Natural Ventilation in Double-Skin Façade Design for
Office Buildings in Hot and Humid Climate
Pow Chew Wong
A thesis submitted in fulfillment of the requirement for the degree of
Doctor of Philosophy
at
The University of New South Wales Australia
I
Acknowledgements
I would like to thank my Supervisors, Professor Deo Prasad and Professor Masud Behnia, and my co-Supervisor Mr. Steve King, for their unfailing guidance and patient for me throughout my quest in fulfilling one of the hardest tasks in my life yet.
My thanks and appreciation also go to those who gave up their valuable time and effort to guide, advise and teach me, especially to Dr. Nathan Groenhout, Dr. Graeme S. Wood, Mr. KW Ng, Mr. Sherman Heng and Ms. Gabi Duigu.
My greatest love and appreciation would go to my wife, Joyce, who has been supporting me day and night, giving me un-surpassing encouragement and love to help me to complete this Thesis. My love also goes to my three children; Jonathan, Janice and Jessica, for their understanding in lighten my load during the hectic period of completing my work.
Lastly, I would like to thank my family overseas for their great patient and support, which has made my study somewhat much more enjoyable and a great sense of fulfillment.
II
Abstracts
The specific climatic conditions and to certain extent the preferred living style in the hot and humid climate of Singapore, most of the electricity consumed in buildings goes towards air-conditioning and refrigeration, especially in work places like commercial and institutional buildings, which are mostly designed to be fully air-conditioned. It is argued that by combining appropriate natural
ventilation strategies with advanced technologies in building façade design will be able to reduce energy consumption in high-rise office buildings in the tropics.
This research seeks to find a design solution for reducing the energy usage in high-rise office buildings in Singapore. There are numerous methods and techniques that could be employed to achieve the purpose of designing energy efficient buildings. The Thesis explores the viability of double-skin façades (DSF) to provide natural ventilation as an energy efficient solution for office buildings in hot and humid environment by using computational fluid dynamic (CFD) simulations and case study methodologies.
CFD simulations were used to examine various types of DSF used in office buildings and the behaviour of airflow and thermal transfer through the DSF; the internal thermal comfort levels of each office spaces were analyzed and compared; and an optimization methodology was developed to explore the best DSF
configuration to be used in high-rise office buildings in the tropics. The correlation between the façade configurations, the thermal comfort parameters, and the
internal office space energy consumption through the DSF is studied and presented.
III
in terms of its energy efficiency through cross ventilation strategy is proposed in this Thesis. A series of comprehensive and user-friendly nomograms for design optimization in selecting the most appropriate double-skin façade configurations with considerations of various orientations for the use in high-rise office buildings in the tropics were also presented.
IV
Table of Contents
Acknowledgements I
Abstract II
Table of Contents IV
List of Figures XII
List of Tables XVII
List of Graphs XIX
List of Acronyms XXI
Chapter 1: Introduction 1
1.1 Introduction 1
1.2 Sustainable development 2
1.3 Energy consumption in commercial buildings 4
1.4 Air conditioning in office buildings and human comfort 5
1.5 Façade design for office buildings 6
1.6 Energy consumption for office buildings in Singapore 7
1.7 Natural ventilation and indoor environment 9
1.8 Thermal comfort standards 10
1.9 Adaptive thermal comfort model and natural ventilation in buildings 11
1.10 Thermal comfort analysis and double-skin façades 12
1.11 Research questions 13
1.12 Scope of research 14
1.13 Methodology 15
V
Chapter 2: Thermal Comfort in Hot and Humid Climates 17
2.1 Evolvement of thermal comfort studies 17
2.2 Definitions of thermal comfort 18
2.3 Important parameters in thermal comfort 19
2.4 Measurement of thermal comfort 20
2.4.1 Psychrometric chart 22
2.4.2 Thermal comfort indices 23
2.4.3 Thermal comfort studies 26
2.5 Questions of adaptability and human comfort 27
2.6 Adaptive thermal comfort model 29
2.7 The ASHRAE Standard 55-2004 31
2.8 Passive solar design in a hot and humid climate 34
Chapter 3: Natural Ventilation Designs in a Hot and Humid Climate 38
3.1 Introduction 38
3.2 The choice of natural ventilation 39
3.3 Natural ventilation and indoor air quality 40
3.4 Development of sustainable designs in buildings 42
3.5 Natural ventilation designs 43
3.5.1 Natural ventilation strategies and techniques 45
3.5.2 Natural ventilation designs in the tropics 48
3.5.3 Natural ventilation research 52
3.6 Natural ventilation and office buildings 54
3.6.1 Natural ventilation and bio-climatic office building designs 55
3.7 Energy consumption for office buildings in the tropics 56
3.7.1 The tropical climate of Singapore 57
3.7.2 Energy usage for office buildings in Singapore 59
VI
Chapter 4: Double-Skin Façades and Natural Ventilation 61
4.1 Intelligent façades 61
4.2 Double-skin façades (DSF) 62
4.2.1 Introduction 62
4.2.2 Classification of double-skin façades 64
4.2.3 Thermal transfer through double-skin façades 70
4.2.4 Design considerations for double-skin façades 71
4.3 Natural ventilation in double-skin façades 73
4.4 Implementation of double-skin façades in office buildings 74
4.4.1 Examples of double-skin façade buildings 74
4.4.2 Fire protection in double-skin façades 78
4.4.3 Sunshading in double-skin façades 79
4.4.4 Effect of nigh-time ventilation in double-skin façades 80
4.4.5 Condensation in double-skin façades 80
4.4.6 Review of the limitations of double-skin facades 81
4.5 Case study for double-skin façade buildings 82
4.5.1 Stadttor (City Gate) at Düsseldorf, Germany 82
4.5.1.1 The façade system (double-skin corridor façade) 83
4.5.1.2 Natural ventilation 84
4.5.1.3 Conclusion 84
4.5.2 Occidental Chemical Center at Niagara Falls,
New York, USA 85
4.5.2.1 The façade system (double-skin multi-storey façade) 86
4.5.2.2 Ventilation systems 86
4.5.2.3 Conclusion 87
4.5.3 Super Energy Conservation Building, Kiyose City,
Tokyo, Japan 87
VII
4.6 Concluding remarks 89
Chapter 5: Computational Fluid Dynamics 90
5.1 Simulating naturally ventilated double-skin façade 90
5.1.1 Building simulation programs 90
5.1.2 Simulating buildings with double-skin façades 92
5.1.3 Coupling CFD and building energy simulations 93
5.1.4 The choice of using CFD program 94
5.2 CFD software 95
5.2.1 Theoretical background for CFD software 95
5.2.2 Grid resolution in CFD software 96
5.2.3 Verification and validation in CFD 97
5.2.4 Constraints for CFD simulation 97
5.3 Research into CFD simulation for building design 100
5.3.1 CFD simulation in building design 100
5.3.2 CFD approaches in indoor environment simulation 105
5.4 A case study of a CFD simulation for double-skin façade 107
5.4.1 Thermal considerations 108
5.4.2 Fluid dynamics considerations 108
5.4.3 Modelling of the façade 109
5.4.4 Findings 109
