized PV module are connected in series. Modules, as a result of the cooperation of the solar cells, have a DC output that needs to be converted to higher voltage AC. This is achieved by an inverter, connected to several modules usually, but also to one or all available modules depending on the application. In general series connections are desirable since they make wiring smaller and of the same size everywhere and because of better conversion efficiency at higher voltages. Only modules with similar insolation values should be connected in series into zones. The negative effects of non uniform insolation are a result of the described counter acting principles. On one hand series connection creates higher voltage, more efficient conversion by the inverters and is cheaper to produce, on the other hand it is much more sensitive to specific modules being shaded or not equally illuminated. In a series connection of all modules the total output will equal the output of the least producing module. Therefore thoughtful zoning of the modules with respective inverters must be made. Partial shading of modules results in similar problems to those caused by non uniform illumination between modules and module zones discussed above. Here the series connection of several solar cells is responsible for both the higher performance and voltage output but also for the sensitivity to unequal performance between the single units of the system.

In this case in a simple series connection a solar cell that is fully shaded in a panel/module that is otherwise well illuminated stops current generation for the complete module and acts as a resistance. To overcome this problem cell strings within modules are equipped with by pass diodes. These by pass channels will enable current to by pass the series-string including the non illuminated/problematic solar cell and the module will therefor make use of the rest of the cells. However to have a performance drop proportional to the amount of shaded solar cells or approximately to the shaded area of a module, each single cell would need a bypass diode, which is practically never the case and would be very expesive and complicated. Therefore groups of more cells are equipped with a by pass diode disabling only their part of the complete surface. Thin film solar cells are much less sensitive to partial shading and are therefore usually not equipped with bypass diodes. The monolithic fabrication process of the modules makes it more difficult anyway to use integrated bypass diodes. Due to the non modular design, (as seen in crystalline cell panels), the shadow effects are more difficult to predict and overcome in thin film structures. Unlike c-Si modules, series connected cells in thin film modules are shaped like very thin wide stripes. This has a big advantage when the designer is able to correctly place the thin cell strips according to the predicted shadow shapes. Imagining a series of very long and thin stripes for example, a transversally long shadow would approximately shade only a small part of each stripe, and all stripes almost uniformly. In that case prediction of usually vertical shadows on a facade would lead to the installation of the thin film stripes horizontally. Organic solar cells (polymer based) show a very good performance in low light conditions but also suffer from partial shading. On the other hand however it is possible to inexpensively integrate organic several by-pass diodes during the roll to roll process which enables a very good partial shading performance.

fig. 4.46 and 4.47 are front elevations of the corrugated thin film PV shape seen in section fig. 4.48. Illumination under different angles creates partial shading. In fig. 4.46 the solar cells are horizontal connected in series. Every horizontal cell is equally shaded and therefore total output is only reduced approximately by the shading percentage. In fig 4.47 solar cells are vertical connected in series. A number of solar cells is completely shaded while others are illuminated. Due to the in series connection the total output is drastically reduced. Fig 4.46 is the preferable positioning. a.

fig. 4.45 Horizontal and vertical partial shading shapes for thin film (both cell direction versions a,b) and crystalline solar modules (c,d). -Module a shows a significant output reduction. -Module b has a reduction almost proportional to the shadow -Module c has approximately 1/2 the output of no shading. -Module d has almost 0 output. (source: Author)

(fig 4.46, 4.47 source: Author) (fig 4.48 source: espacenet patent repository)

fig. 4.46

Parallel connection of zones

zone in Series

fig. 4.47

inverter

fig. 4.49 Panel connection layout and Zoning. Modules with similar partial shading percentages can be organized in single Zone with in Series connection. Parallel connection of different zones to avoid Output losses due to low performing zone. (source: Author)

fig. 4.48

in shadow

in shadow 48

4.11 Custom, non rectangular solar panel shapes in Facades today Solar panels installed (or integrated) on facades today are either standard sized modules with specific dimensions and a rectangular shape, or custom modules in different dimensions and shapes. Combinations of the two are also often used. In large, high budget projects as usually happens with architectural elements, custom designs can be more cost efficient, while smaller scale structures tend to rely on standard solutions to reduce the cost as much as possible. Arrangement of solar modules in a facade either as an integral part of the design of the facade or as a retrofit/ additional structure has to be carefully designed in order to achieve maximum efficiency for the lowest investment. One common strategy is to fill the largest part of the facade with standard (size and shape) modules and for the parts or edges where different angles, shapes and sizes are needed, custom modules can be used. If the parts where custom shapes are needed are not easily visible or do not receive enough sunlight to have an impact on the overall performance it is not uncommon to use dummy modules which are not connected or just metal panels/ plastic panels of similar dark color. Crystalline silicon modules consisting of almost rectangular (with chamfered edges in some cases of monocrystalline cells) polygonal or circular cells can have a non rectangular shape. Cells are connected in series and positioned on a grid with a constant distance between them that can vary to a large extend depending on the application and possible need for semi transparency. A commonly created shape in facades where the main part is covered by standard modules is that of a rectangular panel, one side of which needs to be cut diagonally. Depending on the angle and size this can produce a shape starting from a trapezoid with two opposite facing edges being almost parallel and reaching angles that create triangular modules. In-series cell connections -analysed in the partial shading chapter- necessitate an efficient solution for the cells close to the edges that need to be cut diagonally. For rigid crystalline panels this means that the last cell of each cell row needs to be cut diagonally or that the cell needs to be left out completely. In case of a smaller/cut cell the series connection imposes the use of a dummy (not connected) cell since the lower performance of one single small cell would constitute a performance â&#x20AC;&#x153;bottleneckâ&#x20AC;? for the entire panel. Therefore in terms of active surface area 49

the panel practically looses relatively more performance than the difference in surface area between the rectangular and the trapezoid or triangular shaped panel. The division of a panel in multiple small cell units, which have proportions closer to a 1/1 ratio in width and height is the reason for the aforementioned strategy when creating custom shapes. This division creates an advantage when cells become smaller since the area lost due to the missing last cell of each row is smaller. On the other hand however the design of the cells and their arrangement within the panel (not to mention the solar cell material/ technology itself) makes it much more problematic with partial shading unless complicated and expensive bypass solutions are used. Thin film modules are also usually used in standard sizes and shapes either in rigid panel applications or as longitudinal, thin, flexible modules. Especially the flexible modules giving additional possibilities and design freedom rarely need to be cut to custom shapes. Due to their limited width they are usually placed parallel to each other. The very thin longitudinal shape of the thin film solar cells responds very differently to partial shading and has different problems with custom shapes compared to crystalline cells. The thin cell strips are more difficult to shade and as a result to reduce their performance, since any perpendicular shadow will only cover a small amount of the cell and usually a similar percentage of several parallel cells. Given the in series connection of these modules, as is the case for crystalline panels, the overall performance is practically determined by the most shaded cell strip. In the same sense when creating custom shapes with diagonally cut sides/edges the performance of the module is limited to the shortest cell strip. This considerably limits the possible angles and shapes when maximum pv surface area is the ultimate goal and the material has to be cut. On the other hand flexibility enables bending or rolling the solar panel around small radii which can in some cases be a way to overcome the problem of diagonal edges. Additionally the longitudinal thin cell strips can also be placed in such a way, so that they form parallelograms of equally sized strips. These solutions mainly refer to existing mature crystalline and thin film technologies. Third generation PV cells and the ability to easily and cheaply print on many different substrates will enable inexpensive printing of electrical circuits that can overcome the inherent in series connection limitations, but also folding of solar cells in very small radii. The ability to fold and excessively bend cells and circuits considerably increases the potential for custom shapes of any angle and curvature, even with the existing thin strip pattern.

fig. 4.50 Custom shape Panels . Leaving out the cells that are partly cut. Smaller cell division creates higher total active surface. Constantly reducing cell size could lead to panel performance close to proportional to rectangular/custom area ratio. (source: Author)

fig. 4.51 Custom shape Panels . Using unconnected dummy cells or other material of same shape instead of leaving out the cells. (source: Author)

fig. 4.52 Thin film module. Performance limited by shortest cell strip due to in series connection. Possible solutions by dividing cells strips or adding strips in other direction parallel to edge. (source: Author)

fig. 4.53 Circular custom shapes. Active surface is lost due to shape restriction. (source: http://www.polysunny.com/images/products/2012050815181596326291w500h500upolysun/20w18v-round-solar-module.jpg)

fig. 4.54 Triangular custom shapes. Active surface is lost due to shape restriction. (source: http://img.edilportale.com/ products/prodotti-5478-reld1f49a30-ff93f24d-cbdf-a24b5df1ee5f.jpg)

fig. 4.55 Custom shaped products. Example of hexagonal flexible panel. High division of cells used to cover complete surface. (source: http://blog.is-arquitectura.es/2012/04/01/ vidrio-fotovoltaico-con-silicio-amorfo-para-edificios/)

CdTe (Cadmium Telluride)

4,5-13 %

0,2-0,6

25-30 %

30-44 %

15 %

0,6-0,7

<0,6

17-20 %

6-11 %

5-10 %

0,8-1

12-17 %

11,5-15 %

Euro/Wp

Module Efficiency

25-30 %

16,5 %

13-20 %

13-15,2 %

20,3 %

20-22 %

Max Efficiency (Lab)

fig. 4.56 Table with cell types characteristics (source: Author)

Dye Sensitized solar cell (DSSC)

Multijunction (CPV)

(HIT) High Performance Hybrid silicon

Thin Film:

CIS (Copper Indium Selenide)

Thin Film:

Amorphous Silicon

Polycrystalline

Crystalline:

Thin Film:

Monocrystalline

Crystalline:

PV-Cell Technology

Very High

low

high

medium

Diffuse Light Performance

less prone to overheating -0,7% per Degree C +freezing problems

-0,25% per Degree C

-0,34% per Degree C

-0,25% per Degree C

-0,2% per Degree C

-0,15% per Degree C

-0,43% per Degree C

-0,38% per Degree C

Temperature Degradation

Uniform appearance. Easy to make and semitransparent.

Used with Concentrator systems for very high efficiency.

Typically black, grey or blue. Transparency by gaps between cells.

Uniform appearance. Colours usually dark brown to black. Fine transparency possible.

Uniform appearance. Colours usually gray, brown and black. Fine transparency possible.

Usually medium or dark blue. Transparency by gaps between cells.

Typically black, grey or blue. Transparency by gaps between cells.

Other Characteristics

5_PV System Peripherals

5.1 Grid Connection Types of PV systems according to their connection to the an existing electricity grid PV systems can be used as: - Stand-alone (or off-grid) systems, i.e. for supplying electricity to buildings or other electricity consuming structures, which are not connected or not always connected to a public electricity grid, comprising a. Purely PV systems, in which the only energy source is the photovoltaic device. b. Hybrid systems which comprise in addition to PVs, also other energy sources either â&#x20AC;˘ again renewables (wind energy, geothermal etc.), or â&#x20AC;˘ conventional (diesel generators, turbines etc.) - Grid-connected systems in which the electrical energy produced by the PV cells in addition to being consumed locally (if applicable) is fed to a public electricity grid. In such systems the energy can flow in two directions, namely: a. From the PVs to the grid when not needed by local consuming devices (either steadily as in PV parks located away from any consuming devices or when there is an excess of PV production) b. From the public grid to the consuming devices (essentially in buildings or other urban structures) when the PV energy is not sufficient for the actual needs - Systems combining the two possibilities selectively over time (although they represent a very rare case) 5.1.1 Fully stand-alone systems Fully stand- alone systems are designed to be powered by a PV array only and operate independent of the electric utility grid. They are generally designed and sized to supply certain DC and/or AC electrical loads. They need an energy storage means for facing the fluctuations in solar energy production and load requirement. They are rather unusual in urban environment and in large buildings, but find often use in developing countries for supplying electricity for substantial daily needs, like: - Pumping systems (supplying water to villages, for land irrigation or animalsâ&#x20AC;&#x2122; watering, - Refrigeration systems: particularly to preserve vaccines, blood and other consumables vital to healthcare programs - Lighting: for homes and community buildings such as schools and health centers to enable education and income generation activities to continue after dark, - Battery charging stations: to recharge batteries, which are used to power appliances ranging from torches and radios to televisions and lights, and - Solar home systems: to provide power for domestic lighting and other DC appliances such as TVs, radios, sew ing machines, etc. 53

5.1.2 Hybrid stand-alone systems Hybrid stand alone systems are more common than the fully off-grid ones, in particular if additional renewable energy sources are available. They are basically used when the storage systems designed on the basis of an initially defined period of insufficient sunshine are not in position to cope with exceptional periods of poor weather or excess load, so that an alternative source is required to guarantee power production. PV-hybrid systems combine a photovoltaic generator with another power source typically a diesel generator, but occasionally with another renewable supply such as a wind turbine. The PV generator would usually be sized to meet the base load demand, with the alternate supply being called into action only when needed. This arrangement offers all the benefits of PV in respect of low operation and maintenance costs, but additionally ensures a secure supply. Hybrid systems can also be the correct choice in situations where occasional demand peaks are significantly higher than the base load demand. It makes little sense to size a system to be able to meet demand entirely with PV if, for example, the normal load is only 10% of the peak demand. In this sense, a diesel generator combination sized to meet the peak demand would be operating at inefficient part-load for most of the time. In such a situation a PV-diesel hybrid represents a good compromise. 5.1.3 Grid connected systems Whereas the photovoltaic systems discussed earlier are usually small decentralized systems in the lower and middle kW range, both: - large central photovoltaic power stations in the higher kilowatt to megawatt range and - most of the PV building in contemporary installations are designed to feed directly into the medium -or high- voltage grid for the first one and the low voltage grid for the latter. This is greatly enhanced by substantial financial incentives provided from the states all over the world in form of strongly subsidized prices for feeding energy to the public grid, although this policy is facing at the moment restrictions, lower prices and has essentially changed. On the other hand the upcoming zero energy building requirements make renewable energy production unavoidable and for large buildings a grid-connected system represents the first choice option. 5.1.4 Systems selectively combining (over time) the two types This option provides the user of the building with the possibility to select at any point of time between the following operating options: - permanently and primarily feeding any excess energy to the grid and substantially or partly disregarding the SOC (state of charge)of the anyway existing batteries - prioritizing the SOC of the batteries and feeding only any surplus energy to the grid - disconnecting the grid and running the whole system as a stand-alone one (an option being more or less feasi ble depending on the designed size of the batteries) - compromise all above options in practically any desired extent allowed by the capabilities of a possibly sophisticated and thus adaptable controller. 54

5.2 Peripherals. Devices This term refers to all devices necessary for the efficient and safe functioning of a PV system other than the PV array as such. The term “peripheral” is certainly meant not in a qualitative sense, since their constitute an indispensable part of the PV system. Thus, for a PV system to reliably supply electricity, components additional to the main device, i.e. the PV array, are necessary. These include: - The cables for transferring the electrical current from the array to the further processing devices - A controller of a specific type depending on whether the system is stand-alone (=off-grid), grid-connected, a mixture of them or hybrid (with additional local power generating device, like e.g. a diesel-generator couple). Often a MPPT (maximum Power Pont Tracker) is included either as a separate device or a function of the controller in the system. - An inverter when AC current is desired (most of the cases) - Storage means especially for stand-alone systems (e.g. batteries) - Various switchboards, current fuses, interrupters and other safety and metering devices. All these devices are often called as a group: “the Balance of System or BOS”.

5.2.1 Cables Depending on the distances between the various parts of the PV system, the height of the voltage and the density of the currents the cables play a more or less important role in the design, the costs and the efficiency of the whole system. In buildings where production, consumption and grid connection are comparatively next to one another, the selection of the connecting cables are important only when low voltages are used (e.g. 12V) for large energy transfers, in particular in DC circuits. Proposals for wireless connections are known in which metallic structures supporting or connecting the PV modules are used for transferring current, avoiding thus the need for cables. Although this is a basically interesting solution, it restricts the design possibilities when compared to cables which are “more flexible” in guiding them through the various structures and “paths” in particular as to building integrated PV devices. However if they can be integrated in anyway necessary parts of e.g. a façade they represent a more “elegant” solution (example of Thesis S. Broermsa, TU Delft 2008: http://repository.tudelft.nl/view/ir/uuid:9812d42c-c924-47c9-be70bf0f3f4548db/).

5.2.2 Controllers Typical controllers in PV systems are responsible for multiple functions, the most important being: - to operate the battery within the safe limits defined with respect to overcharging and deep discharging by the battery manufacturer or by the operation mode (charge controllers) - as both the battery voltage and the PV generator voltage vary over a wide range during operation due to the changing state of charge and boundary conditions such as temperature and insolation, a corresponding eletronic part is needed which has: a. either the simpler form of a matching DC/DC converter (MDC), or b. the mostly used MPPT (Maximum Power Point Tracking). The matching DC/DC converter (MDC) simply de-couples the characteristic curves of the PV generator and the battery. The modern sophisticated MPPT controllers feature a smart tracking algorithm that maximizes the energy harvest from the PV and also provides load control to prevent over discharge of the battery by tracking the array maximum power point voltage (Vmp) as it varies under different weather conditions.

5.2.3 Inverters (general) PV arrays produce DC current. Inverters, i.e. electronic devices transforming the DC current into AC of the desired frequency (usually 50 or 60 Hz), are needed in essentially two cases: - if AC voltage is required for either: a. supplying corresponding appliances, or b. feeding electricity to a common AC public grid or for stand-alone DC systems integrated in a modern MPPT cntroller. Inverters for grid-connected systems They represent the second most important component of a grid-connected PV system (after the panels). In contrast to inverters intended only for stand-alone operation, those intended for parallel operation must respond just as well to the grid characteristics as to the solar generator performance. As all of the current flows through the inverter, its properties fundamentally affect the behavior and operating results of the photovoltaic system. Apart from the efficient conversion of direct to alternating current, the inverter electronics include also protective devices which automatically disconnect the system if irregularities in the grid or the solar generator occur. Today most inverter models are additionally equipped with data loggers and measurement computers, which allow the power, voltage, current and other operating parameters to be recorded continuously. This data can be read out at intervals via a serial interface with a laptop computer and analysed.

Inverters for stand alone systems Because of the specific operating conditions of stand-alone inverters, different design aspects have to be considered. In a typical domestic power supply, the ratio of the peak power to the average power is about 25:1, so the inverter must have a high efficiency of approximately 90%, particularly in the partial load range (5–10% of the rated power). Modern inverters must satisfy this condition together with a sinusoidal output voltage and the capacity to withstand short-term, double to triple overloading. The most important requirements on inverters for stand-alone photovoltaic systems are: • Large input voltage range (−10% to +30% of the rated voltage). • Output voltage as close to sinusoidal as possible. • Little fluctuation in the output voltage and frequency. • ±8% voltage constancy, ±2% frequency constancy. • High efficiency for partial loading; an efficiency value of at least 90% at 10% partial load. • Ability to withstand short-term overloading for appliance starting conditions, for example, two to three times the rated current for 5 s for refrigerators and washing machines. • Lowest possible overvoltages for inductive and capacitive loads. • Half-wave operation possible, caused by power reduction with a diode, for example, in hair dryers. • Able to withstand short circuits. (source: “Photovoltaics Guidebook for decision makers” Achim Bubenzer) 5.2.4 Storage means The fact that the solar energy production suffers from fluctuations in three aspects: - Diurnal, i.e. difference in irradiation during the 24 h period of a day, depending also on the orientation (east, west etc.) of the PVs - Daily, i.e. difference in irradiation between subsequent days of a period - Seasonal, i.e. difference in irradiation between seasons, ( winter, summer etc.), has initiated from the very beginning of using solar energy the discussion concerning the need for storing the selected energy for later use in periods when it is not available or not available in the desired quantity/density (also load fluctuations are involved here). Starting from the very basic approach of considering every possible electricity storing means (visible in fig. 5.1 graph, known as the Ragone plot) and considering the main restrictions as to storing electric energy gained from PVs, it becomes evident that: a. high energy density and b. reasonably high power density is needed. This excludes from the very beginning the capacitors of any kind due to their inherent short time storage capabilities and their poor energy density. 57

fig. 5.1 Graph of storage means

As a result, for short and medium term storage (a few days, usually 2-4) batteries represent the most common storage means, solving reliably at least two of the above fluctuation problems, but not the seasonal one. To address also this kind of fluctuation different solutions exist in experimental phase or are searched for, but most of them are based in transforming the electric energy to other forms of energy in rather complicated processes, like storing in fly wheels via mechanical transformation, producing and storing hydrogen for feeding fuel cells, gravity energy storage using pumps, superconducting magnetic energy storage, compressed air storage, super capacitors etc. As the ease of use and the efficiency of the batteries are still unbeatable, they are almost exclusively used particularly in small and medium systems. Batteries are mainly classified as primary or secondary. Primary batteries convert chemical energy to electrical energy in an irreversibly way, which means that when the stored energy is drawn out of them they cannot be recharged (typical examples include Zinc carbon batteries and alkaline batteries). Secondary batteries (commonly called: rechargeable batteries) convert chemical energy to electrical energy in a reversibly way, which means that they can be recharged from another source (charger device) by reversing the chemical reaction by using a slightly higher charging voltage. Typical example includes lead acid batteries and lithium ion batteries. The oldest and the most mature battery technology are the lead acid batteries, known from the usual car batteries. Other secondary battery types are nickel metal hydride (NiMH) and nickel cadmium (NiCd) known from older portable devices, like mobile phones, the first having a much better energy density with compared to the second ones and the more modern Li-ion batteries. However they have a high rate of self discharge and are more and more restricted due to the environmental impact of cadmium and their â&#x20AC;&#x153;memory effectâ&#x20AC;?, meaning that they lose their usable energy capacity if they are repeatedly charged after only a partial discharge. For all these reasons NiCd and NiMH batteries are not good candidates for storage in PV systems. Lithium ion and lithium ion polymer batteries (substantially differing from simple lithium batteries which are for single use only) have a high energy density and are therefore already broadly used in light weight storage applications. However, due to their high prices and low maturity, they are still not used in PV systems. 58

Newest developments and market aspects affecting the storage means for solar electric energy New market aspects influence the development of new more efficient batteries for the following reasons. Grid providers are recently more and more interested in motivating solar energy producers to install or increase existing electricity storage capacities in view of the increasing problem of fluctuations in energy flow in the existing grids in which more and more solar energy is fed. However as this is linked with substantial “bumps and dips” inherent in this kind of energy delivery, due to its intermittent character, the electricity grids will need to become larger and more complex with a small “fill factor” at night or overcast days and “bottle-necks” on sunny days without much consumption (weekends). California is an example of this problem, due to the high rate of increase in PV installations in the last years and grids being unable to cope with this development, so that blackouts or “rolling black-outs” (affecting selected areas and times of the day) are very frequent. Increasing solar energy storage becomes more and more beneficial for the producer, as many grid providers are introducing specific price policies providing different (higher) prices for energy sold at times within the day or weak when there is a surge in the grids and offering substantially lower prices in the opposite load situations. Thus, the producer could store “cheap” energy of any kind (his “own” solar or even from the grid) and consume it at times of shortness/high prices. As the main drawback for storage is the price, the whole investment can be assessed on this basis. This fact is a direct impulse for innovative technology centers (universities and companies) to work more intensively in this field based on the new more promising market circumstances. For instance, BYD, the Chinese battery maker in which Warren Buffet’s Berkshire Hathaway owns a significant stake, is heavily promoting new battery types with improved efficiency and a reasonable value/price relation. Bloomberg Energy predicts that battery storage costs will fall 57% a kWh by 2020. States (in particular in Europe and China) are starting subsidizing also this sector of investment (energy storage). Within this “atmosphere”, known but not yet mature storage means and completely new ones are under development and a few promising results are already visible as the following overview summarizes. One of the newest and most promising types of batteries are redox flow batteries, almost combining the properties of the usual batteries and the fuel cells, but their development is just starting. As their electrolytes exchange only ions through a membrane, there are no signs of decay and provide thus a very long life. In particular a team around Micheal Aziz at Harvard University (http://www.popularmechanics.com/science/ energy/solar-wind/3-clever-new-ways-to-store-solar-energy-16407404) is experimenting with flow batteries working in the reverse concept of a fuel cell based on reactions that convert the chemical energy in small organic molecules (an example being methanol) into electricity. Aziz found out that if he could craft a fuel cell that also runs in reverse—essentially converting energy back into chemical reactants—the resulting flow battery could store solar power using inexpensive, organic fuel. Tom Meyer, a professor of chemistry at the University of North Carolina, announced recently a new strategy of converting solar energy to hydrogen, instead of electricity. The basis of this method is water, which is being divided into its elemental form. After that hydrogen is collected in a tank and burnt at night. The basic relevance to solar energy is to use solar energy to convert carbon dioxide into methanol, its combustive cousin. At night, a power plant burns the methanol as fuel, converting it back into carbon dioxide, which it would capture and store for later. The next time the sun rises, the process begins again, effectively recycling carbon and potentially reducing harmful emissions. The specific project however remains in research phase. 59

Other methods of storing solar power for a clouded day involve converting the sunâ&#x20AC;&#x2122;s energy into heat, which is then captured in thermal storage tanks. Abengoa, a renewable energy firm based in Spain, has already built several solar plants that store excess energy in molten salt, which can absorb extremely high temperatures without changing state. Todayâ&#x20AC;&#x2122;s practicable storage solutions As all above high technology storage means are not yet mature or/and very costly, the practically available choices for PV storage are lead acid and Li-ion batteries, both of them suffering from the drawback that their electrodes undergo chemical degradation during charging and discharging. A typical lead acid battery consists of a number of individual cells, each having the nominal voltage of approx. 2V internally connected in series (so that a 12V battery is made out of 6 cells) within a housing. Each of the lead electrodes are juxtaposed with grid shaped plates for providing an increased reacting surface.

fig. 5.2 Lead acid chemical structure

From the chemical point of view (fig 5.2), the positive electrode is formed of lead oxide and the negative one from pure lead with sulphuric acid (H2SO4) as electrolyte acting on them. In the phase of discharge (meaning an external load demands electricity), electrons follow the path through the external circuit from the negative to the positive pole, causing the chemical reaction between the lead in the plates and the electrolyte in between, which is depleted, thereby reducing the State of Charge (SOC) of the battery. When the battery is connected to an external potential for recharge (in this case, the electric solar energy from the PVs), the electrons follow a reverse path.

Gel type lead acid batteries, in which the electrolyte is in a gel form, perform better than the â&#x20AC;&#x153;floodedâ&#x20AC;? ones and provide the additional advantage of not needing a vent and not suffering from the danger of contaminating their surrounding by releasing gas or even electrolyte when charged or in particular overcharged. The next improvement over the gel type are the AGM (Absorbent Glass Mat) batteries, in which a woven glass mat is used between the plates to hold the electrolyte. They are thus leak and spill proof, possess all the advantages of the gel batteries and in addition maintain the voltage better, self discharge slower and last longer, representing a state of the art choice with a reasonable value for money ratio.

Use of the storage means in a PV system As mentioned in the previous chapter dealing with the types of PV systems, batteries are mainly used in the standalone PV systems, because there is no other source of power to support the PV array.

fig. 5.3 Graph Daily PV output and electricity demand (red). (source: http://joeljean.com/projects. html)

A typical solar irradiance profile is shown in fig. 5.3 during a day on average. An off-grid system would be unable to utilize this excess solar energy produced during the day for the periods of no solar production without a storage means. Batteries store that excess energy from the sun during the day and are consequently discharged during periods of low solar irradiance. (further details about the use of the batteries and the complete PV system are included in the Final System Solution in Chapter 13)

â&#x20AC;&#x192; 61

6_Polymers as PV Substrates on Facades

Polymers instead of glass Lightweight, low cost and high transparency are very important characteristics when trying to approach the best suited material for PV integration on a building skin. Lightweight is strongly linked to low cost during all procedures from fabrication until the installation and finally disassembly of a facade element. Transparent materials are chosen in order to have semi transparent final structure (when PV cells are laminated on the material) that allows the view to the outside and produces energy at the same time. The energy producing structure can be seen as a constantly shading glass/polymer integrated element, a light control device, as a buffer that is able to either act as a second skin to slightly preheat the in between air or just as a wind protection that enables natural ventilation in areas where this would be difficult. Practically the most suitable transparent material for PV integration would be a glass pane that is much lighter than glass, much easier to produce in different shapes (especially when thinking of curved and double curved shapes) and significantly cheaper. An improvement of its low impact resistance due to its brittleness as well as improved heat related behavior would also be an additional benefit. The development of polymers in the last decades/ years has lead to specific materials that come very close to these characteristics and constitute important candidates for replacing traditionally glass constructed elements. The first applications of glass replacing composites can be found in aircrafts, where weight reduction and good mechanical properties are the ultimate goals for an efficient design. Buildings have requirements that are more biased towards durability in longer periods of time and therefore glass is still considered the first choice for facade applications. On the other hand low weight and better mechanical properties in specific areas could become key factors for polymers to become popular if they succeed in overcoming some of their -what has until recently been seen as- inherent problems.

6.1 Lightweight and Transparent Polymers Polymers offer possibilities that even now are still to a large extend unexploited due to the practically unlimited combinations of raw materials and additives, which is not the case for other materials with much less possible variations and more predefined properties. Different combinations of foils, fibers, coatings, fillers and additives can result in completely different final material properties in terms of mechanical performance and visual or haptic result. The performance of polymers in relation to solar radiation in different parts of the spectrum as well as their insulation properties are of great interest for facade applications where they could replace glass panes or semi transparent elements of an active shading device.

