Roland Krippner (Ed.)
Building-Integrated
Solar Technology
Architectural Design with Photovoltaics and Solar Thermal Energy ∂ Green Books
Imprint
Editor: Roland Krippner, Prof. Dr.-Ing. Authors: Gerd Becker, Prof. Dr.-Ing. Ralf Haselhuhn, Dipl.-Ing. Claudia Hemmerle, Dr.-Ing. Beat Kämpfen, Dipl. Arch. ETH SIA M. Arch. Roland Krippner, Prof. Dr.-Ing. Tilmann E. Kuhn, Dr. Christoph Maurer, Dr.-Ing. Georg W. Reinberg, Arch. vis.Prof. DI. M. Arch. Thomas Seltmann Project Management: Fabian Flade, M.A. Jakob Schoof, Dipl.-Ing. Editing and Layout: Jana Rackwitz, Dipl.-Ing. Jakob Schoof, Dipl.-Ing. Translation: Susanne Hauger Proofreading: Emma Letizia Jones Graphics: Ralph Donhauser, Dipl.-Ing. (FH) Simon Kramer, Dipl.-Ing. Simon Axmann, B.A. Annika Ludwig, B.A. Fabiola Tchamko, B.A. Ka Xu, B.A. Frido Flade GmbH FP-Werbung
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Contents
Prologue
6
Introduction and history Solar technology and building culture From solar house to energy self-sufficient building The actors in solar construction Conclusions and outlook
8 8 9 18 19
Buildings as catalysts for energy transformation From energy transition to energy transformation The merging of energy sectors Solar technology as design challenge
20 20 21 22
Physical and geometric principles Foundations of solar energy use Design tools
24 24 27
Technology and systems – photovoltaics The functioning of photovoltaic installations Solar cells and photovoltaic modules Inverters as system headquarters Battery storage systems Planning and design Installation, commissioning, operation and maintenance Requirements, standards and regulations
28 28 30 36 37 38 44 47
Technology and systems – solar thermal energy Operation of solar thermal installations Applications of solar thermal energy Solar thermal energy in the context of the building shell Building shell components with solar thermal functionality Storage and other system components System concepts for solar thermal energy installations Requirements for integrated collectors
52 52 54 56
Integration of solar installations Basics Additive or integrated installation? Building inventory Design integration Structural integration
64 64 64 65 66 68
Designing and building − photovoltaics From the igloo to the tree The building shell as part of the energy system Influence on the design process
72 72 73 76
57 60 62 63
Designing and building – solar thermal energy Climate-appropriate construction Types of solar energy use Indirect use of solar power via thermal collectors Building design with thermal collectors Implementation of thermal collectors in practice Outlook: the future of solar thermal energy Case study 1: Residential complex in Salzburg Gneis-Moos Case study 2: Residential building Schellenseegasse, Vienna
80 80 80 82 86 88 89 90 91
Economy and ecology A look at the life cycle of solar installations Financing Profitability of PV installations Profitability of solar thermal installations Ecological assessment Energy certification and green building labels
92 92 92 94 98 100 102
Built examples Building-integrated solar technology in detail Kindergarten, Deutsch-Wagram Single-family house, Glattfelden Office and residential building, Darmstadt Office building, Kemptthal Office building, Kasel Museum of Archaeology, Herne Apartment building, Bennau Day care centre, Marburg Residential and office building, Romanshorn Education centre, Niestetal Convention centre, Lausanne Centre for Photovoltaics, Berlin
104 104 106 108 110 112 114 116 118 120 122 124 126 128
Appendix Acknowledgements Authors Index of illustrations Literature Regulations, guidelines, standards Glossary Subject index Register of companies and individuals
130 130 130 132 133 135 136 138 140
Prologue
Roland Krippner
In the past few decades, architecture and solar energy have been engaged in an increasingly close, occasionally tense interrelationship. Even though predictions such as “Solar Building to Become a Megatrend” [1] have not been fully borne out, energy-efficient design, construction and modernisation have nevertheless earned a central role in legislation, and a significant place in the curricula of universities and training institutes. The implementation of the EU’s building guidelines (2010/31/EU) now marks the next big step. As yet, the member states of the EU have only partly defined the form that the minimum energy standard mandated in the guidelines for all new buildings in the EU will take. But it is already apparent that supplying buildings with renewable energies, primarily solar heating and solar electricity, will become increasingly important over the coming years. In light of these developments, it is surprising how reticently the discussion of the production of solar energy in buildings has been broached both in public and in professional circles. In the overcrowded and somewhat unmanageable market of books written for architects and engineers, active solar technology is at best a niche topic, and relevant publications on the subject are often already years behind. Texts specialising in the respective energy system technologies dominate. In addition, the reception of the subject is determined largely by photovoltaics. Solar thermal energy, by contrast, remains proverbially underexposed; yet it is perhaps the very thing that could make a significant contribution to a “thermal transition” in Germany. In 2015, 32.5 per cent of the electricity used in Germany was from renewable sources [2]. In the thermal sector of the same year the number was merely 9 per cent. The CO2 emissions for the thermal energy consumed in Germany 6
have remained essentially constant over the last ten years or more [3]. It would be a mistake to reduce the discussion of solar energy in and on buildings merely to its technical aspects. Model calculations and initial pilot projects are already making it clear that photovoltaics and solar thermal energy will have to be fully incorporated into the design of any zero-energy and PlusEnergy buildings of the future that exceed the size of single-family houses. The (flat) roof areas of such buildings alone are generally not large enough to produce the requisite amounts of energy. The design of active solar technology and its integration into buildings therefore constitutes the major challenge facing all architects who wish to contribute to planning the building inventory of the future. In light of the above, the book Building-Integrated Solar Technology for the first time addresses the intersection of architecture and active solar technology within a broader textual framework. Unlike previous texts, it places its investigations of solar thermal energy and photovoltaics on an equal footing, and discusses their individual design potentials. The title of the book relates to established terminology from the solar industry. In the field of photovoltaics, the phrase “building-integrated photovoltaics” (BIPV) or “construction-integrated photovoltaics” has already established itself; but solar thermal energy, too, represents a central building block in the development of a decentralised energy supply for the future. For this reason, the Solarenergieförderverein Bayern e. V. (the Bavarian Association for the Promotion of Solar Energy) categorises its associated operations under the heading “building-integrated solar technology” (BIST), a term 0.1 Umwelt Arena, Spreitenbach (CH) 2012, René Schmid Architekten
which is still relatively new among scientists, and whose definition still suffers from a certain lack of conceptual clarity. A major characteristic of building-integrated solar technology is that, as a rule, the collector surface and/or the PV generator represent a design-determining element of the building shell. In principle, two different strategies can be observed in handling collectors and PV modules. The solar technology system can be concealed in the roof area, in which case it has little effect on the design of the building. Particularly in the context of historically important building complexes, this is a possible way to employ solar technology. In such projects, admittedly, the opportunity to effect the bold transformation of buildings from energy consumers to energy producers is often squandered. Such a goal is better achieved when the solar installation is integrated visibly into the roof, and especially into the facade. The latter approach, however, makes much higher demands on the design not only of the solar installation itself, but also of the entire building. Only in qualitatively sophisticated architecture can solar technology have an enriching influence on contemporary building culture. For this reason, this book focuses on buildings in which architects have specifically employed solar technology as a design-determining element (Figure 0.1). The structure of the book is designed so that it provides rapid access to essential content useful for both experts and laypeople alike. After a brief historical overview of building-integrated solar technology, the chapter “Buildings as Catalysts of Energy Transformation” establishes the societal and urban development context of the present challenge. Chapters 3 through 5 deal with the physical and geometrical foundations of solar energy usage, and provide a more in-depth discussion of the important technical characteristics of photovoltaic and solar heat-
ing systems. In this section, authors from the Deutsche Gesellschaft für Sonnenenergie e. V. (The German Society for Solar Energy), as well as the Fraunhofer Institute for Solar Energy Systems (ISE), bring to bear the expertise they have acquired through both research and practice. The application of the material to real projects is covered in chapters 6 through 8 of the book. Building on general assessments of the “work of integration”, two architects who have spent years making outstanding contributions to the field report from their wealth of experience and give advice on the design and implementation of building-integrated solar installations. A further chapter points out the important economic and ecological aspects of solar energy use. The book then concludes with a selection of representative buildings, complete with architectural drawings and information on the technologies and products used. It is hoped that this book will serve as an impetus for architects and engineers, students, employees of public agencies and, last but certainly not least, building clients, to explore and employ the numerous fascinating technical and design possibilities of solar technology, and to develop them further through new approaches in their own projects. Notes [1] German Solar Energy Association (BSW: Bundesverband Solarwirtschaft e. V.): Press release on the occasion of Munich trade fair BAU 2007. www.solarserver.de [2] Agora Energiewende (ed.): Die Energiewende im Stromsektor: Stand der Dinge 2015 (The Energy Transition in the Electricity Sector: The State of Affairs 2015). www.agora-energiewende.de/fileadmin/Projekte/2016/Jahresauswertung_2016/ Agora_Jahresauswertung_2015_web.pdf (Status as of 31.08.2016) [3] PwC (ed.): Energiewende-Outlook: Kurzstudie Wärme (Energy Transition Outlook: Brief Study on Thermal Energy). www.pwc.de/de/energiewende/assets/pwc-ewo-kurzstudie-waerme-2015.pdf (Status as of 30.08.2016)