5.5 Review of several building simulation software packages 110
5.5.1 Apache software 110 5.5.2 Flovent software 111 5.5.3 Microflo software 111 5.5.4 Phoenics software 112 5.5.5 Airpak software 112 5.5.6 Conclusion 113 5.6 Airpak CFD software 114
VIII
5.6.1 The Airpak CFD software 114
5.6.2 Buoyancy-driven flows and natual convection in Airpak 116
5.6.3 Radiation simulation in Airpak 117
5.6.4 Solution procedures in Airpak 117
5.6.5 The validation of Airpak software 119
5.7 Conclusion 122
Chapter 6: Research Methodology 123
6.1 Introduction 123
6.2 Research strategies in architectural research 124
6.2.1 Literature review 125
6.2.2 Research approach: Qualitative versus Quantitative 127
6.3 The knowledge gap and research questions 130
6.4 Research methods for a tropical double-skin façade 132
6.4.1 Building simulation methodology 132
6.4.2 Computational Fluid Dynamic and Airpak software 133
6.4.3 The CFD Modelling 134
6.4.3.1 Stage 1 – Single office 134
6.4.3.2 Stage 2 – Office blocks 136
6.4.3.3 Stage 3 – Optimization 137
6.4.3.4 Stage 4 – Nomograms 137
6.5 Goals for the research 137
6.6 Limitations of the research 138
IX
7.1.2 The construction materials used for the model 140
7.1.3 The heat sources in the model 141
7.1.4 The boundary conditions of the model 141
7.2 Strategies in modelling 143
7.3 Analysis of preliminary models 144
7.4 Discussion 149
7.5 Initial findings 151
7.6 Progressive modelling 154
7.6.1 Comparison of results for single-skin and
double-skin façades 157
7.7 Conclusion 161
Chapter 8: Multi-Storey Building Modelling 162
8.1 Stage 1 of the multi-storey modeling 162
8.1.1 Simulation results for South facing DSF system 165
8.1.2 Simulation results for North facing DSF system 169
8.1.3 Analysis of results and findings for Stage 1 170 8.2 Stage 2 of the multi-storey modelling 172
8.2.1 Simulation results for South facing DSF system 175
8.2.2 Simulation results for North facing DSF system 177
8.2.3 Analysis of results and findings for Stage 2 178 8.3 Stage 3 of the multi-storey modelling 180
8.3.1 Simulation results for South facing DSF system 183
8.3.2 Simulation results for North facing DSF system 185
8.3.3 Analysis of results and findings for Stage 3 186 8.4 Comparison results for different orientations 188 8.5 The complete 18-storey office building 190
X
Chapter 9: Parametric Studies of Optimization 193
9.1 Strategies for natural ventilation optimization 193
9.1.1 Investigation of different opening locations on
the outer pane of DSF system 194
9.1.2 Investigation of different opening sizes on
the outer pane of DSF system 198
9.1.3 A new type of DSF configuration for hot and
humid climate 199
9.1.4 Investigation of different shaft heights of DSF system 200
9.1.5 Investigation of different air gap sizes with
optimum shaft height 206
9.1.6 Comparison of ‘Fan’ and ‘Shaft’ ventilation methods 209
9.1.6.1 Analysis of ‘Fan’ and ‘Shaft’ ventilation methods 214
9.1.7 Investigation of sun shading devices to the DSF system 214
9.1.8 Summarizing of results and findings for optimization 217 9.2 An improved DSF system for the tropics 218
9.3 Limitations of the research 220
9.4 Nomograms for natural ventilation designs with DSF system 221
9.4.1 Formulation of the nomograms 221
9.4.2 The application of the nomograms 225
9.4.3 The limitations of the nomograms 231
9.5 Conclusion 232
Chapter 10: Contributions and Future Works 234
XI
10.4 Recommendations for future works 236
10.5 Final note 238
References
240
Appendix A
257
Selected benchmarking simulation results.
Appendix B
272Selected optimization simulation results.
Appendix C
276Selected referred papers submitted to International Conferences and International Journals.
XII
List of Figures
Chapter 1
Figure 1.1 Energy consumption by sectors Figure 1.2 Electricity consumption by sectors
Chapter 2
Figure 2.1 Thermal exchanges between the human body and its environment
Figure 2.2 Representation of graphical comfort scale
Figure 2.3 Olgyay’s bio-climatic chart in metrics, modified for warm climates
Figure 2.4 Psychrometric Chart
Figure 2.5 The PPD as a function of PMV
Figure 2.6 The psycho-physiological model of thermal perception: the adaptive model
Figure 2.7 Neutralities predicted and compared with results of field experiments
Figure 2.8 Acceptable ranges of operative temperature and humidity Figure 2.9 Air speed required to offset increased temperature
Figure 2.10 Acceptable operative temperature ranges for naturally conditioned spaces
Figure 2.11 Cross ventilation
Chapter 3
Figure 3.1 Natural ventilation through buildings Figure 3.2 Ventilation rate for good indoor air quality
Figure 3.3 Relation between airflow rate, pollution level and energy demand
XIII
Figure 3.5 Wind patterns around buildings
Figure 3.6 Airflow patterns through rooms for various sizes and positions of openings
Figure 3.7 Velocity of airflow is increased outside the room if the inlet is larger than the outlet (a); velocity of airflow is increased inside the room if the inlet is smaller than the outlet (b); internal partition positions will affect the airflow patterns (c & d)
Figure 3.8 Wind patterns altered by different layouts of groups of buildings
Figure 3.9 Louvers can deflect the airflow upwards or downwards
Figure 3.10 A canopy over a window tends to direct airflow upwards (d); a gap between the canopy and the wall will create a downward pressure (e); airflow within a room will improve if a louvered sunshade is used
Figure 3.11 The inlet of the wind tower can usually be closed to keep out dust or cold air
Figure 3.12 Wind catchers in the oriental courtyard houses of Iraq Figure 3.13 Wind towers in the Bastakia district of Dubai
Figure 3.14 Map of Singapore
Figure 3.15 Electricity consumption among different sectors in Singapore
Chapter 4
Figure 4.1 Typical double-skin façade construction Figure 4.2 Plan and Section of box window façade Figure 4.3 Examples of double-skin façades
Figure 4.4 Section through the multi-storey façade of the Victoria Ensemble in Cologne
Figure 4.5 Consolidated classification tree diagrams Figure 4.6 Heat transfer through double-skin façade
XIV
Figure 4.7 Exterior views and the cavity space within the double-skin façade
Figure 4.8 Exterior view and façade details for One Peking Road Figure 4.9 Exterior view of Jiu Shi Tower
Figure 4.10 City Gate at Düsseldorf, Germany
Figure 4.11 The corridor façade system showing the inner vertically pivoted windows and the façade cavity
Figure 4.12 The Occidental Chemical Center at Niagara Falls
Figure 4.13 The Super Energy Conservation Building at Kiyose City, Tokyo
Chapter 5
Figure 5.1 Isometric view of the office room Figure 5.2 Overview of the solution method
Figure 5.3 Simulation model used for the validation constructed in Airpak
Chapter 6
Figure 6.1 Section through the model (with external space at the left) Figure 6.2 Rear elevation of the model
Chapter 7
Figure 7.1 Sectional elevation of the single office module Figure 7.2 Longitudinal section of the single office module Figure 7.3 Isometric view of the CFD model
Figure 7.4 The single office module with studied openings A, B, C, D & E
Figure 7.5 Location points for taking the simulation results (section of model)
XV
