Environmental Skin by Christopher Strahle

AN ENVIRONMENTAL SKINS Enhancing Thermal Performance with Double-Skin Facades for Hawaii’s Climate.

Christopher G. Strahle Research Document Presentation Committee Members Chair: David Rockwood, PhD Stephen Meder, Arch.D, LEED Kris Palagi, AIA, M.Arch, Manfred Zapka, PhD, PE, LEED-AP, CEM School of Architecture University of Hawai’i at Mānoa

ABSTRACT Highly glazed commercial buildings in Hawaiì present overheating challenges due to high outside temperatures combined with solar gains. In order to optimize thermal performance and reduce excessive cooling loads, the thermal behavior of this type of building requires careful investigation. As an increasing interest in double-skin facades as a successful methodology for controlling building performance continues to be explored in Europe , its feasibility within Hawaiì’s climate has yet to be discovered. In this study, double-skin façade design strategies are examined in Hawaiì’s climate focusing on enhancing thermal performance on an existing building model. This research adopts a CFD simulation approach to model heat and air flow transfers in various double-skin façade design scenarios. The impact of solar radiation, surface temperature, cavity height and air flow rate on temperature and velocity fields inside the channel of the double-skin facade is analyzed. This research focuses on the investigation of context based design for double-skin facades, particularly focusing on design considerations during the design process. In conclusion, this investigation will help to identify the potential of this specific system within Hawaiì’s climate and its ability to improve thermal performance within existing buildings.

CONTENTS

I. INTRODUCTION

THE PROJECT Investigation of context based design for double skin facades, particularly focusing on climatic considerations during the design process.

The Problem: Hawaiiâ&#x20AC;&#x2122;s climate present overheating challenges due to high outside temperature, solar gains and internal heat gains which lead to an excess in cooling loads. A Solution: Can double skin facades act as a ventilated thermal buffer in an attempt to improve thermal performance?

GOALS In this study, double skin faรงade design strategies are investigated to enhance building envelope performance by modeling energy performance of different design scenarios on the existing building model. Finally a discussion and conclusion section follows in which the point of view of the author is given and comments are made

Intelligent Buildings:

What makes a building Intelligent?

Intelligent features:

What are the genetic characteristics?

Thermal Performance:

What is thermal comfort, current standards?

Parameters for Design:

What are the parameters of thermal comfort?

The Double Skin Facade:

How can this system effect performance?

Typology:

Faรงade characteristics?

High-Rise Development:

How did it get so bad?

Characteristics:

Common characteristics of high-rise stock?

Retrofitting:

How do we retrofit these buildings?

II. PROJECT RESEARCH

A NEW ERA OF INNOVATION

What if you could design a building that had the ability to self-adjust itself in order to optimize the environmental conditions within?

Intelligent Buildings:

What makes a building Intelligent?

“Intelligent design means striving to have our buildings in harmony (and integrate) with nature, to protect its qualities, and to recognize its dynamic (and unpredictable) qualities, whether assets or liabilities.” “Adaptation is essential for survival and success: This is true for our buildings as it is for all other aspects of life.”

WHAT IS AN INTELLIGENT BUILDING? An intelligent building is one that maximizes the efficiency of the occupants while at the same time minimizing the cost associated with running the building.” - David Boyd An intelligent building is a house with no style. It is possible that such a building would be positively unsightly, or that there might not even be anything to see.” - Ole Bouman, Archis Building which have fully automated building service control systems.” - Cardin “Buildings where the fabric is used to serve as ‘half of the buildings service’.” - Building Services Intelligence to a building owner is a good business decision.” - Alan Abramson A building becomes intelligent as soon as it is fully rented.” - New York Developer A building that responds to its function and environment through technology.” - Rab Bennetts An Intelligent Building is one that creates an environment that maximizes the efficiency of the occupants of the building while at the same time allows effective management of resources with maximum lifetime cost.” - Robathan

INTELLIGENT ENVIRONMENTAL SKIN

“one which integrates various systems to effectively manage resources in a coordinated mode to maximize: occupant and building performance; investment and operating cost savings; and, flexibility.”

INTELLIGENT

ENVIRONMENTAL

SKIN

[in-tel-i-juhnt] Adjective Ability to vary its state or action in response to varying situations and varying requirements.

[en.viren’men(t)l] Adjective Relating to the natural world and the impact of its conditions.