0.1
7
Introduction and history
Roland Krippner
• Solar technology and building culture • From solar house to energy self-sufficient building • The actors in solar construction • Conclusions and outlook
Solar technology and building culture
Even in the many cases in which they are integrated in a functionally correct and structurally coherent way, it must be said that their integration into the aesthetic aspect of design is often less successful. Solar systems have occasioned an enormous expansion in the technical building repertoire, but – as has been the case for other new products and innovative materials – this expansion must be converted into a conceptual architectural framework. In the incorporation of collectors and PV modules, both technical features and structural and aesthetic concerns must be taken into account. The term “building-integrated solar technology” encompasses both of these aspects in equal measure, and this book likewise addresses them together. Since the early days of active solar energy use in the 1970s, the intersection of architecture and solar technology has been emphasised, and the “willingness to engage in creative approaches from design to architecture” [3] advocated. A large number of realised projects over the past decades have shown, however, that it is precisely this call that has all too often gone unanswered. Given the many positive examples in existence, this would be quite puzzling were it not for the fact that these exemplars are simply too little known. It follows that building-integrated solar technology – that is to say, those design solutions in which the relevant systems form a significant part of the building – continues to lead a niche existence in the field of energy-efficient construction. Reasons for this include professional barriers between the participating players (engineers versus architects), as well as the failure of manufacturers to maximise innovation potential in the production process [4]. Furthermore, thanks to guaranteed feed-in compensations and sharp price reductions on the international
The use of solar energy in and on buildings will be a central theme in future construction. Thermal solar collectors and photovoltaic (PV) modules are already natural components of energy-efficient buildings and innovative shell constructions. The systems available on the market feature efficiency and elegance in equal parts, as demonstrated in trade fairs and in numerous design awards. It is no accident that the past decades have seen photovoltaics and solar thermal energy achieve their status as symbols of progress. Solar collectors and PV modules are important elements of solar construction, but they are not the only ones. In the building-specific use of solar energy one must differentiate between direct (passive) and indirect (active) principles [1]. The first step is to exhaustively employ the direct measures in order to reduce energy needs and to ensure a comfortable internal climate. This encompasses fundamental planning strategies such as a sensibly organised layout, a compact building volume design, appropriate choices of materials and an optimised construction of the building shell, which should in turn reference regional building traditions [2]. Building on this, the next step utilises solar heating and photovoltaics via indirect measures to contribute a sustainable source of energy, replace fossil fuel energy supplies and reduce CO2 emissions. Building-integrated solar technology as a design challenge
Regardless of whether they are of standard format, or custom made for projects of sophisticated design, installation systems always alter the appearance of buildings, and therefore of the cities and the countryside as well. 8
From solar house to energetically self-sufficient building
market, there is a disproportionate focus on photovoltaics. As a consequence, the equally energy efficient and economical installation systems of solar thermal energy are underutilised, especially in ambitious architectural concepts. In addition, the substantial economic downturn in the European solar energy sector over the past few years has led in turn to radical structural changes in building-integrated solar technology. At present, only a few design specialists and systems manufacturers are active on the market. To date, this has not yet negatively impacted on the number and quality of sophisticated solutions, as evidenced, for example, by the “BuildingIntegrated Solar Technology Architecture Prize” of 2014. Paths to a smart solar architecture
For sustainable architecture and urban development, a decentralisation of the energy supply is of critical importance. Referencing Ernst Friedrich Schumacher’s “Small is beautiful” (1973), the German journalist Franz Alt speaks of “Dächertec statt Desertec (roof tech instead of desert tech)”. With this, he emphasises the crucial role that the activation of building shell surfaces by solar technology will play in a future Germany with a technically and economically feasible, 100% renewable energy supply [5]. With the implementation of the European directive governing the overall energy efficiency of buildings, which takes effect in 2019 and 2021 respectively and demands that new buildings conform to the “Nearly Zero Energy” standard, building-integrated solar technology will (again) make clear gains in currency and relevance. The increased research and marketing activities over the past years, coupled with building labels such as “Plus-Energy House”, “Efficiency House Plus” or “SolarActivehouse”, have already had similar effects. The challenge now lies in viewing these developments as a building culture mission – not only in new construction, but also in the energy refurbishment of the existing building inventory.
From solar house to energetically self-sufficient building Building-integrated solar technology is not a new phenomenon. Already toward the end of the 1930s, prototypical developments of solar houses with solar energy roofs were to be found, particularly in the USA. The first “active solar house” was the MIT Solar House I in Cambridge, Massachusetts (1939), where flat-plate collectors are extensively integrated into the pitched roof. In 1948, the architect Eleanor Raymond, working with the energy engineer Maria Telkes, completed the Dover Sun House (also known as MIT Solar House VI) in Dover, Massachusetts. On the south side of the upper storey it features the first complete collector facade of vertical components and is touted as the first solar-heated residence in the world [6]. Up until the mid 1970s, the USA oversaw the construction of further experimental buildings in whose design architects increasingly became involved. This followed the recognition, beginning in the mid-1950s, that the design of active solar houses necessitates close interdisciplinary teamwork among architects, heating engineers and prospective residents [7]. Occasionally, an American prototype would become known in central Europe: In his search for ideas for an energy-efficient building company, the engineer Klaus Daniels set out on a research trip through the USA in the mid-1970s to familiarise himself with the energy and economic potential of active solar technology [8]. In Germany, where the same time period saw an increasing preoccupation with alternatives to fossil fuel energy sources, the topic was approached on multiple levels. The year 1974, during which the first solar facilities were installed, is taken to be the “inception” date of such models. In a piece written in 1979, the author Axel Urbanek suggested that, as this “technical-sociological development” was still in its early stages, the “predominantly aesthetic” demands on the new construction challenges 1.1 Joinery workshop, Freising-Pulling (D) 2010, Deppisch Architekten
1.1
9
Introduction and history
1.14
1.15
building refurbishment project: the modernisation of a simple industrial warehouse in Erfurt (2001). In this case, the photovoltaics have been detached from the building face and integrated, in the form of eleven rows of polycrystalline glass modules, into an eleven-metre-high galvanised steel construction set in front of the facade (Figure 6.13, page 70). The project, developed by the author together with Peter Bonfig, with expert advice from Prof. Thomas Herzog at the Technical University of Munich, is as much a visible symbol for a technology of the future as it is solar protection with optimised solar electricity use. The third call for competition entries under the heading “Architecture and Solar Power – Building-Integrated Photovoltaic Installations” went out in 2005. Despite the long interval between this competition and the previous one, only seventeen projects were entered. Nevertheless, on this occasion the submission designs were of a significantly higher quality overall. First prize was won by the refurbishment of two nine-storey residential buildings in Freiburg (2001, Figure 6.9, page 69) that were built in the second half of the 1960s. The architects Rolf + Hotz transformed the closed south facade into a full height unbroken photovoltaic installation. The horizontally arranged glass modules form a suspended, rear-ventilated facade. On their long sides they are fixed to the aluminium supporting structure with visible black clamping profiles. The back is covered with a black foil, so that even from afar each rectangular frameless element is individually outlined by a dark border. One of the five recognition awards of this competition was awarded to the Paul Horn Arena in Tübingen (2004, Figure 6.5, page 66). Here, Allmann Sattler Wappner have furnished its entire south facade with vertically oriented modules in four different sizes. The architects have employed green polycrystalline PV cells in a glass-foil construct, referencing both the location and the general concept of the building. A distinct white border formed by the foil laminate on its back structures each module and the overall appearance of the facade. Each frameless PV panel is fastened with four multi-part light-alloy stand-offs, which makes it possible to replace individual units if needed.