Figure 7.8 Thermal Environment Conditions for Human Occupancy, ANSI/ASHRAE Standard 55-2204
Figure 7.9 Ventilation Comfort Chart of Singapore Figure 7.10 Standard curtain walling model
Figure 7.11 Double-skin façade model
Figure 7.12 Nomogram showing the acceptable thermal comfort conditions (shaded area) for standard curtain wall system
Figure 7.13 Nomogram showing the acceptable thermal comfort conditions (shaded area) for double-skin façade system
Chapter 8
Figure 8.1 Model geometry of Stage 1 of the 6-storey building
Figure 8.2 Boundary conditions and ranges of parameters used in the CFD simulations
Figure 8.3 Location points for monitoring the simulation results (Stage 1) Figure 8.4 Thermal Environment Conditions for Human Occupancy from
ANSI/ASHRAE Standard 55-2004
Figure 8.5 Location points for recording the simulation results (Stage 2) Figure 8.6 Model geometry of Stage 2 of the 18-storey office building Figure 8.7 Location points for monitoring the simulation results (Stage 3) Figure 8.8 Model geometry of Stage 3 of the 18-storey office building Figure 8.9 The model of the complete 18-storey office building
Chapter 9
Figure 9.1 Investigation of different opening locations (L1) for the outer pane of DSF system
Figure 9.2 Schematic drawing showing selected points for monitoring simulation results
Figure 9.3 Investigation of different opening sizes for the outer pane of DSF system
XVI
Figure 9.4 A new type of double-skin façade model for hot and humid climate
Figure 9.5 Model configurations for simulations
Figure 9.6 Location points for monitoring the simulation results (for the extended shaft model)
Figure 9.7 Isometric view – 3.6m shaft with openings at outer pane of DSF Figure 9.8 Velocity vectors – study of air velocity magnitudes and its moving
directions
Figure 9.9 Temperature contours – study of temperature distribution within the office spaces
Figure 9.10 Velocity particle traces – study of air flow patterns within the office spaces
Figure 9.11 Pressure contours – study of external and internal pressures acted onto the building
Figure 9.12 Configurations of the model for simulations Figure 9.13 CFD models for ‘Shaft’ and ‘Fan’ configurations
Figure 9.14 Locations of record for thermal comfort parameters (example for the mechanical fan at the top of the double-skin façade)
Figure 9.15 Study of the effects of sun shading device within the DSF sir gap Figure 9.16 An improved new type of double-skin façade model for hot and
humid climate
Figure 9.17 Three ‘Axis’ of the nomogram
Figure 9.18 ‘Limits’ and ‘Linear Spacing’ of the nomogram Figure 9.19 ‘Non-comfort Zone’ of the nomogram
Figure 9.20 ‘Comfort Zone’ of the nomogrma
XVII
List of Tables
Chapter 4
Table 4.1 Double-skin façade buildings with various ventilation types and façade systems
Chapter 5
Table 5.1 Comparison of typical functions of ES and CFD programs for building performance studies
Chapter 6
Table 6.1 Reproduced from Glesne, C., & Peshkin, A. (1992): Becoming
qualitative researchers: An introduction
Chapter 7
Table 7.1 Simulation results A Table 7.2 Simulation results B Table 7.3 Simulation results C
Chapter 8
Tables 8.1 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 1 - South facing DSF system)
Tables 8.2 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 1 - North facing DSF system)
Tables 8.3 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 2 - South facing DSF system)
Tables 8.4 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 2 - North facing DSF system)
Tables 8.5 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 3 - South facing DSF system)
XVIII
Tables 8.6 Simulation results for P1, P6 and P11 of different boundary conditions (Stage 3 - North facing DSF system)
Tables 8.7 Comparison of selected simulation results for different orientations of DSF system
Chapter 9
Table 9.1 Table showing sample of simulation results for different opening locations (L1)
Table 9.2 Simulation results at locations P1a, P6a and P11a (various air gap sizes)
Table 9.3 Simulation results at locations P2a, P7a and P21a (various air gap sizes)
XIX
List of Graphs
Chapter 5
Graph 5.1 Measured hourly outdoor temperatures
Graph 5.2 Measured results (Series 1) vs. Airpak simulation results (Series 2)
Chapter 8
Graph 8.1 Comparison of Operative Temperatures for South facing DSF (Stage 1)
Graph 8.2 Comparison of Operative Temperatures for North facing DSF (Stage 1)
Graph 8.3 Comparison of Operative Temperatures for South facing DSF (Stage 2)
Graph 8.4 Comparison of Operative Temperatures for North facing DSF (Stage 2)
Graph 8.5 Comparison of Operative Temperatures for South facing DSF (Stage 3)
Graph 8.6 Comparison of Operative Temperatures for North facing DSF (Stage 3)
Graph 8.7 Comparison of Operative Temperatures for four major orientations of DSF system
Chapter 9
Graph 9.1 Graph showing the temperatures (oC) comparison for different opening locations
Graph 9.2 Graph showing the air velocity (m/s) comparison for different opening locations
Graph 9.3 Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (various shaft heights)
XX
Graph 9.4 Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (various shaft heights)
Graph 9.5 Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (various air gap sizes)
Graph 9.6 Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (various air gap sizes)
Graph 9.7 Comparison of ‘Fan’ and ‘Shaft’ configurations in relation to thermal comfort parameters
Graph 9.8 Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (1.5m shaft)
Graph 9.9 Indoor Operative Temperatures (oC) taken at locations P1a, P6a and P11a (2.5m shaft)
Graph 9.10 Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (1.5m shaft)
Graph 9.11 Indoor Operative Temperatures (oC) taken at locations P2a, P7a and P12a (2.5m shaft)
Graph 9.12 Comparison of indoor Operative Temperatures (oC) for different DSF systems
XXI
List of Acronyms
ACH Air change hour AHU Air handling unit
ASEAN Association of Southeast Asian Nations
ASHRAE American Society of Heating, Refrigerating, and Air-Conditioning Engineers
ASHVE American Society of Heating and Ventilation Engineers BMS Building management system
BRE Building Research Establishment CFD Computational fluid dynamic DBT Dry-bulb temperature
DOE Department of Energy DSF Double-skin façade
EREN Energy Efficiency and Renewable Energy Network
ES Energy simulation
ET Effective temperature GDP Gross domestic product
HVAC Heating, ventilation and air-conditioning IAQ Indoor air quality
LES Large Eddy Simulation MRT Mean radiant temperature NV Natural ventilation OT Operative temperature PD Percentage dissatisfied
PLEA Passive and Low Energy Architecture PMV Predicted mean vote
PPD Predicted percentage dissatisfied RANS Reynolds Average Navier-Stokes RH Relative humidity
XXII
RNG Random number generator SET Standard effective temperature VAV Variable air volume
VOC Volatile-Organic-Compound WEF World Economic Forum
CHAPTER 1
INTRODUCTION
This Chapter provides an overview of the backgrounds for energy
consumption, façade design, natural ventilation and thermal comfort issues in high-rise commercial buildings in the tropics, and set out the research
questions, scope and methodology of the research and the structure of the Thesis.