[skin] Noun The thin layer of tissue forming the natural outer covering of the body of a person or animal.

THERMAL PERFORMANCE & THE FACADE

To be able to evaluate thermal comfort, target criteria for the relevant thermal performance for the design must be established, both design and analysis must be implemented to present a successful solution.

Thermal Performance:

What is thermal comfort, current standards?

Environmental Conditioning:

Methods of space conditioning?

Parameters for Design:

What are the parameters of thermal comfort?

FAร ADE INTERACTION

The faรงade of a building forms the interface between the environment outside and the user inside. The objective in the design of the faรงade is to find the optimum compromise between the internal and external environment and the requirements of the planned building use.

INTELLIGENT FEATURES

These intelligent features have been found to represent the components within built examples of intelligent buildings and what is being called the â&#x20AC;&#x153;genetic characteristicsâ&#x20AC;? which establish the makeup of the intelligent skins.

Intelligent features:

What are the genetic characteristics?

Integration :

Methods of integrating these features?

THE DOUBLE SKIN FACADE

The consideration that building envelopes can respond and interact with environmental conditions, the evolution of the double skinned façade had been the forefront of the design of ‘Intelligent Environmental Skins’. “The double skin façade system involves the addition of a second glazed envelope that has the ability to maximize opportunities for thermal control and improving a buildings energy performance.”

Building Performance:

How can this system effect performance?

Typology:

Façade characteristics?

DEFINITION OF DOUBLE SKIN FACADE “Essentially a pair of glass “skins” separated by an air corridor. The main layer of glass is usually insulating. The air space between the layers of glass acts as insulation against temperature extremes, winds, and sound. Sun-shading devices are often located between the two skins. All elements can be arranged differently into numbers of permutations and combinations of both solid and diaphanous membranes”. - Harrison and Boake “A façade that consists of two distinct planar elements that allows interior or exterior air to move through the system. This is sometimes referred to as a twin skin.” - Arons “A pair of glass skins separated by an air corridor (also called cavity or intermediate space) ranging in width from 20 cm to several meters. The glass skins may stretch over an entire structure or a portion of it. The main layer of glass, usually insulating, serves as part of a conventional structural wall or a curtain wall, while the additional layer, usually single glazing, is placed either in front of or behind the main glazing. The layers make the air space between them work to the building’s advantage primarily as insulation against temperature extremes and sound.” - Uuttu “A second skin façade is an additional building envelope installed over the existing façade. This additional façade is mainly transparent. The new space between the second skin and the original façade is a buffer zone that serves to insulate the building. - Claessens and DeHerde “an envelope construction, which consists of two transparent surfaces separated by a cavity, which is used as an air channel. This definition includes three main elements: (1) the envelope construction, (2) the transparency of the bounding surfaces and (3) the cavity airflow.” - Saelens

CLASSIFICATION

Comparative matrix based on faรงade classifications by Battle McCarthy and the specific ventilation, solar control, and construction strategies; identifying the different classifications of the double-skin faรงade and the different cases that can be considered.

THERMAL INSULATION

Energy Balance - Energy flow paths

- Heat transmission

- Material properties

- Efficiency of shading

Compared to single-layered facades, the double skin construction can achieve a comfortable degree of thermal insulation; but when the two layers are designed with poor performance fully glazed skins, the cooling loads will increase in proportion to the area of glazing.

AIRFLOW The thermal behavior of double-skin facades is significantly affected by the airflow within the system. The flow of air towards, around and within the assembly greatly affects the aerodynamics of the overall performance system.

Fan Assist • Pressure differences caused by mechanical systems Driving Force • Pressure differences caused by wind Stack Effect • Pressure differences caused by thermal buoyancy

• Type of double skin façade • Geometry of the façade • Layer composition • Ventilation strategy

CASE STUDIES

GWS HEADQUARETERS Berlin, Germany Client: Architect:

Gemeinnutzige Siedlungs Wohnungsbaugesellschaft Sauerbruch & Hutton Facility: Office Tower

SUVA INSURANCE BUILDING Basel, Germany Client:

Facility:

Schweizerische UnfallVersicherungs-Ansalt Architect: Herzog & de Meuron Office Building

OCCIDENTAL CHEMICAL Niagara, New York Client: Architect: Facility:

Hooker Chemicals Cannon Design Inc. Research

The purpose of the case study section is to provide references for built examples and to identify the various intelligent features implemented within current building envelopes. In relation to the research project, emphasis has been placed on the methods of active responsive control of the buildingâ&#x20AC;&#x2122;s facade as it relates to thermal comfort regulation.