In contrast to this, the single-family house in Hegenlohe by Tina Volz and Michael Resch (2005, Figure 6.1, page 65) features an elevated PV array atop its shallowangled southwest-facing pitched roof. The horizontally oriented polycrystalline modules are arranged in six rows, and the entire installation is partitioned into two sections conforming to the interior zoning of the building. The module rows project slightly beyond the ridge and eaves, and their stand-off mounting is apparent. With this house, the architects have succeeded in creating an outstanding example of a residential building that embodies a compelling symbiosis between solar technology and architecture.
14
International dynamics – developments since 2008
Starting in 2008, SeV began to hold its competition on a three-year cycle, broadened its scope to include solar thermal energy, and opened it – initially billed as a “European Prize” – to full international participation. In 2008, 40 projects were submitted, of which the jury reviewed 38 entries from eight countries. The European Prize for Building-Integrated Solar Technology was awarded to a new office building for Marché International in Kemptthal near Zurich (2007) by Beat Kämpfen (see Built Example, page 112f); while second place went to an inner city residential and office building in Darmstadt (2006) by Opus Architekten (see Built Example, page 110f). Both projects showcase exemplary energy roof solutions in which not only the overall building concept but also the specific details in the solar technology are convincingly expressed. The jury also awarded three honourable mentions. Two of them were given to buildings that pursue very different and novel approaches to their integration work. In the Sino-Italian Ecological and Energy Efficient Building (SIEEB) for the Tsinghua University in Beijing by Mario Cucinella Architects (2006) – a powerful and architecturally unusual composition – the fin-like PV structures that project from the building, floor by floor, constitute an important and design-defining element (Figure 1.16). The institute building has a U-shaped footprint and is oriented on a north-south axis. Its design incorporates multiple overlapping uses and many references to traditional Chinese symbolism. The varying functional levels of
From solar house to energetically self-sufficient building
the building shell are expressed effectively in both construction and design. The Solar House, with which the team of the Technische Universität Darmstadt won the Solar Decathlon in Washington (USA) for the first time, also represents a new building type. In this experimental building, the solar technology systems integrated into the roof and the facade contribute significantly to the goal of energy selfsufficiency. On the building sides, the amorphous silicon modules are fastened to wood slats and integrated into folding shutter-like wooden doors. This ensures that the slat orientation can be adjusted both for full sun exposure, and to control the amount of light entering through the facade (Figure 1.14). In 2011, the architecture competition for Building-Integrated Solar Technology began accepting projects from all over the world for the first time. Even though most of the 84 entries came from German-speaking regions, the response from a total of thirteen countries attested to the expansion of the event. This time, the architecture prize was awarded to a building type in which design requirements are usually relatively rare: a joinery shop in Freising-Pulling (2010). The building, designed by Deppisch Architekten, is a paredback, elegant structure with a gently inclined north-southoriented roof whose entire surface is covered with photovoltaics. The PV array terminates precisely at the roof edges and has no visible penetrations, which lends the entire building shell a smooth, planar appearance (Figure 1.1, page 9).
The deltaZERO buiding in Lugano by DeAngelis Mazza Architetti (2nd prize, 2009) is a residential high-rise, with a primary construction of reinforced steel acting as its internal storage mass, and a steel and glass facade (Figure 6.11, page 70). On its southern side, integrated full storey-high solar collectors support heating and hot water supply. In this building, the zero-energy concept is combined with a purist design style. The architects have succeeded in incorporating the solar thermal components into the facade in an exceptionally elegant manner, both technically and structurally. The refurbishment of a historical brewery in Bad Tölz by Lichtblau Architekten (3rd prize, 2009) shows that, despite the increased complexity it entails, the successful transformation of original tile roofs into energy roofs is entirely feasible. The central element is the fully glazed solar roof with integrated, mutually coordinated modular systems for light, air, heating and electricity – a symbiosis between old and new that beautifully demonstrates the potential of solar technology in existing buildings. New approaches to the renovation of existing buildings and to energy self-sufficiency have likewise been taken in the three projects that received recognition awards in the competition. In the refurbishment of the headquarters of Energie Steiermark in Graz (2010), Ernst Giselbrecht + 1.14 Solar House for the Solar Decathlon in Washington (USA) 2007, Team Deutschland/TU Darmstadt 1.15 home+ for Solar Decathlon Europe in Madrid (E) 2010, Hochschule für Technik, Stuttgart 1.16 Institute building in Beijing (CN) 2006, Mario Cucinella Architects
1.16
15
Physical and geometric principles
Gerd Becker
• Foundations of solar energy use • Design tools
Foundations of solar energy use
The strength of the incoming solar radiation depends heavily on the time of year (Figure 3.1). The seasons are a consequence of the fact that Earth’s axis of rotation is tilted with respect to its orbital plane (ecliptic) around the sun. The angle of the tilt, or declination, varies cyclically over several tens of thousands of years, but for the purposes of building design, it can be assumed to be constant at 23.45°. In June, when the sun shines predominantly in the northern hemisphere, it is summer in Europe, Asia and North America. Conversely, the southern summer falls between November and February, when the southern hemisphere is more oriented toward the sun. In Munich, on June 21, the sun reaches an elevation of 66° above the horizon, but on December 21 it peaks at 19°. In winter, therefore, the sun’s radiation not only follows a much flatter trajectory but must also traverse a much longer path through the atmosphere. This weakens it even further. Due to Earth’s elliptical orbit around the sun as well as its declination and rotation about its own axis, rays from the sun strike objects on the Earth’s surface at different angles depending on the season and time of day. The incident directions can be described using the solar azimuthal angle αS (the sun’s angular departure from the north-south axis) and the sun’s elevation angle γS (Figures 3.2 and 3.3). There are complex mathematical equations with which to calculate the elevation and azimuth, but it is more practical to employ sun path diagrams. These charts are available for many locations and make it easy to determine sun elevation and azimuth for any given date or time. The internet is another source of sun path diagrams for any desired location, and corresponding software [1] is just as freely available (Figure 3.3).