1.1
Introduction
The amount of energy used and spent in the modern world has been escalating in an alarming way. Air-conditioning accounts for the major portion of the total energy consumption used for the operation of most of the present high-rise buildings. The situation is even more alarming in the case of high-rise buildings in a hot and humid climate where greater energy consumption is desired to provide comfort in the man-made environment. However, the call for energy efficient building design is growing and the situation is particularly critical for the design of office buildings, because the energy consumed by this building type constitutes the most energy usage intensive built environment within the building industry sectors.
This thesis seeks to find a design solution for reducing the energy usage in office buildings, in particular those in the tropics. There are numerous methods and techniques that can be employed to achieve the purpose of designing an energy efficient building and the latest development in façade technology of double-skin façade system claims to be able to reduce energy usage
substantially by allowing natural ventilation especially for commercial buildings. With that in mind, the thesis explores the viability of double-skin façades in providing natural ventilation as an energy efficient solution for office buildings in a hot and humid environment.
Double-skin façades (DSF) are multiple layer skin constructions, with an external skin, an intermediate space and an inner skin. The external and
internal skins can be either single glaze or double glazed glass panes of float glass or safety glass. An adjustable sun-shading device is usually installed in the intermediate space for thermal controls. This research has involved the study of various types of DSF used in office buildings, and the behaviour of airflow and thermal transfer through the DSF, and the internal thermal comfort levels are analyzed through the use of computational fluid dynamic (CFD) simulations.
1.2
Sustainable development
Environmental damage and current climate change concerns are directly linked to human activity. The economic blueprint for industrialised societies was first publicly questioned in 1968 by the newly founded international think-tank, the Club of Rome. In 1972 members of this group published the now-famous report, “The Limits to Growth”, putting forward the idea that economic development must be combined with environmental protection. In 1984, the United Nations Assembly gave the then Prime Minister of Norway, Gro Harlem Brundtland, the mandate to form and preside over the World
Commission on Environment and Development, also known as the Brundtland Commission. The work of the Commission led to the release in 1987 of the report entitled Our Common Future, also called the Brundtland Report, which popularized the term ‘sustainable development’ and its definition as ‘ meets the needs of the present without compromising the ability of future generations to meet their own needs’. Today the Commission’s work has been recognized for having promoted the values and principles of sustainable development.
At the 1992 Rio Earth Summit, heads of states committed their nations to exploring ways of achieving “development which fulfils current needs without
and social factors. This concept of sustainable development is based on three principles:
- Consideration of the “whole life cycle” of materials
- Development of the use of natural raw materials and renewable energy sources
- Reduction in the materials and energy used in raw material extraction, product use and the destruction or recycling of waste
The Kyoto Summit in 1996 was designed to achieve more concrete measures after the Rio Summit’s emphasis on social and cultural factors. Under the Kyoto Protocol, participating nations pledged to bring average greenhouse gas emissions over the period 2008 to 2012 back to 1990 levels. To keep to this agreement, the industrialised countries need to make progress in three areas:
- Reductions in energy consumption
- Replacement of energy from fossil reserves by energy from renewable sources
- Carbon storing
The principles of the Rio Declaration are connected with the formulation of a development plan for the 21st century, known as Agenda 21. The
recommendations in Agenda 21 are:
- protection of the earth’s atmosphere
- integrated land-use planning and management - combating deforestation
- preservation of fragile ecosystems
- promotion of sustainable development in a rural and agricultural context
- maintenance of biodiversity
- an environmentally rational approach to biotechnology - protection of the oceans and coastlines
- protection of water supplies and quality
- environmentally acceptable treatment of waste, including toxic chemicals, radioactive and other dangerous waste, solid waste and waste water
In 2002, the World Summit on Sustainable Development, or commonly called Earth Summit 2002, was help in Johannesburg, South Africa. The participating nations had renewed their commitment to the principles defined in the Rio Declaration and the Agenda 21 objectives. They pledged to develop national sustainable development strategies to be implemented before 2005.
Implementation of the measures agreed at Kyoto has wide-ranging implications in terms of land use, urban planning and architecture. The attempt to reduce the consumption of energy and natural resources, bring down greenhouse gas emissions and produce less waste will have a particularly significant impact on the building and civil engineering sectors.
The application of sustainable development principles to building is one of the most efficient responses we have to the need to reduce greenhouse effect and the destruction of our environment. Such a response is based on three
complementary, closely linked tenets: - Social equity
- Environmental caution - Economic efficiency
1.3
Energy consumption in commercial buildings
Global consumption of primary energy to provide heating, cooling, lighting and other building related energy services grew from 86 exajoules in 1971 to 165 exajoules in 2002. This is an everage annual growth rate of 2.2% per year (Price et al., 2006). Energy demand for commercial buildings grew about 50% faster than for residential buildings during the same period.
conditions, and behavioural factors. The rapid urbanization that is occurring in many developing countries has important implications for energy consumption in the building sector.