Polymer sheets

Visual Transmittance

Density kg/m3

PC 8mm

86%

1170

PC multi-wall 16mm

58%

PMMA solid 8mm

92%

1190

GFRP solid natural color

56%

1580

PEN

90+%

1390

PET

90%

1390

ETFE 200μm 65% silve printing

57%

1700

ETFE 200μm clear

92%

ETFE 250μm white

37%

PE 200μm with UV stabilizer

88%

Foils

fig. 6.1 Table for different polymers and sheet versions with transparency and density characteristics. (source: Author, from CES software)

1200

Transparency Transparency is the characteristic we refer to when a material allows the visible part of the light spectrum to pass through, even though a material may be transparent for a specific wavelength and opaque for the range we are able to see. The way a polymer handles infrared and UV Radiation (longer and shorter wavelengths compared to the visible) strongly influences the characteristics it has as an efficient building skin material and possible glass replacement. The transparency, translucency or opaqueness of a material also largely depends on its dimensions and can therefore not easily be seen as a material property but as a characteristic of a certain element made of a specific polymer. On the other hand it is possible to broadly define the materials that are possible to be more or less transparent. The chemical structure of each polymer and the fillers added to the final material play an important rule in the visual perception of the structure. The chemical/molecular structure of thermoplastics for example ranges from completely irregular (amorphous) to almost crystalline (semi-crystalline). Molecular structure is related to both the polymer but also the production process itself, that can affect its molecular arrangement. Semi-crystalline thermoplastics such as polyethylene (PE), polypropylene (PP) and polytetrafluoroethylene (PTFE) are either milky in appearance or completely opaque, whereas amorphous thermoplastics like polyvinyl chloride (PVC), polystyrene (PS), molymethyl methacrylate (PMMA) and polycarbonate (PC) can be highly transparent.

fig. 6.2 Polymer sheets of different transparency degrees and colors (source: www.sabic-ip.com/sfs)

Thermosets are normally also transparent due to the dense cross-linking preventing the molecules from acquiring a regular arrangement. In the case of elastomers, even though their molecular structure would allow transparency of the material, this is not the case due to the fillers used (carbon black and oil for example). (source: â&#x20AC;&#x153;Construction Manual for Polymers and Membranesâ&#x20AC;? Detail) Further Analysis about polymer material of choice in Chapter 12 of this Thesis)

The most common problems related to the use of polymers as glass replacement for external facade applications are UV-Resistance and weatherability in general as well as low heat/flame resistance, which has an impact in fire protection. PVC for instance has the disadvantage that permanent exposure to UV radiation causes its surface to become yellow over time. fig. 6.3 UV radiation causing Polymer sheet yellow color over time. UV protective layers needed (source: http://www.palramapplications.com/wp-content/uploads/2012/04/poly2.jpg)

6.2 Photovoltaics on Polymers/Membranes Applying solar cells to polymer structures is a process that has been developed mainly during the last two decades. At first experiments with crystalline solar cells were carried out, using Polycarbonate and PMMA/Acrylics (known under the brand name Plexiglas). As already analysed PET (Polyethylene terephthalate) and EVA (Ethylene vinyl acetate) are commonly used as the backing material and encapsulation medium in PV modules. In a later stage the development of second generation inorganic thin film solar cells enabled the integration of modules on flexible polymer substrates. In case of membranes (ETFE etc.) either special pockets can be fabricated in order to receive the thin film PV module or the solar cell module is laminated on the material. Development of low temperature lamination methods but most importantly the emerging third generation organic (polymer based) solar cells, enabling roll to roll fabrication processes and printing of solar cells on a large variety of materials, makes polymer based PV structures very easy and cheap to fabricate. In order to fabricate solar modules with a polymeric substrate instead of laminating a finished solar panel or strip on a polymer, the most important aspect is the compatibility of the substrate with the high temperature deposition and processing steps. This depends largely on the solar cell type with second generation inorganic but mostly third generation organic cells having an advantage.

fig. 6.4 Thin Film PV on ETFE membrane roof (source: http://www.detail-online.com/uploads/ pics/1241128800_149_500_335.jpg)

fig. 6.5 PV film pattern cutting source: Detail Polymers

fig. 6.6 Different polymers and glass characteristics comparison (source: “Industrial plastics” E.Lokensgard,2004)

fig. 6.7 Polymer comparison. (source ‘Materials and design’ by Ashby and Johnson, www.matweb.com

7_Methodology (four phases) towards final Design Proposals The process of approaching the final design solutions is divided in four separated steps, during which an alternative approach is attempted, compared to the standard process of improving solar incidence angles, using tracking or concentrators in combination with medium to high performance crystalline solar cells. In this case the possibility of making use of the advantages of second and third generation inorganic or organic thin film solar cells is researched. The advantages of low cost (with a potential for a further significant cost reduction through roll to roll fabrication and printing methods on cheap substrates), flexibility, extremely low weight and very good diffuse light and partial shading performance, make a different approach for efficiency improvement a necessity. Within this research the effect of three dimensional shape optimization and solar cell surface area maximization is researched and tested. A future where the available facade/envelope area will be the only constraint to power production for a building and the basis on which architects and engineers will attempt to optimize the efficiency of cheap solar cells is a very challenging perspective. The four phases can be seen as the filtering of a vast amount of shapes and possible light control and shading designs incorporating solar cells. In a first phase the properties of the possible structures are analysed, in terms of light control and shading characteristics and in terms of solar cell integration. Only structures that can be seen as an additional layer parallel to the facade plane of a large building are included, since horizontal awnings perpendicular to the facade plane for each story have a very low potential in terms of surface maximization as long as the structure does not extend to much in relation to the windows. Of importance for the successful filtering of the large amount of possible designs are a good combination of specific characteristics. The most important aspect is the PV surface maximization potential which is roughly calculated for every solution and evaluated. Solutions with a surface area as low as that of a flat surface/sheet in front of the chosen facade module are excluded as inefficient. Another important aspect is the ability to withstand wind loads in relation to the weight of the structure. Designs that are extremely sensitive to wind forces, as for example specific flexible fabric or membrane solutions are also not constituting candidates for the final design. This is also related to the ability of the structure to be retracted (or at least slided) and provide thereby complete transparency when needed. Fixed structures may have an extremely high wind resistance to weight ratio (as for example efficient tensile structures) but constant shadow of specific parts of the structure would demand more artificial lighting. In this phase the most efficient solutions will be chosen depending on the above mentioned preset requirements. The second phase is a shape research of the chosen designs and the large amount of variations of their shape. Parametric tools (Grasshopper) are used in order to find the potential of a large variety of shapes in terms of surface maximization. Specific size limits will be set (thickness limit of the structure and module sizes) and all the possible shapes will be compared in terms of PV SURFACE POTENTIAL. The ration of each designâ&#x20AC;&#x2122;s surface area to its facade footprint equivalent (PV structure area/space it covers in a facade) will be extracted parametrically in order to find the optimum layout that would still have an adequate transparency and view to the outside. Specific shapes and folding patterns in some cases inspired by origami structures will show their surface efficiency. Two more characteristics that will be important in this phase for the evaluation are the ability of a shape to fold and unfold from a large size covering the complete facade module to a very small size in order to allow for transparency when needed. This will lead to a group of designs that are surface efficient, but not foldable due to their specific fold/edge pattern and another group of shapes which are deployable/retractable. For specific applications and locations the simplicity of the rigid (sliding) structures will be chosen as preferable, while for other cases the big advantage of retractability will be chosen despite its additional complexity.

Since the available surface area for solar cells is not enough to ensure that sunlight will efficiently illuminate to an acceptable extend the complete area of each design, a solar study will be performed as a next step (Phase 3). Each one of the chosen shapes and shape variations will be analysed and tested in terms of solar exposure, the amount of solar energy reaching the complete surface area, and partial shading percentages within a year. The results will give an indication of which of the surfaces that have been proved more or less “area efficient” in the previous stage are also efficient in terms of solar energy they receive. As someone would expect denser designs have higher shading percentages, but the most important data given by these measurements is the relative performance between comparably “area efficient” designs. The results of the solar study will show the real potential of the three dimensional shapes included in the research. The total solar energy reaching each one of these shapes compared to the energy reaching their facade equivalent (footprint of the shape on the facade plane) is a first indication of what can be achieved by a three dimensional design. A theoretically extremely well performing solar cell technology (not in terms of total maximum efficiency) that is not affected at all by partial shading and therefore has a performance drop almost linearly relative to the shading percentage (which with current solar cell technology is not the case) would be able to have the gains depicted on the solar energy results. After the evaluation of the above mentioned theoretical potential and a short analysis of the impact of partial shading and solar cell connections on the total performance, the fourth phase will include real testing in the PV Lab of the TUDelft Faculty of Electrical Engineering. Current flexible thin film modules by HyetSolar will be used. Since it is impossible to shape these modules exactly into the shapes that proved themselves as the most efficient ones in the previous phases, characteristic curvatures and folding patterns with varying density and angles are created. Wooden basis models of different shapes constructed in the TUDelft Workshop will become the rigid support which will give shape to the flexible modules before they reach the testing machine. The variety of different densities and shapes, but also the positioning of the structure in various angles will give a good indication of how strongly the module efficiency is affected by the 3d shape compared to is flat equivalent. Results showing low efficiency reduction by partial shading and good performance of the three dimensional PV structures compared to their footprint, even for currently used solar cell types, would be an important finding showing an already tangible potential for 3d structures. The final Phase of this process is the integration of the shapes derived from the previous evaluation into the ACTIVE NEXT FACADE. Possible integration possibilities will be analysed as well as different ways to create a retractable adaptive version of the best performing three dimensional PV surfaces. Ways to combine the advantage of low weight, flexibility and the ability to fold and unfold with a good wind resistance will be presented, given that this constitutes the main threat for these structures.

7.1 Four Phase Methdology diagram

Phase 1

Analysis of possible light control structures Light Control/Shading and PV Integration characteristics Important evaluation characteristics: 1. PV Surface Area Potential 2. Wind Resistance to Weight ratio 3. Adjustability/Retractability for complete transparency.

Phase 2 Filter

Shape research in terms of Surface Area Potential Surface Comparison through parametric tools Output of Shape Research: Most surface Area Efficient shapes Important evaluation characteristics: 1. Maximum PV surface area for a given Facade space

Phase 3 Filter

Solar study of Phase 2 Shape Results Solar Exposure, Partial Shading percentage Comparison Output of Solar Study: Highest Solar Energy receiving shapes Optimal shape for PV with linear performance drop due to partial shading. (theoretical) Important evaluation characteristics: 1. Solar exposure (percentage) 2. Partial Shading (percentage) 3. Incident Solar Radiation (total and per m2)

Phase 4

PV Lab Shape Testing. Real Thin film module Performance evaluation Output of Solar Study: Real life performance of selected shapes Real performance drop due to partial shading Potential of three dimensional PV shapes in Facades

The four Methodology Phases will lead to the final shapes of the light control/shading structures. As a final step the design possibilities for the integration of the structures in the decentralized facade module of the â&#x20AC;&#x153;ACTIVE NEXT FACADEâ&#x20AC;? Concept will be analysed. Integration of the results into the Active Next Facade Analysis and presentation of different designs possibilities

Total PV system solutions and use of gained electricity within the Next Facade

8_Phase 1

Analysis of possible light control structures Light Control/Shading and PV Integration characteristics

Introduction. Facade Integrated PV cells

Façades constitute the part of today’s buildings that is mostly responsible for its energy losses and therefore also energy consumption for heating or cooling. Today’s facades also cost about half of each entire building. However they also show a high potential of not only becoming more efficient through new materials and technologies (and therefore lowering the thermal losses) but also of incorporating devices that can transform them into an energy producer and an efficient inner climate regulator. Until only recently active solar energy devices where placed in a similar way as ground mounted devices, using the roofs (flat or tilted) of buildings. This gave a relative freedom to place the panels or collectors in good angles towards the sun without many self-shading problems or angle limitations. In case of gable roofs of homes, inappropriate orientation or dense areas of similarly high buildings there could be some restrictions. Either inappropriate roof structures in specific cases or the need for even higher energy yield from solar energy led people to also add panels (or collectors) to properly oriented facades (of course governmental incentives are still the main reason for making these structures cost effective). These where added as separate structures mounted/fixed on the existing walls (or other structures, balconies etc) by using light metal frames in most cases. In a bigger scale large and especially high buildings (often office buildings etc) with a relatively small roof/footprint area compared to façade surface area, which is often exposed to the sun rays for many hours due to the lack of adjacent high shading objects (buildings trees etc), become an interesting challenge for higher energy yield. The trend of using more and more active solar devices on buildings in the future as well as using the produced energy locally is better understood (explained) in the next chapter, which investigates the political and economical side of the topic. Several examples of façade added (!) active solar systems of this phase can be seen in buildings, where solar panels or collectors are attached to existing or new facades. Facade integration Options Building Integrated Photovoltaics are as already mentioned a relatively recent trend referring to custom shaped or standard PV panels replacing conventional building elements and therefore reducing the total cost on one hand and integrating the device in a more “elegant” and architecturally appealing way on the other. Officially photovoltaic modules are considered to be building integrated, if the modules form a building component providing a function as defined in the European Construction Product Directive CPD 89/106/EEC. (from “Normierung Gebäudeintegrierter PV-Module” Erban C.) According to this definition a similar sub category can be described referring to building facades. Many examples of façade integrated active solar devices have emerged during the last years, with applications in the form of: - Shading , which is very often used because of the similar angles under which a structure/shape both shades and provides good energy yield as a PV surface. Also because of design freedom and adjustability. (examples: lamellas, canopies etc.) - Cladding material/panel. Either in a rainscreen with limited demand for weather tightness, or as conventional curtain wall cladding. - Glass integrated cells. Either in windows (or glass facades) between glass panes or on the external layer (within the glass) of a double skin façade. In both cases these also provide shading. - Other structures. For example instead of balcony railing. 73

Solar cells behind external glass/windows. Until recently practically no examples of internally installed solar cells were known, not taking into account PV powered devices that operate at very low light conditions. Even installations of solar cells in the cavity of double skin facades, where usually wind protected sun shading systems are used, have not been realized. On the contrary there are several examples of solar cells embedded in the external glass layer of big double skin facade glass panes or other roof or facade installed glass panels. The reason solar cells are indeed often using a glass cover or are laminated into glass-glass laminates, but practically never used in the inside of buildings is related to the reflections caused at the interfaces between air, glass and in the inside between glass and air again. The reflectivity and therefore the light that is not reaching the next material layer (in this case the final one being the solar cell) strongly depends on the two adjacent materials refractive indexes and number of interfaces, as well as the thicknesses and wavelength. Solar cell optimized layers use adjacent materials with refractive indexes very close to each other and anti-reflective coating in order to minimize the losses. At the same time uneeded interfaces between materials and air (gaps) are avoided. As a rule of thumb at every interface between air and glass about 4% of the solar energy is absorbed or reflected (if the incident angle is not exceeding a specific value where reflectivity rises dramatically), meaning that about 92% will go through an external glass pane. However bigger problems are created due to the temperature rise and the partly shaded solar cells if the external building skin is not completely transparent and perfectly exposed. Recently due to the development of thin film and especially organic polymer based solar cells a few more projects are attempting to include internally placed solar cells. Examples include ideas of solar cells integrated in conventional shading structures but also ideas of PV curtains where small organic/thin film modules are integrated within the fabric. Photovoltaic fabric is also an interesting idea with a few examples including PV strings that can absorb light from more directions due to their cylindical (3d) shape. These applications are all still experimental, but are based on the properties of new third generation solar cell that are extremely good at low light and diffuse light conditions, less sensitive to perfect insolation angles and to temperatures.

fig. 8.1 PV-thermal hybrid solution using reflectors placed behind glass pane. (source: “Advancement in solar photovoltaic/ thermal (PV/T) hybrid collector technology” V.V. Tyagia, S.C. Kaushika, S.K. Tyagib)

fig. 8.2 PV fabric in woven courtin solution for the inside (source: “Power Textiles” http://www.wired.com/2008/06/ mit-lecturer-de/ )

8.1 PV cells and light control/Solar shading The tendency of using more and more glass in facades and creating increasingly transparent external skins has lead to the necessity of controlling the incoming radiation in an efficient way in order to avoid overheating and glare problems and at the same time preserve a good view through the glass from the inside as well as the feeling of a light and transparent element from the outside. Reducing the use of artificial lighting is an additional goal for achieving an energy efficient building skin. Several light control/shading devices have been developed in order to deal with these problems and needs, using a different layout, mechanism and location on the building envelope to achieve the desired effect. The use of solar cells as the element within the light control device that provides the shading slightly limits the possible designs but has the attractive characteristic of combining energy production and climate/light control in one device. The same device is able to increase the heat gain and reduce heat losses depending on the external conditions and at the same time provide energy to the building through solar radiation. An additional interesting aspect is that the desired angles of incident solar radiation on active photovoltaic surfaces are in practice often well suited for efficient sun protection too, without creating problems with the view to the outside. However the use of photovoltaics as the shading element within a light control device does also slightly limit the possible designs compared to the plethora of options when dealing with non active surfaces. One example mentioned before is the performance drop caused on the solar cells due to the additional external glass layer (adding two new interfaces where part of the energy is lost/ reflected), if the active shading device is used internally, behind a glass pane. Different light control and shading solutions have advantages for specific locations and orientations and have to be carefully chosen according to the context. Additional adjustability, adaptability and moving parts can ensure good performance in changing conditions in terms of light control as well as better angles for the solar cells. On the other hand a balance has to be found when taking into account the cost, complexity and maintenance of such systems. Therefore often lower performing non movable systems are still used.

fig. 8.3

Fixed Solar Panel integration

Facades which are not using any active shading device incorporate solar cells in the opaque part either in the window spandrel between the windows of two floors or as full floor height panels between windows. The advantage of separating the light control device from the active energy device (solar cells) is the cost reduction and the ability to use more solar cell surface area without the need for transparency. If the transparency of a shading integrated active system is higher (therefore percentage of cell surface lower) than the equivalent opaque area available on the facade, then it is more efficient and considerably cheaper to use the available opaque parts. Additionally solar panels can replace the cladding material in this case. The cavity space behind the panels needs to prevent overheating of the modules and can also be used for ventilation air preheating. On the other hand, when dealing with highly transparent facades and when the goal is to cover as much facade area as possible with pv cells, opaque panels are not an option any more.

FIxed Solar Panel integration

fig. 8.4 Fixed panels in horizontal installation (source: Author, based on Cost-Effective “Resourceand Cost-effective integration of Renewables in existing high-rise buildings”)

fig. 8.5 Fixed panels in vertical installation (source: Author, based on Cost-Effective “Resourceand Cost-effective integration of Renewables in existing high-rise buildings”)

8.2 External roller blinds. Shading characteristics

External roller blinds

External Roller blinds consist of a basic roller mechanism around which the shading material is rolled. The material can either be a flexible fabric, or a rigid material (plastic or metal) with horizontal hinges that enables it to be retracted and curved around the specific radius of the roller. Shading is achieved by the semi transparent characteristics of the material. A side railing structure is usually used to prevent flattering, buckling and noise due to wind forces, even though this remains a problem for higher buildings due to the flexibility of the sheet. Ensuring limited sun penetration with a uniform sheet/ surface in front of the facade has the advantage of very smooth shading and an outside view that is not hindered in any direction. However the fact that the sheet covers the entire glass surface does reduce transparency in all directions which has an impact on the view and on the total illumination of the internal space. In contrast to other solutions which only block the view to the sun, even diffuse or reflected light from other directions (not the sun direction) or the view to other directions is filtered through the semi transparent surface. However partly deployed blinds can, if needed, only cover the top part of the glass pane and therefore only block the sun without reducing transparency to other directions. Photovoltaic integration

fig. 8.6 Patent application for PV in roller blinds (source: Patent Repository Pantent number US5433259A)

fig. 8.7 Patent application for double roller PV. (source: Patent Repository Pantent number US2008163984A1)

Not many examples of external roller blinds with integrated PV cells are currently implemented. The characteristics of the shading surface do not allow brittle crystalline cells to be used. Only thin film solutions are practically possible due to their flexibility and mechanical properties. Patent applications of roll-able polymer sheets with integrated thin film or organic PV cells are frequently appearing and will soon become available as products. The minimum possible radius for a flexible thin film solar cell does not create problems with the rolling mechanism. On the contrary the lamination of the cells on transparent plastic foils (given that semi transparency is needed) will have to prove to be strong enough to avoid failure, since the different curvatures between the layers during the rolling/unrolling process can cause their connection to suffer. Varying tensile forces in the material during its operation, changing curvature and different expansion coefficients are threats for a reliable PV roller blind system.

Wind forces are another problem for PV integrated roller blinds. Vibration can cause problems but more importantly strong tensile forces have to be exerted on the sheet to prevent excessive movement. Saddle shapes and other forms inspired from tensile structures could be a solution against wind forces. One possible way to achieve this is to slightly rotate the bottom part of the sheet in relation to the top roller The need for extra wind resistance makes shapes deriving from tensile structures necessary. The saddle shape (fig.8.8, 8.9), where diagonally opposite corner points have the same height which is higher or lower compared to the other two, ensures resistance to external forces. At the same time it can serve as a way to also improve the incident angles of the sun on the PV surface and therefore also achieve a tracking effect. Even though rotation of the bottom part (or top part even though this would be much more difficult) is possible in order slightly track in different angles according to the sun position, tensile forces will constantly change unless an automatic tensioner is installed in the roller. Otherwise stresses on the material could in the long term cause problems to the laminated PV layer (fig 8.10). Additional ways to make a thin shading sheet resistant to wind, improve its rigidity without adding significant weight with supporting structures is (apart from strong tensile forces) to either give the material the highest possible rigidity to still be able to be rolled in a relatively big radius roller and after deploying it, shape it into more rigid designs (corrugated shapes), or to find ways to attach the flexible material to the glass pane or on a transparent polymer panel. Corrugated and prismatic shapes will be analysed in the design decision phase since maximum PV surface area is the first reason for shaping the active solar sheet in three dimensions, with extra rigidity being an additional benefit.

fig. 8.9 Aquiring saddle shape through unequal support length (source: Author, based on patent application US2009014130A1)

fig. 8.8 Saddle shape in front and top view. Shape needed for additional surface rigidity. Saddle created through rotation of top or bottom edge. (source: Author)

fig. 8.10 Danger of Thin film delaminating and buckling through stresses. (source: http://ej.iop.org/images/0960-1317/23/3/035040/Full/ jmm449047f1_online.jpg)

The idea of being able to use an extremely lightweight and thin retractable polymer foil with integrated photovoltaics is practically possible only if the foil is under strong tension or attached to a rigid base structure, since wind protection in front of the PV cells will reduce their efficiency and again add weight. Inflatable structures are also possible in order to achieve the needed resistance and rigidity, however they are usually not easily retractable or compatible with the use of rollers. The aforementioned rigid structure can be the glass pane itself or a 3 dimensional transparent polymer sheet (polycarbonate etc.). To firmly attach the external rollable polymer film to the glass or polymer and be able to retract it again vacuum, static electricity or a pressing mechanism can be used. Static cling foils already exist as shading products in the car industry or for glass windows in buildings. They are however usually applied after careful cleaning of the surface and often with the use of a liquid (water or other liquid) to ensure good visibility without air bubbles. External use has to be further researched since dust and other particles can be trapped between the foil and the glass/plastic surface. The foil can be attached and detached through an electrical switch which controls the “suction” effect between the surfaces. Micro distance holders within the glass or plastic surface can also limit the problems caused by trapped air or dust, but definitely make the solution more expensive. Additionally a horizontal or vertical sliding metal element with a soft plastic/rubber edge can press the foil on the glass and afterwards make the “swiping” movement towards the roller in order to firmly attach the foil to the window (fig. 8.12) . When this element is released the roller can again retract the foil.

fig. 8.11 Top part roller and thin film surface. (source: Author)

Vertically moving mechanism in rail. Horizontal rod pressing foil on glass pane.

Glass pane

fig. 8.12 Foil attachment on glass or polymer using pressure, static electricity or vacuum (source: Author)

fig. 8.13 Top and bottom roller layout with double sided thin film PV. Adjustment through overlapping. (source: http://www.redrok.com/neat.htm patent repository)

Double rollers are also an option to achieve better wind resistance. One roller on the top part of the glass pane and one on the bottom can be connected with the shading material which is wrapped around the rollers with tensile force ensuring to some extend its rigidity. In this case two layers of material constitute the filter to the outside and the system is usually not retractable. Adjustability of the transparency is achieved by rotating the rollers and therefore creating different overlapping patterns of the two foil layers. Also moving the less transparent part of the sheet to the top part of the window is possible if the device is designed with this effect in mind. Complete overlapping of the opaque patterns of the two layers can create more transparency while less overlapping shades the inner space more and produces more energy in case of PV integration. However the solar cells on the second (inner) layer, which need to be on the back side of the material, have a slight illumination loss due to the additional transparent layer in front of them before the sun rays reach their surface. Lamination of PV cells on both sides of the same material or double sided pv cells add more complexity to the design. The problems related to this idea are the same as the ones for simple external roller blinds.

fig. 8.14 Table

Vertical Rolling (flat)

Horizontal Rolling (flat)

-All orientations possible (Advantage for south)

-All orientations possible

-Smooth uniform shading in all directions (equally)

-Retractable, complete transparency possible

-Medium/Low Complexity

-Low cost

-No Tracking possible (otherwise flexible category)

-Only Flexible semi-transparent thin Film solar cells possible

-Very Lightweight, (also depends on wind resistance)

-Very low wind resistance (needs strong tensile forces and/or rigidity through shape, saddle shape etc.)

-Low PV Surface Area Potential

-Low PV Surface Area Potential 80

8.3 Roller based folding structures. Shading characteristics.

External tensile folding roller structures

External roller awnings and roller based folding structures are principally a similar solution to the roller blinds, since almost the same materials and retracting method/mechanism (roller) is used. The main difference is the angle of the shading surface in relation to the glass pane or facade in specific facade parts. Either the complete surface or only part of it can acquire a specific angle which can be very useful for less transparent materials in order to improve the view to the outside in different directions. For high solar altitudes and south orientation the surface can become almost horizontal and the angle can be theoretically adjustable if needed. The point at which the shading material will fold if the device is designed in order to have a vertical and a diagonal/tilted part can also vary according to the conditions. The substructure which supports the sheet and creates the folding angle also adds rigidity to the material against wind due to the forces exerted on the folding edge. On the other hand long overhangs and angles closer to the horizontal plane make bulky and heavy supporting arms necessary. Photovoltaic integration Integration of photovoltaics in retractable awnings is similar to the integration in external roller blinds solutions since the sheet materials are practically the same and the only difference is the supporting substructure that enables different angles. Structures for awnings made of flexible material can despite their weight and size serve also as adjustable tracking mechanisms, at least for one axis tracking. The arms used for the operation of the device can theoretically be controllable in order to orient the shading surface to optimal solar incidence angles. On the other hand tracking only around the horizontal axis means that changes in angles will only be necessary for varying sun altitudes without improved performance for azimuth change.

horizontal tension rods

Foil attachment on glass

Foldable hinged metal rods stretching PV foil

fig. 8.15 Creating Rigidity in foil through folding and tensile forces or through attachment on glass. (source: Author)

Horizontal Roller designs Conventional external roller blind or awning solutions usually operate in the vertical axis. There are however also applications where the shading surface is retracted horizontally. Horizontal movement limits the ability of the device to be partly deployed in order to shade efficiently for high solar altitudes (when the sun is high during the summer midday) and enable a non filtered view. On the contrary however it can be partly deployed to shade part of the glass pane width for lower sun angles in the morning and evening. Main advantages and disadvantages of roller solutions in general. The most important advantages of roller based mechanisms is the fact that the surface is completely retractable when solar shading is not needed. At the same time adjustability of the amount of desired shading material on the facade is also very important. The complete device is very lightweight when complicated supporting arm structures are not used for adjustable angles. The most important advantage however related to solar cell integration, which in a next stage will be further optimized, is the fact that a roller based device can cover the complete facade surface with semi transparent solar cells when direct sunlight enables high energy yields, and due to its retractability, not reduce daylight when the facade needs to be completely transparent (for example in the afternoon or in cloudy conditions, when a rigid shading panel would force the users to use artificial light). Disadvantages of these systems are related to the same properties that ensure adjustability, namely flexibility of the shading sheet materials whether that is a fabric, a plastic membrane, or a rigid material with hinges. In all cases wind loads have to be carefully dealt with and the retractable material with the solar cell lamination has to withstand in many cases very strong tensile forces, temperature changes and curvature changes. fig. 8.16 Table

Vertical Rolling folding

Horizontal Rolling

-All orientations possible (Advantage for south)

-All orientations possible

-Smooth uniform shading in all directions. View hindered by horizontal folding structure.

-Retractable, depending on folding structure size

-Medium Complexity

-Medium/low cost

-Some Tracking possible with adjustable folding

-Only Flexible semi-transparent thin Film solar cells possible

-Lightweight, if folding structure kept at low weight

- Medium wind resistance. (high tensile forces)

-Relatively high PV Surface Area Potential

-Relatively high PV Surface Area Potential 82

8.4 Horizontal blinds/louvers. Shading characteristics. Horizontal louvers or blinds can either be fixed or retractable. The lamelas themselves can also have a fixed angle or can be adjustable through rotation around a horizontal axis. Fixed solutions are cheaper and easier to implement. They are also easier to be constructed strong enough for wind resistance, even though they donâ&#x20AC;&#x2122;t have the advantage of retractable solutions to be completely lifted and protected when the wind load is too high.

External blinds/louvers horizontal

Shading with fixed and opaque horizontal (or close to horizontal) louvers is mostly suited for south orientations and usually better lower latitudes where the sun reaches high angles. In this case the view to the outside is not significantly hindered. During the summer the shading elements block the high altitude sun rays from entering the building, thus reducing the chance of overheating and the cooling demand. During the winter the lower sun altitude angle enables the sun rays to penetrate through/between the shading elements, heat the inner space and therefore reduce the heating demand. Structurally fixed (but also adjustable) louvers can be of different materials but usually metal is used. The supporting structure is most of the times a metal frame or just vertical mullions attached to the main facade structure or slabs.

fig. 8.17 Horizontal adjustable louvers. (source: http://www.wicona.se/sv/Produkter/Solskydd/)

fig. 8.18 Horizontal adjustable louvers. (source: http://www2.reynaers.com/SharePointApps/Arch_cat/EN/layer/images/BS100%20 solar_3D_detail1.jpg)

Adjustable and retractable external blinds can adapt better to different external light conditions. They create more or less transparency or shading depending on the internal needs and sun angle. When the conditions allow complete transparency they can be lifted in order to improve the view to the outside and gain the maximum possible physical light, thereby reducing the need for artificial lighting. Adjustment of the shading devices is usually achieved by rotation around a horizontal axis, placed either centrally or off-center relative to the louver surface.

Scale/size of the shading elements plays an important role in the overall architectural impression given and in the view to the outside. Smaller sizes create a smoother shading pattern and look often similar to the effect created by flexible roller blind materials. They also require a denser arrangement and makes the structure more complicated and vulnerable compared to larger element designs. The shading angles on the other hand are not dependent on the size of the lamelas but on the proportions and relative position of the units. 3d Shading studies can show how each design performs in practice. In both cases however, either for moving or fixed shading elements, horizontal blinds remain a solution suited more to south facades. Photovoltaic Integration

fig. 8.19 Louvers installation (source: â&#x20AC;&#x153;Cost effectiveâ&#x20AC;? project)

Integration of solar cells on external louvers is the most popular way for architects and engineers to combine energy production with solar shading and light control. In contrast to roller blind integration which is still under development (and can only become a viable solution with flexible solar cells that withstand thousands of bending cycles and tensile force variations), integration on external horizontal louvers has been possible for several solar cell types for many years. Size of the shading units is important for a cost effective design since very small modules become very expensive because of both the higher price of smaller separate solar modules and the necessary additional connections. Fixed systems have to be carefully designed after 3d simulations for the specific location and orientation in order to avoid partial shading. Very high sun altitudes in specific locations makes the spacing between the modules very large if the optimal ~35o of the surface (in relation to the ground plane) is the goal. Therefore often slightly steeper angles are used to increase the PV area and shading efficiency. Moving systems are rotating around the horizontal axis either centrally located in the shading unit or off-center. Tracking is therefore only following the suns altitude changes, which are very slow. Therefore manual adjustment is more cost efficient that an automated system. Crystalline cells profit more from the tracking effect than thin film solutions.

fig. 8.20 3d model. Preliminary Research, foldable triangular louvers idea (by Author)

In both cases the materials used can vary, although most common solution is a glass-glass laminate that has a specific spacing between cells to allow some light to pass through. 84

45o

45o altitude, close spacing shaded

60o

60o altitude, close spacing shaded

60o

68o

68o altitude, designed to avoid partial shading through spacing

fig. 8.20 Adjustable Louvers spacing and partial shading

In order to avoid partial shading of the PV sunshades the spacing between them and the angle have to be carefully calculated. The highest position of the sun throughout the year is usually used as the reference in order to make sure the units are never shaded. For fixed louvers either only the part that is constantly illuminated throughout the year is covered with PV cells or the spacing and angle is enough to ensure complete illumination. The mechanism used for adjustable external louvers usually moves more units parallel to each other at once, if not the complete column of modules for one floor hight. If the rotation axis is centrally placed on the louvers then a frame or just vertical mullions incorporate vertical rails and a system to rotate the axis, either with gears or with an arm hinge connection. fig. 8.21 Adjustable Louvers mechanism, metal levers moving all louvers at once (source: Author)

fig. 8.22 Adjustable Louvers mechanism using rotating gear/axis (source: Author)

(Figures Source: Shadoglass, Glass Solar Shading Systems) fig. 8.23

Central rotation axis. PV cells used on front/lower part of the lamella in order to avoid partial shading from unit above.

fig. 8.24

Rotation from vertical rail and hinged sliding lever system without supporting axis.

fig. 8.25

Rotation mechanism of fig. 8.24 in three different angles in side view

fig. 8.26 System using rotation within

vertical metal profile

fig. 8.27 Lever system side view. Hydraulics used for operation

8.5 Vertical Louvers. Shading characteristics. Vertical louvers either in fixed designs or with an adjustability around the vertical axis are very efficient shading devices for east or west facades but also for blocking the early morning and late evening low sun reaching the facade surface from the sides. The movable fins can be oriented towards the desired view and at the same time block or allow the sun penetration. Rotation can even lead to a position parallel to the facade where complete shading of the space can be achieved or smooth semi transparency if the shading surfaces are not completely opaque.