Solar energy on Earth occurs in direct (radiation) and indirect (e.g., wind energy) forms. For buildings, the radiation represents a particularly significant energy source. It can be used both actively and passively. Passive and active applications
Passive use describes the employment of specific structural measures to collect, store and distribute incoming solar radiation, generally without the use of technical installations. Active use encompasses all technical measures that are utilised for the capture, distribution and, at times, storage of solar energy. There are two different types of active use of solar energy to distinguish: solar thermal energy and photovoltaics (PV). In solar thermal energy, collectors convert solar radiation into heat; in contrast, PV modules use it to produce electricity. The energy yields of these systems are primarily influenced by local conditions and can vary strongly with geographical location. Secondly, the inclination angle and the exposure of the installations themselves determine energy yields. In these cases, the differences between solar thermal and photovoltaic systems must be taken into account. Sun elevation and solar azimuth
Earth’s elliptical orbit around the sun takes one year. The Earth-sun distance varies slightly between 0.983 and 1.017 astronomical units (AU; 1 AU = 149.6 million kilometres, which represents the average distance from the Earth to the sun). In addition, the Earth rotates around its own axis once per day. This creates the daytime variations that have a significant influence on the use of solar radiation. 24
Foundations of solar energy use
Spring in the northern hemisphere
Solar power and energy
Nuclear fusion processes occurring in the sun’s interior generate energy in the form of electromagnetic radiation. The wavelengths of this radiation range from 10 – 20 metres to several kilometres. The power at the sun’s surface measures 62.5 MW/m2, which corresponds to a temperature of 5777 K. Since this power is radiated outward into space in all directions, only a tiny fraction of it arrives on Earth. No losses occur in the journey through the vacuum between the sun and the Earth. The incoming power density of the radiation that reaches the upper edge of the atmosphere is known as the solar constant. Because of Earth’s elliptical path about the sun, and because of the resulting time-dependence of the sun-Earth distance, the solar constant varies very slightly. Its median value is 1367 W/m2. On its way through the atmosphere, the sun’s radiation is reduced by reflection, absorption and scattering. The cumulative effects of these depend on how far the sunlight must travel through the atmosphere, which is expressed by the air mass coefficient (AM). Radiation striking the atmosphere from the outside has not yet traversed any air, so its AM coefficient is zero. On the equator the value is AM 1.0, while in central Europe it is AM 1.5. The composition of the sunlight – its spectrum – depends on the air mass, or density of the air (Figure 3.4).
Summer in the northern hemisphere
June
N March Winter in the northern hemisphere
Autumn in the southern S hemisphere
N
N December Sun
S Winter in the southern hemisphere
Autumn in the northern hemisphere
S N
Spring in the southern hemisphere
September S
Summer in the southern hemisphere
3.1 Zenith North
S S
Sun elevation S
Solar azimuth (from the north or south) South 3.2 West
Typical power/energy values of annual solar radiation
Meridian
North
Elevation angle γS Sun paths a
South c
b
Azimuth αS
East
2.25
UV Light
2.00 1.75 1.50
Direct and diffuse solar radiation AM1.5; ASTM G173 Global radiation on a surface located opposite the zenith and tilted at 37°; AM1.5; ASTM G173; 1000 W/m2 Extraterrestrial radiation; AM0; ASTM E490; 1367 W/m2 Spectral sensitivity according to DIN 5031-1
1.25
100
1.00
80 Infrared
3.1 Position of the Earth with respect to the sun in different seasons 3.2 The incident angles at which solar rays strike an object are described by the solar azimuthal angle and the solar elevation angle 3.3 Seasonal sun path at 50° northern latitude a June 21 (summer solstice) b March 21/September 21 (equinoxes) c December 21 (winter solstice) 3.4 Extraterrestrial (AM 0) and global (AM 1.5) spectrum of sunlight. The energy content of the radiation depends strongly on wavelength.
0.75
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0.50
40
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Spectral sensitivity [%]
3.3 Radiation intensity [W/(m2·nm)]
For reasons already described, the solar radiation on the Earth’s surface (global radiation) is smaller than the solar constant. Its median value is 1000 W/m2. The global radiation can be divided into three main parts: direct radiation, indirect radiation (diffuse radiation) and a smaller component that stems from reflections from the surrounding environment. In central Europe, more than 50 % of the total annual radiation consists of diffuse radiation (Figure 3.8). The effects of haze in urban and industrial regions increase this percentage further. For technical use however, the decisive factor is primarily the direct radiation percentage, though to a limited extent, diffuse light is also converted to energy. In physics, the equation relating energy and power is given by energy = power ≈ time interval. Therefore, the solar energy available per year or per month at any given location is the sum of the products of all momentary radiation power levels and their associated durations. For every location on Earth there exist typical solar energy values that vary from year to year (about ±10 % in
0 280 600
1000
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2200 2600 Wavelength [nm] 3.4
25
Technology and systems – photovoltaics
Seal
Aluminium frame
Solar cells and photovoltaic modules Glass EVA composite
Solar cells
Reverse side foil of synthetic composite 4.4 +
Efficiency records [%]
Solar glass PVB foil /cast resin Cell layer PVB foil /cast resin Backing glass
–
Solar glass Cell layer Reverse side foil composite
Solar glass Cell layer Reverse side VSG
Solar glass Cell layer Glass panel/foil composite Air space Inner insulating glass pane
Solar glass Cell layer Glass panel/foil composite Air space Inner insulating glass pane VSG
4.5
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Stacked cells (concentrator) Silicon monocrystalline Silicon polycrystalline Silicon micromorph/amorphous
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CIS, CIGS CdTe Dye cells Organic cells
Mono and polycrystalline silicon cells
Solar cells based on crystalline silicon (Si) dominate the market with a current share of 90 %. Silicon is a nontoxic material that has been long known and proven in the field 4.6
30
A single crystalline solar cell can now supply about 4 Watts with a typical cell voltage of about 0.5 Volts. To create larger units with conventional voltages as readyto-install building components, many solar cells are combined to form a PV module. In a standard module, sets of 54, 60 or 72 cells connected in a series combine to form a single or occasionally double string (Figure 4.5). A typical crystalline standard module has a power output of between 150 and 300 Watts corresponding to a surface area of 1.2 – 2 square metres – with approximate dimensions of 1.6 metres ≈ 1 metre. At 15 – 25 kg mass it can be managed by a single person. Geometrically, thesolar cells in a single module are arrayed in 4 – 6 rows, sandwiched between a glass pane in front and a composite plastic film on the back. During the manufacturing process the solar cells are embedded bilaterally into a transparent synthetic medium called ethylene vinyl acetate (EVA). This ensures that the cells are protected from weather, mechanical stresses and moisture. The facing glass is a special hardened solar glass with a low iron oxide content, and is therefore especially transparent. Most modules are equipped with an aluminium frame that protects the vulnerable glass edges and is used for mounting (Figure 4.4). The use of frameless modules is also unproblematic, provided that special care along with the appropriate mounting clamps are used in the installation process. The electrical contacts are made through sockets fixed to the rear of the modules. These are equipped with the standard connection cables and with polarised and touch-safe plug contacts. Materials other than EVA that can be used to encapsulate solar cells in a standard module include polyvinyl butyral (PVB), Teflon and cast resin. These alternatives are employed, for example, when glass is used as backing instead of foil. With the appropriate permits, such double glazed modules can serve as overhead glazing or as a component of the facade (Figure 4.12, page 33). Solar modules come in standard and special varieties. In contrast to the custom-made special modules, standard modules are produced inexpensively in bulk and are used in photovoltaic installations that place no unusual demands on the modules. They are installed with standard mounting systems on the roof or on open surfaces. There are many materials and concepts for solar cells (Figure 4.8) that differ from one another in form and colour as well as properties and performance characteristics (Figure 4.15, page 34). The following sections will present a few important types of solar cell.