The two most important sources of energy demand in the U.S. commercial buildings are space heating, ventilation, and air conditioning (HVAC) systems, which accounted for 31% of total building primary energy use; and lighting, which accounts for 24% of total building primary energy use (USDOE, 2005). The results for large commercial buildings in many other countries are thought to be similar to those for the United States, although no such statistical
breakdown is available for other IEA member nations or for the developing world.
The above statistic has called for a great attention to reduce energy usage for commercial buildings to in turn reducing the emission of Green House Gases. This is also the thought behind the aims of this Thesis to focus unto proposing an effective way to reduce energy consumption of office buildings in the tropics to give a ‘little’ contribution to the building sector.
1.4
Air conditioning in office buildings and human comfort
The Larkin Building built in 1960 at Buffalo, USA by Frank Lloyd Wright is thought to have been the first air-conditioned building in the world where cooled air was pumped into the building via specially designed air-ducts. The popularity of the International Style that followed saw the upsurge of buildings with strong geometric forms, with an emphasis on large windows and a curtain wall system.
This preferred style at that time, and the advances in technology caused the dissociation of the buildings’ indoor environment from their surrounding climate. An office building which is not constrained by daylight, ceiling height
and plan depth could have a deeper plan, lower ceiling height and greater floor-to-envelope ratio, which has a great impact on the occupants and the effect on human comfort is tremendous.
1.5
Façade design for office buildings
The late 19th century saw a time of accelerated economic growth that led to a global building boom. Real estate values skyrocketed, especially in the city center areas and together with the advancement of building technology like steel skeleton construction and the invention of elevators, this led to the creation of the first high-rise building. Skidmore, Owings and Merrill (SOM) in New York achieved the ‘true’ curtain wall system for office buildings during the middle of the 20th century. Since then, glass curtain wall buildings have appeared everywhere, under the influence of the so call International Style, until in the late 20th century office buildings with glass façades had become a normal feature in all the cities around the world and the building facades for our offices had degenerated into monotonous surfaces.
Since the awareness of the need for energy efficiency increased dramatically in the wake of the oil crisis of the 1970s, the design of smooth glass containers for most of our office buildings, which rely heavily on artificial means to provide an acceptable internal environment, has come under intense scrutiny. The building façade has become even more increasingly important in recent years in the areas of research and development as a result of growing awareness of the importance of sustainable living. High-rise buildings are within the critical category, as more than 80% of the façade for these types of building are constructed using some sort of glazing for their envelopes. The urgency to improve the energy usage of our ‘glass-box’ office buildings with original
required external energy to enter the indoor environment but at the same time to expel any unwanted built-up heat within it. The present technologies have allowed very complex façade systems to be developed and to function
according to the clients’ requirements so as to dramatically reduce the energy usage of large buildings.
The availability of technologies, the desire for an all-glass facade and the commitment to improve the energy usage of large buildings had lead to the development of double-skin façade, which originally is a European Union architectural phenomenon believe to be able to improve indoor air quality through natural ventilation without the acoustic and security constrains of naturally-ventilated single-skin facades.
1.6
Energy consumption for office buildings in Singapore
The 2000 World Competitiveness Yearbook, complied by the World Economic Forum (WEF), ranked Singapore 25th out of 45 countries in terms of energy intensity or the amount of commercial energy consumed per dollar of GDP. This could be due to the fact that Singapore depends heavily on
air-conditioning to cool its buildings all year round. The hike in recent oil prices and the global decline in the supply of fossil resources, together with the growing international concern about carbon dioxide emissions and greenhouse effects, have all resulted in a call for an effective use of energy resources.
The 1998 Kyoto Protocol of the United Nations Framework Convention on Climate Change, to which Singapore is a signatory, established a legally binding obligation on the developed countries to reduce their emissions of carbon dioxide and other greenhouse gases by an average of 5.2% below 1990 levels by the years 2008-2012. Singapore’s energy consumption growth over the period 1980-1995 was 11.9%. The average annual growth in GDP over the same period was around 7.6%. All this means that Singapore will come under
increasing international pressure to reduce its CO2 emissions and to directly lower its energy consumption.
Energy consumption in Singapore can be attributed to the three main sectors of industry, residential and commercial buildings, and transport (Figure 1.1). In the hot and humid climate of Singapore, most of the electricity consumed in buildings goes towards air-conditioning and refrigeration, especially in work places like commercial and institutional buildings, which are mostly designed to be fully air-conditioned. Figure 1.2 shows the distribution of electricity consumption in the building sector, with commercial and industrial buildings constituting close to two-thirds of the total electrical energy consumed.
Energy Consumption in Singapore by Sectors
29% 34% 37% Transport Industries Buildings (Residential/ Commercial)
Electricity Consumption in the Building Sector 18% 25% 57% Public Residential Buildings Private Residential Buildings Commercial/Industrial Buildings
Figure 1.2 Electricity consumption by sectors (Source: Power Supply Pty Ltd)
1.7
Natural ventilation and indoor environment
Most vernacular buildings in the world were naturally ventilated designed, even though some of the buildings have been compromised by the additions of internal walls and mechanical systems. Natural ventilation has become an increasingly attractive method for reducing energy use and costs, and for providing acceptable indoor air quality in order to maintain a healthy,
comfortable and productive indoor climate. In favorable climates and building types, natural ventilation can be used as an alternative to air conditioning plants with savings of 10%-30% of total energy consumption.
However, using natural ventilation to prevent overheating within a building presents a great challenge to maintaining acceptable indoor air quality (IAQ) standards. Controlling indoor air quality appears to be more of a concern during winter periods when interior spaces need to be heated to provide
acceptable thermal comfort and most of the windows in a building may be closed. The control of airflow rates then becomes the ultimate consideration.
For summertime cooling, important considerations are internal heat loads, external solar gains, building characteristics such as thermal mass and insulation levels, the overall building floor area, and site layout. Controlling airflow rates is not as much of a concern here, as long as the occupants are comfortable. The higher the airflow rate the greater the cooling effect.
1.8
Thermal comfort standards
There are a number of international thermal comfort standards that have make substantial contribution to the knowledge of thermal comfort. The main
thermal comfort standard is ISO 7730, which is based upon the predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) thermal comfort indices (Fanger, 1970). The standard also provides methods for assessing the local discomfort caused by draughts, asymmetric radiation and temperature gradients. Other thermal comfort standards include ISO 8996, which describes six methods for estimating the metabolic heat production and it is an important requirement in the use of ISO 7730 and the assessment of thermal comfort. ISO 9920 provides a database of the thermal properties of clothing and garments that based upon measurements on heated manikins, and ISO 7726 supports thermal comfort assessment with measuring instruments.