Vertival louvers

It is possible to make vertical louvers retractable in order to achieve complete transparency by sliding the units to the side, often covering only a small vertical part of the facade due to the slender shape of the louvers. The material of the shading units can be practically any of the commonly used facade materials. Semi transparency is often achieved with translucent glass fins or perforated metal sheets. The supporting structure can be a frame attached to the existing facade or only horizontal transoms on which the vertical axis of the louvers is supported. If the facade design enables it, the vertical fins can also be placed on the edge of the building slabs or on the existing horizontal parts of the facade frames.

fig. 8.28 Vertical translucent shading Louvers (source: http:// www.coltgroupamerica.com/shadovoltaic-photovoltaic-systems. html)

Wind is again a threat especially for movable louvers as it is for horizontal sun shading systems and therefore the supporting structure and hinges or other moving elements have have to be properly designed and installed for noise and vibration -free operation. Photovoltaic Integration

fig. 8.29 Vertical rotating PV Louvers (source: http://www.coltgroupamerica.com/shadovoltaic-photovoltaic-systems.html)

Integration of solar cells on vertical louvers is principally similar to the respective horizontal solutions. Fixed designs are not as common as for horizontal blinds or for non active simple vertical shading louvers. In order to achieve a relatively good output fixed photovoltaics would have to be placed at least under a diagonal angle in relation to the facade plane, usually towards the south. In order to avoid excessive partial shading of the elements they would need to be installed with a relatively large spacing. Despite spacing or even tracking it is impossible to avoid shading for a long time of the day for non south facades. PV cell technologies with better performance in diffuse light conditions like thin film or organic cells are definitely better suited for less optimal orientations.

Tracking louvers moving around the vertical axis can retain a better position in relation to the sun rays and are able to track the sun throughout its daily movement. Even if vertical opaque sunshades are better suited for east or west

orientation when considering the shading efficiency as a ratio of the shading percentage and unhindered external view (transparency), photovoltaic integration can in some cases lead to a vertical arrangement as the most efficient solution even for south facades, especially if off-peak performance or more uniform output within the entire day is a primary goal. Vertical positioning of the solar cells compared to the perfect horizontal inclination when using horizontal tracking louvers can have advantages for high latitudes where the sun reaches only low angles and therefore a fixed element in this direction does not have significantly limited performance.

Ability of system to be retracted for transparency

Angle adjustment

Partial shading of Louvers for large angles

fig. 8.30 Vertical adjustable louvers system and shading threat (source: Author)

Horizontal Louvers Fixed

-South orientation preferable

-Partial Transparency. View hindered in specific angles by opaque louvers depending on size and spacing.

-Not Retractable

-Low Complexity

-Low cost

-No Tracking

-Most types of PV cells possible. Opaque and semi-transparent both possible.

-Relatively Lightweight (wind resistance dependent)

-Relatively good wind resistance (size/material dependent

-Very low PV Surface Area Potential

fig. 8.31 Table with horizontal louvers characteristics (source: Author)

Vertical Louvers Fixed

Horizontal Louvers Adjustable

-All orientations (south orientation preferable) -Partial Transparency. View hindered in specific angles by opaque louvers depending on size and spacing. Adjustability -Retractable, depending on louvers thickness -Medium Complexity -Medium cost -Tracking possible, Sun altitude only -Most types of PV cells possible. Opaque and semi-transparent both possible. -Medium Lightweight (wind resistance dependent) -Medium wind resistance (large amount of moving parts), size/material dependent -Very low PV Surface Area Potential (for full surface illumination)

Vertical Louvers Adjustable

-All orientations (East/West and North orientation preferable) -Partial Transparency. View hindered in specific angles by opaque louvers depending on size and spacing. Adjustability -Retractable, depending on louvers thickness -Medium Complexity -Medium cost -Tracking possible, Sun Azimuth only -Most types of PV cells possible. Opaque and semi-transparent both possible. -Medium Lightweight (wind resistance dependent) -Medium wind resistance (large amount of moving parts), size/material dependent -Very low PV Surface Area Potential (for full illumination

fig. 8.32 Table with vertical louvers characteristics (source: Author)

8.6 Sliding flat panels. Shading characteristics. Sliding facade panels are a very popular and relatively inexpensive way of shading. They are practically in terms of design simplicity the first step in a range varying from simple fixed panels on the opaque part of the facade until complicated systems that use rotation, rollers and sliding in different combinations to achieve better light control. Panels sliding on rails are either completely opaque or translucent/semi-transparent made of a variety of different possible materials.

Sliding panels shading

fig. 8.33 Sliding panels facade in vertical installation (source: http://www.arcat.com/photos/coltint/139856.jpg)

The shading effect can be very similar to rollable flexible sheet when the material has similar light filtering properties. It is a shading method that does not rely on a large number of small opaque or semi transparent units with completely transparent spacing between them creating a specific shading pattern. On the contrary the structure, similar to flexible roller blinds, creates a smooth uniform shading through one simple sheet. The advantages in terms of the view to the outside and the resulting shadow are therefore similar to those of the roller blind sheets. Additionally sliding panels are much stronger and simpler in making them wind resistant. Mechanisms have relatively low maintenance compared to other movable solutions and can be integrated within the window structure as additional rails. The disadvantage of the sliding panel principle is the size of the elements, and the fact that they cannot be retracted in order to use less facade space when they are not needed as a protective layer for the glass part. Photovoltaic Integration.

fig. 8.34 2Degree Facade in BAU 2009 exhibition using sliding PV panels (source: http://www.fat-lab.de/archive/portfolio/bau-2009)

A relatively large amount of projects with sliding pv panels has been realized. The pv cells are in this case not integrated on a separate structure but form a complete solar panel of specific dimensions. Usually glass-glass laminates are used in order to achieve semi-transparency through the spacing between the solar cells, which can be either crystalline cells or thin film cells. The way to achieve semi transparency in these cell technologies has been analysed in the related chapter. Since parallel movement of the panels to the facade plane has no effect on the angle of the sunrays hitting the photoactive surface, sliding does not constitute a tracking method. Therefore the position of the panels as long as they are not covered or shaded by other building elements or external adjacent objects has no impact on the energy output.

The next phase of the Graduation Thesis will concentrate on the PV surface area maximization through three dimensional solutions. Flat sliding panels used as shading elements and active solar energy devices have in this context the disadvantage of not only covering less surface area due to their flat design (which is anyway the case for all of the aforementioned designs since they are aiming at total direct surface illumination in most conditions), but also being restricted to either completely cover the glass part of a facade, or slide to the side and cover the opaque part of the facade. This is not the case for retractable panels either as roller blinds or using folding mechanisms. In a theoretically optimal situation where the conditions are perfect for energy production and shading is needed simultaneously (as a result of the direct sunlight) rigid panels are not able to cover the complete surface. For real conditions however and given the prices of solar cells today, this is only a problem in relation to the specific illuminated surface area optimization study.

area available for fixed PV etc.

area covered by sliding panel

fig. 8.35 Sliding panels and the disadvantage of constantly covering only part of the facade surface. (lower surface are potential) Comparison to roller version.

fig. 8.36 Digital 3d model showing potential for different panel layers including PV integration created during preliminary research phase. (source: Author)

Horizontally sliding panels

Vertically sliding panels

-All orientations possible.

-Smooth uniform shading in all directions.

-Retractable (through sliding, therefore covering facade surface)

-Low Complexity -Medium/Low Cost

--Medium Complexity. More complex than Horizontal sliding due to the need to constantly carry/support panels weight.

-No Tracking

-Medium/Low Cost

-Most types of PV cells possible.

-No Tracking

-Relatively Lightweight (dependent on Material and wind resistance)

-Most types of PV cells possible.

-Relatively good wind resistance (dependent on material and sizing) -Low PV Surface Area Potential

-Relatively Lightweight (dependent on Material and wind resistance) -Relatively good wind resistance (dependent on material and sizing) -Low PV Surface Area Potential

fig. 8.37 Table

8.7 Sliding 3d panels and rigid Origami. Shading characteristics.

Sliding 3d panels shading

Sliding three dimensional surfaces are an option that has similar characteristics to simple flat and rigid panels. It is not a common shading solution, especially when considering complex shapes, with curvatures or folding patterns in both directions. The main reasons for shaping a sliding surface meant for shading purposes in the third dimension would be either architectural/aesthetical reasons or an increase in structural rigidity and therefore wind resistance in combination with lower weight (and less material). Due to the limitations or high cost of glass when used for free shapes and the need for semi-transparency, the materials used for such an application would probably be metal (perforated for semi-transparency) or polymer based. Since the 3-dimensional shape is rigid and does not fold or bend, even prismatic or other shapes can be

fig. 8.40

fig. 8.38 Horizontally corrugated 3d shape sliding on facade. (3d model by Author)

made of a single material, without connections. The use of one single material and the potential to shape it in a structurally very efficient form can make the system very lightweight if a thin and light material is used. Shading characteristics are similar to those of a semi-transparent flat sheet although very dense designs can create variations in transparency and create darker parts. Viewing angles are either slightly or significantly limited depending on the overlapping of the surface for large deviations from the direction vertical to the facade.

fig. 8.39 Honeycomb (inverted pyramid) shape example. (3d model by Author)

Photovoltaic Integration.

Shading equivalent surface a x length

Energy equivalent surface b x length b

fig. 8.41 Additional PV surface created through 3d shaping of surface

Even though the use of a three dimensionally shaped surface as a sun shading element that would slide in the horizontal or vertical axis along a facade -covering a transparent part of it- can only be justified when it enables the use of a more lightweight material (that would otherwise suffer from buckling), when considering PV integration the same idea becomes more interesting. One first decision is related to the type of solar cells which are going to be used for such a structure. A crystalline solar cell solution would impose a design that -as usually is the case with louvers- optimizes specific surfaces in order to avoid partial shading during the most important hour range throughout the day during a whole year. At the same time the design and size of each cell makes non orthogonal surfaces either completely inefficient or only slightly less efficient. Thin film solutions on the other hand enable a more lightweight design and allow for folding or bending of the cells down to a specific radius. The main difference however would be in the design of the three dimensional surface in order to achieve higher efficiency. There is the potential of using inorganic or organic third generation thin film solar cells for surfaces that are not perfectly exposed to the sun during all hours within the day. Very good performance of these cells under diffuse light and various angles makes the maximization of the active solar surface of a 3d shading panel a challenge, in order to explore the possible benefits. On the other hand the sliding principle and rigid design retains its inherent disadvantages described for the flat sliding surface design, namely the limited active surface that can never cover the complete facade module. 3d designs can in that sense overcome this problem but not reach the surface values of retractable units. They provide however a simpler, single material wind resistant solution. The possible 3d shapes are unlimited. Optimization can lead to designs based on surface maximization, maximum view to specific directions, or structural rigidity for the minimum weight.

fig. 8.42 Different patterns for 3d shape structures (3d models by Author)

Horizontally sliding 3d shapes

Vertically sliding 3d shapes

-All orientations possible depending on 3d shape

-All orientations possible depending on 3d shape. Advantage for South orientation due to ability to only shade top part of the glass pane/window.

-Smooth uniform shading. Dense designs creating partially more shadow. Under large angles limited view due to overlapping surfaces. -Retractable (through sliding, therefore covering facade surface) -Medium/low Complexity -Medium Cost -No Tracking

-Smooth uniform shading. Dense designs create partially more shadow. Under large angles limited view due to overlapping surfaces. -Retractable (through sliding, therefore covering facade surface) -Medium Complexity. More complex than Horizontal sliding due to the need to constantly carry/support panels weight.

-Most types of PV cells possible. Crystalline requiring constantly exposed active surface. Thin film suited for surface area maximization.

-Medium Cost

-Lightweight (if shape is optimized for good rigidity/ weight ratio and not for maximum surface)

-Most types of PV cells possible. Crystalline requiring constantly exposed active surface. Thin film suited for surface area maximization.

-Very good wind resistance (especially with rigidity optimization) -High PV Surface Area Potential with new PV cell technologies compared to footprint. However no complete facade covering potential.

-No Tracking

-Lightweight (if shape is optimized for good rigidity/ weight ratio and not for maximum surface) -Very good wind resistance (especially with rigidity optimization) -High PV Surface Area Potential with new PV cell technologies. Shapes limited to window dimensions. No complete facade covering potential. fig. 8.43 Table

8.8 Linear Foldable structures and parallel Origami folds. Shading characteristics.

Foldable structures shading

fig. 8.44 Horizontally folding facade panels (source: http://www.world-architects.com/en/pages/products/ colt-sun-protection

Foldable shading structures use hinge connections between rigid elements in order to achieve a deployable shading device that can cover a larger or smaller facade surface area depending on the internal climate needs and the external conditions. They can be seen as a rigid panel solution with hinges that adds adjustability, but also a three dimensional arrangement, which will be further analysed. When the system is retracted the surface is packed into a very slender shape, usually vertical to the facade plane. The folding principle however is not always necessarily applied in linearly growing structures, but also in various shapes, often inspired by origami structures. One important characteristic often related to the shape change of these structures is the movement according to a Poisson ratio, which shows that the width of the structure is changed in relation to the length change. Shorter arrangements become thicker while longer arrangements become thinner for the same structure surface. The shading effect created by folding structures largely depends on the specific shape and the way it folds and unfolds. Simple linear deploying panels act as normal panels when they are deployed completely, and as horizontal overhangs or vertical louvers (depending on the direction of the system) when shrunk to their smallest shape. The fact that during their folding and unfolding process foldable structures take different shapes and dimensions on both directions can be used in favor of the light control efficiency and energy performance. Possible materials and supporting structures are similar to sliding panels, which means that a large variety can be used. Hinges and mechanisms to ensure the movement of the system add complexity and need to be carefully designed to withstand wind forces without noise and vibrations. On the other hand the division of the total surface in smaller panels and the folding geometries enable the use of thin and lightweight panel materials.

fig. 8.45 Vertically folding shading panels (source: http:// www.stylepark.com/en/colt/folding-shutter-front)

Photovoltaic integration Not many examples of foldable photovoltaic structures on facades are known. Most foldable photovoltaic panels use the hinged connections, for a more compact shape during transport. The main reason for this lack of foldable designs is partial shading of panels during the folding positions and the changing position of the sun throughout the day. Most PV integration strategies related to shading systems start with designs that expose the active shading surface area to the sun in such angles that overshading of each unit to the other ones is avoided completely or at least to a very large extend. This is related to the properties of crystalline silicon cells which show extreme performance drop when partially shaded or not facing direct light at all. On the other hand thin film (inorganic or organic) cell technology is much more efficient under diffuse light conditions and partial shading. Therefore it constitutes a good candidate for such a structure in the future. The compact design of a partly folded sun shading system with an active solar surface equivalent to a much larger facade area has a potential of making optimal use of the given facade area of a building. On the other hand the inefficient ratio of used pv material to the energy gains (due to the design) makes this a design for the future, when the solar cells price is less important than the total efficiency per square meter of facade area.

a Shading equivalent surface a x length

Energy equivalent surface b x length

fig. 8.46 Shading and energy producing equivalent surface area. PV area per projection increased through folding.

The study of partial shading percentages in foldable structures (performed in the following chapters) is a very useful process in order to better understand how shape can improve the total efficiency by exposing more PV material to the sun in all conditions.

fig. 8.47 Preliminary research 3d model. Vertical folding panels and climate box horizontally integrated

100

8.9 Linear Deployable Origami Shapes.

Foldable structures shading

fig. 8.48 Origami folding patterns. Size reduction.

fig. 8.49 Origami inspired light control facade structure. Al Bahar Towers in Abu Dhabi (2012). (source: http://www.archiscene.net/ firms/aedas/al-bahar-towers/)

101

Origami shapes or thin sheet folding patterns in architectural applications are mainly used as a technique to improve the rigidity of a thin sheet material that can retain its low wait and thickness, but at the same time withstand significantly higher forces than its flat equivalent. Another characteristic is the fact that some of these designs are developable/foldable with a very high ratio between folded and unfolded size and a very strong anisotropy (meaning that it is very easily folded in one direction by exerting only a low force on it, but much stronger in withstanding forces in the other direction). Both of these properties are useful for a linearly deployable facade element used for light control. Shading characteristics of such a structure made of a semi-transparent material are similar to those of sliding 3d panels and rigid origami described above. Overlapping surfaces under specific angles and more shading in denser parts of the surface create variations in an otherwise smooth and uniform light filter. Specific patterns can be optimized for shading/view in predefined directions or for high rigidity. Non linear Deployable Shapes Similarly to linearly folding and unfolding 3d shapes, other patterns can be used in order to control the light through the size difference seen as a projection of the shape on the facade. Shapes that open and close like flower structures, surfaces that fold vertically to the facade plane with fixed folding axis or a deployment motion that includes rotation are only a few examples of how a three dimensionally folded sheet can contract and expand in order to control the incoming light. Precisely defining the shading characteristics of these structures would require an analysis of each specific pattern since every shape creates different effects. On the other hand mechanisms that do not include a motion of all surface points in order to make the shape completely retractable, but rather a folding around fixed axes, will certainly have a partly hindered view to the outside. (fig 8.49)

Photovoltaic Integration Similarly to rigid 3d shapes, integration of PV cells in developable origami-like 3d structures is not usual in architectural practice. The main examples of this idea can be seen in small scale applications, like easily transportable foldable solar cell modules. However real origami based PV structures that fold and unfold in order to make use of the big size difference between the two conditions as well as the structural advantage due to the folds, are developed extensively for space vehicles, space stations and satellites. The advantage of low weight and extremely high size ratio between the folded and unfolded state is very important in this context. Due to the hinged edges of the prismatic shapes electrical connections are slightly more difficult. Similarly, depending on the solar cell type partial shading has to be taken into account. Crystalline and thin film organic or inorganic PV cells are theoretically both possible, but the low performance of crystalline cells under average and low illumination, bad angles or partial shading, as well as their brittle material properties make second and third generation thin film cells a better choice. In this case the direction of the cell stripes has to be carefully chosen depending on the shape. Apart from the ability to fold into small sizes, which is very useful for maximum facade transparency, and their optimization potential for added structural rigidity, origami shapes have a very high potential for maximizing PV surface area for a given facade space. Prismatic patterns can add significant amounts of active solar surface, which will be further researched in a later stage.

fig. 8.51 Origami linear folding

fig. 8.50 Origami patterns with PV integration used in space due to size (weight) reduction through folding. (source: http://www.heraldextra.com/news/local/photos-byu-engineers-use-origami-for-spaceprojects/collection_a07bded8-58e9-11e3-a1930019bb2963f4.html#0)

102

Linear Foldable Structures. Horizontal

-All orientations possible depending on 3d shape. -Uniform shading by semi-transparent panels. View hindered by hinge structures. Transparency and view in specific directions dependent on number, size and spacing of hinges. -Partially or fully retractable. Dependent on folded/ unfolded structure size ratio and panel thickness. -Medium/high Complexity depending on 3d shape. Less complex than vertical due to easier support by bottom rail.

Linear Foldable Structures. Vertical

-All orientations possible depending on 3d shape. Advantage for South orientation due to ability to only shade top part of the glass pane/window. -Uniform shading by semi-transparent panels. View hindered by hinge structures. Transparency and view in specific directions dependent on number, size and spacing of hinges. -Partially or fully retractable. Dependent on folded/ unfolded structure size ratio and panel thickness. -Medium/high Complexity depending on 3d shape.

-Medium/High Cost

-No Tracking. Only slight tracking possible due to changing angles in different folding positions.

-Most types of PV cells possible. Crystalline requiring constantly exposed active surface. Thin film suited for surface area maximization. Second generation thin film longitudinal strip cells exclude specific triangular prismatic shapes for surface maximization.

-Medium Lightweight (dependent on panel material, shape optimization and hinge number/material)

-Medium weight (dependent on panel material, shape optimization and hinge number/material)

-Relatively good wind resistance (dependent on shape

-Relatively good wind resistance (dependent on shape)

-Very high PV Surface Area Potential with new PV cell technologies.

-Very high PV Surface Area Potential with new PV cell technologies. Shapes limited to longitudinal panels and according to window dimensions.

fig. 8.52 Table

103

Non Linear Origami deployable shapes

-All orientations possible depending on 3d shape. -Uniform shading by semi-transparent panels. View hindered by fixed hinge axis structures. Transparency and view in specific directions dependent on number, size and spacing of fixed elements. -Not fully retractable. Light control dependent on folded/unfolded size ratio. -Medium/high Complexity depending on 3d origami shape. -Relatively high Cost -No Tracking. Only slight tracking possible due to changing angles in different folding positions. -Most types of PV cells possible. Crystalline requiring constantly exposed active surface. Thin film suited for surface area maximization. Second generation thin film longitudinal strip cells exclude specific triangular prismatic shapes for surface maximization. -Medium weight (dependent on panel material, shape optimization and fixed hinge number/material) -Relatively good wind resistance (dependent on shape) -High PV Surface Area Potential with new PV cell technologies. Dense designs for higher area create transparency problems. fig. 8.53 Table

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8.10 Flexible Shading. Bending, twisting

fig. 8.54 Preliminary research 3d model. Surface twisting

The use of flexible materials as shading elements has always been extremely common in Architecture, mostly in the form of fabric and more recently due to the rapid development of polymers also in the form of foils and membranes. The main advantage modern buildings make use of is the low weight, low cost and simplicity they offer. As active and adaptive mechanisms for light control the most common systems making real use of their flexibility are roller blinds. The ability to retract the light filtering (or blocking in case of opaque materials) surface into very small sizes through rolling is very important for ensuring efficient inner climate control and maximum transparency. On the other hand the properties of the material are only partly exploited when only using the principle of rolling around a rotating axis. Designs of lightweight shading systems have been created, that are able to twist or bend in order to control the light or to some extend track the sun. Tensile force is used to stabilize the thin fabric or membrane in order to withstand wind forces. Even using relatively strong tensile forces, adaptive and moving fabric or membrane structures on facades tend to face significant problems with wind loads especially on facades of higher buildings. Most solutions are therefore either fixed (non adaptive) tensile structures using the known methods of creating rigid tensile shapes through tensile surfaces and compression elements (struts etc), or protected through an additional external glass layer or other protective structures.

fig. 8.55 Preliminary research 3d model. External flexible corrugated PV curtain.

Material shapes not belonging to the categories of membranes of fabrics can also exhibit a flexible behavior, enough to also adapt to the external conditions and be used as light control devices. These can be bending or twisting polymer sheets with specific wind resistant basic shapes, as for instance flexible corrugated sheets. Also structures with a combination of flexible and rigid parts can make use of the flexibility in order to deform and acquire a specific position in relation to the sun.

fig. 8.56 Vertical flexible strips twisting to control light and improve exposure to sun.

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Photovoltaic integration Despite the widely spread use of flexible materials in architectural shading devices, especially in the form of fabric but recently also as polymer membranes, solar cells have rarely been integrated on flexible facade elements, especially when these use their flexible characteristics to control the incoming light. The PV integrated designs developed until today mainly as patent applications have been previously discussed in the roller blind chapter, since this is the most common way of using material flexibility in favor of light control. In non rollable solutions adaptability is achieved by bending or twisting the surface in order to change the exposure angle of the shape either for tracking reasons or for changing the percentage of light passing through. An extremely limited amount of this kind of applications has been used in practice however. Mainly ephemeral lightweight structures, large span membrane roofs, pavilions, exhibition spaces or military tents are the structures where solar cells on flexible materials have been successfully used in a building scale. In only very few cases these structures are able to constantly adapt to the external conditions with movement of the flexible material. The reason is the difficulty of a fabric or polymer-PV laminate to withstand multiple bending cycles and tensile force fluctuations. On the other hand structures that are not exposed to strong wind forces and therefore need lower tension to achieve adequate stability are better candidates for bending/ twisting PV systems. Solar cell types that can be used for flexible designs are practically only inorganic or organic thin film (second or third generation) cells. It is however possible to use crystalline cells in designs that are only partly flexible and also contain rigid parts. Flexible parts are in this case used as elastic hinges.

fig. 8.57 Use of PV cells in flexible ephemeral structures. Pavilion electrical car charging design. (source: http://www.dezeen.com/2013/11/14/volvo-pure-tension-pavilion-charges-an-electric-car-by-synthesis-design-architecture/)

fig. 8.58, 8.59 Preliminary research bending panel designs

The aforementioned light control system categories that use hinges in order to fold and unfold can in a similar way use bending instead of folding even if the folded state can not be as small in size as folded elements. Therefore flexible systems will have similar shading characteristics but a better view to the outside due to the absence of a separate hinge material. 106

Systems of the same type (similar retraction method, similar shape) that use bending instead of folding will not be analysed in tables again, since the shading characteristics are similar. Differences can be seen in two important categories. Wind resistance and rigidity is significantly reduced and therefore creates problems in higher buildings. On the other hand the view to the outside is not hindered by hinges, as explained above. In order to avoid repeating a similar analysis in a characteristics table only systems that use flexibility in a unique way that can not be easily imitated using hinged rigid shapes will be included.

fig. 8.60 Table

Horizontal twisting fabric/membrane strips

-All orientations possible. Slight advantage for South orientations. -Uniform shading by semi-transparent material. Spacing creates transparent stripes similar to horizontal louvers. -Partially or fully retractable if moving on rails. Otherwise view hindered only in specific angles by overlapping surfaces. -Low/medium Complexity depending on tensile forces needed and simplicity of twisting mechanism. -Medium Cost -Tracking possible. Theoretically possible to rotate without twisting and with twisting only partial tracking. Altitude tracking. -Mainly organic or inorganic thin film solar cells possible. Otherwise semi-rigid solutions with crystalline cells. -Lightweight -Bad wind resistance (dependent on tensile forces) -Very Low PV Surface Area Potential. 107

Vertical twisting fabric/membrane strips

-All orientations possible. -Uniform shading by semi-transparent material. Spacing creates transparent stripes similar to horizontal louvers. -Partially or fully retractable if moving on rails. Otherwise view hindered only in specific angles by overlapping surfaces. -Low/medium Complexity depending on tensile forces needed and simplicity of twisting mechanism. -Medium Cost -Tracking possible. Theoretically possible to rotate without twisting and with twisting only partial tracking. Azimuth tracking -Mainly organic or inorganic thin film solar cells possible. Otherwise semi-rigid solutions with crystalline cells. -Lightweight -Bad wind resistance (dependent on tensile forces) -Very Low PV Surface Area Potential.

It is relatively difficult to achieve additional benefits from active solar sheet twisting compared to complete surface rotation or partial folding of one part of the surface with hinges. Bending of only one part of the surface is not as efficient in terms of tracking as complete surface rotation and creates less uniform illumination of solar cells. This would be acceptable if the potential of high PV surface area would exist, in order to balance the above mentioned problem. This is however not the case for simple sheets. On the other hand bending can have some important advantages. Firstly the ability to adapt to different facade surfaces of any three dimensional shape and curvature. Secondly the use of only one simple material without a separate material for hinges, which has an impact on the view from the inside or outside. A third important advantage is the ability to create lightweight structures through specific optimized bendable shapes. However the fact that the more the material can be bent in order to achieve better tracking or more efficient light control and transparency the worse the wind resistance becomes, poses significant in many cases problems. One of the first examples of flexible facade structures with PV integration will be presented, in order to highlight the use of these characteristics in favor of the energy efficiency of a modern building.

Bending/Twisting Shading surfaces

-All orientations possible. -Uniform shading by semi-transparent material. -Not retractable unless sliding used. In this case however the bending mechanism becomes very complex. -Medium Complexity if fixed. High complexity if sliding.

Result The Shading and Light Control Systems evaluated as the most efficient for further development are the ones that fulfill to a variety of predefined criteria visible in the created tables. The characteristics however, with the largest influence on the selection are the PV surface area potential and the ability to retract the structure into small sizes for transparency. Good wind resistance for low weight and low complexity are also important advantages.

-Medium Cost -Partial tracking possible. -Mainly organic or inorganic thin film solar cells possible. Otherwise semi-rigid solutions with crystalline cells. -Lightweight (dependent on ability to slide) -Bad wind resistance (strong tensile or compressive force needed). -Very Low PV Surface Area Potential for twisting sheet. Medium to high Surface Potential from bending.

fig. 8.61 Table

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8.11 Reference Project. Soft House by Keneddy Violich Architects The Soft House building is a project completed in 2013, designed by an international team lead by Sheila Kennedy, demonstrating some innovative concepts at the International Building Exhibition in Hamburg. It uses solid softwood structure that sequesters carbon, and a movable textile infrastructure that harvests solar energy and provides solid-state lighting. Of interest for the current thesis is the choice of flexible fabric elements for the facade incorporating multi junction thin film solar cells. Integration of flexible facade modules that make use of the materials properties not only to achieve a curved predefined shape, but to actually move, follow the sun or control the light, is a novel concept and implementation that until now was usually only seen in theoretical concepts or small scale standard devices (pv sunscreens for example). The external facade skin is made of a tensile structure starting with a fiber reinforced composite board part on the top of the building (which retains some elastic properties), which extends diagonally in front of the main building facade to form textile strips with integrated solar cells. The solar tracking motion is achieved by rotating the bottom part, where the strips are attached to a corrugated frame/beam, practically through twisting the textile to the east and west direction. Additionally the rooftop composite board structure is able to raise or lower the complete flexible strip system depending on the period of the year and thus the sun altitude. During extremely windy conditions the facade is able to retract flat against the roof for protection. The twisting movement of the strips also acts as a light control system for the inside of the building and enabling specific viewing angles. Adjustments to the responsive facade system are made both seasonally and daily through a central Building Management System. “The multi junction solar cells used generate about 60 kilowatt-hours (kWh) of electricity daily, or about 16 kWh per housing unit, significantly above half the anticipated household energy needed, with the balance coming from IBA’s supplementary clean energy grid. The façade demonstrates how historically “hard” energy infrastructure such as non renewable energy, glass-based solar panels, and sun-tracking machinery can be transformed by design that uses soft, lighter-weight, low-carbon materials linked by energy and information networks.” Kennedy S. article “Building façades that move, textiles that illuminate”

fig. 8.62 “Soft House” view of PV strip supporting frame with curvature for limiting partial shading of the cells. (source: http:// www.kvarch.net/projects/87)

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fig. 8.63 “Soft House” facade (source: http://www.detail.de/architektur/themen/bewegliche-pv-membranen-soft-house-in-hamburg-021023.html)

PV strips top part adjustable inclination for seasonal tracking Horizontal position for wind protection

PV strip twisting for light control and tracking.

fig. 8.64 â&#x20AC;&#x153;Soft Houseâ&#x20AC;? section (source: http://www.kvarch.net/projects/87)

fig. 8.65 Four different inclination positions of flexible PV Strips depending on season and weather (source: http://www.kvarch.net/projects/87)

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9_Phase 2

Shape research in terms of Surface Area Potential. Surface Comparison through parametric tools.

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9.1 Solar cell efficiency and 3d shapes Active surface area maximization With the solar cell surface optimization in micro-scale through prismatic shapes, pyramids (facing out or to the inside) etc., as an inspiration it is not over-optimistic to imagine a larger scale three dimensional arrangement that would improve the efficiency of PV cells through light-trapping, multiple reflections and significantly increased photovoltaic surface area.

fig. 9.2

fig. 9.1

Three dimensional arrangements of light absorbing structures are also very often seen in nature. Tree leaves or flowers have the tendency to shape into optimized forms in order to achieve two goals that are also very important within the development of the design of this thesis, namely the best light absorbing surface form and at the same time the shaping of this extremely thin and lightweight surface in order to acquire the necessary rigidity. An additional advantage is the ability to expand and contract the whole surface by moving only limited control points/elements and thereby avoid a very complicated structure. Trees as a whole can also be seen as three dimensional light absorbing mechanisms, since their photo-reactive surfaces (leaves etc) are not arranged in a single sun facing layer that absorbs the entire energy, but in many small scale elements that absorb an amount of light and let the sun-rays pass through to reach other layers at different parts of the tree. This can be seen as the opposite approach compared to the increasingly popular solar concentration method used in high tech solar devices, where concentration of the sun rays through a lens or parabolic/ curved mirror enables the use of a very small surface PV cell, thus making expensive high performance cells possible.

fig. 9.1, 9.2 Solar cell surface texturing in microscale for increase in efficiency. Pyramid and inverted pyramid shape most efficient (source: http://www.pveducation.org/pvcdrom/ design/surface-texturing)

fig. 9.3 Leaf structure with corrugations. Natureâ&#x20AC;&#x2122;s approach of solar exposure optimization and structural rigidity of thin surfaces.

fig. 9.4 Tree structure, branches and leaves creating light absorbing layers in 3 dimensions to increase solar energy use.