Solar cells and photovoltaic modules
3.5 Wp
100– 250 Wp 1 kWp –10 MWp
60 –100 cm 15.6 cm
100 – 200 cm 1 kWp – 500 kWp Cell string
Cell
Module
String
Generator 4.7
of electronics, and is readily available in the Earth's crust. Since it does not occur in its pure form, however, it must be extracted at high temperatures from molten quartz sand. By chemical processes the raw silicon is then purified until it is nearly 100 % pure. The resulting silicon is subjected to various processes in blast furnaces to yield monocrystalline or polycrystalline silicon. In the manufacture of cells, crystalline silicon blocks, or ingots, are cut into thin wafers. These are then furnished with an additional phosphorus-doped cell layer, as well as an antireflective coating and contact points, to create the finished solar cells. • Monocrystalline silicon cells are mostly square with rounded corners (semisquare). The side length of the square cells is 12.5 or 15 centimetres. Since the cell material consists of only a single crystal, the cell surface is homogeneously dark blue to black. The electrical quality of monocrystalline solar cells is very high, reaching efficiencies of between 15 and 19 % (Figure 4.6). • Polycrystalline cells can be identified by the shimmering shades of blue in their crystal structure. They are square with a side length of 10, 12.5, 15 or 15.6 centimetres. Their efficiencies typically lie in the 14 to 17 % range. Polycrystalline silicon is easier and cheaper to produce than the monocrystalline variety, which is why Crystalline silicon solar cells
Monocrystalline
Polycrystalline
polycrystalline PV modules dominate the world market with a 50 % market share. High performance cells Manufacturers and research institutes are continually working to improve solar cells. High performance cells stand out in that their efficiencies are markedly higher than those of most solar cells. They are based on the use of very pure silicon as well as on an improved cell structure coupled with innovative contacts, for example contacts on the rear face. This raises the cell efficiencies to more than 22 %. Other manufacturers combine different technologies, such as applying an additional amorphous silicon layer to monocrystalline wafers to raise efficiencies to more than 21 %. One possibility for producing high performance cells inexpensively is the so-called PERC concept, in which polycrystalline cells are processed further by automated standard means. PERC stands for “Passivated Emitter Rear Cell”; meaning the
4.4 Cross section of a framed standard solar module 4.5 Typical layering (from outside in) of photovoltaic modules for building integration 4.6 Efficiency curves: verified records of laboratory-made miniature solar cells 4.7 Modular construction of a solar generator 4.8 Typology and characteristics of the three solar cell generations Organic solar cells
Thin film solar cells
On glass pane
Foil and strip-shaped solar cells
Special types • High-efficiency cells • Hybrid cells
• Amorphous silicon • Micromorph silicon
• Wafer technology: round to square individual slices • Slice thickness 0.2 mm, edge length 10.0 –15.6 cm • Approx. 85 % market share, mature technology • 14 – 20 % • 13 – 17 % Cell efficiency Cell efficiency
• Vacuum technology, galvanic: normally full-surface substrate layering • Layer thickness 0.5 – 5.0 μm, cell width 0.5 – 17.0 mm or strip width 1 – 36 cm • Approx. 15% market share and rising • 6 –10 % • 8 –14 % Module efficiency Module efficiency
• CdTe • CIS, CIGS
• Amorphous silicon • CIGS
Foil /glass substrate • Dye solar cells • Polymer solar cells • Oligomer solar cells
• Printing process or similar • Nanostructure • Pilot stage • 2 – 3 % efficiency
4.8
31
Technology and systems – photovoltaics
emitter and the rear face of the cell are designed with a protective layer, which reflects the light striking it back to the wafer. This allows additional energy to be utilised, making efficiencies of about 19 per cent possible. Thin film solar cells
4.9
4.10
4.9 The thin film modules form a homogeneous surface. University of Erfurt (D) 2011, AIG Gotha 4.10 Coloured solar cells are used as a stylistic element. Office building in Bordeaux (F) 2013, BDM Architectes 4.11 The flexible foil modules are fully integrated into the air-filled ETFE foil cushions. They lie flat on the mechanically pre-stressed middle layer of three layers of foil. The modules are grouped together inside the cushions by cables, which exit these through the underside. Carport, Waste Management Department, München (D) 2011, Ackermann Architekten 4.12 Laminated safety glass with polycrystalline PV cells; Research Centre AGC Glass Europe in Gosselies (B) 2013, Assar Architects
32
The substantial material and energy requirements for the manufacture of crystalline silicon cells have been reflected in high production costs. In the 2000s, the growing cost pressure led to increases in the development and production of thin film cells, for which the material and energy needs were smaller. Although it was assumed early on that the market share of thin film technologies would continue to rise, the cost savings achieved in the interim in the crystalline sector, coupled with its higher efficiencies, made a technological transition unlikely in the medium term. The thin film market share in 2015 stood at 10 %. Thin film technology is interesting from a technological and usage standpoint because of a number of characteristics. Among these are reduced sensitivity to temperature and shading, flexibility, better utilisation of the spectral bandwidth of sunlight, geometrical freedom, possible transparency levels of the material, a homogeneous appearance, integration advantages, and a tunability to to desired light spectra. These advantages, however, must be weighed against a reduction in efficiency compared to crystalline modules, as well as a more rapid degradation in efficiency over the modules’ lifetimes. The optical qualities of the thin film modules are especially noticeable. Their individual cells are narrow strips, which, in contrast to the typical grid pattern of crystalline modules, look homogeneous from afar (Figure 4.9). Because of this, thin film modules on roofs are often less conspicuous and easier to integrate into the architecture of the building. Nowadays however, crystalline modules with dark frames and foil backing, and a similar visual appearance, are also available on the market. Amorphous silicon (a-Si) and micromorph (μ-Si) silicon cells Amorphous silicon is the star of thin film technology. Small amorphous modules are incorporated in their millions into calculators, watches, torches, etc. A disadvantage of amorphous cells is their low efficiency of about 6 %. The development of stacked cells has led to a higher efficiency of up to 7 %, and in the case of micromorph cells up to 12 %. In stacked cells, cell layers (two in tandem cells, three in triple cells) with differing spectral sensitivity are stacked to increase efficiency. Amorphous silicon cells have good temperature behaviour in that their efficiency drops only slightly when they are hot. Modules with this type of cell are therefore especially suited for integration into buildings where there is limited rear ventilation. The very thin cell material facilitates the production of flexible modules (Figure 4.11). In such cases, the facing glass is left off, and instead the cell material, suspended in a fluoropolymer and EVA bond, is deposited onto a flexible metal foil. In the past this type of module was attached directly to roof
Solar cells and photovoltaic modules
membranes, for example, making it usable even on roofs that were structurally unsuited to standard modules, such as lightweight flat roofs. Micromorph solar cells are a combination of microcrystalline and amorphous silicon in a tandem configuration. Compared to amorphous cells, micromorph cells achieve a markedly higher efficiency. The efficiency decline that occurs in many thin film modules within the first 1000 hours of operation, known as the initial degradation, is much smaller in micromorph cells. Visually, the two technologies are barely distinguishable. Many manufacturers have made attempts to gain entry to this technology field. However, since the expected efficiency increases and production cost decreases in comparison to the rival crystalline technology have so far failed to materialise, many companies have been forced to curtail their production. Copper indium diselenide cells (CIS) Among the thin film technologies, CIS technology currently achieves the highest efficiencies at 14 %. The manufacturing process, however, is complex, and indium in particular is an expensive material. Their dark grey to black colour makes the cells optically appealing. CIS modules have a lower temperature dependence than crystalline, and lose about a quarter less efficiency (about 0.1 % per °C) when heated. In well-ventilated installations this corresponds to annual energy losses of about 0.6 %, and in less favourable applications (facades or roof integrations without rear ventilation, for instance), of 1.5 %, compared to the standard test conditions (see electrical