BUILDING STOCK

High-Rise Development:

How did it get so bad?

Characteristics:

Common characteristics of high-rise stock?

Retrofitting:

How do we retrofit these buildings?

ENERGY CONSUMPTION

In Hawaii it is increasingly observed that existing commercial high-rise buildings with highly glazed facades present overheating challenges due to high outside temperatures and solar gains. The increase need for air conditioning adopted to provide comfortable temperatures within work hours corresponds with the peak temperatures in the day.

BUILDING CHARACTERISTICS Height There is no absolute definition of what constitutes a “tall building”. It is a building that exhibits some elements of “tallness” in various categories. It’s not just about height but about the context in which it exists.

Lease Span The lease span of a building is the clear distance from the service core to the external envelope. It is dependent on the functional requirements and size of the floor plate and is an important consideration for space planning.

Window Area & Type A buildings window system provides visual connections between the exterior and interior of the building but also carry important issues of managing heat gain and loss, as well as controlling natural daylighting from entering the building. As the heat gains through opaque walls are low due to the current high standard of thermal insulation required, it is the window-to-wall ratio and the combination of glazing types and sun shading system that effect the magnitude of the solar gains.

BUILDING RETROFITS

As with every construction, office buildings are subject to physical and functional decline. Regular building maintenance can slow down this process, but after a certain time larger interventions become inevitable. Types of Strategies:

Methodology:

• The Stabilization • The Substitution • The Double Skin Façade

• • • •

Building Performance Evaluation – Determine building for retrofit solution Energy Audit – Comprehensive and detailed energy use study Design Solution – Design the most appropriate building retrofit solution Implementation – Implement and validate the designed building solution

III. PROJECT INVESTIGATION

BUILDING & ENVIRONMENT

Due to the fact that the fundamental role of buildings is to protect its occupants from external climactic conditions, developing an appropriate building envelope is an important part of enhancing a building overall thermal performance. Focus is placed on the role of the faรงade as its plays an important part through which these conditions can be controlled.

First Hawaiian Center:

Building brief?

Climate as Context:

What are the existing environmental issues?

FIRST HAWAIIAN CENTER

The First Hawaiian Center, located at 999 Bishop Street in downtown Honolulu, is the tallest building in Hawaii and the corporate headquarters of First Hawaiian Bank. . Designed by the architectural firm Kohn Pederson Fox Associates (KPF), the tower is composed of two distinct forms, one which faces the sea and the other which faces the mountains.

CLIMATE AS CONTEXT If a building is designed and constructed to accommodate the local climate (i.e. by using appropriate building components and operation strategies), then the achievement of occupant comfort and efficient operation in the building will be greatly increased.

SOLAR EXPOSURE

With an understanding of the basics, a detailed solar path analysis of solar rays that will impact the First Hawaiian Center helps to inform schematic design decisions. The amount and intensity of solar rays that hit the faรงade of the building throughout the year play a major role in determining the amount of solar access and exposure the building will endure.

SOLAR RADIATION

The following simulations have been performed to determine the cumulative yearly value of solar radiation that strikes the buildings envelope of a given orientation, in this case, north, east, south, west. These radiation simulations help to determine which part of the buildings envelope receives the most solar radiation and which sections of the faรงade should be under evaluation.

WIND FLOW

Hawaii benefits from steady, gentle trade winds typically moving at 15 to 20 mph from the northeast to the southwest. A wind rose can be used to characterize the direction, speed, and frequency of wind. It gives detailed information about wind direction and frequency for a month or a whole year.

FORM & ENVELOPE

To begin to understand these specific areas of the building one must run through the process of first isolation those areas of interest resulting in a specific faรงade boundary model.

Faรงade Model:

Simplified boundary condition?

Analysis:

Micro-climate conditions of the existing model?

Envelope Strategies:

What does the faรงade need to do?

FAĂ&#x2021;ADE MODEL

To begin to understand these specific areas of the building one must run through the process of first isolating those areas of interest. The building has to be simplified in order to obtain a simulation model. In the case for such a large building with many similar wall types, a small portion of the buildingâ&#x20AC;&#x2122;s faĂ§ade has been chosen.