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On the opposite end, research focusing on low cost production methods (as the roll to roll process for thin film/organic solar cells) and cheap/simple materials with lower demand in perfect insolation angles and direct light conditions, follow a completely different direction that is probably more suited for building/ facade integration due to its simplicity, low cost and design freedom. In this sense this design freedom and the ability to create three dimensional large surface shapes can become the optimization medium and the performance “push“ for these technologies.

In this case the most relevant solar optimization scales can be considered the facade, building, or facade module scale, even though as described above the microscale can also become an interesting influence. Urban scale solar optimization can be seen as the broader goal of achieving a uniformly illuminated area through efficient design. Therefore facade solutions/structures in the building scale that promote the idea of easy adaptability to optimized building volumes and “transformability” of less optimal flat facades into an efficient three dimensional skin are highly preferable.

Research and experiments have been conducted in this direction in order to find better performing alternatives to flat solar panels. The first hindrance in this procedure is the above mentioned non uniform illumination of non flat surfaces and partial shadow of modules. New third generation materials, inexpensively printed by pass diodes, a large increase in photovoltaic area or adaptive characteristics are possible answers to these concerns.

The common characteristic of the optimization in all scales is that in almost no case the optimal shape is in the end the perfectly flat surface. Cost/efficiency and not optimal performance is until now the main factor imposing flat and rectangular solutions.

9.2 Shape optimization in different scales The process of optimizing the shape of a surface for getting the best out of the solar radiation has many different applications and can be traced back to the very distant past when people just used their experience and knowledge to improve their living comfort in practice. Today solar optimization of surfaces has reached a level where human experience and knowledge can only define some basic guidelines for better performing shapes, while for most of the work in many cases we rely on specific computer software. Optimization can aim at maximizing solar heat gain, physical light penetration into spaces but also in more extreme scales (when compared to the human scale) such as optimization of the micro-scale of materials which can for example be perceived as texturing of a foil or a solar absorbing material, or on the other hand the forming of a landscape or positioning of the building volumes within a city. The different spots within the whole range of varying surface sizes constitute focus points for sciences/fields like material science and chemistry up to architecture and urban planning. The important ability of integrating the knowledge from the optimization in a different scale level into the currently researched structure can be decisive for a successful design.

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Approaching the scale of interest of this Thesis, it can be observed that today most architectural solar surface solutions with an ambition of shape optimization are using algorithms and parametric design in order to achieve optimally illuminated shapes. They usually focus on optimal incident angles on surfaces or on solving the partial shading problem by shaping the surfaces in such a way that they are acceptably illuminated throughout a day or a longer period. Practically the same procedure is followed for daylight analysis or for analysing the views to the outside and hindrances by adjacent buildings and objects. This does often not take into account the different PV technologies that perform better or worse in diffuse light conditions and partial shading and the possibility to maximize the solar active area and optimize shape at the expense of perfect incident angles and material cost. Projects and researches on exactly this topic, focusing on an alternative way of reaching additional efficiency have been researched in order to better understand the principles and mechanisms that could lead to a new way/strategy of using and integrating solar cells.

fig. 9.5 Microscale optimization. Honeycomb arrangement in texturing. (source: paper “19.8% efficient ‘‘honeycomb’’ textured multicrystalline and 24.4% monocrystalline silicon solar cells” Jianhua Zhao, Aihua Wang, and Martin A. Green)

fig. 9.6 Building scale optimization through 3d shaping of building surface for predefined solar exposure. (Source: “THE SUN ADAPTIVE ENVELOPE” project Ying-Chen Lin, Hamze Khalil El Sarout Che-Wei Yeh Wang, and Martin A. Green)

fig. 9.7 City scale optimization through 3d shaping the landscape and building blocks and building profiles. Maximization of solar exposure and optimal angles at specific parts. Insolation or PV performance goal. (Source: “Parametricism-A New Global Style for Architecture and Urban Design” Patrik Schumacher, London 2008)

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Research References

9.3 MIT 3d solar cells project (figures used from MIT 3d solar cell report paper and publications)

An example of research focusing on the potential of three dimensional solar cells is that of Marco Bernardi, Nicola Ferralis, Jin H. Wan, Rachelle Villalon and Jeffrey C. Grossman1, conducted by the departments of Materials Science and Engineering, Mathematics and Architecture within an interdisciplinary project of the MIT. This project uses genetic algorithms to produce different three dimensional shapes for a given footprint and compares them to each other and to the equivalent flat module. Various prismatic shapes derive from the solar optimization through this process. These shapes consist either entirely of solar cells in different shapes or of a combination of cells and mirrors. The main advantages of the three dimensional arrangement shown in this research derives from both the additional PV surface area, the multiple surface orientations favoring off peak solar gains and the reflections between the surfaces compared to the single reflection of a flat module. Advantages of the three dimensional shapes are extremely evident in higher latitudes and in diffuse light conditions compared either with the flat panel footprint equivalent or even with a sun tracking panel of the same size as the footprint. Additionally three dimensional structures seem to have a significantly more uniform output throughout the seasons of a year as well as during different hours of a day, which is in contrast to the high peak power hours of high performing PV cells (which is also very often a problem for the grid).

Results show that even though, as expected, the flat or almost flat panel is the best option when trying to optimize the energy output per PV surface (which can be interpreted as the maximum gain per euro spent), the total yield per footprint area is much higher for any of the chosen three dimensional PV structures. Especially high and origami shaped designs that gain more surface even on the vertical axis compared to flat vertical panels show very good performance characteristics and much more uniform output throughout the day or year. Mirrors and reflective surfaces within the 3d structures show a high potential of significantly improving the Energy per Solar cell surface area ratio, where high surface shapes with sharp angles suffer. Comparison of the performance of different shapes for different latitudes in increments of approximately 10o is performed in order to highlight how the advantage of 3d shapes over flat panels changes depending on the location. From these results it is obvious that the higher the latitude and lower the available direct sunlight, the more 3d shapes outperform flat panels for the same footprint. As a final step, since the research concluded that a prismatically formed structure of as large height as possible would perform the best, possible applications were proposed such as an electrical bike charging station.

The testing procedure included laboratory but also real outside conditions tests since the distances between the light source and the panels should remain approximately constant, which is sometimes difficult for different shapes under laboratory testing machines. Several shapes were tested, starting from a solar cell cube of varying heights, origami like tower designs and different combinations of triangular cells placed within the same given footprint. Apart from solar cell surfaces, also prismatic mirrors were used to see how reflections can influence the final energy output.

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fig. 9.8 3d Solar cells tower test model in origami pattern

fig. 9.9

Shapes combining solar cell and mirror prismatic surfaces in order to increase reflections and total surface area. fig. 9.10

Graphs showing the efficiency ratio between flat panel and three dimensional cube in different latitudes. Significant gains for higher latitudes. fig. 9.11

Table. Comparison of 9 different arrangements in relation to a flat panel on the ground of equivalent footprint area. Energy/Solar Cells Area best for flat panel (price related) while on the contrary Energy/Footprint area best for 3d shape 1. 116

The designs developed as a result of the 3d solar cell structures research aim mainly at giving solutions to the energy need in dense urban areas where installation of solar panels of similar output on the horizontal plane would be impossible. Thinking of building roofs it is possible to integrate such structures since the surface area limit is also in this case the main constraint and the height can be theoretically as large as the construction and wind resistance allows. On the other hand the vertical facade plane is significantly limited in possible thicknesses of additional structures and many other important aspects like view and transparency for example limit the possible design solutions. fig. 9.12

Development of different tower designs for performance comparison. Height increase leading to better performance for given footprint. fig. 9.13

Development of different tower designs for performance comparison. Hight increase leading to better performance for given footprint. Example of an electric bike charging tower/station.

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9.4 Other projects and research related to 3d shaped solar modules As already mentioned, the fact that the development of flexible Thin Film PV technology has resulted in efficiencies which come close to the ones achieved by c-Si (especially for specific conditions, like very hot climates or low direct light conditions), opens new possibilities for their use which are strongly enhanced by their additional advantage of flexibility. It is exactly this flexibility property of Thin Film PV cells that opens a new area of research and innovations as the attribute “shape/form” finds new limits (or even no limits) when compared to rigid c-Si PV’s, for which flat has almost been a “synonym”. Forming of flexible PV in 3 dimensions could be initiated for a number of reasons: - for opening new applications where flat modules cannot be used - for creating aesthetically interesting devices - for using PVs on surfaces already having complex shapes - for improving the efficiency of the flexible PVs through optimizing their shape In particular as to the last aspect, both the relevant theories and software tools where already existing, namely those on the field of geometry based performative design. It is thus not surprising that in particular in the field of Building Integrated PV researchers have already provided substantial material in this direction. Important examples in this category are: 1. “Prediction of solar energy gain on 3-D geometries” (http://www.sciencedirect.com/science/article/pii/S037877881300176X), E. Roohollahia, M.A. Mehrabiana and M. Abdolzadehb In this paper 7 specific simple 3D geometries have been investigated (4 convexly curved with varying radius of curvature, one Π-shaped and 2 Λ-shaped) for roof applications by using a calculation method for average radiation on sloped surfaces developed by Klein and Theilacke (K-T method: http://my.safaribooksonline.com/ book/energy/9781118415412/chapter-2-available-solar-radiation/c02_level1_20_xhtml) They found out that a smoothly curved convex surface provides the best annual solar PV energy compared to other flat versions. 2. “Outdoor and diurnal performance of large conformal ﬂexible metal/plastic dye solar cells” (http://www.sciencedirect.com/science/article/pii/S0306261913007009),V. Zardetto, G. Mincuzzi, F. De Rossi, F. Di Giacomo, A. Reale, A. Di Carlo and T.M. Brown In this case flexible Dye Sensitized Solar Cells have been tested on cylindrically curved convex surfaces with two radii of curvature (R=5.5cm and R=13.5 cm) in comparison with flat ones and the result was that an enhancement in Energy gain/footprint area of 7.9% and 13% are obtained for the mildly and sharply curved geometries, respectively.

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TU Delft is also involved in this field. Important examples of relevant work are a. b.

“Computation and Performance” (http://www.bk.tudelft.nl/fileadmin/Faculteit/BK/Onderzoek/Onderzoeksgroepen/Containers_ en_Banners_Onderzoek/Containers/doc/Computation_Performance_01.pdf), “Design explorations of performance driven geometry in architectural design using parametric modelling and genetic algorithms” (http://architecture.mit.edu/pdfs/buelow2.pdf),

In these projects Michela Turrin, Peter von Buelow and Rudi Stouffs, refer to the combination of parametric modeling and genetic algorithms to achieve a performance oriented process in designing a roof with ecological climate control. Also more context specific ones relating to particular buildings have been developed: c.

“Parametric design of the Vela roof: A case study on performance oriented exploration of design alternatives” (http://repository.tudelft.nl/search/ir/?q=title%3A%22Parametric%20de sign%20of%20the%20Vela%20roof%3A%20A%20case%20study%20on%20performance%20ori ented%20exploration%20of%20design%20alternatives%22) A project developed by Turrin, M., Stouffs, R.M.F. Sariyildiz, I.S.

However all the above cited references substantially referred to roof structures or independent modules. The following MSc thesis: 4.

“Curved Photovoltaic Surface Optimization for BIPV: An Evolutionary Approach.Based on Solar Radiation Simulation” (Sheng Cheng) (http://discovery.ucl.ac.uk/18982/1/18982.pdf)

is more related to the present thesis since it relates to facade integrated flexible PV cells by “addressing the problem of the optimization of curved photovoltaic surfaces that may become the alternatives of the traditional flat PV surfaces in BIPV”. The proposed method combines three parts: an evolutionary algorithm (Genetic Algorithm) for optimization, an adaptive simulation tool based on Hay’s anisotropic radiation model, and a comparison module for analysis”. For these reasons a more detailed attention is drawn on this paper: The Genetic Algorithm (GA) is a method (remotely relating to Darwin’s evolution theory) based on a combination of the genotype and the phenotype evolution. The Genotype is based on the characteristics which someone or something (normally a physical entity, human animal etc., in this case a curve) has from its genes (in this case the Bezier chosen parameters). The Bezier’s theory (initially used for designing and defining in mathematical terms the curves of Citroen cars in the 60s) is used for defining in a mathematical way curves by using mathematic equations in which there is one parameter (t) so that by varying this parameter within selected (or given) limits (Bezier limits are 0<t<1) groups of curves are produced. The phenotype evolution refers to what happens to the individuum or the curve (in this case) after intended variation e.g. for some optimization or mutation, change, degradation etc. 119

fig. 9.14

GA optimizes in a second step the first curves (called the first population) and selects 80 parametrically defined curves (Figure 3.4.2., page 23) by varying two predefined points.

fig. 9.15

In Figure 3.4.1 of page 23, a six points (red) starting sinus shape is used, because of its repeating pattern making each curve easy to connect to the next one without any seam.

To perform the further optimization four objectives are defined to produce the phenotype variation. This means in a philosophical/mathematical sense that the first 80 curves are the pure result of their genotype, i.e. the 100% fixed mathematic variation defined without any qualitative aspects. Among the predefined four criteria, the second one (value of k) refers to having as quickly as possible (when amending the shapes based on these criteria data) converging results (=all curves overlap to one final optimum curve), the third criterion concerns the maximum annual energy gain and the fourth the minimum curve surface area (=cost). From one population to the other this method chooses only the best performing ones. In the examples used in the paper (page 33) for different earth latitudes it is interesting to see that near the equator flat is best and for a latitude similar to Delft (L=51.3deg) the sinus curves have their troughs/valleys horizontal (whereas at L=31.2 they are vertical) which is in-line with the fact that in these northern areas tracking makes more sense around a horizontal axis (the curved PVs should adapt a shape so as not to need much tracking). From this point onwards the aforementioned method is used for â&#x20AC;&#x153;Optimization of curved PV lamellas for faĂ§ade shading system of the ECN building in the Netherlandsâ&#x20AC;?.

Shape optimization of PV lamellas performed by the reference thesis project. fig. 9.16

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Through the optimization developed in this paper the final curved PV panel exhibits an improvement by 12.5 per cent increase from flat, but there is also about 2.5 per cent decrease in the mean total annual radiation comparing with that of the flat panel. It is believed that this reduction was a necessary compromise for achieving a better stabilization characteristic. Furthermore, in terms of the distribution of the solar gain, the biggest difference between 16 rows of the flat PV panel is 769.7 MJ·(m²·year)¯¹, i.e. about 17% of the mean total, while after optimization that of the curved PV panel is much improved by being reduced to 463.6 MJ·(m²·year)¯¹ less than 10% of the mean total, which is a not unimportant aspect, since it avoids the need for too much extra energy producing devices and also the criterion whether all the panels produce similar voltage in a changing sun radiation path, which is again important for electric circuitry reasons. The orientation of the building is as expected of great importance for the generation of the final shape. “A big problem for popularizing BIPV projects is that many buildings (or their parts) may have not good positions to the sun so that traditionally they are not considered suitable to be integrated with photovoltaics”, while adjustment of the performance to be more efficient in diffuse or direct radiation is also possible through shape: “…proper radiation mode (e.g. clear-sky or cloud-cover mode) must be predisposed for the different purposes of solar energy usages. For instance, clear-sky mode might be more suitable for the projects in which the direct solar radiation is more important, while cloud-cover mode can provide a global consideration of the solar radiation according to a specific location”

As a conclusion it can be said that even though the last analyzed thesis project as well as other papers focus on the three dimensional shaping of a PV surface in order to improve the performance compared to flat panels, the real approach has significant differences from what is attempted in this project. The difference lies in the fact that the methodology of finding a 3d shape that perfectly exposes its surface to the sun for a given location and orientation is not using surface maximization (within a specific predefined frame) as a first step and criterion (Phase 2). Within this thesis the advantage of increased PV area making use of both diffuse and direct light is a first defining factor for creating the first group of shapes, while the comparison of the ways each shape exposes its surface to the sun constitutes the second step in the form of a solar analysis. In that sense research related to prismatic or curved PV shape optimization on the basis of active surface area can be related to Phase_2, while other optimizations focusing more on optimally exposed 3d surfaces are more related to Phase_3 (Solar Analysis) of this work.

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9.5 Phase 2 shape Generation All figures, illustrations, tables and graphs from this point until the end of the Graduation Report are created by the Author unless otherwise noted.

9.5.1 Parametric definition of surface area Potential The search for the three dimensional shape that would constitute the optimal basis for an active solar surface starts with an attempt to maximize the available surface area. Maximization of the surface can only be defined in this case in relation to the predefined facade area or facade footprint of the shape. Apart from that a maximum thickness/distance from the facade plane has to be defined, since it would me meaningless to include shapes that can infinitely extend vertically to the facade surface and become infinitely thick. The problem can therefore be defined as the finding of the maximum surface area of a shape of a specific width range (in this case from extremely slender up to probably 60cm, which in most real world facade structures would already be too wide/thick as an element). Referring to a surface area efficient shape will from now on always describe an efficient relation/ratio between the folded/curved/prismatic shape surface and its projection (or footprint) on the facade plane. This will answer the question â&#x20AC;&#x153;how much active surface area can I fit in this facade or facade module with this shape patternâ&#x20AC;?. An Evolutionary Solver/Genetic Algorithm (Galapagos Plugin for Grasshopper) using every time a specific shape as the basis and creating different dimension variations (width or hight variations/thickness variations) within the preset limits did produce results about the most surface efficient version of each shape. However this, as someone can easily predict before the computer calculation, will always point to the infinitely dense (or minimum preset width) and maximum thickness version of the shape pattern. This should not be seen as a disappointing result for the efficiency of the shape finding procedure, since the goal of the process is to compare the shapes to each other, not the different variations of he same shape, as the result in this case is easily predictable (most dense design is the most surface area efficient). Furthermore a question arises as to why would someone create the different shape versions since always the denser and thicker versions prove themselves as more efficient and not just compare the basic patterns/shapes to each other. The reason for the specific process has to do with the different degree to which each shapes (or folding patterns) surface area increases due to the same dimension change. 123

fig. 9.17

Examples of different Pattern types in section

fig. 9.18 examples of 3d shape versions

fig. 9.19 Parametric creation of 3d shapes and their vertical footprint surfaces (on facade plane).

Basic Surface Pattern Footprint of surface on facade plane. Constant area of 3.975m2 fig. 9.20 Grasshopper script for creating different versions of basic 3d patterns and extracting surface area ratio

Basic Surface Pattern Definition Changing the dimensions of the basic Pattern/Shape in width density, height density and thickness Three Dimensional Shape Surface area divided by its footprint (facade projection) area. Result is the surface efficiency of each shape. 124

A large amount of different basic shapes have been used as an input for the parametric script for calculating their surface efficiency compared to their flat equivalent. The criteria of choice have been as a first step simplicity of foldings and corrugations in order to allow the creation of a foldable, retractable and moving structure (with more complicated shapes following). Therefore the first patterns of choice are simple zig zag/jagged and corrugated surfaces in horizontal axis and also their respective vertical direction versions. Linear foldable structures having parallel edges and a highly variable density and thickness create already a large amount of different designs. Prismatic shapes and shapes with generally flat surfaces folded into three dimensional surfaces are also transformed into their curved versions, which means that lines (in section) connecting angles/edges are transformed into curves (similarly to a sinus curve around the X-axis) since this transformation results again in a larger surface (due to the increased length of the curve compared to the line when connecting two points). Corrugations and foldings showing a linear pattern in one direction became in a next step sharp edges in both directions resulting in pyramid and honeycomb patterns. These designs are very often seen in respective micro-scale optimizations and surface texturing for improved PV efficiency. In micro-scale they aim however mainly at a larger amount of reflections and at a longer path of the sun-rays through the active PV material. The benefit created by reflections can also be considered a positive point with a possible impact on the efficiency even for the facade scale this thesis deals with.

The basic pattern of â&#x20AC;&#x153;zig zagâ&#x20AC;? corrugation translated into its sinus curved version results in an increase from 1.20 to 1.30 surface efficiency ratio, which is higher for denser versions. In their densest version (four times denser division) the values are 2.86 and 3.04 respectively. fig. 9.21

fig. 9.22

As predicted corrugations and folding edges in both horizontal and vertical direction (creating pyramid-like surface patterns and honeycombs) have an advantage in surface area efficiency over simply linear folded or corrugated designs of the same pattern division scale (pattern units which are repeated to build up the specific module footprint).

fig. 9.23

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Pattern versions (dimension changes) Footprint facade

Basic shape 1/1

1/2 width of pattern units = twice as dense horizontally

1/4 width of pattern units=four times as dense horizontally

x1.5 width of pattern units=1.5 times wider/stretched horizontally (total width of shape always constant)

x2 (double) width of pattern units=two times wider/stretched horizontally (total width of shape always constant)

1/2 thickness of complete shape (more “shallow” 3d surface)

1/4 thickness of complete shape

1.5x thickness of complete shape (“deeper” 3d folding)

x2 thickness of complete shape

126

Examples of Patterns and different versions

Basic Inverted Pyramid pattern/shape. Surface area: 5.691 m2 Efficiency (3d Surface to footprint ratio): 1.431

One fourth width pattern of inverted Pyramid Surface area: 11.795 m2 Efficiency (3d Surface to footprint ratio): 2.96 127

Half width pattern of inverted Pyramid Surface area: 7.33 m2 Efficiency (3d Surface to footprint ratio): 1.843

One fourth thickness pattern of inverted Pyramid Surface area: 4.104 m2 Efficiency (3d Surface to footprint ratio): 1.032

Double thickness pattern of Pyramid Surface area: 9.062 m2 Efficiency (3d Surface to footprint ratio): 1.431

One fourth width pattern of inverted Pyramid Surface area: 11.795 m2 Efficiency (3d Surface to footprint ratio): 2.96

Half width pattern of inverted Pyramid Surface area: 7.33 m2 Efficiency (3d Surface to footprint ratio): 1.843

One fourth thickness pattern of inverted Pyramid Surface area: 4.104 m2 Efficiency (3d Surface to footprint ratio): 1.032 128

9.5.2 Origami shape characteristics and models testing Additionally prismatic, origami shapes and patterns were used. The most characteristic and very popular pattern of this group is the Miura-Ori shape. Apart from its very high surface efficiency (1.58 surface ratio between three dimensional surface and its footprint, which steeply increases when folded into a denser arrangement) it is also known for its ability to linearly fold into a shape multiple times more slender compared to its basic shape. As an additional bonus this fold significantly increases the rigidity of the surface not only compared to the flat equivalent, but also compared to simple parallel folds or corrugations. Forces vertical to the pattern plane are dealt with extremely well. It is not without reason that this type of folding is often used in lightweight sandwich panels made of paper or plastic in order to achieve a very high rigidity to weight ratio. On the other hand the strong anisotropy in the design enables easy shrinkage and expansion of the shape in only one axis. Furthermore other origami inspired shapes were modeled and used as the input for the parametric script to extract their surface area efficiency. However from an early stage many of them could be easily described as not efficient enough, since the surface efficiency ratio reached comparable values (to the previous shapes) only for extremely dense versions. One more negative point is the difficulty to fold or unfold these patterns or the extremely large amount of edges, which in the real application will inevitably translate to hinges in a moving/adjustable device. Specific patterns were modeled with paper (fig.9.24-9.29) and others only using 3d software (Rhino) in order to see the motion of the folding and unfolding surface, the required hinges in a real structure, but more importantly the shadows created within the surface by the sun. The assumption that a more surface area efficient shape also receives more solar radiation still remains to be proven in the next Phase of the Research and definitely is not the case for any given shape, since denser and thicker shapes or shape variations also exhibit a larger shaded area. The surface efficiency is therefore only the first step to take, which will however give an indication of the potential of the idea of surface increase. A comparison that is of equal importance with the three dimensional shape study, is the relation of these shapes to the flat facade equivalent of the same footprint area but also to louvers designs. Optimal inclination of approximately 30+ degrees of fixed louvers is taken as a benchmark in order to better evaluate the performance of the 3d shapes. It is obvious that in terms of surface area potential both solutions (flat sheet and louvers) are far behind 3d shapes, especially in the case of louvers. On the other hand next steps including solar studies and PV integration will show the strength and advantages of smaller and better oriented surfaces in specific areas. fig. 9.24

fig. 9.25

Lighting of Origami and linear folded shapes from different angles to observe shadow shapes. Shape and size of shadows have an impact on PV integration potential 129

fig. 9.26

fig. 9.27

Lighting of Origami and linear folded shapes from different angles to observe shadow shapes. Shape and size of shadows have an impact on PV integration potential

fig. 9.28

Construction of models in Workshop to observe hinge limitations and dimensions due to material thickness.

fig. 9.29

Construction of models in Workshop to observe hinge limitations and dimensions due to material thickness.

130

9.5.3 Preliminary shadow study. Shadow patterns A first solar study, not for performance evaluation but in order to further investigate the shapes of the shadows within each 3d shape has been performed. The very basic idea of the spotlight moving around the physical model in order to see what shading patterns are created and get a first idea of the shading percentage but also of the possible problems for conventional PV cell connections (already discussed in partial shading chapter) is now translated into a more detailed 3d software shading simulation. The sun has been set in 5 characteristic positions, 2 morning points, one for midday and another two for evening positions. Thereby the shape change of the shadows on the surface can be observed and analysed. This would definitely become a useful tool for designing the PV connection layout even though complex shapes still pose too many problems for conventional PV technologies (first and probably also second generation PV technologies). On the other hand the same process can be useful in different scales and applications of 3d active solar surfaces, for example in order to shade or illuminate specific surfaces in predefined hours during the day and adjust the performance according to the needs of a building depending on the way the prismatic skin is folded. The example of the Miura-Ori pattern shows the change of the change of the shadow shape.

Shadow Pattern Late Morning

fig. 9.30

131

fig. 9.31

Shadow Pattern Early Morning

fig. 9.32

fig. 9.33

Shadow Pattern Early Evening

fig. 9.34

Shadow Pattern Midday

Shadow Pattern Late Evening

132

Compared Shapes chosen for next Stage Solar Analysis.

Curved one side patterns

Corrugated (zig-zag) patterns 133

Corrugated curved patterns (sinus curve-like section)

Pyramid patterns Inverted or facing outside

Origami patterns

Sun exposed surface area of selected shapes and relation to module footprint area (3.975 m2). Surface Area Efficiency. Grouping according to shape category. SUN EXPOSED AREA OF DESIGNED SHAPES AND ITS RELATION TO THE FACADE MODULE FOOTPRINT AREA 2

fig. 9.35 Table

OF 3,975m SHAPE

Flat facade Louvers 30deg horizontal Corrugated vertical 1/1 Corrugated vertical 1/2 thickness Corrugated vertical 1/4 thickness Corrugated vertical 1,5 thickness Corrugated vertical double thickness Corrugated vertical 1/2 width Corrugated vertical 1/4 width Corrugated horizontal 1/1 Corrugated horizontal 1/2 thickness Corrugated horizontal 1/4 thickness Corrugated horizontal 1,5 thickness Corrugated horizontal double thickness Corrugated horizontal 1/2 height Corrugated horizontal 1/4 height Corrugated vertical curved 1/1 Corrugated vertical curved 1/2 thickness Corrugated vertical curved 1/4 thickness Corrugated vertical curved 1,5 thickness Corrugated vertical curved double thickness Corrugated vertical-curved 1/2 width Corrugated vertical-curved 1/4 width Corrugated horizontal-curved 1/1 Corrugated horizontal-curved 1/2 thickness Corrugated horizontal-curved 1/4 thickness Corrugated horizontal-curved 1,5 thickness Corrugated horizontal-curved double thickness Corrugated horizontal-curved 1/2 height Corrugated horizontal-curved 1/4 height origami basic 1/1 origami basic 1/2 thickness origami basic 1/4 thickness origami basic 1,5 thickness origami basic double thickness origami basic 1/2 width origami basic 1/4 width Pyramids 1/1 Pyramids 1/2 thickness Pyramids 1/4 thickness Pyramids 1,5 thickness Pyramids double thickness Pyramids 1/2 width Pyramids 1/4 width Curved one side vertical 1-1 Curved one side vertical double thickness Curved one side vertical fourth width Curved one side vertical half width Curved one side horizontal 1-1 Curved one side horizontal double thickness Curved one side horizontal fourth height Curved one side horizontal half height Inverted pyramid 1-1 Inverted pyramid fourth width Inverted pyramid half width Inverted pyramid double thickness

Exposed area [m2] 3,838 2,576 4,788 4,193 4,031 5,641 6,654 6,654 11,387 4,788 4,193 4,031 5,641 6,654 6,654 11,387 5,157 4,344 4,077 6,162 7,267 7,262 12,094 5,157 4,344 4,077 6,162 7,267 7,262 12,095 5,863 4,505 3,863 8,125 10,208 8,821 15,514 5,691 4,467 4,104 7,288 9,062 7,33 11,796 5,078 7,127 11,902 7,127 5,078 7,127 11,902 7,127 5,691 11,795 7,33 9,063

Surface area increase as to module footprint area of 3,975m2 0,97 0,65 1,20 1,05 1,01 1,42 1,67 1,67 2,86 1,20 1,05 1,01 1,42 1,67 1,67 2,86 1,30 1,09 1,03 1,55 1,83 1,83 3,04 1,30 1,09 1,03 1,55 1,83 1,83 3,04 1,47 1,13 0,97 2,04 2,57 2,22 3,90 1,43 1,12 1,03 1,83 2,28 1,84 2,97 1,28 1,79 2,99 1,79 1,28 1,79 2,99 1,79 1,43 2,97 1,84 2,28

134

9.5.4 Results and Evaluation of Phase 2

Flat facade footprint equivalent area of 3.975 m2. Second graph point showing 30degrees fixed louvers covering considerably smaller surface even compared to flat

Advantage in surface Efficiency for all (Muira-Ori) Origami shape versions. 1/4 width pattern highly efficient.

fig. 9.36 Graph

Results of the first phase show a significant advantage of the chosen Miura Ori Origami Pattern compared to other prismatic shapes and linearly folded or corrugated surfaces. Increase in the pattern density results in a steeper curve of surface area gain, which can be observed in the graph. On the other hand flatter and less dense versions of the specific origami donâ&#x20AC;&#x2122;t have the same significant difference from simpler shapes. Designs with parallel corrugations or folding have lower surface efficiency compared to most shapes with prismatic division in both directions, horizontal and vertical. When compared to each other shapes that have similar patterns and spacing, but have curved parts where others have straight lines (curved panels instead of flat), show a small advantage. Pyramid shapes are approximately in the middle range between very surface efficient prismatic origami layouts and less efficient simple/parallel folding surfaces. Even better performing versions of the origami folds presented here have been developed, using curved segments where the normal origami pattern would use a straight (flat) part. These however would become extremely difficult to construct since they neither have a foldable shape (limited to rigid) nor are the foldable out of a single sheet. 135

Shape versions sorted according to their surface efficiency (increase of surface compared to footprint surface) fig. 9.37 Table

136

9.5.5 Comments on Phase 2 and additional aspects. The results of this research phase give an indication of the most surface area efficient 3d shapes. They are the shapes into which a flat surface should be folded in order to have the maximum active solar area for a given size on a building facade. More efficient shapes are possible but many of the extremely well performing shapes are already very complex especially when in a later stage the demand for a retractable, lightweight and wind resistant structure arises. The shape generation procedure could theoretically follow the steps of other less restricted solutions (compared to a given facade module), where the computer software starts generating random prismatic surfaces with only a few shape restrictions like non overlapping surface layers and starts using genetic algorithms to evaluate each shape version according to the predefined needs (surface area in this case). However this would lead to designs with extreme complexity and density and nearly impossible to be translated into a real world device. Theoretically a very well designed/organized group of restrictions that include structural integrity and a smart (geometrical) definition of shape complexity and density in order to avoid non buildable solutions could lead to a much more efficient shape generation. As explained above surface area efficiency as defined in this chapter (namely the surface area of the 3d shape compared to its projection/footprint on the facade plane) only gives a theoretical potential of the shape and not the real solar energy value that the shape receives. The next step of solar analysis will prove if more surface efficient shapes are practically also better (and which of those, since the general assumption for any denser shape is wrong) in accepting solar energy by the sun during the year compared to their flat equivalents. An additional aspect that needs to be researched in order to evaluate its potential is the possibility of using 3d shapes with tracking systems. Although this would be contradicting to the idea of using only shape for achieving a performance gain through cheap thin and printable PV cells in the future, the fact that tracking could ensure the much better illumination of the complete surface of a very dense shape is a very intriguing thought and could make extreme designs with very high surfaces viable. A second aspect that often comes to mind when thinking of surface maximization is the impact this has on heat transfer. The driving force for heat transfer is the difference in temperature levels between two objects, but the second influencing factor is the surface of the barrier between the warmer and colder side. If the engineer or designer wants to use the developed high surface shape for example for preheating air behind the structure (through the temperature rise of the surface by the solar cells for example) , then surface maximization enhances the effect due to more heating area. The next phase will be based on the current shape evaluation and use the results in order to calculate the real insolation values, which will show the advantage of surface increase or the disadvantage due to the increase of shading percentages within the structures.