characteristics of PV modules, page 35). The higher energy yield for red light means that more energy is produced during sunrise and sunset, which represents a slight advantage in east or west-facing installations. To improve conductivity, in some cases some of the indium is replaced with gallium (CIGS cells). Cadmium Telluride Cells (CdTe) The 13 % efficiency of the glossy dark green to black cadmium telluride (CdTe) solar cells also exceeds that of the amorphous cells. Of all the thin film technologies, the manufacture of CdTe modules once yielded the greatest cost savings. Like CIS modules, CdTe modules have a smaller dependence on temperature so that, compared to crystalline modules, they lose about a third less efficiency when heated. Since they yield more energy in the blue range of the spectrum, CdTe cells are especially useful for energy production when the sky is overcast. For more red-shifted light as at sunrise and sunset, on the other hand, the cell yields are relatively suppressed. The use of the heavy metal cadmium is somewhat controversial. Since cadmium is a waste product generated in the extraction of zinc, its further processing into the nontoxic compound CdTe can be viewed as ecologically unobjectionable. But in the case of fire, it is possible for toxic cadmium to be released into the smoke due to the high temperatures. Studies done by the Bayerisches Landesamt für Umwelt (Bavarian Department of the
Environment) found, however, that at a distance of about 100 metres from a fire the threshold value for posing a health hazard is not reached. At the end of their operational life, CdTe modules must be treated as hazardous waste and, like the products of other module technologies, should be recycled. Organic solar cells
A novel, organic type of solar cell, developed in 1991 in large part by Swiss chemist Michael Grätzel, could become the least expensive alternative to silicon-based technology in the future. While the energy conversion in solar cells to date depends on the semi-conducting p-n junction of silicon, thereby making a cell behave like an illuminated diode, an organic solar cell absorbs light via an organic dye and extracts energy from sunlight using photosynthesis, as a plant does with chlorophyll. In 2013, a maximum efficiency of 12 % was achieved with a very small 1.1 square-centimetre plastic solar cell. These organic tandem solar cells are manufactured at low temperatures of approximately 120 °C by vacuum thermal evaporation of carbon molecules, and deposited in a roll-to-roll process onto a transparent 30 centimetre-wide polymer foil substrate. The main advantage is the light weight of the plastic modules. Modules with an efficiency of 7 % are currently being introduced to the market. The main problem with organic solar cells has thus far been their long-term stability: in practical experiments their
4.11
4.12
33
Technology and systems – photovoltaics
Aluminium frame Glass cover Photovoltaic cell Absorber (metal sheet) Collector tube Rear insulation
Electricity Heat
4.13
Hybrid modules, or combined systems for PV and solar thermal energy
Power [W/m2]
In hybrid modules, solar thermal collectors are combined with PV modules (Figure 4.13). Structurally, the solar cells are located on the surface of a liquid or air-cooled absorber, with which they are thermally coupled. Since conventional PV modules convert about 16% of solar radiation into electrical energy while much of the remainder is converted to heat, hybrid collectors can certainly prove practical. The thermal part of the collector acts largely like a normal flat plate collector without selective layering. The behaviour of the PV part, on the other hand, depends strongly on the application. If hybrid collectors are operated at a uniform low flow temperature, as for example in swimming pools or heat pumps, this can result in cooling of the PV part and consequently to increases in electrical yield. Conversely, using them for
service water heating without a heat pump can have the opposite effect: the cells would be subjected to further heating and their electrical yield could drop significantly. In most cases it is advisable to separate the solar thermal and photovoltaic energy conversion processes whenever there is enough space to do so. So-called combined systems with standard dimensions are often based on proven framing systems for roof windows. As a consequence, window manufacturers offer roof windows, PV modules and thermal collectors all based on the same grid dimensions and the same window frames (Figure 4.14). In addition, for new buildings or complete roof renovations, combination systems often come factory-integrated into the prefabricated roofs. Electrical properties of PV modules
The efficiency of solar modules plays an essential role in energy efficiency. It determines the maximum electrical power that a given cell or module surface can generate from sunlight. Since the solar radiation intensity fluctuates with the weather, an irradiance of 1000 W/m2 is defined as the reference value for the determination of the efficiency.
100
50 %
180
45 % 43%
160
✺ 155 36% ✺ 134
140 ✺ 125
120
30 %
100
25 % ✺ 84
22%
80
40
35 %
✺ 105
20%
60 ✺ 40
40 %
Transparency
efficiency dropped significantly after just a few years. It remains to be seen whether the advertised lifetime of 20 years will truly be achieved, making organic solar cells suited for use in permanent building applications. According to the manufacturers’ claims, the modules have passed relevant long-term simulation tests.
4.14
15 %
15% 13%
10 %
✺ 35
10 %
20 %
20
5%
0
0 a-Si thin film
Polycrystalline
Monocrystalline Highly efficient
10 % Transparency
5 mm Spacing
3 mm Spacing
a-Si thin film 20% Transparency
Monocrystalline Semitransparent
Monocrystalline Highly efficient
Polycrystalline
5 mm Spacing
25 mm Spacing
50 mm Spacing 4.15
34
Module current and voltage Radiation intensity directly affects the current in the module. If the light intensity is halved, the current delivered by the module is also halved. The module voltage is mostly influenced by the module temperature. It increases with low temperatures and can therefore exceed its nominal value by up to 20 % in winter. Conversely, the voltage drops as the temperature rises, and the power generated by the module drops as a consequence. The behaviour of the electrical parameters is specified for specific temperatures. In crystalline modules the decline in the nominal power amounts to 0.45 % per degree of temperature elevation. On a sunny summer day the operating temperature of roof modules can easily exceed 50 °C. Nevertheless, because of the longer exposure and higher solar irradiance values, the yield of solar modules is almost 80 % higher in summer than in winter. Good rear ventilation of the solar generator supplies cooling and ensures good electrical production.
E = 1000 W/m2
4
E = 800 W/m2
3.5 3
E = 600 W/m2
2.5 E = 400 W/m2
2 1.5
E = 200 W/m2
1 0.5 0 0
5
10
15
20
25
30
35
40
45
50
VMPP range Module voltage V [V]
a
Module current [A]
Solar panel characteristic curve The characteristic curve (also known as the current-voltage or I-V curve) of a solar module illustrates the interaction of the parameters (Figure 4.17), and shows all the operating points that can occur under standard test conditions under given loads. The MPP, or Maximum Power Point, which corresponds to the point on the curve for which the product of current and voltage is maximised, can be readily identified. This maximum power (PMPP) and its associated voltage VMPP and current IMPP are stated on the rating label of a module, in addition to two additional characteristic values: the short circuit current IS and the open circuit voltage VO. Unlike other technical devices, PV arrays only rarely operate in their nominal ranges, since standard test conditions are rarely obtained in reality. Current, voltage and power change continually throughout the day, depending on temperature and irradiance.
5 4.5
6 5 4
ϑ = 75°C ϑ = 50°C
3
ϑ = 25°C ϑ = 0°C
2
ϑ = –25°C
1 0 0
10
20
30
40
50
60
VMPP range Module voltage V [V] 4.16
b
Module current [A]
Power In addition to radiation intensity, the solar spectrum and the cell temperature are both critical factors influencing the output power of a cell (Figure 4.16). The Standard Test Conditions (STC) used in determining the electrical performance values in photovoltaics therefore also include a reference cell/module temperature of 25 °C and a solar spectrum defined for a solar elevation of 41.8° and AM 1.5. The efficiency of a module is always a little smaller than that of the cells, since the facing glass does not transmit all the sunlight and the cell coverage of the module area is not 100 %. The efficiency of PV modules is given by the ratio of the PV output power to the solar power delivered to the module surface. Since the combined surface area of all the cells in a module is smaller than that of the module itself, the module efficiency is smaller than the cell efficiency. And since the corners of monocrystalline cells are usually rounded, modules with polycrystalline cells often have module efficiencies on par with those of monocrystalline modules.