Thermal comfort research carried out in Europe and the USA during the mid-20th century was mainly concentrated on using climate chamber studies. The thermal comfort standards of ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, were first established in 1966. Since then,
body. Six key variables were identified as affecting the perception of thermal comfort, namely air temperature, radiation, relative humidity, air movement, clothing and metabolic rate. The standard attempted to provide an objective criterion for thermal comfort by specifying personal and environmental factors that will produce acceptable interior thermal environment for at least 80% of a building’s occupants. The standard defined thermal comfort as ‘the condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation’ (p. 2). It also defines thermal sensation as a conscious feeling, commonly graded into categories of cold to neutral to hot.
The ASHRAE Standard 55 was originally developed to provide guidelines for centrally controlled HVAC (Heating, Ventilation and Air-Conditioning) systems. The general application of the standard has limited the efforts to develop more person-centered strategies for thermal control in naturally ventilated or mixed-mode buildings. Such strategies may provide important social and environmental benefits through energy consumption reduction and increase occupant satisfaction and work efficiency, especially in office buildings.
1.9
Adaptive thermal comfort model and natural ventilation
in buildings
The primary limitation of the original ASHRAE Standard 55 is its “one-size-fits-all” approach where clothing and activity are the only modifications one can make to reflect seasonal differences in occupant requirements. The standard has allowed important cultural, social and contextual factors to be ignored which lead to an exaggeration of the “need” for air conditioning in indoor environment. In view of the standard limitation, many researchers argued that the level of occupant satisfaction with indoor environment and the energy consumption of buildings could be reduced if we allowing people greater control of their indoor environments. This has lead to the development
of adaptive thermal comfort model with consideration for naturally ventilated buildings.
The latest ASHRAE Standard 55 -2004 has incorporated the adaptive thermal comfort model with an analytical method based on the PMV-PPD indices and the introduction of the concept of adaptation with a separate method for naturally conditioned buildings. The standard is intended for use in design, commissioning and testing of new or existing buildings and other occupied spaces (residential or commercial) and their HVAC systems.
One important criterion in applying an adaptive model for thermal comfort like ASHRAE Standard 55-2004 is the possibility of individual control. Occupants of naturally ventilated buildings have possibilities for changing the air velocity in the indoor environment by operating the windows and can often create an acceptable environment even with a relatively high indoor temperature. Psychological adaptation also plays an important part in naturally ventilated buildings because the occupants have a more direct contact with the external weather, and higher temperatures are expected for the indoor environment.
1.10 Thermal comfort analysis and double-skin façades
A number of interesting investigations and findings are reported in the literature pertaining to passive ventilation in buildings and the thermal performance of double-skin facades. Even though most of the research has been done in temperate conditions, it has revealed a close link between natural ventilation design and the function of a double-skin façade.
similar natural convection ventilation studies. Most of these have used the concept of stack effect or the solar chimney and found that passive ventilation in summer is possible even for multi-storey buildings. In particular Priyadarsini et al. (2003) have established the energy efficiency of a stack system used in residential buildings in a hot and humid climate region. Li and Delsante (2001) went a step further to investigate the effects of natural ventilation caused by wind and thermal forces in a single zone building with two openings. Ventilation graphs are plotted using the air change parameters (thermal air change, wind air change and the heat loss air change) for design purposes.
Gratia and Herde (Gratia and Herde 2004) also attempted to look at the impact of double-skin façade facing a southern direction in a temperate climatic condition. Thermal analysis using simulation software for the different seasons of a year was done for a low-rise office building with and without double-skin façade. It was found that significant energy saving is possible if natural
ventilation could be exploited through the use of a double-skin façade.
1.11 Research questions
Natural ventilation strategies and double-skin construction are not new concepts and much research and development has been done to improve on those ideas. In fact most of the vernacular architecture in the tropics uses much natural ventilation concepts in ventilating the indoor environment. Double-skin curtain walls were also first used at the Steiff Factory in Giengen, Germany during the early twentieth century.
Although attempts have been made in recent years to use double-skin façades to introduce natural ventilation into high-rise buildings in Europe, China and Hong Kong, most of these buildings are located in the temperate countries. There are still questions remains to be answered that formed the basic research questions for this thesis as follow:
• Whether double-skin façades are able to provide acceptable indoor conditions for the high-rise buildings occupants under natural ventilation strategies in hot and humid climate regions.
• If that is possible, the question will be what is the ‘opening window’ regime during the day would need to be, so that natural ventilation could be introduced.
• This will also lead to the question of the possibility for formulating some useful guidelines for designing double-skin façade for high-rise buildings in the tropics.
The above are the main questions raised in this Thesis that hoped to answer satisfactory and it was discussed in greater length in Chapter 6.
1.12 Scope of research
This research seeks to find a design solution for reducing the energy usage in high-rise office buildings in the tropics, and more specifically in the tropical island of Singapore. There are numerous methods and techniques that could be employed to achieve the purpose of designing energy efficient buildings. The thesis explores the viability of double-skin façades to provide natural
ventilation as an energy efficient solution for office buildings in a hot and humid environment by using computational fluid dynamic simulations and a case study methodology.
1.13 Methodology
Computational Fluid Dynamic (CFD) has become a useful tool for designers in the study of indoor and outdoor environmental conditions in building design especially for the close observations of thermal and energy transfer between different environments. Furthermore, the parameters such as air velocity and relative humidity solved by CFD are critical for designing an acceptable indoor comfort environment. CFD techniques have been applied with considerable success in building design and their advantages in analyzing ventilation performance have been reported by Murakami (1992) and Liddamant (1992). Papakonstantinou et al. (2000) have demonstrated that numerical solutions for ventilation problems can be obtained quickly and in good agreement with experimental measurements.
CFD simulations for a series of modular office spaces will be studied and analyzed and the emphasis will be on the use of natural ventilation strategies in providing acceptable indoor environment conditions in a tropical environment like Singapore. The results will be compared against the findings from the case studies for double-skin façade buildings completed in recent years, to study the differences and to learn of any constructive lessons.
A comprehensive methodology in simulating the high-rise office buildings is proposed and a new type of double-skin façade configuration for the use in the tropics is recommended. A series of initial design rule of thumb in the form of nomograms are proposed at the end for designers who wanted to design high-rise office buildings using double-skin façade in the hot and humid climate.
1.14 Structure of Thesis
The first three Chapters of the Thesis will look at the research and findings of thermal comfort studies and standards formulated in the world, the various natural ventilation
strategies and their positive impact on high-rise office building design in reducing energy consumption, the use of double-skin façade technologies in achieving energy efficient building designs, and how all these could help in providing an option for the future of high-rise office building design in the tropic. These Chapters will explain the gap in the research and the supportive arguments for carrying out the whole
painstaking work for this thesis.