137

138

10_Phase 3

Solar study of Phase 2 Shape Results Solar Exposure, Partial Shading percentage Comparison

139

140

Performance evaluation of shapes in terms of solar energy and partial shading. Phase 3 deals with the analysis of the shapes compared according to their surface area potential in the previous chapter. The assumption that a significant increase of the surface area achieved through folding or bending (into curved shapes) of an active solar surface can (for specific shapes) lead to increased performance and energy output of PV cells (which are integrated on these 3d shapes) has to be proven in a first step through a solar analysis with a specialized software and in a next step through testing. The amount of solar energy actually reaching the different surfaces and the amount of partial shading within a day and year (on average) show the real potential of a surface to create an efficient basis for solar cells in the future. Better performing structures in terms of incident solar energy after this analysis would be the best shapes for a real world application of solar cells only in case solar cell technology completely overcomes partial shading problems, since PV performance needs to be completely and linearly proportional to shading percentages. Currently this is not the case, even though new technologies and printability of circuits on cheap substrates or a possible size reduction of the cell units can approach less and less partial shadow sensitive modules. For this reason this will not be the last phase of the shape evaluation. A large amount of different 3d shapes was produced in a previous stage and 56 of them were compared and evaluated in terms of surface area for further solar analysis. They all share a total size of approximately 2X2 meters when projected on the facade plane. This can be seen as the facade surface the PV-shading structure covers. The selection of the footprint/projection size for the shapes is important to be relatively close to the real facade module size, in order to have precise results for the real application. In a further approach and in view of one version of the Alcoa NEXT façade having 3-4 narrower glass parts (of a size of approx. 0.85m incorporating in one of these glass parts the installations of the decentralized climate control devices) the performance of a few of the above shapes were also assessed in such a narrower module. (results of these modules not included) 10.1 Solar Analysis Process For the assessment of the different shapes, which will constitute the external light control/shading surface Autodesk Ecotect Analysis has been used. The parametric design software used for the previous phase in the surface area evaluation exports each one of the 56 3d shapes in 9 groups created through a variation of the dimensions of 9 specific “basis shapes”. The parameters chosen for the Ecotect calculation were: Location: London Lat=51.4 deg: Lng: 0.0 deg Orientation: γ=0deg, i.e. south facing Inclination of PV surface (tilt): β=90deg, i.e. vertical on the facade

141

The 56 tested shapes have been organized in 9 groups according to their basis shape which is being subsequently transformed. The three dimensional shapes are compared to 2 reference shapes which are important as more conventional alternatives to the three dimensional approach of this Thesis. The first reference shape is the flat surface covering the same facade area as the tested 3d shapes (projection), which is approximately 2X2 meters. This can be seen as the flat semi transparent solar panel installed on the facade plane or a thin film membrane/fabric shading solution of the same size. The second reference shape is the conventional solution of fixed louvers with an inclination/tilt of approximately 30 degrees and a spacing that ensures complete illumination even for the maximum sun altitude. The louvers are covering the same facade surface in total as the other shapes but their projection on the facade plane is as expected significantly smaller in area even compared to the flat panel surface (which leads to their last place in the previous surface evaluation phase). In this analysis the comparison with the reference shapes is important mainly in terms of total Wh due to the impact of surface area. Wh/m2 is expected to be very good for perfectly oriented surfaces.

Reference shape Louvers fixed at 30o Performance

142

Shape Group

Number of Variations

Corrugated Vertical

Corrugated Horizontal

143

Average daily Radiation of Basic Shapes only

144

Corrugated Vertical Curved

Corrugated Horizontal Curved

145

146

Origami (Miura-Ori)

Pyramids

147

148

Curved One Side Vertical

Curved One Side Horizontal

149

150

Inverted Pyramids (honeycomb)

151

The solar radiation received by each one of the 56 created shapes has been calculated and the respective graphs have been created. The average radiation reaching each part of the 3d shapes throughout the year is depicted in the accordingly colored shapes. Only the basis versions are included within these report pages. (All the 56 shape versions are included in the appendix). Graphs have been created showing the average daily incident radiation of the shape within a year (and for each month separately) and the percentages of average shading in the shape and average exposed area. A non transparent non reflective material has been chosen in order to simplify the analysis procedure. The light passing through a semi transparent layer could in specific shapes and angles reach a second PV layer and create additional output. Furthermore reflections within the PV covered shapes are probably an additional benefit that has to be further investigated.

152

10.2 Ecotect results Ecotect provided a plethora of results, the most important of which are: - The average daily incident solar radiation on each version (56 in total) of a specific basis shape in graph and data presentation - The percentage of unobstructed sky (non shaded parts) within each version of a specific basis shape in graph and data presentation - The percentage of shading within each version of a specific shape in graph and data presentation - Screenshots of the views of the shapes with a color range representing the daily average solar exposure not only for the total shape but also for each part of its geometry. Elaborating the ecotect results These results were then used to produce excel worksheets in order to evaluate the performances of aforementioned 56 shapes in a sequence of steps as follows:

Step_1 : Examining the shapes one by one From the extracted data 56 excel sheets were produced (one of them can be found in the next page, whereas all of them are contained in the appendix with the calculations). In each such excel sheet the available annual solar energy (kWh/year) for each shape has been divided by the footprint area of each module (as mentioned above around 4 m2) so that the most important value of energy/ footprint area on the façade was calculated in kWh/year∙m2 (criterion 1: column highlighted in red). It has to be pointed out that the above mentioned “annual solar energy” value does not represent the electric energy delivered by PV cells, since the specific type and efficiency of the used PV technology (e.g. thin film, organic etc.) is not taken into account in these solar irradiation calculations with Ecotect and the software is unable to predict the precise partial shading effects (on performance) in a complex PV module connection. In a further step, the available annual solar energy (kWh/year) for each shape was divided by the exposed area of the PV (i.e. the total PV surface on each module) so that a value of energy/surface of used PV was calculated in kWh/year∙m2 PV (criterion 2: column highlighted in light orange). This value is representative for the cost of the PV cells in each module, assuming that their price is analog to their area. In a third column both the above criteria were considered together in a so-called combined energy index (criterion 3: blue column, being based on multiplying the corresponding values of the previous criteria 1 and 2).

153

The figure below depicts the excel sheet for the shape: “Curved one side vertical fourth width”, being that with the maximum energy/footprint area on the façade in kWh/year∙m2 (criterion 1).

Curved one side vertical fourth width shape. Dense version of the Curved One Side Basis Shape 154

Step_2 : Producing lists of the performance of the 56 examined shapes and sorting these lists according to already defined qualitative criteria. In this step further series of four excel sheets were produced in which the 56 shapes were listed against the previously defined 3 criteria, as “unsorted”, “sorted by criterion 1, 2 and 3”, respectively. In these lists two further columns have been added comprising: a) The “STANDARD DEVIATION of daily energy gain”. This further criterion representing the statistical value of the standard deviation of the values of the “average daily TOT.Wh” of each shape (see bottom of the figure of the previous page). The importance of this value is that a small standard deviation (from the mean value) indicates a small fluctuation of the average daily energy gain throughout the year, which helps in designing the whole solar energy system in view of the expected loads and in calculating the characteristics of necessary energy storage media. b) An index value based on the division of the total annual energy gain through the said standard deviation, a criterion for a good energy gain in a more or less steady delivery in the timeline. Next pages depict in sequence: - The “unsorted” list for the 56 shapes. - The same list sorted as to criterion 1: maximum annual energy gain/m2 of footprint. - The same list sorted as to criterion 2: maximum annual PV performance (kWh/year∙PV surface = PV material cost factor). - The same list sorted as to criterion 3: combination of criteria 1 and 2 (based on the product of the corresponding two values).

155

COMPARISON OF SHAPES IN VIEW OF 5 CRITERIA (1: Energy gain; 2: PV performance (cost); 3: Combined 1 and 2; 4: standard deviation; 5: combined 1 and 4)

Flat facade Louvers 30deg horizontal Corrugated vertical 1/1 Corrugated vertical 1/2 thickness Corrugated vertical 1/4 thickness Corrugated vertical 1,5 thickness Corrugated vertical double thickness Corrugated vertical 1/2 width Corrugated vertical 1/4 width Corrugated horizontal 1/1 Corrugated horizontal 1/2 thickness Corrugated horizontal 1/4 thickness Corrugated horizontal 1,5 thickness Corrugated horizontal double thickness Corrugated horizontal 1/2 height Corrugated horizontal 1/4 height Corrugated vertical curved 1/1 Corrugated vertical curved 1/2 thickness Corrugated vertical curved 1/4 thickness Corrugated vertical curved 1,5 thickness Corrugated vertical curved double thickness Corrugated vertical-curved 1/2 width Corrugated vertical-curved 1/4 width Corrugated horizontal-curved 1/1 Corrugated horizontal-curved 1/2 thickness Corrugated horizontal-curved 1/4 thickness Corrugated horizontal-curved 1,5 thickness Corrugated horizontal-curved double Corrugated horizontal-curved 1/2 height Corrugated horizontal-curved 1/4 height origami basic 1/1 origami basic 1/2 thickness origami basic 1/4 thickness origami basic 1,5 thickness origami basic double thickness origami basic 1/2 width origami basic 1/4 width Pyramids 1/1 Pyramids 1/2 thickness Pyramids 1/4 thickness Pyramids 1,5 thickness Pyramids double thickness Pyramids 1/2 width Pyramids 1/4 width Curved one side vertical 1-1 Curved one side vertical double thickness Curved one side vertical fourth width Curved one side vertical half width Curved one side horizontal 1-1 Curved one side horizontal double thickness Curved one side horizontal fourth height Curved one side horizontal half height Inverted pyramid 1-1 Inverted pyramid fourth width Inverted pyramid half width Inverted pyramid double thickness

Combined PV Standard Energy Enegry gain kWh/year/m2 Deviation gain/standar performance and PV footprint on the (kWh/m2 of PV of daily energy d deviation facade performance gain (B/E) surface) index 381 394 150 2014 189 374 582 218 2477 151 424 352 149 2310 184 401 380 152 2137 188 394 389 153 2087 189 457 322 147 2554 179 496 296 147 2832 175 460 275 127 2544 181 541 188 102 3063 177 426 353 150 2319 184 401 380 152 2132 188 395 389 154 2086 189 463 326 151 2614 177 511 305 156 2981 171 471 281 132 2648 178 556 194 108 3182 175 433 333 144 2367 183 401 367 147 2155 186 394 384 151 2096 188 476 307 146 2643 180 524 286 150 2952 178 484 265 128 2645 183 595 195 116 3292 181 434 334 145 2372 183 402 368 148 2160 186 394 384 151 2091 188 477 308 147 2650 180 526 288 151 2982 176 481 263 127 2607 185 602 197 119 3418 176 408 277 113 2212 184 395 348 137 2131 185 368 379 139 1960 188 484 237 115 2736 177 526 204 107 3009 175 482 217 105 2697 179 597 152 91 3396 176 461 322 148 2658 173 407 362 147 2277 179 380 368 140 2081 183 525 286 150 3103 169 577 253 146 3458 167 489 265 130 2827 173 587 198 116 3475 169 408 319 130 2237 182 506 282 143 2872 176 613 204 125 3432 179 463 258 119 2567 180 408 319 130 2247 182 518 289 150 2993 173 600 200 120 3425 175 486 271 132 2719 179 390 272 106 2404 162 374 126 47 2339 160 299 162 48 1887 158 472 207 98 2831 167

156

In the table “Comparison of shapes in view of 5 criteria” all 56 shapes are listed starting with the two reference shapes, namely the “flat surface” and the “louvers 30deg horizontal” and continuing with the previously mentioned 9 groups of basis shapes and their different shape version (basic 1_1 shape followed by the mentioned 6 or 3 “dependent” shapes produced on the basis of the 1_1 one by changing the “width/height” (i.e. the wave length) and the “thickness” (i.e. the amplitude). As mentioned above, the Standard Deviation values (fourth column, white background) are created on the basis of the fluctuation values of the daily average “TOT.Wh” for each month of the year as explained under step_1 above and the corresponding 56 excel sheets (see the relevant “STANDARD DEVIATION” value at the bottom of these sheets) Finally, the last column (“Energy gain/standard deviation (B/E)”) is an index value representative for good overall yearly energy gain per m2 of footprint on the façade with a small deviation of daily energy production, i.e. a comparatively steady energy gain (low fluctuation). Comments on the performances of the 56 shapes can be made as long as these shapes have been ranked in tables produced by sorting the above results in view of each criterion. 10.2.1 Results Table “Sorted by energy gain” Criterion 1 (per facade footprint area). Comments: Focusing initially at the best “performers” as a group, a first observation leads to the fact that the best performing shape has some distinguishing distance from the next group of the 4 following best shapes, the latter being very close to one another. From this position downwards bigger “steps” in decrease of performance are observed. The best performing shape (“curved one side vertical fourth width”, (figure in step 1) has an increased performance of +61% compared to the reference “flat façade”, +64% compared to the reference “louvers 30d horizontal” and +2,5% compared to the mean value for the following group of the next 4 best shapes lying very close to one another (values 595-602). This shape retains a reltively good performance also in the further two criteria values (criterion 2: 204; criterion 3: 125) which are the second bests within the group of the first 10 best performing shapes. It can thus be considered as the “winning” shape if the overall energy gain is the crucial aspect in the shape selection without completely neglecting the other aspects (as to the standard deviation assessment, see details in the corresponding chapter below). In general shapes with the most undulations/folds (12 undulations of approx. 17cm in width=amplitude in the 2 X 2 m studied module) perform better than any others with less, occupying 8 out of the 10 best places in this comparison. This means that the selection meets the set requirements of increasing as much as possible the exposed area without providing substantial self-shading problems. This fact is further enhanced if considering that the “one side curved” (meaning those exposing only convex surfaces to the sun) are slightly better, apparently because of less “deep concave” valleys when compared with the corrugated ones with an almost sinusoid form. It has however to be pointed out that “high amplitudes” (i.e. shapes with an increased “thickness”, in particular those with a “double thickness” form), although not being among the 9 bests, build a “compacted” group occupying positions 10-17 showing in the same time good values at to criterion 2 (=PV material cost). 157

COMPARISON OF SHAPES IN VIEW OF 5 CRITERIA (1: Energy gain; 2: PV performance (cost); 3: Combined 1 and 2; 4: standard deviation; 5: combined 1 and 4) sorted by energy gain: criterion 1 STANDARD

DEVIATION Combined PV Enegry gain of daily e nergy kWh/year/m2 performance gain and PV footprint on (kWh/m2 of the facade performance (column A of PV surface) excls of e ach index

Energy gain/standard deviation (Column B/ column E)

shape)

Curved one side vertical fourth width Corrugated horizontal-curved 1/4 height Curved one side horizontal fourth height origami basic 1/4 width Corrugated vertical-curved 1/4 width Pyramids 1/4 width Pyramids double thickness Corrugated horizontal 1/4 height Corrugated vertical 1/4 width Corrugated horizontal-curved double thickness origami basic double thickness Pyramids 1,5 thickness

613 602 600 597 595 587 577 556 541 526 526 525

204 197 200 152 195 198 253 194 188 288 204 286

125 119 120 91 116 116 146 108 102 151 107 150

3432

179

3418

176

3425 3396 3292 3475 3458 3182 3063 2982 3009 3103

175 176 181 169 167 175 177 176 175 169

Corrugated vertical-curved double thickness

524

286

150

2952

178

Curved one side horizontal double thickness Corrugated horizontal double thickness Curved one side vertical double thickness

518 511 506

289 305 282

150 156 143

2993 2981

173 171

2872

176

Corrugated vertical double thickness Pyramids 1/2 width Curved one side horizontal half height Corrugated vertical-curved 1/2 width origami basic 1,5 thickness origami basic 1/2 width Corrugated horizontal-curved 1/2 height Corrugated horizontal-curved 1,5 thickness Corrugated vertical-curved 1,5 thickness Inverted pyramid double thickness Corrugated horizontal 1/2 height Corrugated horizontal 1,5 thickness Curved one side vertical half width Pyramids 1/1 Corrugated vertical 1/2 width Corrugated vertical 1,5 thickness Corrugated horizontal-curved 1/1 Corrugated vertical-curved 1/1 Corrugated horizontal 1/1 Corrugated vertical 1/1 origami basic 1/1 Curved one side vertical 1-1 Curved one side horizontal 1-1 Pyramids 1/2 thickness Corrugated horizontal-curved 1/2 thickness Corrugated vertical 1/2 thickness Corrugated horizontal 1/2 thickness Corrugated vertical-curved 1/2 thickness Corrugated horizontal 1/4 thickness origami basic 1/2 thickness Corrugated vertical 1/4 thickness Corrugated vertical-curved 1/4 thickness Corrugated horizontal-curved 1/4 thickness Inverted pyramid 1-1 Flat facade Pyramids 1/4 thickness Louvers 30deg horizontal Inverted pyramid fourth width origami basic 1/4 thickness Inverted pyramid half width

496 489 486 484 484 482 481 477 476 472 471 463 463 461 460 457 434 433 426 424 408 408 408 407 402 401 401 401 395 395 394 394 394 390 381 380 374 374 368 299

296 265 271 265 237 217 263 308 307 207 281 326 258 322 275 322 334 333 353 352 277 319 319 362 368 380 380 367 389 348 389 384 384 272 394 368 582 126 379 162

147 130 132 128 115 105 127 147 146 98 132 151 119 148 127 147 145 144 150 149 113 130 130 147 148 152 152 147 154 137 153 151 151 106 150 140 218 47 139 48

2832 2827 2719 2645 2736 2697 2607 2650 2643 2831 2648 2614 2567 2658 2544 2554 2372 2367 2319 2310 2212 2237 2247 2277 2160 2137 2132 2155 2086 2131 2087 2096 2091 2404 2014 2081 2477 2339 1960 1887

175 173 179 183 177 179 185 180 180 167 178 177 180 173 181 179 183 183 184 184 184 182 182 179 186 188 188 186 189 185 189 188 188 162 189 183 151 160 188 158

158

Additionally it can be observed that two “pyramid” shapes occupy positions 6 and 7 with good energy values (587 and 577), whereas the “double thickness” one shows good values for both criteria 2 and 3, namely the best ones within the first 9 best performing shapes. A further observation at the top group provides the interesting result that whether the undulations are arranged “vertically” or “horizontally” is not a crucial difference since in the group of the first 10 best performing shapes 4 are vertical (including the origami) and 4 horizontal. In general “curved” surfaces are performing better than those with “edges” occupying 4 out of the first 5 ranking positions. An interesting fact which shouldn’t stay uncommented is that among the 10 best performing shapes, three particularly complicated ones are contained, namely: - The dense origami (“origami basic ¼ width”) in position 4 and - The two pyramid shapes in positions 5 and 6, all three with very satisfying values between 577 and 597, the ‘pyramid double thickness”, i.e. that with increased amplitudes delivering the best criterion 2 result under the first 9 best performers. This means that if architectural aesthetic aspects would encourage the use of such prismatic origami-like shapes, this could be done without compromising the energy gain performance, but rather enhancing it. Considering the standard deviation results in the “Criterion 1” table (fourth column, white background), there appears to be an almost linear reverse analogy between these values and the energy performance (first column, red background) in that the best performing shapes show the worst standard deviation results. This is not surprising, since complicated shape patterns with a substantially increased exposed surface (Nr.1 performing shape “curved one side vertical fourth width” has: 11,902 m2) compared to the “flat” reference one (which has: 3,975 m2) provide very high solar energy gains in good insolation periods, whereas in overcast winter days the difference between them and any other “simple” shape in small. This means that, if the sun is shining with large percentages of direct radiation the “high performers” provide large amounts of energy, both in absolute terms and also in comparison to any “simpler” shape, but when there is practically no or very tittle sun radiation it makes a considerably smaller difference whether the shape is “simple” or more “sophisticated”. Simple shapes have thus low standard deviation values as having low maxima and “high performing shapes” show substantial fluctuations resulting from their extreme performance in sunny days. (This, as explained from the beginning of Phase 3 only concerns solar energy hitting the surfaces and not the PV performance) This criterion can thus only be used within each group of shapes if energy values very similar and a more steady energy producing pattern is wished, “sacrificing” to some extend the total energy gain for that quality aspect. This fact is furthermore supported by the results of the last (fifth column, white background) in which both the energy gain and the standard deviation are ”coupled”. In that sense one could theoretically chose in view of these values a comparatively good performing shape (not the best but maybe within the first 10) which is also slightly better than the best ones in achieving somehow lower fluctuations. However the differences in this last column are so minimal that such a selecting method appears relatively inefficient (=high loss in energy with small gain in stability). 159

COMPARISON OF SHAPES IN VIEW OF 3 CRITERIA (1: Energy gain; 2: PV performance (cost); 3: Combined 1 and 2) Sorted by criterion 2

Louvers 30d horizontal Flat facade Corrugated horizontal 1/4 thickness Corrugated vertical 1/4 thickness Corrugated vertical-curved 1/4 thickness Corrugated horizontal-curved 1/4 thickness Corrugated vertical 1/2 thickness Corrugated horizontal 1/2 thickness origami basic 1/4 thickness Corrugated horizontal-curved 1/2 thickness Pyramids 1/4 thickness Corrugated vertical-curved 1/2 thickness Pyramids 1/2 thickness Corrugated horizontal 1/1 Corrugated vertical 1/1 origami basic 1/2 thickness Corrugated horizontal-curved 1/1 Corrugated vertical-curved 1/1 Corrugated horizontal 1,5 thickness Pyramids 1/1 Corrugated vertical 1,5 thickness Curved one side vertical 1-1 Curved one side horizontal 1-1 Corrugated horizontal-curved 1,5 thickness Corrugated vertical-curved 1,5 thickness

374 381 395 394 394 394 401 401 368 402 380 401 407 426 424 395 434 433 463 461 457 408 408 477 476

582 394 389 389 384 384 380 380 379 368 368 367 362 353 352 348 334 333 326 322 322 319 319 308 307

Combined Enegry gain and PV performance index 218 150 154 153 151 151 152 152 139 148 140 147 147 150 149 137 145 144 151 148 147 130 130 147 146

Corrugated horizontal double thickness

511

305

156

Corrugated vertical double thickness Curved one side horizontal double thickness Corrugated horizontal-curved double thickness Pyramids 1,5 thickness Corrugated vertical-curved double thickness Curved one side vertical double thickness Corrugated horizontal 1/2 height origami basic 1/1 Corrugated vertical 1/2 width Inverted pyramid 1-1 Curved one side horizontal half height Pyramids 1/2 width Corrugated vertical-curved 1/2 width Corrugated horizontal-curved 1/2 height Curved one side vertical half width Pyramids double thickness origami basic 1,5 thickness origami basic 1/2 width Inverted pyramid double thickness Curved one side vertical fourth width Origami basic double thickness Curved one side horizontal fourth height Pyramids 1/4 width Corrugated horizontal-curved 1/4 height Corrugated vertical-curved 1/4 width Corrugated horizontal 1/4 height Corrugated vertical 1/4 width Inverted pyramid half width origami basic 1/4 width Inverted pyramid fourth width

496 518 526 525 524 506 471 408 460 390 486 489 484 481 463 577 484 482 472 613 526 600 587 602 595 556 541 299 597 374

296 289 288 286 286 282 281 277 275 272 271 265 265 263 258 253 237 217 207 204 204 200 198 197 195 194 188 162 152 126

147 150 151 150 150 143 132 113 127 106 132 130 128 127 119 146 115 105 98 125 107 120 116 119 116 108 102 48 91 47

kWh/year/m2 footprint on PV performance the facade (kWh/m2 of PV surface)

160

For instance the first shape achieving a better stability than the best performing shape (“curved one side vertical fourth width”) is the “corrugated vertical curved ¼ width” shape (nr. 5 in the ranking) which provides on one hand a 4,25 % improved energy stability (3432/3292) linked with a 3% decrease in absolute energy gain (595/613). In conclusion this last criterion cannot be decisive but is included mainly for the completeness of the solar analysis data. 10.2.2 Results Table “Sorted by Criterion 2” (per PV surface area). Comments: Here the best performing shape, the horizontal conventional louvers with a fixed 30deg inclination, has such a substantial value difference (582 to 394) from even the second best shape (again a very simple “flat curtain” in front of the glass part of the facade) that although this fact could appear frustrating at first sight, it is a known aspect the present thesis takes as a basis, namely that a better exposed small surface will be more efficient in terms of material use compared to any complex 3d shape. The goal on the contrary is increasing the exposed PV surface by creating more or less complex 3D shapes, so that due to the increase in exposed surface more TOTAL energy is collected. It is apparent that by doing this, self-shading problems will arise, which decrease the efficiency of each cm2 of the PV material, in exchange for a higher TOTAL energy output per m2 of Facade space. The same can be said for any limited area that needs to be covered with as much active material as possible to gain the maximum energy. This part is thus to some extend a cost based ranking. It can however be stated already that due to the fact that: - The prices for the thin film or similar technology PV’s will be dramatically decreasing especially after the use of new production methods and polymers based cells and substrates - For fulfilling the already set “zero-energy-building” requirements renewable energy gain has to be maximized - The available façade surface of a building will have to be used for the maximum solar energy production the entirely performance based criteria will become more and more important. 10.2.3 Results Table “Sorted by Criterion 3” (combined). Comments: In this table a combination of criteria 1 and 2, i.e. considering both the total annual energy gain and the PV material efficiency expressed in kWh/per m2 of used PV material is attempted. Fixed louvers are again highly efficient in this category . The main decisive aspect related to the results is the amortization/payback time for the thin PV installation costs. If this is comparatively low (around 1-2 years according to the bibliography with decreasing trend) then after this period the maximum energy gain is more and more relevant (see previous table and corresponding comments) compared to cost related aspects. A combination of TOTAL energy gain and performance per m2 will therefore give an indication of the payback time (at least to the extend that shape can influence it). As already explained however very short payback times of most new PV technologies will make this criterion too, less and less important in the future in favor of absolute performance. 161

COMPARISON OF SHAPES IN VIEW OF 3 CRITERIA (1: Energy gain; 2: PV performance (cost); 3: Combined 1 and 2) sorted by criterion 3 kWh/year/m2 footprint on PV performance the facade (kWh/m2 of PV surface) Louvers 30d horizontal Corrugated horizontal double thickness Corrugated horizontal 1/4 thickness Corrugated vertical 1/4 thickness Corrugated vertical 1/2 thickness Corrugated horizontal 1/2 thickness Corrugated horizontal 1,5 thickness Corrugated vertical-curved 1/4 thickness Corrugated horizontal-curved 1/4 thickness Corrugated horizontal-curved double thickness Corrugated horizontal 1/1 Flat facade Corrugated vertical-curved double thickness Pyramids 1,5 thickness Curved one side horizontal double thickness Corrugated vertical 1/1 Corrugated horizontal-curved 1/2 thickness Pyramids 1/1 Corrugated vertical 1,5 thickness Corrugated vertical-curved 1/2 thickness Corrugated horizontal-curved 1,5 thickness Pyramids 1/2 thickness Corrugated vertical double thickness Corrugated vertical-curved 1,5 thickness Pyramids double thickness Corrugated horizontal-curved 1/1 Corrugated vertical-curved 1/1 Curved one side vertical double thickness Pyramids 1/4 thickness origami basic 1/4 thickness origami basic 1/2 thickness Corrugated horizontal 1/2 height Curved one side horizontal half height Pyramids 1/2 width Curved one side vertical 1-1 Curved one side horizontal 1-1 Corrugated vertical-curved 1/2 width Corrugated vertical 1/2 width Corrugated horizontal-curved 1/2 height Curved one side vertical fourth width Curved one side horizontal fourth height Corrugated horizontal-curved 1/4 height Curved one side vertical half width Corrugated vertical-curved 1/4 width Pyramids 1/4 width origami basic 1,5 thickness origami basic 1/1 Corrugated horizontal 1/4 height origami basic double thickness Inverted pyramid 1-1 origami basic 1/2 width Corrugated vertical 1/4 width Inverted pyramid double thickness origami basic 1/4 width Inverted pyramid half width Inverted pyramid fourth width

374 511 395 394 401 401 463 394 394 526 426 381 524 525 518 424 402 461 457 401 477 407 496 476 577 434 433 506 380 368 395 471 486 489 408 408 484 460 481 613 600 602 463 595 587 484 408 556 526 390 482 541 472 597 299 374

582 305 389 389 380 380 326 384 384 288 353 394 286 286 289 352 368 322 322 367 308 362 296 307 253 334 333 282 368 379 348 281 271 265 319 319 265 275 263 204 200 197 258 195 198 237 277 194 204 272 217 188 207 152 162 126

Combined Enegry gain and PV performance index 218 156 154 153 152 152 151 151 151 151 150 150 150 150 150 149 148 148 147 147 147 147 147 146 146 145 144 143 140 139 137 132 132 130 130 130 128 127 127 125 120 119 119 116 116 115 113 108 107 106 105 102 98 91 48 47

162

If despite the described future tendencies both criteria 1 and 2 should have a high ranking in selecting the best shape (and thus criterion 3 expressing this would be important), the above table contains acceptable cases like the: - Corrugated horizontal double thickness (value 156, position 2), - Corrugated horizontal curved double thickness (value 151, position 10) - Corrugated vertical curved double thickness (value 150, position 13) and - Pyramids 1,5 thickness (value 150, position 14), which have acceptable energy values (511, 526, 524 and 525, respectively) combined with excellent results as to economic use of PV material. It should final be mentioned in this respect that, with the exception of the very first shape and the last four ones, all remaining 51 shapes form practically a dense group with values between 100 and 150 such that this criterion loses its strength and significance for a further reason. Results Step_3 : Examining and comparing the shapes within each group It is apparent that the 56 examined shapes have in addition to any already discussed measurable qualitative performance data, also attributes referring to their architectural appearance. As architects would certainly put particular emphasis on the appearance of a shading system designed to be integrated in front of the glass part of a façade, in practice, a first selection could be based on the desired basic shape as to its aesthetics and the performance aspect would in such a case be the next selecting criterion. For exactly this reasons a further series of 10 excel worksheets were produced with the following concept. In each of the first nine of them, the “shape versions” of each group are listed and compared according to the above mentioned criteria 1, 2 and 3. In a further sheet (table “Shape Group Performances”), the mean values of all the shapes of each group are listed, so that the performance of each shape group as a whole is visible. The differences are not big, since here mean values of each group are contained and as the previous tables have shown, there are substantial fluctuations in performance within the shapes of each group. However, it is apparent for instance that both “one-side curved” groups (either vertical or horizontal) perform well even as a whole goup. This means that if a designing architect would accept this shape as fulfilling his aesthetic needs, he could then turn to one of the next sheets for a more detailed selection within this group.