HEAT GAIN

By estimating skin heat flow one can begin to understand its contribution to the buildingâ&#x20AC;&#x2122;s cooling requirements. The amount of heat that is transferred through a buildings skin due to temperature differences between inside and outside is dependent on the size of the difference, the resistance to heat flow by the skin materials, and the area of the assembly.

HOURLY TEMPERATURE

These graphs display hourly temperature patterns within the space (without HVAC conditioning) on a temperature/time graph, time running in the x-axis and temperature in the y-axis.

SOLAR EXPOSURE

The amount of solar radiation transmitted through the skin of the building is relative to the size of surface area, orientation, and heat transmission characteristics of the exposed surfaces. Since the solar heat gain through glazing can be fairly large, understanding the amount of incident radiation that will strike the faรงade will help to determine specific measures or methods of reducing the solar load.

FAĂ&#x2021;ADE INVESTIGATION

Hawaii's location and the application of this system will require a different set of design solutions to meet performance requirements.

Skin Configurations:

Specific goals and constraints guiding design?

Approach:

How do you approach this model?

Simulations:

What are the resulting figues?

APPROACH

Simulation of the model helps to represent real physical occurrences and simplification of reality into the modeled faรงade condition. As a result, the study has to be simplified in an appropriate way in order to obtain a simulation model.

CONSTRAINING PARAMITERS

During the planning and design process, recommendations for the design of double skin walls are to select appropriate control strategy relating to glazing properties, establishment of shading devices, and height of the cavity. Since these parameters are greatly dependent on the environmental context of the existing building, predicting energy performance early in the design stage can influence design decisions.

SINGLE STORY FACADE

One of the advantages of the single story, corridor faรงade over other typologies is that corridor facades are not limited to the full height of the building. However, they do not utilize the stack effect as much as the multi-story assembly because the linking effect will be broken at each floor.

ENERGY FLOW PATHS

Cavity Zone •

Zonal Temperatures – –

•

Radiant Temperatures Operative Temperature

Heat Balance – – –

Occupied Zone

Glazing Infiltration Internal Gains

Zonal Temperatures – –

•

Radiant Temperatures Operative Temperature

Heat Balance – – –

Glazing Infiltration Internal Gains

Simulation of the design scenario has revealed the specific heat balance and temperature of the system. Using the graphs below, we can analysis this specific system and how it affects the buildings internal temperature and those cooling loads needed to provide a comfortable interior environment. The following section is again presented as two parts; the graphs shows individual breakdowns which are measured in kBtu/h and temperature by 0F for the specific boundary condition of both the cavity and the adjacent occupied space.

AIR FLOW PATHS

Velocity Flow Rate • • • • •

Point of Stagnation Point of Acceleration Point of Lowest Pressure Point of Separation Wake & Turbulence

Temperature Distribution • • • • •

Air Intake Temperature Air Exhaust Temperature Maximum Air Temperature Minimum Air Temperature Mean Air Temperature

The ventilation of the intermediate space involves questions relating to the velocity of air flow, the temperature distribution within the air temperature and the pressure differences that are always the motive force of air currents. The relevant air flow patterns for this specific scenario can be largely determined on the basis of the information contained in the CFD simulation results concerning the ventilation of the facades intermediate space.

MULTI-STORY FACADE

The multi-story faรงade scenario consists of an external air curtain where the air cavity is open at the top and bottom, forming a large open volume. The intermediate space between the inner and outer layer is joined vertically and horizontally by a selected number of rooms.

ENERGY FLOW PATHS

Cavity Zone •

Zonal Temperatures – –

•

Radiant Temperatures Operative Temperature

Heat Balance – – –

Occupied Zone

Glazing Infiltration Internal Gains

Zonal Temperatures – –

•

Radiant Temperatures Operative Temperature

Heat Balance – – –

Glazing Infiltration Internal Gains

AIR FLOW PATHS

Velocity Flow Rate • • • • •

Point of Stagnation Point of Acceleration Point of Lowest Pressure Point of Separation Wake & Turbulence

Temperature Distribution • • • • •

Air Intake Temperature Air Exhaust Temperature Maximum Air Temperature Minimum Air Temperature Mean Air Temperature

HYBRID FACADE

The hybrid faรงade scenario consists of a double skin faรงade system with operable panels that can be open or close according to the needs of the system. With this hybrid system a single story or multi-story condition can be formed by means of opening and closing each individual panel accordingly.