Chapters 4, 5 & 6 will explain the methods used to achieve the goals of formulating some design guidelines for using double-skin façades for high-rise office buildings in a hot and humid climate.
Chapters 7, 8 & 9 will present the results and the findings of the research and propose some options for naturally ventilating high-rise buildings in the tropic.
Chapter 10 will list the achievements of the research work and the limitations and recommend future follow-up to the work that has been done.
Chapter 2
Thermal Comfort In Hot and Humid
Climates
This Chapter provides a general overview of what are thermal comfort and the development of adaptation models for supporting the introduction of natural ventilation strategies to be used in double-skin facades system.
2.1 Evolvement of thermal comfort studies
In the early 1920s Houghten and Yagloglo (1923) attempted to define the ‘comfort zone’ at the ASHVE (American Society of Heating and Ventilation Engineers) laboratories. In England, Vernon and Warner (1932) and later Bedford (1936) carried out empirical studies among factory workers in relation to industrial hygiene. However, the studies of thermal comfort got their real momentum during and after World War II , involving not only fields like engineering, but also the areas of physiology, medicine, geography and climatology. In architecture, Victor Olgyay (1963) was the first to bring together findings of the various disciplines and interpret these for architectural purposes.
Although thermal comfort studies began more than a century ago, more
significant research was carried out by Fanger in 1970, explaining that thermal comfort is an influential factor in human performance and that man’s
intellectual, manual and perceptual performance is at the highest when he is experiencing thermal comfort. An improvement in environmental conditions occurred as a result of people spending most of their lives in an artificial climate. The aim of creating artificial climates was to adaptat the thermal environment so that every individual is in a state of thermal comfort. Ruck (1989) went as far as to say that the human factor is the principal concern in the design of buildings, where human well-being and performance should be considered as much as the human need for a suitable and stimulating environment.
2.2
Definitions of thermal comfort
The definitions for thermal comfort are manifold and it is a difficult task to pinpoint, which is most accurate, and which best explains the state of human response. The following are some acceptable definitions of thermal comfort:
Fanger (1970, p13) defined thermal comfort for a person as the condition of mind that expresses satisfaction with the thermal environment. Due to the biological variations in people, the aim is to create optimal thermal comfort in such a way as to provide that the highest possible percentage of a group’s feels thermal comfort.
Givoni (1976, p3) defined thermal comfort as the absence of irritation and discomfort due to heat or cold, and as a state involving pleasantness.
O’ Callaghan (1978, p43) defined thermal comfort as the study of the effects of climatic impact on human response.
The ASHRAE (2004, p.2) definition of comfort is ‘the condition of mind that expresses satisfaction with the thermal environment; it requires subjective evaluation’. This clearly embraces factors beyond the physical or
physiological. Figure 2.1 shows schematically the thermal exchange between the human body and its environment through radiation, evaporation and convection processes.
Figure 2.1 Thermal exchanges between the human body and its environment (Source: Design Primer for Hot Climate, Konya, 1980, p.26)
2.3
Important parameters in thermal comfort
The variables that affect thermal comfort can be grouped in to three categories, namely environmental, personal and contributing factors.
Environmental factors include air temperature, air movement, humidity and radiation.
Personal factors means metabolic rate or activity and clothing. Contributing factors include food and drink, acclimatization, body shape, subcutaneous fat, age and gender.
Air temperature is the most important environmental factor and is measured by the dry bulb temperature (DBT). This will determine the convective heat dissipation with any air movement. Air movement is measured in m/s
(velocity, v) and it affects the evaporation of moisture from the skin and thus gives an evaporative cooling effect. Humidity in the air will affect the
evaporation rate and is expressed by relative humidity (RH, %), absolute humidity or moisture content (AH, g/kg), or vapour pressure (p, kPa). Radiation exchange will depend on the mean temperature of the surrounding surfaces and is referred to as the mean radiant temperature (MRT). The mean radiant temperature cannot be measured directly but can be approximated by globe temperature measurements.
The metabolic rate may be influenced by food and drink, and the state of acclimatization. Clothing is one of the dominant factors affecting heat dissipation. The unit for the thermal comfort measurement of the clothing effect is clo. This corresponds to an insulation cover over the whole body of a transmittance (U-value) of 6.45 W/m2K (a resistance of 0.155 m2K/W). 1 clo is the insulating value of a normal business suit with cotton underwear. Shorts with short-sleeved shirts would be about 0.25 clo, heavy winter suit with overcoat will give around 2 clo and the heaviest arctic clothing is around 4.5 clo (Szokolay, 1997, p.9).
2.4
Measurement of thermal comfort
The ASHRAE Scale used in laboratories and the Bedford Scale used in field studies are the most frequently applied scales, producing similar results for human comfort experiments. They use a seven-point scale, where 3 means hot and –3 means cold. Table 2.1 below shows the comparison of the two scales.
In laboratory studies factors influencing thermal sensation, especially clothing, are reduced to a minimum, and an independent environmental variable is manipulated, while the dependent variable, comfort level are isolated from external influences (de Dear, Leow and Foo, 1991). In field studies, personal
Table 2.1 Comparison of thermal comfort scales (Source: PLEA Notes: Thermal Comfort, 1997, p.15)
The original ASHRAE scale used numbers from 1 to 7 where 1 meant cold and 7 meant hot. The ‘graphic scale’ used by Woolard in a Solomon Islands study shown in Figure 2.2 below used a scale from 1 to 7.
Figure 2.2 Representation of graphical comfort scale (Source: PLEA Notes: Thermal Comfort, 1997, p.15)
Olgyay (1953) in his bio-climatic chart (Figure 2.3) has the dry-bulb
temperature (DBT) on the Y-axis and the relative humidity (RH) on the X-axis. The aerofoil shape in the middle is the comfort zone. Curves above the aerofoil
show how air movement can extend the upper limits of thermal comfort and lines below the aerofoil show how radiation could extend the lower limits of the comfort zone. According to Olgyay, cool-humid conditions are often referred to as dank and hot-dry as torrid or scorching.
Figure 2.3 Olgyay’s bio-climatic chart in metrics, modified for warm climates (Source: Introduction to Architectural Science, Szokolay, 2004, p. 21)
2.4.1 Psychrometric chart
The atmosphere is a mixture of air and water vapour. The science dealing with this mixture is called psychrometry and the graphic representation of various attributes of this mixture is the psychrometric chart (Figure 2.4). The attributes represented in the chart are dry bulb temperature (oC), absolute humidity (g/kg), saturation humidity, relative humidity lines (%), wet bulb temperature lines (oC), specific volume lines (m3/kg), and enthalpy lines (kJ/kg).