163

Shape Group Performances

SHAPE

MEAN VALUE OF Group kWh/year/m2 footprint on the façade

MEAN VALUE OF Group PV performance (kWh/m2 of PV surface)

MEAN VALUE OF Group Combined Enegry gain and PV performance i ndex

Corrugated vertical

453

315

140

Corrugated horizontal Corrugated vertical curved Corrugated HORIZONTAL curved ORIGAMI PYRAMIDS CURVED ONE SIDE VERTICAL CURVED ONE SIDE HORIZONTAL INVERTED PYRAMIDS

460

318

143

472

305

140

474

306

141

466 489

259 293

115 140

498

266

129

503

270

133

384

192

600 Corrugated vertical

500

Corrugated horizontal 400

Corrugated vertical curved

Corrugated H ORIZONTAL curved

300

ORIGAMI

PYRAMIDS

200

CURVED ONE SIDE V ERTICAL

CURVED ONE SIDE H ORIZONTAL

100

INVERTED PYRAMIDS 0 1

164

In such a case scenario of shape group selection by the architect, the following two tables could be used, the first for the “horizontal one-side curved” and the second for the “vertical one-side curved”. If the designer prefers the “vertical” arrangement of the convexly curved “peaks” of this group, then he can find from this table that it is the “curved one-side vertical ¼ width” that performs the best (613 kWh/year∙m2), but shows as expected and analysed before some deficiencies in the criterion 2 (costs of use of PV material).

Curved one side vertical 1-1 Curved one side vertical double thickness Curved one side vertical fourth width Curved one side vertical half width MEAN V ALUE OF THIS GROUP Best rank i n sorted l ist

PV kWh/year/m2 performance footprint on the (kWh/m2 of facade PV surface) 408 319

Combined Enegry gain and PV performance index 130

506

282

143

613

204

125

463

258

119

498 1

266 22

129 28

700 600 500

Curved one side vertical 1 -‐ 1

400

Curved one side vertical double thickness

300

Curved one side vertical four th w idth

200

Curved one side vertical half width

100

0 1

700

600 500

400 300 200 100 0

165

Curved one side vertical Curved one side vertical Curved one side vertical Curved one side vertical 1-‐1 double thickness fourth w idth half width

If a horizontal arrangement of the “waves” is more desirable from aesthetic (or other non performance related) reasons, the further selection would have to be made on the basis of respective tables of the “horizontal” group. In this case, the “curved one-side horizontal ¼height” will have to be selected if energy gain performance is the leading need. It is obvious that different shapes could be selected for each side or part of a building not only for aesthetic reasons but also for reasons of illumination needs of each room (vertical or horizontal moving of the shading curtain) and possibly also depending on the orientation of the external building surface (east, west etc.). Such aspects although lying beyond the work of the present thesis, could easily be elaborated based on the above explained selection procedure, possibly with more shapes and using Ecotect to deliver data for more orientations.

Curved one side horizontal 1-1 Curved one side horizontal double thickness Curved one side horizontal fourth height Curved one side horizontal half height

408

PV performance (kWh/m2 of PV surface) 319

518

289

150

600

200

120

486

271

132

MEAN V ALUE OF THIS GROUP Best rank i n sorted l ist

503 3

270 23

133 15

kWh/year/m2 footprint on the facade

Combined Enegry gain and PV performance index 130

700 600

Curved one side horizontal 1 -‐1

500

Curved one side horizontal double thickness

400 300

Curved one side horizontal fourth height

200

Curved one side horizontal half height

100 0 1

700

600 500 400 300

200 100 0

Curved one side horizontal 1 -‐1

Curved one side horizontal double thickness

Curved one side horizontal fourth height

Curved one side horizontal half height

166

11_Phase 4

PV-Lab TUDelft Shape Testing. Real Thin film module Performance evaluation

167

168

11.1 Importance of Phase 4 as a next step after the Solar Analysis Phase 3 has been an important step towards the evaluation of the created 3d shapes in terms of solar energy that their surface receives on average every day within a year. The results showing incident solar radiation on every part of the different shapes and partial shading percentages were important to get an idea of the potential of each shape in terms of solar energy received. The best performing shape showed an increase of more than 60% in total incident solar radiation compared to a flat surface of the same dimensions. In a theoretical situation where solar cells are developed to such an extend that they are able to perform in a directly and linearly proportional relation to the light/energy received (thus also to the shading percentage), a further step would not be needed after a precise solar analysis (even without the use of solar cells in the simulation the results would be accurate enough). The solar analysis step would therefore be the final one, since PV performance would be predictable through incident radiation values and partial shading percentages. Through a simplification in order to make this more clear it is enough to observe the solar performance of a partly shaded thin film PV module. Due to the in series connection of solar cells within a module, covering 50% of the modules surface (therefore 50% shading) will not reduce its output only by 50%. The precise decrease in performance for thin film solar cells is highly dependent on the shading pattern and the arrangement of the cells in relation to the shadow. If a shadow covers a large area of a cell for example the reduced performance of this cell will become a â&#x20AC;&#x153;bottleneckâ&#x20AC;? for the complete module performance due to the in series connection. Therefore unequal illumination of cells can create problems if the cell layout and shading is not taken into account. If a shadow perfectly vertical to the cell strips is achieved, then the performance drop can come as close as possible to the proportional decrease, since cells will be equally illuminated. The difficulty of achieving this in a complex prismatic 3d shape makes real PV testing an important evaluation process, at least for integration of current conventional PV cell technology. As explained future development can lead to much less shading sensitive cells and much more efficient connections. (The effects of partial shading have been analyzed in chapter 4.10 of the current thesis.) In order to observe the effect of shaping a surface into three dimensional arrangements and the performance decrease created by non uniform illumination of the surfaces, the solar analysis of Phase 3 is followed by approximately 100 real Thin film PV module Testing sessions in the PV-Lab of the Faculty of Electrical Engineering of the TUDelft. fig. 11.2 Shape version 5 prepared for testing with wooden structure

169

fig. 11.1 Basis Shape corrugated surface

fig. 11.3 PV Lab testing of shape under specific tilt angle

fig. 11.4 Construction of wooden models of varying curvatures

fig. 11.5 Temperature measurement on different spots of the surface

170

11.2 Creating the testing models. Solar panels/modules used. Restrictions. For the PV-Lab testing flexible thin film PV modules of 1.20m X 0.325m by HyET SOLAR have been used. They are based on single junction amorphous silicon Technology. These longitudinal solar cell strips show excellent mechanical behavior despite their extremely thin profile of only 0.5 mm and very low weight of approximately 600 gram/mÂ˛. The ability to bend (and roll) into small diameters is an important advantage for the transportability of the modules, their integration potential on various surfaces and therefore also for the successful application as the active solar surface of three dimensionally shaped surfaces, which will be tested in this case. Given that a large number and variety of different module sizes and shapes has not been available and would be an expensive and extremely time consuming attempt (and would also need an extremely detailed study of the arrangement of the cells/modules within the surfaces to avoid excessive shading of specific cells), the testing process had to be limited to the shape variations of a single flexible module. The transformations of the module in more and less dense corrugated 3d shapes would already give an indication of the effects of different illumination angles, non uniform lighting of the cells and partial shading. In order to shape the given Thin film PV module, the minimum bending diameters predefined by the Manufacturer had to be taken into account in order to avoid damages to the solar cells. In this case minimum diameter has been limited to approximately 7-8 cm. This means that sharp angles are not possible to be created in order to precisely simulate prismatic foldable designs. In relation to the total area however the effect of the curved edges compared to a sharp angle/edge is not a significant difference. For comparison with curved designs the minimum diameter is not a problem. The restrictions in module number and size also imposes the simulation of only part of the three dimensional object that would cover a facade, which means a limited number of folds/corrugations. The height and size of the PV testing machine is also a significant restriction since the height of approximately one meter does not allow for large models especially when tilted for testing different angles.

Minimum PV Bending Radius : 35-40mm

fig. 11.6 Shapes designed according to minimum bending radius of PV Material

171

fig. 11.7 Wooden sub-structure and flat thin film panel by HyET Solar before before attachment

fig. 11.8 PV Laboratory Testing Machine

fig. 11.9 Different curvature shapes

172

Construction of different 3d shapes. The shapes which are compared in the Lab Testing Process are corrugated designs of varying density and thickness (amplitude of curves). They can be interpreted as 3d surface parts similar to the single axis corrugated shape versions of Phase 2 and 3 (for example corrugated curved vertical/horizontal). Seven (7) different wooden 3d shape versions have been constructed in the TUDelft Faculty of Architecture Workshop. Shape density and thickness as defined in previous phases research is in this case translated into the compression of a flexible module into denser and taller/thicker arrangements, since total length is significantly restricted. The specific shapes were chosen as characteristic steps from extremely slender and wide patterns to very thick and dense solutions. The wooden shape profiles were glued to transverse parts in order to constitute rigid frames for supporting the thin film module. The module was attached to each one of these shapes using tape. For simulating different sun angles (and theoretically also installation of 3d structures in an angle compared to the facade plane, which is not the case in this analysis) the three dimensionally shaped PV module is rotated both parallel and perpendicular to its corrugations in order to extract performance values of the structure for vertical and horizontal corrugation tilt/inclination of 30o, 45o and 60o. More test angles between these characteristic values would create only very small differences in test results since thin film technology is not very sensitive to small angle changes, which will also become clear from the results. Angles beyond 60o were for most shapes impossible due to space restrictions from the Testing Machine. Even 60o have been impossible to measure for specific shapes with a longer total size. For creating the needed angles the PV structure is supported by a modified/custom adjustable hinged wooden frame that is â&#x20AC;&#x153;lockedâ&#x20AC;? into the specified tilt angles.

fig. 11.10

173

fig. 11.11

Shape 07 o

Thin Film PV covering only corrugated part of the model fig. 11.12

Shape 06 o

Shape 05 o

Shape 04 o

Shape 03 o

Shape 02 28o

fig. 11.13

Shape 01 22

Thin Film PV

174

11.3 Testing Procedure and shapes For the measurement of each created shape a reference cell approximately at the height of the measured module and a thermometer are used in order to have the correct values of actual illumination (intensity) and temperature as both significantly affect the results. Seven shapes and their corresponding 7 footprints as a flat module part have been tested. Every shape and flat footprint sized module has been tilted in both directions in order to get values for different sun angles. - Shape 01 is the flattest and less dense design with a footprint (projection) size of 29.7cm X 86.86cm. The angle of the linear segments of the triangle/corrugation is approximately 22o - Shape 02 has a footprint (projection) size of 29.7cm X 85.84cm. The angle of the linear segments of the triangle/corrugation is approximately 28o - Shape 03 is the flattest and less dense design with a footprint (projection) size of 29.7cm X 81.33cm. The angle of the linear segments of the triangle/corrugation is approximately 40o - Shape 04 has a footprint (projection) size of 29.7cm X 72.75cm. The angle of the linear segments of the triangle/corrugation is approximately 52o - Shape 05 has a footprint (projection) size of 29.7cm X 60.85cm. The angle of the linear segments of the triangle/corrugation is approximately 64o - Shape 06 is the flattest and less dense design with a footprint (projection) size of 29.7cm X 54.37cm. The angle of the linear segments of the triangle/corrugation is approximately 70o - Shape 07 has a footprint (projection) size of 29.7cm X 46.94cm. The angle of the linear segments of the triangle/corrugation is approximately 76o The seven Footprint modules were tested separately. In order to get the needed size parts of the 120cm X 29.7cm (size measured at cells edge, not at modules adge since there is a plastic part around them and two metal connector strips) module were covered with cardboard. Cell strips have been always covered transverse (perpendicular to their long limension) in order to achieve similar performing cells in the series connection and prevent a significant performance drop due to problematic partial shading. Inclinations, Tilt angles: All shapes and footprint modules were tested under different characteristic angles. Flat position (0o), 30o, 45o, 60o in both directions, parallel and perpendicular to the curvature were measured. Shapes that were unable to fit under the testing Machine under specific angles (usually 60o, in some cases also 45o) were only tested for the possible positions.

175

On the basis of this testing concept, the following groups of data have been extracted: a. 7 Flat PV strips (each one with the footprint area of each shape) in 4 inclinations produced 28 measurements. b. 7 shapes under 4 inclination angles (some of them in both 2 orientations) produced 46 measurements. c. A number of measurements experimenting with temperatures and hight differences for the reference cell.

11.3.1 Collecting the results The results from the tests are provided by the relevant software in a complicated but complete in terms of data format which needed subsequent simplification in particular as to the format of the numbers and comprised measurements or identification data for the following categories:

fig. 11.14 Table

176

11.3.2 Reason for inclination angles Measurements are performed for all seven shapes in different inclinations (0deg, 30deg, 45deg, 60deg) for the following reasons: The thesis deals with arranging thin pv films substantially on vertical parts of a building (façade), i.e. under a β=90deg inclination. The Ecotect results are based on exactly this assumption. However in the TU Delft pv lab facility the technical “sun” is fixedly positioned horizontally (=sun elevation angle: α=90deg) and the model had to be positioned under the illumination table. Consequently real conditions were difficult to be simulated unless some workaround could be found. As can bee seen from the comparison of the two sketches (fig 11.15) a for instance β=30deg inclination of the models under the lab α=90deg sun (left sketch) corresponds (see the same value of 17,305 for the module component of sun radiation) to having a PV on a β=90deg façade and the sun under an α=30deg elevation (situation in the morning/evening and in the winter even at noon).

Laboratory layout fig. 11.15 Angles created as simulation of real conditions.

177

Real Situation Simulated

11.3.3 Transforming and organizing the results The results created by the PV testing specific software have been processed in the following manner: a. The data has been transformed in a â&#x20AC;&#x153;.txtâ&#x20AC;? file and then to an .xls one so that an excel worksheet could be produced. b. Not particularly useful columns and other data have been extracted so that the main data could form the excel worksheet, a part of which has the following layout seen in the table.

fig. 11.16 Table

Run Id

PV Temper Size Irradiance ature (expos (correct (corrected) ed ed) area)

Jsc

Isc

Voc

Impp

Vmpp

Fill Factor

Pmpp/m 2 of PV [W/m2]

Eta

12,46 12,01 12,03 11,71 11,44 10,06 10,14

[W]=mA (Impp)/1000 x V (Vmpp) 8,969 8,596 8,207 7,153 5,851 5,031 4,169

0,345 0,343 0,353 0,352 0,349 0,339 0,349

34,777 33,721 33,982 33,114 32,413 31,208 29,952

3,478 3,372 3,398 3,311 3,241 3,121 2,995

895,45 860,56 806,09 632,77 882,68 848,42

11,96 12,41 11,20 11,25 12,75 11,24

10,711 10,679 9,030 7,121 11,257 9,535

0,344 0,345 0,342 0,339 0,340 0,335

38,722 38,608 32,646 25,745 40,699 34,471

3,755 3,744 3,166 2,497 3,947 3,343

861,82 868,67 735,23 630,87 938,90 899,46

12,41 12,01 11,72 10,65 11,64 11,65

10,692 10,433 8,615 6,717 10,931 10,476

0,337 0,342 0,337 0,337 0,335 0,331

37,489 36,582 30,208 23,552 38,328 36,731

3,749 3,658 3,021 2,355 3,833 3,673

[cm2]

[C]

[W/m2]

[mA/cm2]

[mA]

[mV]

[mA]

[V]

2.579 2.549 2.415 2.160 1.805 1.612 1.392

55.37 55.61 55.91 55.81 55.71 55.62 55.69

639,91 643,77 644,75 646,65 647,13 647,72 649,73

0,394 0,384 0,387 0,391 0,398 0,405 0,399

1014,85 978,00 933,78 845,63 718,54 652,76 554,92

25,59 25,60 24,92 24,01 23,34 22,76 21,56

719,65 715,60 682,18 611,03 511,34 499,96 411,32

jim_01_accuracy 2.766 jim_01_30d_vertical_accuracy2.766 jim_01_45d_vertical_accuracy2.766 jim_01_60d_vertical_accuracy2.766 jim_01_30d_horizontall_accuracy 2.766 jim_01_45d_horizontall_accuracy 2.766

62.81 62.88 62.76 62.35 60.22 59.16

656,12 649,30 634,08 597,87 652,84 649,34

0,456 0,452 0,395 0,322 0,475 0,426

1260,05 1249,16 1092,49 890,57 1313,48 1177,22

24,68 24,81 24,15 23,62 25,24 24,14

jim_02_accuracy 2.852 jim_02_30d_vertical_accuracy2.852 jim_02_45d_vertical_accuracy2.852 jim_02_60d_vertical_accuracy2.852 jim_02_30d_horizontal_accuracy 2.852 jim_02_45d_horizontal_accuracy 2.852

58.64 59.49 59.48 58.79 55.69 56.08

656,10 650,49 647,51 619,57 653,53 616,96

0,445 0,438 0,381 0,309 0,466 0,446

1268,22 1249,67 1085,83 879,87 1330,30 1271,46

24,98 24,38 23,53 22,66 24,54 24,92

jim_01_footprint_accuracy jim_02_footprint_accuracy jim_03_footprint_accuracy jim_04_footprint_accuracy jim_05_footprint_accuracy jim_06_footprint_accuracy jim_07_footprint_accuracy

Pmpp

[%]

Most important columns for the evaluation and creation of relevant graphs are the ETA percentage (conversion efficiency) the exposed PV area in cm2 (not the footprint but the total area of the PV surface in this case) and the Pmpp/m2 of PV, which shows the produced Watts per m2 of PV surface. Again this refers to the total area of the PV cells. Therefore an additional column is important to be calculated showing the total produced Watts per m2 footprint surface area, which is the aim of the process from the beginning.

178

11.3.4 Presenting the results The results have been composed in two major excel files. One with a final and more handy form of the results grouped per “shape” (footprint, shape 01, ...,07) suitable for evaluation. One similar excel worksheet with the results grouped “per orientation” meaning the inclination angles at which the produced models are positioned and measured. Footprint measurements

footprint footprint footprint footprint footprint footprint footprint

[C]

[W/m2]

2.579 2.549 2.415 2.160 1.805 1.612 1.392

55.37 55.61 55.91 55.81 55.71 55.62 55.69

639,91 643,77 644,75 646,65 647,13 647,72 649,73

m 2]

[cm2]

Jsc

[m A /c

Irradiance (corrected)

01-flat 02-flat 03-flat 04-flat 05-flat 06-flat 07-flat

Temperature (corrected)

Run Id (shape Nrorientation of undulations inclination as to fllor)

PV Size (exposed area)

fig. 11.17 Table

0,394 0,384 0,387 0,391 0,398 0,405 0,399

Isc

Voc

Impp

Vmpp

[mA]

[mV]

[mA]

[V]

1014,85 978,00 933,78 845,63 718,54 652,76 554,92

25,59 25,60 24,92 24,01 23,34 22,76 21,56

719,65 715,60 682,18 611,03 511,34 499,96 411,32

12,46 12,01 12,03 11,71 11,44 10,06 10,14

Pmpp

[W]=mA (Impp)/1000 x V (Vmpp) 8,969 8,596 8,207 7,153 5,851 5,031 4,169

Fill Factor

Pmpp/m2 Pmpp/m2 of PV of module exposed footprint surface

Eta

[%]

[W/m2]

[%]

0,345 0,343 0,353 0,352 0,349 0,339 0,349

34,78 33,72 33,98 33,11 32,41 31,21 29,95

3,478 3,372 3,398 3,311 3,241 3,121 2,995

Δeta Increas within e in the Pmpp goup in as to flat view of shape 0deg

Eta versus PV Size (exposed area) 3,600 3,500 3,400 3,300

3,200 3,100 3,000 2,900

500

1.000

1.500

2.000

2.500

3.000

The above table has been produced by sequentially partly illuminating (by covering the rest with an opaque surface) seven different parts of the whole thin PV module (of an original size of approximately 29.7 x 120 cm). All seven parts have the same width (29,7 cm), but the length of each produced “strip” corresponds to the footprint of each one of the seven constructed PV models. For instance for the shape Nr. 5, “05-flat footprint” in the above table, the footprint area was calculated as 0,6085m=60,85cm x 29,7 cm width =1805 cm2 and the PV module was “covered” accordingly so as to have exactly this exposed area. The reason for this part of tests was to have seven reference measurements of flat thin film PV modules, each in the size of the footprint area of each of the seven models, so that a comparison as to the possible “gain” (or loss) of the seven 3d shapes compared to the flat footprint equivalents can be made.

179

The above described seven tests were repeated also for inclination angles (30deg, 45deg, 60deg) other then flat (=0deg) for comparison with the corresponding results of the “real” models under such inclination angles. From the above table is it apparent that by reducing the exposed area of a PV module its efficiency expressed in the “eta” value is decreased in a linear way, this being expected when knowing the typical electrical circuitry characteristics of a PV module. In other words, when reducing the exposed area of the thin film module, the energy gain is not only reduced due to the less exposure as a total amount of produced energy (see column Pmpp in the above table) which is apparent, but most importantly also the “specific” energy production (see the Pmpp/m2 column in the above table) is reduced, which is expressed by the additional “eta” value (efficiency of the module) decrease (from 3,478 to 2995). This was expected but its extent was measured in these tests.

11.4 Models Lab Testing Results

In the following pages all the results for all the measurements of the seven models/shapes are depicted, grouped according to two different aspects: 1) Results grouped per inclination angle and orientation=shape importance Meaning that for each angle of positioning the PVs under the irrdiation table (flat=0deg, 30deg, 45deg and 60deg as to the floor) and for each orientation (vertical or horizontal = ”valleys” of the undulations are directed vertically/horizontally to the floor surface) a table is created such that the seven “shapes” can be compared. 2) Grouped per shape= inclination angle and orientation importance Meaning that for each of the seven shapes a table is created such that the importance of the anlge of positioning the PVs under the irrdiation table (flat=0deg, 30deg, 45deg and 60deg as to the floor) and its orientation (vertical or horizontal=”valleys” of the undulations are directed vertically/horizontally to the floor surface) can be compared. 3) Tables= Results grouped per inclination angle and orientation=shape importance

180

2.766 2.852 3.026 3.201 3.379 3.462 3.549

62.81 58.64 54.34 54.07 57.45 60.16 62.06

Jsc

Isc

Voc

Impp

Vmpp

Pmpp

Fill Factor

656,12 656,10 632,10 648,69 655,91 644,62 640,20

0,456 0,445 0,379 0,363 0,300 0,256 0,234

1260,05 1268,22 1146,41 1162,67 1014,37 885,78 830,56

24,68 24,98 26,92 25,60 24,74 23,86 22,67

895,45 861,82 805,01 811,07 754,97 607,38 606,16

11,96 12,41 12,91 12,61 11,33 11,94 10,23

10,711 10,692 10,391 10,228 8,556 7,250 6,204

0,344 0,337 0,337 0,344 0,341 0,343 0,329

4,000

Eta

38,72 37,49 34,34 31,95 25,32 20,94 17,48

3,755 3,749 3,434 3,195 2,532 2,094 1,748

Pmpp/m2 Increase of PV in Pmpp [W/m2 of as to flat footprint] shape 41,53 41,95 43,03 47,35 47,40 44,97 44,57

1,19 1,24 1,27 1,43 1,46 1,44 1,49

Increase in Pmpp as to flat shape

Pmpp/m2 of footprint performance of shapes

Eta performance of shapes

Pmpp/m2 of PV [W/m2 of module exposed surface]

Temperature (corrected)

01-0deg 02-0deg 03-0deg 04-0deg 05-0deg 06-0deg 07-0deg

0ο angle

Irradiance (corrected)

Run Id (shape Nrorientation of undulations inclination as to fllor)

PV Size (exposed area)

11.4.1 Results Per Inclination

1,60

48,00 1,50

47,00

3,500 3,000

46,00

1,40

45,00

1,30

44,00

2,500

1,20

43,00

Temperature (corrected)

01-vertical-30deg 02-vertical-30deg 03-vertical-30deg 04-vertical-30deg 05-vertical-30deg 06-vertical-30deg 07-vertical-30deg

2.766 2.852 3.026 3.201 3.379 3.462 3.549

62.88 59.49 60.97 54.79 57.76 52.59 61.92

Irradiance (corrected)

Run Id (shape Nrorientation of undulations inclination as to fllor)

Jsc

Isc

Voc

Impp

Vmpp

Pmpp

Fill Factor

649,30 650,49 636,08 648,78 657,32 641,86 640,45

0,452 0,438 0,380 #VALUE! 0,297 0,278 0,241

1249,16 1249,67 1149,27 NaN 1003,00 962,45 853,84

24,81 24,38 26,52 26,58 24,81 23,65 22,75

860,56 868,67 843,27 770,70 693,40 654,05 636,44

12,41 12,01 12,30 12,39 12,30 11,43 10,34

10,679 10,433 10,374 9,547 8,526 7,477 6,584

0,345 0,342 0,340 NaN 0,343 0,328 0,339

Eta performance of shapes

3,500 3,000 2,500 2,000 1,500

181

1,00

41,00

PV Size (exposed area)

1,500

4,000

1,10

42,00

Pmpp/m2 of footprint performance of shapes 48,00 47,00

1,60 1,50

45,00

1,40

43,00

1,30

42,00

1,20

41,00

1,10

40,00

38,61 36,58 34,28 29,82 25,23 21,60 18,55

Pmpp/m2 Increase of PV in Pmpp Eta [W/m2 of as to flat vertical 30ο angle shape footprint] 3,744 3,658 3,428 2,982 2,523 2,160 1,855

41,41 40,93 42,96 44,20 47,24 46,38 47,30

Increase in Pmpp as to flat shape 1,70

46,00

44,00

Pmpp/m2 of PV [W/m2 of module exposed surface]

2,000

1,00

1,22 1,21 1,29 1,37 1,49 1,52 1,62

62.76 59.48 60.87 55.05 57.61 45.1 61.05

Isc

Voc

Impp

Vmpp

Pmpp

634,08 647,51 636,63 649,76 656,55 642,02 642,17

0,395 0,381 0,347 0,306 0,267 0,347 0,216

1092,49 1085,83 1051,27 980,74 902,47 1202,60 767,77

24,15 23,53 27,18 25,25 24,21 22,55 21,74

806,09 735,23 718,27 668,74 686,07 597,37 529,72

11,20 11,72 13,52 12,38 10,86 10,65 10,84

9,030 8,615 9,711 8,281 7,448 6,363 5,743

0,342 0,337 0,340 0,334 0,341 0,235 0,344

Eta performance of shapes

Pmpp/m2 of footprint performance of shapes

3,500

2,500 2,000 1,500

Temperature (corrected)

01-vertical-60deg 02-vertical-60deg 03-vertical-60deg 04-vertical-60deg 05-vertical-60deg 06-vertical-60deg 07-vertical-60deg

2.766 2.852 3.026 3.201 3.379 3.462 3.549

62.35 58.79 59,00 63.89 57.27 39.84 60.02

2,600 2,400

1,400 1,200 1,000

1,20 1,10

1,00

Vmpp

Pmpp

597,87 619,57 640,65 649,33 656,71 638,19 637,89

0,322 0,309 0,278 0,234 0,236 0,217 0,186

890,57 879,87 842,27 750,38 796,00 752,32 658,38

23,62 22,66 25,98 24,21 23,36 21,09 20,88

632,77 630,87 612,03 545,58 550,60 470,05 458,78

11,25 10,65 12,21 11,53 11,40 10,30 10,20

7,121 6,717 7,476 6,291 6,277 4,843 4,680

0,339 0,337 0,342 0,346 0,338 0,305 0,340

Pmpp/m2 of footprint performance of shapes

33,00

1,600

1,30

Impp

2,000 1,800

1,40

Voc

35,00

2,497 2,355 2,470 1,965 1,858 1,399 1,319

27,61 26,35 30,95 29,13 34,77 30,04 33,62

1,28 1,26 1,47 1,42 1,73 1,55 1,81

2,00 1,90 1,80 1,70 1,60 1,50

29,00

1,30

25,00

25,75 23,55 24,70 19,65 18,58 13,99 13,19

Pmpp/m2 Increase of PV in Pmpp Eta [W/m2 of as to flat shape footprint] vertical 60ο angle

Increase in Pmpp as to flat shape

31,00

27,00

1,27 1,23 1,50 1,47 1,61 1,60 1,74

1,50

Isc

2,200

35,01 33,80 40,21 38,34 41,27 39,47 41,25

1,60

Jsc

37,00

3,166 3,021 3,209 2,587 2,204 1,838 1,618

1,70

Fill Factor

Eta performance of shapes

32,65 30,21 32,09 25,87 22,04 18,38 16,18

1,80

Irradiance (corrected)

Run Id (shape Nrorientation of undulations inclination as to fllor)

PV Size (exposed area)

1,000

Eta

Pmpp/m2 Increase of PV in Pmpp [W/m2 of as to flat footprint] shape

Increase in Pmpp as to flat shape

42,00 41,00 40,00 39,00 38,00 37,00 36,00 35,00 34,00 33,00 32,00

3,000

Pmpp/m2 of PV [W/m2 of module exposed surface]

2.766 2.852 3.026 3.201 3.379 3.462 3.549

Jsc

Fill Factor

Pmpp/m2 of PV [W/m2 of module exposed surface]

Temperature (corrected)

01-vertical-45deg 02-vertical-45deg 03-vertical-45deg 04-vertical-45deg 05-vertical-45deg 06-vertical-45deg 07-vertical-45deg

Irradiance (corrected)

Run Id (shape Nrorientation of undulations inclination as to fllor)

PV Size (exposed area)

vertical 45ο angle

1,40 1,20 1,10 1,00

182

2.766 2.852 3.026 3.201 3.379 3.462 3.549

60.22 55.69 60.17 62.67 55.16 38.36 59.76

Jsc

Isc

Voc

Impp

Vmpp

Pmpp

Fill Factor

652,84 653,53 632,70 646,77 656,52 640,50 643,59

0,475 0,466 0,372 0,314 #VALUE! 0,283 0,248

1313,48 1330,30 1124,83 1005,36 NaN 978,73 879,59

25,24 24,54 26,36 24,91 24,56 22,95 22,68

882,68 938,90 823,43 721,76 745,61 622,84 616,99

12,75 11,64 12,49 12,38 11,25 11,01 11,01

11,257 10,931 10,283 8,937 8,387 6,856 6,790

0,340 0,335 0,347 0,357 NaN 0,305 0,340

Eta performance of shapes

Pmpp/m2 of footprint performance of shapes

4,500

50,00 49,00 48,00 47,00 46,00 45,00 44,00 43,00 42,00 41,00 40,00

3,500 3,000

2,500 2,000

1,10 1,00

9,535

0,335

34,47

3,343

36,97

1,346

9,636

0,335

31,84

3,184

39,90

1,487

11,53

8,457

0,340

26,42

2,642

39,15

1,497

717,43

11,32

8,120

0,343

24,03

2,403

44,99

1,757

557,69 624,23

11,47 10,46

6,395 6,532

0,302 0,333

18,47 18,41

1,847 1,841

39,67 46,93

1,609 1,983

Irradiance (corrected)

Eta

Temperature (corrected)

Fill Factor

59.16

649,34

0,426

1177,22

24,14

848,42

11,24

3.026

60.16

625,30

0,350

1059,13

27,14

723,53

13,32

04-horizonatl-45deg

3.201

61.79

645,22

0,309

988,02

25,16

733,22

05-horizonatl-45deg

3.379

54.98

655,22

0,292

985,23

24,05

06-horizonatl-45deg 07-horizonatl-45deg

3.462 3.549

38,00 59.58

636,03 640,67

0,272 0,244

940,37 867,03

22,54 22,63

47,00

2,900 2,700

43,00

2,300

41,00

1,900 1,700

1,500

183

45,00

2,500 2,100

Voc

Impp

Vmpp

Pmpp

Pmpp/m2 of footprint performance of shape

49,00

39,00 37,00 35,00

1,80

1,20

2.766

3,100

Increase in Pmpp as to flat shape

1,30

03-horizonatl-45deg

3,300

1,28 1,26 1,28 1,28 1,47 1,39 1,67

1,40

01-horizontal-45deg

Eta performance of shapes

43,65 42,88 42,58 41,38 46,46 42,53 48,78

1,50

Jsc

3,500

3,947 3,833 3,398 2,792 2,482 1,980 1,913

1,60

Run Id (shape Nrorientation of undulations inclination as to fllor)

Isc

40,70 38,33 33,98 27,92 24,82 19,80 19,13

1,70

PV Size (exposed area)

1,500

Eta

Pmpp/m2 Increase of PV in Pmpp [W/m2 of as to flat footprint] shape

Pmpp/m2 of PV [W/m2 of module exposed surface]

4,000

Pmpp/m2 of PV [W/m2 of module exposed surface]

Temperature (corrected)

01-horizontal-30deg 02-horizontal-30deg 03-horizonatl-30deg 04-horizonatl-30deg 05-horizonatl-30deg 06-horizonatl-30deg 07-horizonatl-30deg