ENERGY FLOW PATHS

Cavity Zone •

Zonal Temperatures – –

•

Radiant Temperatures Operative Temperature

Heat Balance – – –

Occupied Zone

Glazing Infiltration Internal Gains

Zonal Temperatures – –

•

Radiant Temperatures Operative Temperature

Heat Balance – – –

Glazing Infiltration Internal Gains

AIR FLOW PATHS

Velocity Flow Rate • • • • •

Point of Stagnation Point of Acceleration Point of Lowest Pressure Point of Separation Wake & Turbulence

Temperature Distribution • • • • •

Air Intake Temperature Air Exhaust Temperature Maximum Air Temperature Minimum Air Temperature Mean Air Temperature

COMPARATIVE ANALYSIS

Simulations have been performed to understand how each double skin facade can be used to reduce cooling loads of the building by minimizing thermal gains by providing a level of added performance. A comparative faรงade analysis can help this understanding in identifying the most energy-efficient and effective faรงade type for the given application.

COMPARATIVE ANALYSIS 8000 Total Cooling (kBtu)

7000 6000 5000 4000 3000 2000 1000 0

Base Model

Scenario 1

Scenario 2

Scenario 3

The results show that any type of double skin wall performs better than the base model of a single skin double glazed faรงade. It has been found that the addition of a second layer will reduce the models total cooling loads by 60% but it was the ability for the system to maximize air flow throughout the system and drafting away surface gains is what made the difference.

IV. CONCLUSION

FINDINGS Based on these results, recommendations for the design of double skin facades in Hawaii’s climates are:

Air cavity: •

Limiting the air cavity size reduces cooling loads by minimizing maximum air temperature. However, when external driving forces are limited, a multi-story shaft cavity is needed to produce a pressure difference caused by thermal buoyancy creating a stack effect.

Airflow types: •

Since majority of the commercial buildings consumed energy is utilized for cooling, there are possible advantages for hybrid ventilation type. Diurnal changes between hot day temperatures and cool night temperatures could also be used, where mechanical ventilation system could be used during the day and natural ventilation during the night. The thermal behavior of double-skin facades is significantly affected by the airflow within the system. The flow of air towards, around and within the assembly greatly affects the overall performance of the system.

Shading: •

Cavity shading devices can provide some protection against solar heat gain and incorporation of these elements within the cavity is important to the efficiency of the system. All shading devices should be located closer to the external skin. However, close attention should be placed on the design of cavity shading because if not designed properly, they could create resistance and losses in airflow of the system and will cause an unwanted temperature increase. This relationship is based on a case by case basis in order to obtain the necessary outcome.

Glazing: •

Effective window sizing and glazing types will have a significant impact on energy consumption. It is important to understand the specifications when selecting an appropriate glazing type. This research has strictly focused on high reflectance glass, however, low-e and high performance glazing may prove to be beneficial within this specific climate context.

FUTURE RESEARCH It is necessary for the design approach to be comprehensive considering the faced as an integrated part of the building and examined in great detail in order to determine all the parameters that will lead to a high performance solution.

Need Further Development: •

Development of advanced CFD techniques used to validate and predict the physical properties of the cavity more accurately.

•

Mock up testing of proposed solutions and feedback from real buildings.

•

Comparison with external shading device on single skin façade.

•

Research on the possibility of multi-layered façade systems

•

Prediction of energy use for entire building

•

Study application

Design objectives for any façade type are to provide thermal, visual and acoustical performance with minimum energy consumption. Since there are numerous combinations between façade types, ventilation strategies as well as system components, context based design that adapts to local environment conditions is of primary importance.

The fundamental premises are that by designing the buildingâ&#x20AC;&#x2122;s facade to task the environmental control, one can achieve better comfort and efficiency (without operating HVAC systems unnecessarily) for any climate with a well-balanced integrated system rather than a detached curtain. Because of the continuous fluctuations of all environmental factors across time, the building faĂ§ade must be understood not as a simple barrier but rather a selective, permeable membrane with the capacity to admit, filter and/or reject any of these environmental factors.

DISCUSSION