Module current [A]
Solar cells and photovoltaic modules
8 7 6 5 4 3 2 1 0 0
30
Monocrystalline Si Polycrystalline Si
60 CIS
90 CdTe
120 a-Si
140
160
μ-Si Module voltage V [V] 4.17
4.13 Cross section of a hybrid module 4.14 Combined system with PV modules, solar thermal collectors and roof windows; Tract house in Leverkusen (D) 2013, Caroline Wachsmann 4.15 Relationship between transparency and efficiency for various typical cell types 4.16 Irradiation and temperature dependence of PV modules a) Module current as a function of voltage and irradiance b) Module current as a function of voltage and module temperature 4.17 Comparison of characteristic curves for different module types
35
Economy and ecology
Roof covering (e.g. concrete roof tile, tile, metal roofing, fibre cement, slate) on existing battens PV with standard modules, thin film η = 12% PV with standard modules, crystalline η = 15% PV with custom modules Cold facade, full-surface rear-ventilated curtain wall With PV near-standard modules, thin film η = 12% With PV near-standard modules, crystalline η = 15% With PV custom modules Warm facade (post-and-beam construction), double glazed With semitransparent PV, insulating glass construction Warm facade (element facade), double glazed With semitransparent PV, insulating glass construction 0
250
500
750
1,000
1,250 1,500 net costs [€/m2] 9.2
calculated operating income statement, the incomes from the sale of electricity are set against the expenditures for the depreciation of the purchase price (usually 5 % per year over 20 years) and the operating costs. Gains are added to the other revenue sources of the taxpayer and the total is subject to the customary income tax. Losses, on the other hand, reduce the taxable income. In some income situations special depreciations can be used in the first few years of operation to reduce taxes. Usually, expenses for storage batteries do not qualify for a tax credit. If there is no interest in tax write-offs, however, the operation of a PV array can be declared as a not-forprofit hobby by basing the value of the self-consumed electricity not on the equivalent grid subscription price, but on the EEG compensation rate. In this case the 20-year operational prognosis usually foresees no surplus, and the PV array no longer needs to be taken into account for income tax purposes.
per square metre of collector area, building clients receive additional bonuses for simultaneously installing a biomass or heat pump heating system, for replacing a boiler or optimising the heating system, and for connecting their systems to a heat network. In new construction, thermal solar installations are state-of-the-art, and only especially innovative systems are eligible for subsidies: systems that provide more than 50 % solar coverage, for example, or those for apartment buildings, for non-residential buildings, for heat networks or for solar cooling.
Profitability of PV installations Thanks to the cost-reducing pressure exerted by the EEG compensation, the investment costs for standard PV arrays have similarly fallen. Costs
Subsidies for thermal solar installations
150
20
Power
Surface area requirement
kW
p
h/
0 02
15
kW
1
10
100 Yield
Wp
50
500
5
h/k
kW
0
0 0
0.20
0.15
0.10
0.05
Efficiency per unit area [kWp /m²]
200
Surface area requirement [m²/kWp]
Surface area requirement [m²/kWp]
The ministry of economic affairs is using a market incentive programme to encourage the expansion of renewable energies in the thermal sector. The main focal points for the subsidies are the existing building inventory and commercial/industrial process heat. In addition to current investment grants of €50 to €200
0
15% 10% 20% Module efficiency Crystalline standard module Crystalline custom module Thin film Thin film
5%
Standard module Custom module Organic photovoltaics 9.3
94
Technological advances and the effects of scale of sharply increased production quantities caused end user prices to drop an average of 13 % per year in the decade between 2006 and 2016. The largest of these cost reductions occurred for PV modules, which still make up the largest proportion of the cost of installations at about 48 %. Specific cost values are available only for standard arrays installed outside of the building shell on pitched or flat roofs, and these values are in reference to electrical power. They show that large arrays are more costeffective per peak kilowatt-hour than small ones. In early 2016, the average cost of roof-mounted systems of up to 100 kWp lay at about €1,300kWp. The per-square-metre prices could be very different, as they depend on the efficiency of the modules. 9.2 Price ranges of building-integrated photovoltaic systems in comparison to passive building shells: total costs include electrical systems technology and the facade system; for roof-integrations the roof structure and roof lathing are excluded. 9.3 Relationship between power in kWp and area in m2 for photovoltaic arrays 9.4 Yield estimates for building-integrated PV arrays: 1. Determination of the horizontal global radiation at the building site 2. Multiplication with the yield factor 0.9 m2/kWp gives the specific yield of typical roof installations = 100 % 3. Conversion factors (in %) for specific installation situation 4. Subtraction of shading losses, where applicable
Profitability of PV installations
Yield estimates for building-integrated PV arrays Calculating the array power output Step 1: Horizontal global radiation
Step 3: Conversion for specific installation situation (orientation, inclination and system losses) Rear-ventilated integration
Step 2: Specific yield for roof-top array 45°
a
77
90°
55 c South-facing Inclined at 30°
0° 85
85 82 78 92 85 56 East 81 94 91 95
30°
100 % b
15°
Non-rear-ventilated integration
93
94
100
93
99
67
69
90°
68
53
West
855–1135 kWh/kWp /yr
Regional averages
Regional averages
Regional averages
a) 985 kWh/m2/yr b) 1050 kWh/m2/yr c) 1030 kWh/m2/yr
a) 890 kWh/kWp /yr b) 945 kWh/kWp /yr c) 1020 kWh/kWp /yr
a) 470 – 890 kWh/kWp /yr b) 500 – 945 kWh/kWp /yr c) 540 – 1020 kWh/kWp /yr
0° 82
90
95
65
67
South
66
South
with: 15 % efficiency 70 – 130 kWh/m2/yr 75 – 140 kWh/m2/yr 80 – 153 kWh/m2/yr
Step 4: Reductions due to shading losses
Cause
Ambient shading
at greater Surroundings: Nearby buildings distance, only partial shade and vegetation
Effect
Analysis dh
the building or PV array itself: Roof overhangs, projections, sun protection elements, module attachments
Serious consequences, since nearby objects usually cast hard shadows
Temporary Snow, leaves and dirt
Non-critical, since usually short-term or slight and evenly distributed
10 % efficiency
5 % efficiency
47 – 89 kWh/m2/yr 50 – 94 kWh/m2/yr 53 – 102 kWh/m2/yr
22 – 45 kWh/m2/yr 25 – 47 kWh/m2/yr 27 – 51 kWh/m2/yr
Recommendation
γh
Δh
Temporary shading
30°
West
950 – 1260 kWh/m2/yr
Self-shading
15° 82
82 80 76 89 54 East 79 91 88 91 75 91 91 96
45°
γv dv Δt
Losses in yield
< 5% γh < 12° ∫ dh > 5 ≈ Δh γh < 16° ∫ dh > 3,5 ≈ Δh < 10 % Insignificant γv > 63° ∫ dv > 2 ≈ Δt Shaded areas (γh > 12° in easterly – southerly – westerly direction) should not be covered with PV modules or cells If needed, choose electrically inactive dummy modules, coloured / printed glass or other space fillers
Customised electrical wiring can reduce losses if needed
As a rule, cleaning is not cost-effective; check regularly for localised dirt / contamination
Few percentage points; Given in steps 3 and 4 9.4
Compared to the typical roof-top array, building-integrated PV systems are associated with higher costs. Even though inexpensive, mass-produced standard modules can often be used in the plane of the roof, a high-quality design and a meticulous on-site implementation of the connection details result in additional expenses. The stylistic aspects and structural demands of facade integration, on the other hand, often require custom-made modules. This can easily raise the module prices by a factor of 1.5 – 3 as compared to standard modules. Up until now, the mostly customised solutions and the dynamic cost development have not allowed for generally applicable pricing characteristics. The design complexity of facade integration precipitates not only the higher module costs but also a rise in the proportional budgeting of secondary construction costs, which may include such items as expenditures for planning and permits. As a consequence, building-integrated PV arrays can be seen as economical only in light of their multifunctional aspects, for example when they replace the customary roof coverings or facade cladding, or if they eliminate the need for additional sun protective measures. Even in comparison to the building components they replace, the investment costs for PV modules and the associated electrical sys-
tem technology are usually greater (Figure 9.2). The initial additional outlays relativise themselves over the lifetime of the array, however, because the production of solar electricity generates revenue. Yields