Irradiance (corrected)

Run Id (shape Nrorientation of undulations inclination as to fllor)

PV Size (exposed area)

horizontal 30ο angle

Pmpp/m2 Increase of PV in Pmpp [W/m2 of as to flat shape footprint] ο

horizontal 45 angle

Increase in Pmpp as to flat shape 2,200

2,000 1,800

1,600 1,400 1,200

1,000

2.852 56.08 616,96 3.201 60.83 647,79 3.379 55.27 654,46 06-horizontal-60deg 3.462 37.35 631,06 02-horizontal-60deg 2.852 56.08 616,96 07-horizontal-60deg 3.549 59.46 638,53 04-horizontal-60deg 3.201 60.83 647,79 05-horizontal-60deg 3.379 55.27 654,46 Eta performance f shapes 631,06 06-horizontal-60deg 3.462 o 37.35 07-horizontal-60deg 3.549 59.46 638,53 4,000 3,500

4,000 3,000

Eta performance of shapes

Jsc

Jsc 0,446 0,257 0,254 0,259 0,446 0,231 0,257 0,254 0,259 0,231

Isc

Voc

Impp

Vmpp

Pmpp

Isc Voc Impp Vmpp Pmpp 1271,46 24,92 899,46 11,65 10,476 823,39 23,96 603,79 11,20 6,760 857,70 24,18 619,50 11,26 6,978 898,02 22,07 568,42 10,37 5,897 1271,46 24,92 899,46 11,65 10,476 819,41 21,99 579,61 10,61 6,149 823,39 23,96 603,79 11,20 6,760 857,70 24,18 619,50 11,26 6,978 898,02 22,07 568,42 10,37 of 5,897 Pmpp/m2 of footprint performance 819,41 21,99 shapes 579,61 10,61 6,149 50,00

Pmpp/m2 of footprint performance of 45,00 shapes 50,00 40,00

3,500 2,500

45,00 35,00

3,000 2,000

40,00 30,00

2,500 1,500

35,00 25,00

2,000

30,00

1,500

25,00

Pmpp/m2 ofPmpp/m2 PV of PV [W/m2 of module [W/m2 of module exposed surface] exposed surface]

Irradiance Irradiance (corrected) (corrected)

Temperature Temperature (corrected) (corrected)

Run Id (shape Nrorientation of undulations Runas Idto fllor) inclination (shape Nrorientation of 02-horizontal-60deg undulations 04-horizontal-60deg inclination as to fllor) 05-horizontal-60deg

PV Size (exposed PV Size (exposed area) area)

horizontal 60ο angle Pmpp/m2 of PV Eta [W/m2 of footprint] Pmpp/m2 Fill of PV Eta 0,331 36,73 3,673 41,10 Factor [W/m2 of 0,343 21,12 2,112 footprint] 31,29 0,337 20,65 2,065 38,66 0,297 17,03 1,703 36,58 0,331 36,73 3,673 41,10 0,341 17,33 1,733 44,17 0,343 21,12 2,112 31,29 0,337 20,65 2,065 38,66 0,297 17,03 1,703 36,58 Increase in Pmpp as to flat shape 0,341 17,33 1,733 44,17 2,600 Fill Factor

2,400

2,200 2,600 2,000 2,400 1,800 2,200 1,600 2,000 1,400 1,800 1,200 1,600

Increase in Pmpp as to flat shape Increase in Pmpp 1,499 as to flat 1,524 shape 1,924 1,891 1,499 2,379 1,524 1,924 1,891 2,379

Increase in Pmpp as to flat shape

1,400 1,200

184

11.4.2 Assessment of Results per Inclination The results of this category clearly show by comparing shapes which start from less dense and â&#x20AC;&#x153;thickâ&#x20AC;?/tall (shape 01) and reaching denser 3d shapes (shape 07), how the performance changes in absolute percentages and compared to the given footprint. ETA (Efficiency) Graph As can be seen by the eta efficiency graph (left green graph) the efficiency with which the module converts the radiation it receives to electricity drops as we reach denser shapes almost linearly. This is expected since folding/ bending a given surface into denser shapes worsens its absolute efficiency due to partial shading. This can be observed in practically all inclinations. It is important to note that the efficiency concerns the total PV area of the corrugated shape (not footprint area). Total output per Footprint area Graph (Pmpp/m2 footprint) In contrast to the efficiency graph, the total output per FOOTPRINT area depicted in the middle (red) Graph shows despite some fluctuations that the denser/taller shapes provide more Energy per footprint area. Eventhough the 0o graph appears to have a maximum for shapes Nr. 04 and Nr. 05 and shows a decline for shapes Nr. 06 and Nr. 07, when taking all the inclinations and orientations into account shape Nr. 07 (the densest shape) performs the best with a mean value of 43,8 W/m2. Increase as to flat shape Graph In the third graph (right, yellow graph) in which the direct comparison with the flat PVs of exactly the same footprint and under the same inclination/orientation is made, the above fact becomes even more evident and clarifies even the few possible fluctuations of the previous graphs shown from the shapes Nr. 06 and 07 only as to the 0deg results. This graph can be considered as the most reliable one as each exact case is compared with the flat in the same position under the illuminating table and also clearly shows the intention of gaining Energy from denser shapes. All shapes show an INCREASE in performance compared to flat with denser shapes performing the best.

185

11.4.3 Ecotect results compared to lab results Considering the facts that: - The sun elevation angles in the geographical middle and northern European latitudes are between 15deg and 60 deg (=diurnal and seasonal ranges) and - As discussed above these circumstances are simulated by the inclinations of 30deg and 60deg in the lab tests, the following table shows the improvement of the energy production with increasing “thickness” as calculated with Ecotect for the shape “corrugated curved vertical” which Is the most similar to the models tested in the lab.

exposed area

kWh/ye ar/m2 footprint on the facade

Corrugated vertical curved 1/4 thickness Corrugated vertical curved 1/2 thickness Corrugated vertical curved 1/1 Corrugated vertical curved 1,5 thickness Corrugated vertical curved double thickness

4,077 4,344 5,157 6,162 7,267

550 530

394

510 490 470 450

401

410

433

370

476

430 390 350 0,000

5,000

10,000

524

As can be observed the increase in energy production between the “flatest” (“1/4 thickness”) and the “thickest” (“double thickness”) shape is 524/394=1,33, a result fully in line with the 47,3/41,41=1,14 ( for the 30deg), 41,25/35,01=1,18 (for 45deg) and 33,62/27.61=1,22 (for the 60deg), providing a mean value of 1,18 of the lab tests (see previous tables). As expected lab results exhibit slightly lower values combared to the theoretical ecotect values (1,33 vs. 1,18) but the same tendency.

exposed area

The same can be observed for the horizontal orientation in the following graph:

Corrugated horizontal curved 1/4 thickness Corrugated horizontal curved 1/2 thickness Corrugated horizontal curved 1/1 Corrugated horizontal curved 1,5 thickness Corrugated horizontal curved double thickness

4,077 4,344 5,157 6,162 7,267

kWh/year/ m2 footprint on the facade 394 402 434

550 530 510

490 470 450 430 410 390

477

370

526

350 0,000

5,000

10,000

As can be seen the increase in energy production between the “flattest” (“1/4 thickness”) and the “thickest” (“double thickness”) shape is 526/394=1,34, a result fully in line with 48,78/41,38=1,18 (for the 30deg), 46,93/36,97=1,27 (for 45deg) and 44,17/31.29=1,41 (for the 60deg), i.e. a mean increase of 1,29 of the lab tests (see previous tables). 186

The lab tests results for the horizontal inclinations come much closer to the theoretical ones ( 1,34 vs. 1,29) then the verticals did (1,33 vs. 1,18), although this is more due to unavoidable inaccuracies during the testing procedure .The distance below the illuminating table of the lab measured from the floor is relatively short compared to the size of the models. Therefore when the models are inclined at large angles, e.g. 60deg, the upper parts of the PV folds come nearer to the lamps and receive a bigger intensity which cannot easily be corrected by the software. This is apparently a bigger problem for horizontally oriented undulations, since the upper facing large part of the model receives too much light as compared to the models with vertically oriented undulations where this testing problem did not exist (and all the parts where illuminated almost evenly).

11.4.4 Results per Shape In the following pages all the results for all the measurements of the seven models/shapes are depicted, grouped per â&#x20AC;&#x153;shapeâ&#x20AC;?. Footprint values, gained in the previously described tests, are contained at the beginning of the tables for comparison and evaluation reasons.

SHAPE 01

187

SHAPE 02

SHAPE 03

188

SHAPE 04

SHAPE 05

189

SHAPE 06

SHAPE 07

190

11.4.5 Assessment of Results per Shape The 7 “Results per Shape” tables depict the results of the measurements of the seven models created for the different inclinations. They show the effect of different angles for each shape. The left hand graphs (green) depict the change in efficiency for the different inclinations, first tilting the module perpendicular to its corrugations (vertical) for points 1-4 and then parallel to them (horizontal) for points 5-7 or 5-8 of the graph. In both cases we observe a uniform decrease in efficiency (per total PV area) since the angles of incidence are getting steeper and diverging from the optimal 90o. The right hand graphs show a slight but steady increase in energy production per footprint area with increasing the angle of the shapes compared to the flat shapes with same inclinations. The fact that Shape Nr. 07 is the best performing is visible also here when comparing the values of the red graphs to each other. The graphs do not show substantial fluctuations, which means that the shapes perform almost equally good in all orientations. It is interesting to note that the increase in energy production between the 60deg vs. 0deg (always for the more reliable “vertical” tests (see the previous comments on the reliability of the horizontal tests) is 50%, 60%, 40%, 62%, 37% and 50% for the shapes 01-07 respectively, which means that the density and thickness changes of the shape do not show a varying behavior for different inclinations (uniform performance). This can be seen as an additional advantage, since it gives the designer the freedom to select the best performing shape which is not extremely sensitive to small location and orientation changes.

FINAL RESULT It can be concluded that the Laboratory tests have verified the software solar analysis to a very satisfactory extent.

191

192

12_Integration of the results into the Next Active Facade Analysis and presentation of different designs proposals

193

Introduction The 4 Phase procedure starting from the first shape research and ending with the PV-Lab testing provided useful information about the way an external light control/shading device could be shaped 3 dimensionally in order to improve its efficiency compared to flat solutions and conventional louvers for the same facade surface. Specific highly efficient (in total energy output per m2 facade) shapes were produced. They would however only constitute theoretical solutions if an attempt to translate them into a building structure -an external active light control skin- had not been made as a conclusion of the research part. Limitations and boundary conditions have intentionally not been strict from the beginning of the shape finding process in order to first highlight the most efficient arrangements and the principles/directions that lead to more efficient shapes in terms of energy production. In this sense denser arrangements and prismatic shapes with a large number of folds have proven themselves as shapes with a high potential. Using the most efficient shapes of the 4 Phase process the technical feasibility of real facade structures of these shapes has to be tested. Categories are created according to the operation principles and shape of each solution and the way they deal with the predefined requirements. Requirements - Dealing efficiently with the inherent problem of rigidity and therefore Wind Resistance of thin, lightweight or flexible materials and complicated shapes. - Ability of the structure to be retracted into small sizes (or to move to an opaque part of the facade) in order to allow for an unhindered view and avoid energy consumption (artificial light) due to constant shading by the solar cells. - Integration in terms of technical/structural coherency (and architectural quality as added value) in the Facade module of the NEXT Active Facade by Alcoa. - Low weight and thin profile compared to glass semi transparent solutions of the same shape. (use of Polymers as substrate) Four (4) categories of solutions according to their shape and therefore also according to their performance potential have been created, each of which is divided into different technical solutions in order to investigate the efficient operation of the shape on the NEXT FACADE module. Solutions of lower complexity and cost are provided as well as more sophisticated and complex designs. The categories were chosen by the best performing shapes of the 4 Phase Process. For each solution a Material Selection using specialized software (CES Material Selector software) will be performed in order to use the materials fulfilling the structures requirements in the best possible way.

194

Solution Categories Category A One side Curved (only convex) Shape. Maximum Performance according to 4 Phase Research. Highest Incident Solar Energy Value.

1. Flexible pre-shaped curved shape units (strips). Deployment, fixing and acquiring shape through Tension. (Roller mechanism) 2. Flexible shape units (strips). Fixing and acquiring curved shape through Compression. (Roller mechanism) 3. Hybrid system. Thin membrane/foil attachment on rigid transparent 3d shape. (Roller mechanism)

Category B Corrugated (â&#x20AC;&#x153;zig zagâ&#x20AC;?) Shape. Within the 5 best performing shape versions according to 4 Phase Research.

1. Linearly folding structure using rigid hinges. 2. Linearly folding structure using flexible connections.

Category C Origami (Miura Ori) Shape. Maximum Surface Potential (future potential for PV) Within the 4 best performing shape versions according to 4 Phase Research.

1. Folding Origami Structure using separate metal hinges 2. Folding Origami Structure with integrated plastic hinges

195

Solution 1. Tension

Solution 2. compression

Linear Unfolding

Origami Unfolding

196

Category D (not analysed) Rigid Sliding non foldable shapes. Pyramids etc. Practically any efficient shape possible. Surface Area Losses since not complete surface covered (gains only compared to flat panel of same size). Category D is not further analysed since it is only consisting of a rigid polymer semitransparent 3d shape that slides horizontally on a rail system and improves efficiency compared to only half (or less) of the facade surface. The aim of the final thesis proposals is to maximize the efficiency for the complete facade module surface.

Modeling of NEXT FACADE Exhibition Module In order to efficiently realize the integration of the Active PV structure it has been important to first become familiar with the NEXT Facade Module as an architectural element and climate control system, according to what ALCOA has presented in exhibitions. Therefore the module has been first modeled using 3d software (Rhino). The layout of the facade consists of equally sized units of windows of approximately 850mm width, with variable versions of either operable or fixed glass parts. In one of the 850mm wide grid units the climate control box is installed/integrated incorporating the climate devices by TROX. This unit constitutes the main opaque part of the facade and therefore will be the part where horizontally deploying shading devices will retract and accommodate the mechanisms responsible for the movement of the active shading system. In a different version of the NEXT concept the climate box could be placed horizontally either parallel to the facade plane covering the part underneath the external windows, or as a cassette perpendicular to the facade plane integrated in the extension of the slab. In both these cases a vertical folding and unfolding (or roller deployment) would be more efficient in terms of maximum transparency, since vertical arrangement would cover additional window space. For the development of the solutions the standard vertical layout has been chosen in order to restrict the possible solutions.

197

Operable Window unit

Climate Box Fixed Windows

198

12.1 Category A

One side Curved (outwards convex) Shape. This category includes solutions for the possible integration and operation of the most efficient shape in terms of total incident solar energy, which is the surface consisting of a repetition of convex shape units. The challenge in this case lies in achieving a design that can be retracted and retain its low weight characteristics deriving from the thin film PV material and its light and transparent substrate. Decisions: Rigid or flexible convex strips. Rolling or folding. The convex curved units of the shape can either be made of a rigid transparent material or make use of the ability of thin film solar cells to be applied to flexible materials. Rigid semi circular units would need to be perfectly rolled around a horizontal or vertical roller of approximately the same radius in order to have 2 curved units attached to it for every rotation. This solution would require a very large roller due to the large radius of the strips (when seen in transverse section). Between the strips a hinge would be needed either made out of metal or plastic/elastic material. On the other hand folding of the curved strips (through their hinges) into a more slender shape instead of rolling would require a lot of space for rigid segments/strips and a very complicated structure of rails in order to transform the curved units into flat ones (given that they are made flexible) and stack them close to each other in front of the Next Climate Box. All three solutions developed try to avoid using completely rigid semi circular strips in order to gain space. For the same reason they use a roller for retraction of the active PV surface which in all cases is flexible to a different extend in every case. 12.1.1 Solution 1 The first solution/proposal for the realization of the â&#x20AC;&#x153;One side curvedâ&#x20AC;? shape includes a horizontal roller on the top part of the Next Facade module. The active solar material is flexible to a specific extend but strong enough to be stabilized under tension. The horizontal strip units are preshaped with a starting radius of approximately the radius of the roller, in order to be perfectly attached to it during operation of the device. The total perimeter of the roller is either exactly the size (arc length) of the curved unit or even smaller. In order to deploy the flexible material that uses either metal or plastic hinges between the semicircular strips. The complete surface is pulled by vertical cables with mechanisms on the bottom part of the facade module. As the material is pulled down by the cables (with the shape of the units still being smaller in radius according to their pre-shaped curvature) the roller can be locked at specific heights. At the moment the roller locks (stops rotating according to the tensile force of the cables) the cables start deforming the flexible strips. The cables need to be attached to every horizontal hinge in order to ensure the equal deformation of every strip. The deformation of the strips is a transformation from their preshaped condition (smaller radius) into a perfectly semicircular shape of larger radius. When the cables create the adequate tensile force to shape the material into the final curvature, the complete system is stabilized in order to withstand wind forces. The curvatures of the surface and the horizontal hinges create already a surface which is significantly more rigid that a flat sheet of the same material using the same tension 199

Horizontal strip under tension to acquire longer radius than preshaped condition

Hinges for all designs either as separate metal parts or part of the polymer curved shape with only metal axis for rotation. 200

Operation in 4 Steps (Solution 1)

Position 1 One curved strip unit with preshaped radius. Slight Radius increase only due to roller resistance

Position 2 Two curved strip units with pre-shaped radius. Slight Radius increase only due to roller resistance

Position 2 Three curved strip units with preshaped radius. Slight Radius increase only due to roller resistance. In case of excessive roller resistance possible force provided by electric motor.

Position 4. Final Position Curved strip units under tension by vertical cables pulling on the horizontal hinges. Deformation of the units. Radius reaches the predefined (efficient) value due to tension.

4 201

Semi transparent PV external view Roller on the top part of the module Vertical Tension Cable

202

12.1.2 Solution 2 Solution 2 is an alternative to the first proposal using the reverse effect in order to achieve the final most efficient shape. The layout of the system is very similar with the roller and cables in the same arrangement. The flexible material is pre-shaped into a flatter (bigger radious) design compared to the final shape. The roller in this case probably needs to be of bigger size than in the previous situation. The PV material is deployed as a surface consisting of slightly curved units pulled down by the vertical cables. When the surface reaches the bottom of the module the curved strip units start deforming under the tensile force by the cables attached to the TOP hinge of each curved segment strip (force is tensile but the effect is perceived like a compression of the strip). Through this force the horizontal strips start curving more and more into smaller radii until they reach a stable condition when acquiring the desired shape. In both Solution 1 and 2 the horizontal flexible strips are deformed in order to acquire a semi circular shape with a shorter or longer radius. In case the material cannot have the desired properties in order to both be rollable around a relatively small roller and exhibit the needed rigidity/forces in order to be safely fixed in the final position only trough forces on the material itself, then the vertical curved strips at both ends of each horizontal strip can have increased curving resistance and safely fix the structure in the needed position. In case of solution 2, since it starts from a flatter shape in order to achieve the final curvature, could also be folded (and stacked parallel to each other, similar to the foldable designs following) as flat horizontal units on the top part of the module and be pulled down as flat segments which are finally pressed and deformed in the same way into the optimal curve. 12.1.3 Solution 3 Hybrid Device Solution 3 is an idea that could be applied to several other shapes and tries to make use of the properties of flexible thin film PV structures and at the same time contribute to the energy efficiency of the building not only in terms of energy production and shading but also by preheating air before it reaches the climate devices in the respective box unit. (as this has an impact on the efficiency of the complete system) In order to have the lightest and simplest possible PV surface and at the same time retain the advantages of 3d shapes in terms of solar energy reaching the structure a hybrid system had to be developed. Not only would a thin flexible membrane/foil surface not be efficient enough due to its flat shape, but it would also have significant problems with wind resistance. The hybrid device consist of both a completely rigid transparent 3d surface with the shape and curvatures that achieve the best results according to the analysis and testing, and a very thin and lightweight PV foil in front of it, that is retractable by a small diameter roller (diameter can be significantly smaller compared to less flexible previous versions). The PV foil needs to be attached to the 3d shape behind it and acquire the specific shape in a way that would optimally look like the process of Vacuum Forming. When the shading is not needed any more the flexible PV can be detached from the surface and retracted into the roller in its flat shape again. The fact that it is currently difficult to create elastic stretchable PV material (although it has been experimentally tested many times) makes the attachment of a thin PV surface difficult for corrugations and prismatic arrangements in both directions. On the other hand for designs based on one axis corrugation and simple curvature it is possible to efficiently attach the external surface on the inner 3d shape. 203

Operation in 4 Steps (Solution 2)

Upper Hinge pulled downwards

4 204

Compression (shape change) mechanism. (Solution 1,2) Tension cables or side rails

205

Apart from a vertical rail solution, a system with vertical tension cables can be used. In this case the shape changing mechanism is detached from the horizontal strips that need to be rolled on the top part of the facade. The tension cable motion in the horizontal and vertical direction ensures locking of the strips into a stable, compressed and windproof position.

Stabilizing cable movement

Vertical rail for compression

206

Solution 1,2 Building Integration

3d Model showing interior space of an office building using the proposed facade system in two positions, completely closed for maximum performance and shading and partly retracted for allowing a better view and more physical light entering the space. 207

The climate box constitutes the opaque part of the facade and regulates the inner climate. The box operates behind the PV shading system, which in this case uses rollers to retract into the top part of the facade.

3d Model showing section of an office building using the proposed facade system. Different arrangements of the climate boxes and open or closed PV shading systems can create interesting facade patterns. 208

Three ways that can be used to achieve this are vacuum between the two surfaces, static cling and pressure by an external object. Vacuum requires a sealed system and additional installations which makes it a solution very difficult to realize. Static cling is an idea that relies to a large extend on a perfectly smooth and clean surface and a structure that would make the first attachment of the foil by swiping over it following the curvature of the shape behind it. This is also considered a complicated solutions to develop. Pressing the PV surface into the voids of the basis structure in order acquire its shape is a more realistic solutions that has been developed. The one sided convex curved segments are in this case placed vertically as a rigid transparent polymer (instead of glass for lower weight and because it is much easier to take curved shapes) structure that constitutes a second skin for the facade and can have adjustable openings or gaps on the bottom and top in order to adjust the air input to the climate box (which in this case would need to be customized for this design). A roller box is placed at the edge of the module close to the climate box, which is responsible for deploying and retracting the flexible PV surface. At the external side of the facade module in front of both the rigid polymer surface and the PV sheet a number of vertical cables under tension or thin rods a placed. These move perpendicular to the facade plane and the other two surfaces. The movement of the cables or rods (which are exactly in front of the curved segment recessed connection lines) towards the rigid surface presses the PV surface firmly on the convex curves of the 3d shape. It has to be noted that this pressing process has to be performed for each segment separately starting from the last curve (and approaching the first one) and not simultaneously for all curves, because the PV surface between the wires/rods is shorter than the total length of the curves it needs to be attached on and will otherwise not reach the inner/recessed parts of the curves (or friction between the cable and the PV surface will be created by parallel movement, which will destroy the cells). Vertical cables/rods moving on rails perpendicular to the facade (pressing PV surface) Flexible PV surface

209

Horizontal tension cable for unrolling PV active surface. Rigid Polymer high performance 3d shape

Operation in 8 Steps (Solution 3)

8 210

850

Climate Box devices

211

Combination of: Low weight foil 3d shape Wind resistance + Air preheating potential

150

260

50 43

300

PV Foil

Rigid 3d Polymer (transparent)

Vertical Cable/rod 212

12.2 Solutions 1, 2 (Category A) Material Selection Selecting the polymer material for the bending 3d structures As designed, the sun-shading strips of these two solutions, must be flexible enough so as to comply with the compression/expansion movements when in use (see relevant parts and figures describing this). They have to be also strong enough and durable in repeated cycles of bending. Properties requirements description: Flexible (easy to bend) expressed by: Flexural modulus (low is good) At the same time the material needs to be strong enough in bending (not arriving easily at its bending yield limit = external fibers collapse due to extreme tensile deformation beyond elastic limit and material brakes) expressed by: Flexural strength (high is good) Additionally during repeated opening-closing use the material should not show fatigue signs, expressed by: Fatigue strength at 10.000.000 cycles (Mpa) (high is good) Thermal properties: High working temperature, depending on the maximum envisaged temperature, e.g. 80 °C Thermal expansion should be low, so that it does not significantly change shape during thermal changes. (may not be crucial for small changes) Other Properties (Light, Optical): UV resistance necessary since it is exposed to light Transparency needed (optical quality is the optimal solution, simply transparent also acceptable) The Material selection Process starts with the CES Level 3 Polymers (677 hits) Subsequent limit functions were applied for the following qualities: Stage 1: UV radiation (sunlight) resistance: “good” and “excellent” accepted -> 260 Materials left Stage 2: Optical properties: “transparent” and “optical quality” accepted: ->42 Materials left Stage 3: Fresh water exposure: “excellent” accepted: -> 33 Materials left Stage 4: Flammability: all except “highly flammable”: -> 22 Materials left Stage 5: Melting point >150degC, and Maximum service temperature>80degC: -> 7 FINAL Materials left 213

CES Level 3 Polymers (677 hits)

Graph 12.1

Stage 1: UV radiation (sunlight) resistance: 260 Materials Stage 2: Optical properties: 42 Materials left Stage 3: Fresh water exposure: 33 Materials left Stage 4: Flammability: 22 Materials left Stage 5: Melting point >150degC: 7 FINAL Materials left 7 Possible Materials (graph 12.2) : 1. PEN (Unfilled Amorphous) 2. PVDF 3. PVDF 2 4. ETFE (Unfilled) 5. FEP (Unfilled) 6. PCTFE (Unfilled) 7. Polyester Liquid Crystal

214

For the 7 Materials that fulfilled all the predefined requirements a graph was created on the basis of the following index: Flexural strength (modulus of rupture) â&#x2C6;&#x2122; Fatigue strength at 10^7 cycles / Flexural modulus depicted on the y-axis Price â&#x2C6;&#x2122; Density depicted on the x-axis This index compromises all desired properties vs. the price

Graph 12.2

The result of the graph can be described as clear, as none of the materials performs substantially better than PEN, which is additionally the cheapest one. Furthermore this material is also selected for the thin film PV substrate, (see next part) so that using a unique material for the thin film and the supporting structure gives more manufacturing possibilities, namely producing both in one process step. 215

216

12.3 Solution 3 (Category A) Material Selection Material Selection for rigid part. Selecting the polymer material for the rigid 3d structure. (the material selection applies also for Categories B and C, since they use a rigid Polymer material with the same requirements. Starting from the CES Level 3 Polymer, first a limit function (stage 1) has been applied for the following qualities: UV radiation: good and excellent Optical properties: transparent and optical quality Fresh water exposure: excellent Flammability: all except highly flammable This gave 13 out of 677 polymers in total. In stage 2 a graph was created (graph 12.3) with the following characteristics: x-axis :price y-axis: flexural strength/density Results PVC (rigid or high impact), PEN and PCTA all have similarly good performance and prices increasing in the mentioned order, whereas PEI performs slightly better but at double price compared to PEN and 10 times the price of PVC. In stage 3 an additional graph was created The yield strength has also been taken into consideration since for flexing also the tensile strength is relevant for the outer fibers. Therefore as a y-axis the following was used : flexural strength* yield strength/density There was practically no change compared to stage 2 Final result: PVC (just the rigid variant not necessarily high impact, since there is not a big difference in performance) and PEN PEN has optical quality and is used very often as a Thin Film PV substrate, but costs double the price (3 €/kg than PVC (1.3 €/kg). PVC has the advantage of a lower “Embodied energy, recycling” meaning that it needs less energy to be recycled (23 MJ/kg) , whereas PEN needs 27 MJ/kg, which is however not a significant difference. Weights are practically all the same with a density of 1.3 kg/dm3. If a 2 x 2 m panel has a thickness of approximately 5-8 mm, it has a volume of about 20 dm3 and a weight of 26 kg. In case of a complex 3d shape this could become about 3 times higher due to the surface efficiency of ~3 for the best shapes. The price difference is even in this case probably not the most important aspect due to the relatively low weight. PEI, which is the best material as to strength, having a flexural strength of 150 MPa can also be considered a very good choice (PEN 78, PVC 70) even though it is categorized as transparent and not “optical property”. 217

Results Stage 1

Stage 1 LIMIT

UV radiation: good and excellent 677 Polymers

Optical properties: transparent and optical quality Fresh water exposure: excellent Flammability: all except highly flammable

Results Stage 2 GRAPH

Graph 12.3

The relation of â&#x20AC;&#x153;flexural strength/densityâ&#x20AC;? was selected as the main criterion because the structure should be break resistant (=flexural strength) but light in weight (=low density).

Results Stage 3 GRAPH Same as in Stage 2 218

Solution 3 (Category A) Material Selection for thin PV surface (membrane) part. In this solution in which the thin film PV is pulled in front of the rigid immovable transparent pre-shaped part on the façade and guided so as to perfectly conform to the 3d shape forming rigid part, it is reasonable to produce the thin film PV already on a substrate (anyway needed in the usual thin film production) that is suitable also for this large scale pulling/guiding/attaching mechanism. In this case it has been necessary to use a different material selection approach, namely not using CES in a broad “open end” procedure, since there is not so much freedom in selecting materials suitable to be used for the carrying substrate of a thin film PV, in particular in view of their fabrication processes and restrictions. As a result the process is based on selecting the best suitable material within those that are already used by the relevant industry. The group of such materials substantially contains: PEN Polyethylenenapthalate PET Polyethylenetherenpthalate PE (LD) Polyethelene (low density) PVDF Polyvinylidenefluoride PS Polysterene PI Polyimide PTFE Polytetrafluorethelene ePTFE expanded Polytetrafluorethylene PEDOT Poly (3,4-ethgylenedioxidy-thiophene), (which is a polyester) PSS Polysterensulfonate CES Material Selection Software was used for these materials (those included in CES Level 3 Polymer 2013, in this version without extensions PEDOT and PSS are missing) for allocating their crucial properties. Thereby a final selection could be reached within this restricted group. The table “Main Properties of Materials usually involved in providing the Substrate of Thin Film PV’s” based on the researched CES Software Properties depicts this approach followed and serves as a good comparison basis. The apparent requirements in the case of the developed Solution for transparency and a high melting point (for reasons of roll-to-roll production feasibility as the most “low cost” thin film fabrication method) excludes in a first filtering step PI, PE, PTFE and ePTFE. PE’s poor UV resistance was a further obstacle for this material, just like the very high price of PI, having a flexural strength which is not so important here, since the film will be held under a very strong underlying structure (wavy rigid part fixed on the façade). The 60deg C maximum working temperature of PET does not provided enough limits for sun shading applications such that this is also excluded. PEN, PVDF and PS are left, being almost equally “good” in flexibility. 219

Main properties of materials usually involved in providing the substrate of thin film PVs flexural mean Flexural maximum thermal strength melting working expansion UV price density price modulus Mpa (high i s point (low i s temperature (low i s good) resistance €/kg (kg/m3) €/m3 good) degC good) degC (mean) μstrain/degC (mean) (mean)

optical property (optical quality i s best) optical quality optical quality

material's acronym

full name

PEN

Polyethylenenapthalate

3,65

1390

5073,5

2,5

180

270

170

good

PET

Polyethylenetherenpthalate

1,82

1390

2529,8

2,7

270

117

good

PE (LD)

Polyethelene (low density)

1,5

932

1398

0,25

110

250

poor

PVDF

Polyvinylidenefluoride

13,9

1780

24742

2,25

166

160

190

Polysterene

1,87

1050

1963,5

3,2

190

135

fair

optical quality

Polyimide

130

1430

185900

2,28

250

261

392

excellent

opaque

PTFE

Polytetrafluorethelene

13,4

2200

29480

0,5

261

325

145

good

opaque

850

0,3-‐1,0 (depending (depending on degree 17000 on degree of of expansion) expansion)

315

252

good

opaque

ePTFE

expanded Polytetrafluorethylene

translucent

excellent translucent

Result As PEN has the highest maximum working temperature (180degC), highest melting point (270degC) and is simultaneously the cheapest choice, it appears the best selection for the thin film substrate. It is also worth noting that it was also selected for the sun shading strips of the solutions 1 and 2, this being of additional value (see previous Material Selection Process).

220

12.4 Category B

Vertical Corrugated foldable Shape. This category deals with the linear foldable non-curved patterns created in the previous phases. They exhibit good overall performance and have the simplest and easiest to construct layout since no curved surfaces are included and additionally there is no need to find sophisticated solutions for wind pressure due to the use of rigid elements. The decision about the hinges between the longitudinal units constituting the shape is mainly related to aesthetical reasons and simplicity of construction. A large number of metal hinges may not be desirable by the architect. Plastic hinges or a main polymer structure and a metal axis as the connecting element that enables the rotation is also possible. Vertical arrangement of the system has been chosen since the foldable structures developed have a larger retracted shape compared to the roller based systems combined with flexible materials. In order to have a wind proof system that folds and unfolds in a uniform manner every inner bottom hinge is connected vertically with three (3) small rollers/wheels running inside a bottom and top rail. These three rollers placed on top of each other have been used in order to be firmly pressed from one side (opposite side for one of them) and free from the other side (pressed by the inner walls of the rail system). This way the system has no play which would be a problem with wind loads and at the same time no excessive resistance when sliding. For making sure that the rigid vertical PV units donâ&#x20AC;&#x2122;t move when reaching their final position a hinged metal structure is created between every inner edge at the top and bottom of the structure. In their final position these metal parts are nearly parallel to each other and therefore will not allow further â&#x20AC;&#x153;openingâ&#x20AC;? of the vertical units angles. At every second hinge on the bottom and top a coil spring system is used connecting the hinges to each other. This way the structure has a constant uniform tendency to fold into its smaller size. A counter force from the side of the operable window (right side when viewed from outside) in the form of a tension cable around an electric motor pulls the structure to its closed/deployed state by connecting it only to the last hinge. The system is designed in such a way that it is constantly under tension in order to increase wind resistance.

Cable in constant tension for stability hinged flat bars for preventing excessive angles

Coil spring for tension. 3 Roller system in rails. Metal hinge

221

Operation in 4 Steps (Category B)

222

The vertical Panels fold in front of the climate box of the Next Facade in order to cover a space that is already opaque when the PV surface is retracted.

A combination of tension cables with coil springs is used for adding stability to the structure and hinged bars between the main hinges on the bottom prohibit further extension of the structure. The extended shape of the bars needs to be under a slight angle in order to bend/fold under forces parallel to the rails.

Three rollers/wheels are used in the railing system in order to ensure sliding without any play and noise from wind pressure. The top and bottom roller is tightly attached on the one side of the railing inner walls and the middle roller on the opposite side. Pressure form both sides creates stability in the system and attachment of each roller only to one side allows sliding (rotation) without additional friction. 223

Current thin film technology integration. PV integration in the developed shapes has to be carefully designed in terms of the direction of the cell strips. Even though there will be (and there are already) solutions with completely smooth tinted glass or plastic by using very small checkered patterns or stripes (and in the future also organic cells with 100% smooth semi-transparency), for a proposal that also takes into account current thin film cells it is safe to use the cell strip pattern. As already analysed in the partial shading chapter the in series connection of longitudinal cells requires the relatively uniform output of each cell in order to avoid significant losses. Any completely shaded cell will minimize the modules power output as long as there is no by-pass circuit. This can even cause damage to cell by overheating. It is therefore important to orient the cells perpendicular to the shadow, if the shadow (in this case created by the shape itself has a longitudinal shape). The advantage of shapes folded or curved in a linear way with folding and curves parallel to each other lies in the easily predictable shadow shape parallel to the corrugations (at least for the biggest part of the surface). Taking this shape into account, the cell strips can be placed transverse to the long vertical shape units and thereby be always shaded practically to the same extend. As a result performance decrease is almost linearly proportional to the shading percentage for these simple shapes, or as low as it can be for the specific technology and shadow pattern. For other corrugated shapes including parallel folding or curvatures (in one axis) the cells follow the same transverse direction (perpendicular to the curvature or folds). In the case of Origami or other complex 3d shapes an extra analysis has to be made in order to minimize the losses. Other cell shapes and minimizing their sizes could be one solution when dealing with current conventional technologies. On the other hand third generation polymer based cells can include large numbers of printed bypass circuits and very small cell units in order to minimize the losses.

224

760

200

850

Climate Box devices

225

Four horizontal rows of hinges are used for ensuring stability/rigidity and the necessary rotation of the vertical PV strips. The hinge is always positioned on the internal part/side of the folding angle in order to reduce its total size and create a specific spacing between the panels when completely folded. In this condition the PV system covers precisely the part in front of the climate box module incorporating the different devices, which constitutes the opaque part of the facade system.

Operable window part Fixed window part

220

260

50 43

200

Tension cable Rigid Polymer (semi-transparent) Horizontal hinged bars 226

12.5 Category C

Origami (Miura Ori) Shape. The Origami shape selected is the most efficient 3d shape in terms of total surface compared to its footprint and between the top performing shapes in terms of average solar radiation received within a year. Important characteristics of the specific folding pattern are its ability to (nearly) linearly fold into much more slender shapes, its very high rigidity and resistance to forces perpendicular to its plane (when rigid hinges are used) and finally its anisotropy (easy to unfold in one direction and high resistance in the other axis) and expansion of the complete surface according to a Poissonâ&#x20AC;&#x2122;s ratio. This means that expansion in the horizontal axis creates a (slight) contraction in the vertical axis. It also has the advantage of theoretically being able to unfold the structure by compressive force in the transverse direction (or fold through tension). An additional result of these characteristics is the fact that every unit of the complete surface unfolds uniformly and exactly to the same extend as the rest of the structure. This has not been the case in any of the other linearly folding structures, where additional systems had to be developed in order to ensure uniform folding and unfolding of the complete structure and constant tension between hinges for added wind resistance. In that sense we can conclude that the origami shape is highly efficient also structurally and as a folding and unfolding mechanism. When trying to construct the Miura Ori Origami in a real rigid (facade) structure with hinges that perfectly folds and unfolds two main aspects have to be taken into account. The first one is related to the slight change in height, when the structure contracts. The total height becomes slightly or significantly shorter depending on the specific pattern and the chosen angles of the prismatic surfaces. As a result a system similar to Category B, where two rails (one on top an one on the bottom of the module) ensure the folding and unfolding movement of the shape is in this case not enough, since restricting the slight movement of the structure in the vertical axis prohibits any movement in the horizontal axis. To solve this problem two additional vertical metal rails have to be constructed on the top of the module. These are connected to a number of hinges on the top row of the origami surface. The number of the connection points depends on the wind forces and the overall rigidity and weight of the structure. The total origami structure therefore always has one part where it is supported or hanging from, which should not allow for a vertical displacement of the connections (which can be on top-hanging- or one bottom -supported-) and another part where only a displacement vertical to the facade plane is prohibited while vertical movement is allowed. In order to not rely on the structures weight for stabilizing the structure vertically (one goal is to achieve low weight) the top connections are under tension by coil springs and cables installed on the upper part of the vertical rails. The second very common problem this proposal has to deal with is the thickness of the panels compared to the hinge design and position. Origami shapes of this kind succeed in contracting to extremely slender shapes compared to the deployed position only if the folding units have a very small thickness and if the hinges fold them to a position very close to each other. The hinges therefore need to be small and on the outside part of the fold. Since half of the foldings are facing outside the hinges also need to be installed in an alternating manner. Furthermore the hinges should not be close to the corner point where four (4) different prisms-surface units meet. The thicker the surface units of the origami are the more distance has to be left between them in order to fold into slender shapes.

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Operation in 4 Steps (Category C)

4 228

Top Part connections

Bottom Part connections

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Vertical rails ensuring vertical displacement of top hinges during horizontal folding and unfolding of the structure.

Road view. Future idea of integrating media screens or advertisement panels in combination with PV cells only on the units facing down. The total impression will be that of a uniform screen for tall buildings.

V-shaped sliding lower support

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Vertical Rails Top connetion (height change when folding)

V-shaped sliding lower support. Bottom connection

231

The separate hinge version includes metal hinges. Despite their small size the fact that two hinges per edge have to be used, the view to the outside and the overall appearance is affected, creating a complicated pattern of metal parts. On the other hand the use of the polymer material as a hinge with only a thin metal (or even polymer) axis on each edge can create a smoother, cleaner and more transparent system. The size and position of the hinge(s) in both cases has to be carefully chosen in order to allow for efficient folding of the panels. A longer distance from the origami shape corners ensures an easier folding into thinner shapes.

Rigid PV Polymer 8mm

35mm

V-shaped sliding lower support

560mm

100mm

260mm

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Category C Building Integration

3d Model showing interior space of an office building using the proposed origami facade system in two positions, almost closed for more performance and shading and mostly retracted for allowing a better view and more physical light entering the space. 233

The climate box constitutes the opaque part of the facade and regulates the inner climate. The box operates behind the origami PV shading system.

3d Model showing section of an office building using the proposed origami facade system. Different arrangements of the climate boxes and open or closed PV shading systems can create interesting facade patterns. 234

12.6 Comparing and evaluating the developed Solutions/Proposals Different ways of transforming the best performing shapes into operating structures have been developed and presented. A comparison of these solutions can be considered the final step for selecting the most efficient solution in total even though it can be said that each Category shows its own strong points and advantages. Category A Solution 1 and 2 Both proposals use the most efficient shape in terms of total solar Energy reaching its surface. It has therefore a significant advantage compared to a flat surface of the same facade (projection) area and PV technology. The proposed mechanism is based on the simple and safe solution of rotating rollers, which can also be considered a space efficient approach due to the small size it retracts to. An other important aspect is the fact that these solutions make real use of the flexibility of the PV material and its Polymer substrate, since the final shape and stability is achieved through bending of the material. The same property is also the one used in order to roll it into a smaller radius when retracting. The shape itself has a low amount of hinges hindering the view to the outside and only parallel to each other. Plastic hinges made as part of the surface material using only an inner axis for rotation further improve the feeling that the surface is transparent and made of only one material. Disadvantages that can be addressed (apart from the aesthetical aspects which can theoretically be a problem for any of the shapes depending on the architects or clients taste) are the fact that fatigue of the material can in the long term become an issue if it is not selected carefully and if the lamination is not of the needed quality. Also noise problems need to be solved since the unrolling of a pre-shaped material of a certain rigidity compared to a fabric/foil can create non uniform/smooth movement.

Solution 3 Dealing with the same shape, Solution 3 exhibits the same strong points deriving from the very good performance achieved in the solar analysis. The operation and mechanism of this solutions is principally different from the first two discussed, since a completely rigid polymer is used as a second skin for the facade module, while the PV surface is used in the form of a rollable membrane. This can be characterized as a hybrid system, differentiating the shape from the active PV surface and only connecting them when the PV cells are needed. Two most evident advantages of this are firstly the very simple and extremely lightweight construction of the PV material and PV related mechanism, since it is comprised of only a roller and a thin PV material. A second strong point is the ability to use the rigid transparent polymer skin to create a buffer zone where air can be preheated before reaching the climate devices in the box unit and thereby increasing efficiency of the system. If the attachment of the membrane to the rigid Polymer is successful and precise wind resistance is practically good enough for any conditions. In contrast to the first two solutions as a complete system Solution 3 can be described as relatively complicated structure and prone to problematic operation if not perfectly executed, due to dust or water trapped between the two layers or non perfect attachment of the foil. Additionally more moving parts are required, since the movement of the system is divided in two steps, that of the roller unrolling and that of the cables/rods pressing the membrane on the rigid shape. Additionally pressing of the membrane on the surface needs to happen for every corrugation separately in order to avoid excessive tension and friction. Despite these disadvantages the idea of attaching a thin, flexible and lightweight membrane of active material on a rigid surface of optimized shape is very attractive as a principle because it deals with the inherent problems of thin flexible structures, namely rigidity in its final position and therefore wind resistance, in an elegant way. 235

The process of acquiring the specific shape, which will be the optimized shape for the given location and orientation, through a deformation that reminds of the vacuum forming process can be a very interesting architectural expression for a facade. In terms of the final selection and evaluation however, the possible problems in the perfect operation of the system place this solution slightly below the first two. Category B

The Solution (one solution using either plastic or metal hinges) of Category B developed is based on the vertical corrugated folding pattern which in its dense versions is among the best performing shapes. The fact that rigid elements are used with characteristics similar to glass increases the total rigidity and stability of the sun shading system and therefore makes it less prone to deformations and problems due to wind pressure. At the same time a large variety of perfectly operating versions of similar structures in the built environment ensures that there will be efficient answers to any possible problem related to the folding or retracting mechanism. Solar cells are perfectly protected and since they are not bending in any case there are no fatigue threats for the surface or other durability concerns. On the other hand this solution does first of all not make use of the flexible characteristics of new generation PV cells, but only of their ability to perform well under partial shading while being light and thin. Additionally a large number of hinges is used (either completely metal hinges or only a central meta axis with a plastic hinge) at the top and bottom of the module which can have an effect on the noise created if not dealt with carefully. Even though design/aesthetics related aspects are not part of the evaluation process it can be said that this solution is conventional, very commonly seen in simple shading devices and therefore not perfectly highlighting neither the new possibilities of PV technology and flexible substrates, nor clearly showing that the shape is a result of a performance optimization.

Category C

Category C is based on the most efficient shape in terms of surface increase compared to its footprint area, which is the Origami (Miura Ori) pattern. It is also one of the best performing shapes in terms of total solar radiation reaching its surface. The strongest point of this solution lies in the fact that it expresses in the clearest way as an architectural element the idea of densely folding a surface to increase its surface for efficiency reasons. Other versions of this pattern and different angles used in similarly complex prismatic shapes which change their geometry according to the location and orientation, truly highlight the idea of surface maximization. This however can only be considered an architectural approach in the evaluation of this Solution. Structurally the multiple folds create high rigidity and the specific design enables efficient folding into small sizes, even though it cannot always reach the slender shape of a simply corrugated shape. Another big advantage for the operation of the system is the fact that forces exerted on only one corner point of the structure are able to uniformly fold and unfold the complete shape, even if the force is exerted perpendicular to the movement of the structure. (pressing from top and bottom for example.). Disadvantages of the Origami based solutions are mostly related to their very high complexity and extremely large number of hinges which has an effect on the view to the outside unless a solution using only the transparent plastic is used. Precision of the structure is also very important to avoid problems in the folding and unfolding procedure as well as very thin profiles/prisms and hinges in order to achieve a slender folded shape. Noise can be also a problem due to the large amount of hinged connections. Finally the fact that flexibility of the PV material is not used for the operation of the system can also be considered a negative point.

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13_Options for the complete PV system based on the final designs (Possible use of the gained Energy) 13_1 Energy Produced and consumed (loads) The present thesis aims at producing as much solar electricity as possible through the development of a PV structure integrated within the NEXT Facade decentralized Facade module by Alcoa. This type of facades is characterized by the fact that climate devices are incorporated within the façade structure so that a decentralized climate control can be achieved with the advantages presented in previous parts of the present thesis. In view of this characteristic of the NEXT facade, it would be highly desirable to directly use the PV energy produced individually in each module for operating the above mentioned decentralized climate devices (provided by the company TROX). Thus, a stand-alone system could be a first choice or at least a first idea to be elaborated. A first crucial aspect in possibly selecting such a stand-alone system is the assessment of the energy storage means which are unavoidable for this kind of PV systems. The following basic calculations can contribute to the feasibility assessment of this solution. Assuming that two modular façade units of 2m each in width form the front opening (in case of high floor to ceiling heights probably 4 modules or two higher modules can be used) of a modular office room of e.g. 4m X 4m of floor surface, the following assumptions are made: The expected daily average energy production for the best performing shape can be calculated as follows. According to the previous chapter for 2 modules in the above defined modular office room the Ecotect calculations provided the following energy production data: • At maximum: 11.801 Wh (in July) resulting to: 11.801 X 2 modules = 23.602 Wh • At the minimum: 1914 Wh (for December), which means: 1914 X 2 = 3.828 Wh, • 6.658 X 2 Wh = 13.316 Wh in a year around daily average. As these energy calculations represent the irradiation energy, they must be multiplied by the efficiency of the specific thin film used so as to get the possible final energy gain. From a semi-transparent thin film efficiencies from 6.5% to 13.7% can be expected. 8% can be accepted as a relatively “safe” value to use. Efficiency data from source: graphs (http://engineering.case.edu/centers/sdle/sites/engineering.case.edu.centers.sdle/files/x26-overview.pdf) and (http://pubs.acs.org/doi/ipdf/10.1021/nn4052309)

This leads to an energy production of 306Wh, 1.065Wh and 1.888Wh (minimum, mid, maximum) - A two days autonomy is selected, meaning that the storage means should be in position to cover the expected loads for 2 consequent fully overcast days in which there is no PV production at all.

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The standard/steady consumptions in this office space are: 1. 2 modern solid state memory drive PCs with 50W each 2. A decentralized sill ventilation unit FSL-B-ZAU by TROX is used with a ventilation recovery capability with following specifications, can be used with 10 W

fig 13.1 Trox Unit specifications Table (source: http://www.trox.de/en/downloads/e6adefe3503acbc7/8616486b2b04499bec5a4d1e7bf4039f/pi_fsl_11_en_2_capricorn.pdf?type=prod uct_info)

3. Illumination needs calculated as follows: • Recommended Light Level for office (500-750 lux=lumen/m2)-> selected 600 lux, recommended for computer workstations • Selected lighting fixture: Compact Fluorescent Lights (CFL) of 20W consumption, delivering 1200 Lumen each (Note that for the selected CFL the luminance tends to be about 40 to 70 lumens per Watt of power draw, whereas incandescent lights are more like 10-17 lumens/Watt) • Cu = coefficient of utilization = 0.9 • LLF = light loss factor = 0.9 • Floor area: 16m2 • Number of lights needed= 600 lux ∙ 16m2 / 0.9 ∙ 0.9 ∙ 1200 lumen = 10 lamps • Total electricity consumption for lighting: 10 X 20 W = 200 W The office will be occupied for approx. 10 hours per day No further consumptions are taken into consideration in this type of stand-alone system calculation. As a result the total load is rated at: 260W

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13.2 Needed Capacity of Storage means for Stand alone Option On the basis of the above assumptions, the capacity of the energy storage means can be calculated when further assuming: - A 0.95 for the efficiency of the charge controller (see previous chapter for details) in duty for charging the batteries - 0.95 for the efficiency of the used stand-alone inverter (see previous chapter for details) for supplying the AC current needed for the above appliances - 3% energy loss for the DC cables between the PV and the above controlling devices (AC losses are neglected due to the proximity of the whole decentralized system) Needed capacity: 260 Wh x 10h/day ∙ 2 (days autonomy) / 0.95 ∙ 0.95 ∙ 0.97 = 5940 Wh Concerning the type of battery on which the present calculation is based, for the present step conventional cheap lead-acid batteries are considered, but other options are discussed in the corresponding research chapter dealing with the peripheral devices for PV systems. As 2 PV modules supply electricity to the selected modular office space, it is better to connect them in such a way, that a 42V current is delivered, a fact beneficial in particular for a stand-alone calculation (leading to fewer batteries). A possible modern battery useable in PV systems has the specifications of: - 80% battery discharge depth (should not be too high for longer battery life expectancy) - 42 V - 50 Ah Based on all the above data the number of batteries needed is calculated as follows: 5940 Wh / 0.8 ∙ 42 V = 176 Ah, which means: 176 Ah / 50 Ah per battery = 3.5 This means approx. 3-4 batteries are needed. This result, namely installing e.g. 3 batteries in each office, is evaluated as being at the limit of practical use for both economical and space reasons. It has to be pointed that the above exemplified consumption includes only basic needs (PCs, lights, climate unit in its basic function).

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13.3 Assessment of the calculated stand-alone option For the batteries part improvements are possible. A small improvement would be to use the DC current produced by the PV array (layout shown in the fig 13.2) for the lighting fixtures operating correspondingly under DC (possible with new LED lighting systems) and to remove load from the inverter to some extend, but this would be of marginal significance. Charge Control

Battery

DC Load

Inverter

AC Load

fig 13.2 Option of using DC from PV cells separately.

A 72V/50Ah lithium ion prismatic cell battery with the specifications shown in the fig 13.4 could be used to reduce the space requirements, since in such a case only 2 batteries would be needed (5940 Wh / 0.8 â&#x2C6;&#x2122; 72 V = 103 Ah, 103 Ah / 50 Ah per battery = 2). This solution is more expensive (such batteries cost around 600â&#x201A;Ź) and is based on the assumption that the modules can be connected so as to provide 72V which is normally feasible, otherwise the controller could adapt the voltage to that of the battery (with some small loss). New batteries are developed (in particular also in view of hybrid or fully electric cars), producing solutions which are very promising (fig 13.5, graph, in which modern batteries have enormous efficiencies as to the usual lead-acid ones) They are at the moment expensive, but as this is normal with new electronic products their price will certainly be much lower when broadly produced and marketed.

fig 13.3 Lithium Ion Prismatic cell Battery

fig 13.4

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fig 13.5 Graph showing different storage typesâ&#x20AC;&#x2122; energy density/mass per volume

The above Ragone plot depicts the relation in energy density versus mass (y-axis) and volume (x-axis) of the battery types which are theoretically usable in PV systems. Thus high volumetric and mass energy density means less volume and less weight, respectively, both important parameters for choosing the battery type. In view of this diagram, lead acid are the worst in both volumetric and per mass (gravimetric) energy densities, whereas lithium ion batteries, in particular the prismatic lithium polymer ones show ideal material properties which could make them the optimal storage option. They are however still in their research phase so far and are thus very expensive. Fast development in the sector shows that much more efficient battery types will in the future become available for reasonable prices.

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For the stand-alone option the energy storage problem seems to constitute an inhibitory factor at least under current market conditions. However it cannot be seen as the main obstacle since there are further considerable reasons. More importantly the energy producing capacity of the 2 connected PV modules (ranging from a minimum of 306 Wh per day to a maximum of 1.888 Wh with a mean of 1.065Wh) cannot be considered as sufficient for the expected total loads of 5940 Wh per day. Further aspects influencing the preferable system selection. (against stand alone) As the whole building would surely be connected to the grid for a series of apparent reasons, it appears uneconomical to use only costly and cumbersome lead-acid batteries for storing PV energy when a grid-connected solution would benefit from: • Supplying any possible excess energy to the grid, especially if beneficial prices are paid to PV energy producers according to usual subsidizing policies for feeding energy to the public grid • Simplification of the whole structure • Providing both the architect and the real estate specialist with the option of using the space at any future point for any different, much more energy consuming purpose or being more flexible in possible retrofit solutions within the expected lifetime of the building Thus the selection of the proposed solution should take as given that a grid-connection is provided and should be used when this is reasonable, indicated, or unavoidable.

13.4 Options in order to use a partly autonomous system

If despite the aforementioned problems an autonomous system should be developed, for instance for ecological or even promotional reasons, the following options could lead to this direction: a) Using sophisticated batteries, like for instance prismatic lithium polymer batteries (their advantage can be seen in fig.13.5) which at the moment are expensive and cannot yet be seen as a mature technology. b) If an additional renewable energy source could be available a hybrid stand-alone system could be a feasible choice. If for any reason great load peaks for a short period of time are be expected, even a conventional energy source could make sense in such a hybrid stand-alone system. c) Designing the whole building in such a way and interconnecting the possible PV energy production on different façade orientations (west, east etc. possibly also that of PVs on the roof) so that the load fluctuations with in the day are compensated to some degree. Thereby the size of the back-up batteries is substantially decreased. 242

When looking for a reasonable solution for combining a grid connected system and the desire to make as much as possible direct local use the produced solar energy, Systems selectively combining (over time) the two possibilities are the best choice.

PV Modules

Breaker

Power meter

Utility

Breaker

Batteries

Building Appliances

fig 13.6 System selectively combining stand-alone and grid layout.

This option gives (considering a connection scheme as shown in fig 13.6) the user of the building the possibility to select at any point of time between the following operation options: • Permanently and primarily feeding any excess energy to the grid and substantially or partly disregarding the SOC (state of charge) of the anyway existing batteries. • Prioritizing the SOC of the batteries and feeding only any surplus energy to the grid • Disconnecting the grid and running the whole system as a stand-alone one (an option being more or less feasible depending on the designed size of the batteries) • Compromising all above options in practically any desired extent allowed by the capabilities of a possibly sophisticated and thus adaptable controller. The question that arises is how this option could be applied in the present case. 13.5 The possible concept

The controller should be programmed to prioritize the energy distribution in the followng manner: The battery and electrical connections should be designed so that all devices directly positioned or functionally linked to the NEXT façade module should be provided with all possible solar energy produced by the two thin film modules (according to previously described arrangement).

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In this case, given that the option of a grid connection is provided by the accordingly programmed controller, it appears reasonable to first deliver with solar energy all devices in absolute proximity to the next faรงade and in particular those which constitute a part of it or/and are functionally linked to it, namely: 1. The drives/motors for moving the sun shading system 2. The TROX-cassette devices 3. Possible further climate devices

1. The drives/motors for moving the sun shading system (consumptions) In the solutions proposed for moving the sun shading thin film PV curtains small motors are needed. Assuming for the sun shading PV carrying polymer material a thickness of 5mm and a density of 1.3kg/dm3, then for a total size of 2m x 2 m, 26kg can be calculated as the Sun-shading structures weight. A small (preferably DC) motor of 75W can raise each shading device in less than 30 sec, whereas the two shading devices of each office module of 16m2 will be raised in a sequential way. 2. The TROX-cassette devices (consumptions) As already calculated above the recovery boxes FSL-B-ZAU of TROX need only up to 20W at the maximum (range 9W-20W). They have a water coil for heating and cooling with a capacity of 800W/400W, respectively 3. Possible further climate devices (consumptions) If a decentralized heat pump is desired, which makes sense in a decentralized facade system like the NEXT FACADE, a modern Heat Pump can be used with a COP (efficiency) of 5, which is at the lower limit of efficiency (COPs of up to 7 are known in particular for mild climates, with winter temperatures >0deg). Therefore a consumption of: 800/5= 160W (winter) and 400/5= 80W (summer) can be calculated for the heat pump.

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The energy for the sun shade lifting motor has not to be taken into account in this calculation since it is only an instant load (i.e. only a few times a day and in the worst case, the other consumptions could stall their function for a few seconds when the motor lifting the curtain has to work). Thus for the whole TROX and Heat Pump function 180W (winter) and 100W (summer) are needed. This is translated for a working day of 10hours into 1.000Wh - 1.800Wh of energy. Use of Energy Conclusion Comparing these needs with the PVs’ energy producing assumptions/calculations (ranging from a minimum of 306 Wh per day to a maximum of 1.888 Wh with a mean of 1.065Wh), it can be concluded that it makes sense to use the produced solar energy for the whole “NEXT environment”, climate and sun shading moving related. Batteries will definitely be needed if the stand alone “mode” will be absolutely prioritized, but the energy from the grid could supplement the PV energy in a simpler and more efficient configuration. It has to be mentioned that the use and design for the storage means has to be based on space availability and expected beneficial prices for feeding electricity into the public grid. This means that for instance if no beneficial prices were paid at some point of time, then using larger batteries makes sense, in particular for an office building in which energy can be accumulated in weekends and holidays for use during workdays.

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14_Summary of Results of the 4 Phases and Design Every step of the chosen methodology gave useful results for evaluating the real potential of shaping solar modules into three dimensional shapes in order to increase performance of new PV cell technologies per covered area. Surface Area Comparison The generation of shapes of different patterns and densities within a parametric environment resulted in 56 surfaces, the best of which (“Origami shape (Miura Ori) 1/4 width version”) reached a surface area 3.9 times larger than the area of their footprint (projection) on the facade plane or of a flat module covering the same space. This can also be translated as 6 times more area compared to a conventional arrangement of PV louvers. Solar Analysis The solar analysis of the 56 shapes gave important insight on the real solar energy reaching the three dimensional surfaces created in the previous phase. The best performing shape (“Curved one side vertical 1/4 width version”), has an increased amount of solar energy reaching its surface of +61% compared to the reference “flat façade” and +64% compared to the reference “louvers 30d horizontal”. PV Laboratory Testing The Laboratory testing phase has been the final step of the performance comparison and evaluation in order to observe real performance decrease caused by partial shading and non optimal angles of 3d surfaces. A total of approximately 100 tests performed with seven (7) 3d shapes (simulating a corrugated design) of different density positioned in a multitude of different angles gave important results for judging the real potential of the three dimensional PV solutions. In practically all angles the densest PV arrangements proved to be more efficient in TOTAL OUTPUT than their flat equivalents of the same footprint area. As expected efficiency of the PV surface decreases almost linearly proportional to the density of the PV shape corrugations due to partial shading. Integration of the results into the NEXT Facade as shading structures Three different shape versions have been developed (Categories A,B,C), based on the best performing shapes with the highest potential of constituting a retractable external shading skin that produces energy. For every shape proposals of possible operation have been made. While the Origami (Miura Ori) pattern exhibits the idea of folding a surface to create more active area (while being retractable) in the most efficient and clear way, it remains a very complex solution that additionally does not make use of the flexibility of the material. On the other hand the convex (facing out) curvature units of the “one side corrugation” combine the maximum performance through shape according to the solar analysis, a more predictable shading pattern and the use of the flexible characteristics provided by emerging solar cell technologies. This category can therefore be considered the most efficient solution in total. Complete PV system selection Finally a possible total PV system is proposed, selectively combining direct local use the produced solar energy with a grid connection. The achieved energy output is thereby used for running the sun shading mechanism, the climate control devices and a possible heat pump. 247

15_Conclusion, Recommendations and further Potential (Reflection) Starting with a research that addressed several topics related to solar cells and their integration in buildings and facades more specifically by using the example of the NEXT Facade, this Graduation Project attempted to trace the potential of three dimensional shapes as active solar skins (used as shading devices) compared to conventional flat solutions. Imagining a future of cheap, lightweight solar modules which are easy to curve, twist or fold an alternative approach compared to high tech tracking or concentrating solutions was developed, where shape and surface area increase can lead to better performance. Performance characteristics and properties of new generation solar cells especially in terms of diffuse light performance and partial shading are defining factors for a possible success of these structures in the future. A four phase methodology was developed in which (at a first phase) the potential of PV surface area increase for a given facade space was researched. Best performing shapes in terms of maximum surface area were compared through a solar analysis to find the energy their surface receives on average and PV lab testing gave an indication of real performance for three dimensional structures. Finally the best performing designs were developed as structures and integrated in the decentralized NEXT Facade concept Module created by Alcoa. Using the same methodology in a later work or project a multitude of different parameters and restrictions could be added and predefined in order to finally end up with an interesting variety of prismatic or curved three dimensional shapes. Only by changing the location and orientation of the developed structure already completely different 3d shapes emerge as most efficient. The absolute interdependence of the actual shape and the context has the potential of generating a large variety of different performance based forms that are both pleasing to the eye, efficient and unique. The methodology itself could in the future include a more efficient shape generation process as a starting point, where a genetic algorithm process would â&#x20AC;&#x153;circleâ&#x20AC;? through millions of shapes and shape versions in order to find the optimal performing ones according to the predefined needs and criteria. Surface area maximization, solar performance and PV performance, which in this case constitute three different steps could in the future be merged into one automatized process with specified boundary conditions. Additionally the effect of reflections between the prismatic or curved surfaces of a 3 dimensional PV shape could be of importance considering the final results, making specific shapes slightly more efficient than they look in the current calculations. The PV testing Phase could in a future stage become a much more accurate filter of the total amount of well performing shapes. Overcoming cost restrictions accurate 3d models of exactly the designed curvatures and folding angles could be constructed out of polymer materials and thin film inorganic or organic PV cells. Time consuming real world testing in external conditions instead of only laboratory measurements is an additional step that could reveal more performance characteristics and possible advantages or disadvantages of three dimensional PV surfaces compared to what the present Thesis achieved. As a final point, it could be argued that the charming process of connecting and correlating form, performance and context, into a unique for every combination result is in itself worth the effort and future development despite any emerging difficulties during implementation. Such a process has the potential of being translated into a future alternative approach not only as a different way of generating more electrical power from a given surface, but also as a different (more performance based) way of using form/shape in architecture.

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