In Germany, ideally oriented, well-ventilated and essentially unshaded PV roofs generate an average of 890 – 1,020 kWh of solar electricity per peak kilowatt per year; for south-facing PV facades the range is about 600 – 700 kWh. For crystalline modules with a high cell surface density and an efficiency of 15 %, this corresponds to approximately 130 – 150 and 90 – 105 kWh/m2/year, respectively. Modules with different cell types or with customised variations in colour or partial transparency will exhibit lower specific yields proportional to their efficiencies (Figue 9.3). Given the location, surface orientation and integration situation, the average expected yield over the longer term can be estimated using regional global radiation and yield values (Figure 9.4) as well as proportional conversion factors. In building integrations where the array lacks rear-ventilation or is insulated – as in insulating glass or sandwich panels, for example – the energy yield will be reduced, since these modules are 95
Built examples
Office building Kemptthal, CH 2007 Building client: Marché Restaurants Schweiz, Kemptthal Architect: kämpfen für architektur, Zurich Energy planner: Naef Energietechnik, Zurich Photovoltaics: Beat Kämpfen, René Naef (Design) SunTechnics Fabrisolar (Implementation) The Marché International Support Office is the first office building in Switzerland with a zero energy balance. Its compact, unpretentious volume was designed according to the principles of passive solar architecture. A distinctive mixture of customary timber construction, innovative PCM technology and photovoltaics characterises both support structure and building shell. Insulation thicknesses based on passive house standards minimise the heat losses of the building, whose structural framework is built entirely (with the exception of the two concrete stairwells) of pre-fabricated solid wood panels. Half of the predominantly glazed south facade is comprised of multi-layered special glass, which functions as translucent insulation, protection against overheating and, thanks to an incorporated phase change material (PCM) of salt hydrate, thermal storage. The facade construction provides a thermally and visually pleasing interior and offers excellent workplace ambience. The entire surface of the skillion roof is dedicated to electricity generation and delivers 100 % of the required electrical energy. The installed array consists of anthracite-coloured, standard glass-glass thin film modules. The architects succeed in creating an unobtrusive but exceedingly elegant detailing of the roof and its edges. The small-structured scalelike overlaps in the modules result in a well-balanced, aesthetically convincing structured roof surface that sets a new standard. 112
aa
a
a
Office building in Kemptthal
1
3
4 2
5 Site plan, scale 1:2,500 Cross section · Ground floor plan scale 1:400 Vertical section of roof / facade scale 1:20 1 thin film photovoltaic modules 2 wall construction: 25/90 mm Douglas fir formwork 30 mm rear ventilation / battens black underlay 15 mm HDF panel 80/40 mm battens alternating with 80 mm glass wool insulation 225 mm ribs alternating with 225 mm glass wool insulation
3
4 5
6
7
2 ≈ 35 mm triple-ply panel floor construction: underlay 280 mm cellulose insulation 30 mm triple-ply panel 160/40 mm ribs alternating with 160 mm glass wool insulation 30 mm triple-ply panel vertical awning Triple glazing in timber frame, Ug = 0.5 W/m2K 40/120 mm larch handrails on 80/40/8 mm steel L profile 40/40 mm larch grate on 240/80 mm squared timber
6
7
Solar installation technical data Type of integration
Integrated into skillion roof, rear-ventilated
Supporting structure
Timber battens and counter battens
Installed power
44.6 kWp
Installation size
485 m2
Exposure
South, 12°
Anticipated energy yield
40,000 kWh/yr
Modules
First Solar frameless thin film glass-glass module
Number
649
Dimensions
120 ≈ 60 cm
113
Built examples
Daycare centre Marburg, D 2014 Building client: City of Marburg Architect: opus Architekten, Darmstadt Energy concept: ee concept, Darmstadt
The City of Marburg has constructed a day care centre on the Vitosareal, located in the south of the city. The centre’s location in the park, its connection to the historic buildings in the vicinity and the gentle eastward rise of the local topography were all important design parameters. Furthermore, the building was to be the first day care centre in Germany to meet the “Efficiency House Plus” energy standard. The result is a building that is distinctive on the outside as well as in its interior design. While the ground floor
is built in the massive construction style, the upper storey and roof are light timber frame constructions. Both the roof and the facade of the “folded” building are optimised with regard to natural lighting and to activation of the exterior surfaces. The active solar surfaces face southward, while the glazing on the northern side is primarily designed to admit daylight. Six bands of photovoltaic modules, oriented to the south with an inclination of 17°, determine the characteristic sawtooth-like roof construc-
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tion. This structure continues vertically on the southwest facade, whose opaque surfaces are likewise fully utilised for generating electricity. The wide-aspect, rearventilated glass-glass modules are arranged in a taut grid pattern. The monocrystalline cells and dyed metallic brazing tapes cause the inner structure to recede in favour of a homogeneous surface effect, and the resulting impression from a distance is one of a perfectly detailed, part black and part transparent glass facade.
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Daycare centre in Marburg
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Site plan, scale 1:2,000 Cross section ¡ Upper level floor plan scale 1:400 Vertical section of roof / facade scale 1:20 Horizontal section of facade scale 1:20 1 solar roof construction: black monocrystalline photovoltaic modules (VSG) 80/80 mm battens plastic sheet seal 21 mm formwork 360 mm spars /cellulose insulation vapour barrier 18 mm OSB panel 28/60 mm timber frame covered with acoustic felt fleece padding 35/20 mm pine battens
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plastic sheet seal wall construction: black monocrystalline photovoltaic modules (VSG) vertical and horizontal aluminium support structure plastic sheet seal (PE) 15 mm OSB panel 320 mm timber studs / mineral fibre insulation vapour barrier 18 mm OSB panel 38 mm block plywood fleece padding 35/20 mm pine battens ventilation flaps for night-time cooling aluminium louvres
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Solar installation technical data Type of integration
Roof and facade, integrated
Supporting structure
Aluminium / timber supporting structure
Installed power
52.32 kWp
Installation size
304 m2 (Roof) 81 m2 (Facade)
Exposure
South (roof) Southwest (facade)
Anticipated energy yield
38,500 kWh/yr
Modules
ertex solar (monocrystalline)
Number
354
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Built examples
Convention centre Lausanne, CH 2014 Building client: École Polytechnique Fédérale de Lausanne (EPFL) Architect: Richter Dahl Rocha & Associés, Lausanne Photovoltaics: Richter Dahl Rocha & Associés with Catherine Bolle (design) Solaronix, Aubonne (implementation)
The newly built SwissTech Convention Centre is part of an expansion of the EPFL campus. The building features a roof with crystallike planes, covered with aluminium shingles and projecting up to 40 metres beyond the building face. The interior encompasses an open foyer with building-height glass facades, seminar rooms and a multi-functional conference hall for up to 3,000 people. While large-scale glass facades often appear as monotonous, smoothly reflective surfaces, the architects in this case opted for a different approach: the west facade of the building boasts the largest solar array of dye-sensitised solar cells to date. The cells, also known as “Grätzel cells”, were developed in the early 1990s and patented in 1992 by the chemist and EPFL professor Michael Grätzel. A total of 300 square metres of glass-glass modules, with solar cells in yellow, green and red hues, are installed on the outside of the glass facade. The full-height glass louvres consist of individual aluminium-framed panels, each of which encompasses four 50 ≈ 35 centimetre cells. The louvres are affixed at slightly varying angles and at varying distances from the plane of the facade. The additive PV facade not only functions as a solar shade, but produces appealing lighting effects in the foyer. In combination with a light facade construction, the cell technology opens up new design possibilities, particularly for glass facades. 126
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Convention centre in Lausanne
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Site plan, scale 1:10,000 Cross section · Ground floor plan scale 1:1,500 Horizontal and vertical sections of facade, scale 1:20 1 steel facade support 2 14 mm insulated glazing + 17 mm SZR + 8 mm insulated glazing, held laterally by pressure plates
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anodised aluminium cover 50/50/5 mm steel tubing glass-glass solar panels framed in anodised aluminium, each 2,100 ≈ 410 mm panel consisting of 4,350 ≈ 500 mm modules with 13 strip-like 2-cm-wide Grätzel cells
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Solar installation technical data Type of integration
Additive facade
Supporting structure
Additional steel construction on post-andbeam facade
Installed power
3 kWp
Installation size
280 m2
Exposure
Southwest
Anticipated energy yield
2,000 kWh/yr
Modules
Solaronix electrochemical thin film (Grätzel dye) solar cells, Glass-glass modules framed in aluminium
Number
1400
Dimensions
35 ≈ 50 cm
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