Psychrometric process can be traced on the chart by locating the status point on the chart of known quantities and all other quantities can then be read from the
point to the uppermost scale, it will give the sensible heat/total heat (HS/H) ratio.
Figure 2.4 Psychromertic Chart
(Source: PLEA Notes: Thermal Comfort, 1997, p. 13)
2.4.2 Thermal comfort indices
Most of the indices of warmth were developed within the first 50 years of the 20th century, both empirically and analytically, and were established from controlled chamber studies with fit young Americans and Europeans. Inevitably they specify an optimum value that has been assumed to apply equally to all people.
Empirical indices and analytical indices were developed mainly for defining limits of comfort, setting exposure thresholds, and determining the optimum control measures for thermal comfort. Examples of some of the empirical indices are:
• Effective temperature (Houghten and Yagloglou, 1923),
• Operative temperature (Winslow, Herrington & Gagge, 1937),
• Wet bulb globe temperature (Yaglou and Minard, 1957),
• Equivalent temperature (Dufton, 1932 & 1933),
• Equatorial comfort index (Webb, 1960),
and some of the examples of the analytical indices are:
• Predicated 4-hour sweat rate (McArdle and collaborators, 1947),
• Index of thermal stress (Givoni, 1963),
• Predicted mean vote (PMV),
• Standard effective temperature (Nishi and Gagge, 1977),
• Index of thermal sensation (Gagge).
Macpherson (1962) suggested that there were many factors not recognised by the various indices and that the most important of these was acclimatization. The static models like the PMV approach denies the role of acclimatization. O’ Callaghan (1978) developed models for thermal comfort in three areas of human response, namely physical, physiological and sociological. The physical model defined the body as a thermal system, in which heat exchange between the body and the environment through the skin and clothing occurs. The physiological model explained the subjective responses to the thermal environment and the involuntary actions that occur when the body is outside the neutral state, like sweating and shivering. The sociological model denotes the factors that mostly prevent the application of the accepted human comfort criteria to the environmental conditioning of interiors.
Fanger’s (1970) comfort equation is probably the most meticulous and detailed analysis of human thermal relationships with the proximal environment. He stated that the thermal balance of the body is influenced by air temperature,
resistance. The dependence of these parameters on each other in providing thermal comfort was discussed by Kut (1970).
The PMV (predicted mean vote) and the PPD (predicted percentage dissatisfied) form the basis of the formulation of ISO 7730:1994,
Determination of the PMV and PPD indices and specification of conditions for thermal comfort. According to the standard, the PMV equation can be written
as: PMV = (0.303 x e-0.036xM + 0.028) x {(M-W) – -3.05 x 10-3 x [5733-6.99(M-W)-pa] – -0.42 x [(M-W)-5815] – -1.7 x 10-5 M(5867-pa) – -0.0014 x M(34-ta) – 3.96 x 10-8 x fcl x [(tcl+273)4 – (tr+273)4] – -fcl x hc x (tcl - ta) }
where: fcl is the ratio of man’s surface area while clothed to man’s surface area while nude; ta is the air temperature oC; tr is the mean temperature oC; pa is the partial water vapor pressure Pa; hc is the convective heat transfer coefficient W/(m2K); tcl is the surface temperature of clothing oC.
The PMV equation above can be calculated for different combinations of metabolic rate, clothing, air temperature, mean radiant temperature, air velocity and air humidity. The tcl and hc can be solved by iteration. The PMV index predicts the mean vote of the votes of a large group of persons on the following 7-point thermal sensation scale:
+3=hot, +2=warm, +1=slightly warm, 0=neutral, -1=slightly cool, -2=cool, -3=cold
The predicted percentage dissatisfied (PPD) is found as a function of the predicted mean vote (PMV) from the equation below and can be represented as the graph in Figure 2.5.
PPD = 100 – 95 x e-(0.03353 x PMV4 + 0.2179 x PMV2)
Figure 2.5 The PPD as a function of PMV
(Source: ISO 7730:1994, Determination of the PMV and PPD indices and
specification of conditions for thermal comfort, p.3)
Auliciems (1981) went on to propose a psycho-physiological model of thermal comfort, which is also the basis of his adaptation hypothesis. Later on, the two-node model of the JB Pierce laboratories and the ET* (and SET) indices derived from this form the basis of ASHRAE Standard 55-1992: Thermal
environmental conditions for human occupancy.
2.4.3 Thermal comfort studies
Dry bulb temperature is the most useful measure for the specification of comfort. For the measurement of the magnitude of discomfort or stress, the other environmental factors like humidity, radiation and air movement should be taken into consideration. Most of the thermal comfort models used the DBT (dry bulb temperature) as an index of thermal comfort or neutrality. Drysdale
simplest index of all is the DBT; and also under ordinary conditions in still air the DBT in itself is a better index of warmth than is effective temperature and any other composite index.
In recent years, there have been further findings in the effectiveness of the thermal indices developed to represent thermal comfort in specific climatic zones. Williamson et al. (1995) found that the PMV overestimates warm discomfort, especially in warm climates. Karyono (1996) found that people in South East Asia (hot and humid climate) prefer up to 6K higher temperature than suggested by Fanger, and this is explained as the result of adaptation of people to higher outdoor temperatures.
De Dear, Leow and Foo (1991) carried out a study on both air-conditioning and naturally ventilated buildings in Singapore and found that air-conditioned buildings showed similar results but naturally ventilated buildings showed 3K warmer than Fanger’s values. De Dear, Leow and Ameen (1991) had also carried out climate chamber experiments on thermal acceptability in Singapore. They found that the upper limit of the acceptable comfort zone at 70% RH was 27.6oC and at 35% RH was 27.9oC. The results were in line with the
predictions of the current international comfort standard, ISO 1984, despite the fact that the empirical bases of the standard were subjects from much colder climates in northern Europe and the US.
2.5
Questions of adaptability and human comfort
Physiological neutrality or thermal equilibrium does not necessarily mean comfort but other factors such as past experiences, socio-cultural factors, habits and expectations will influence perceptions of thermal comfort. The original ASHRAE standard 55 was developed through laboratory tests of perceived thermal comfort with the limited intent of establishing optimum HVAC levels for fully climate-controlled buildings. Therefore the standard was initially
applied universally across all building types, climates and populations. As a result, even in relatively mild climatic zones it was hard to meet the standard’s requirement of thermal comfort without mechanical systems. Many argued that the standard ignored the importance of cultural, social and contextual factors. It was argued that giving people greater control of indoor environments and allowing temperatures to more closely track patterns in the outdoor climate could improve levels of occupant satisfaction with indoor environments and reduce energy consumption. This argument was supported by the research done by de Dear and Brager (1998), which they found that when occupants have control over operable windows and are accustomed to conditions that are more connected to the natural swings of the outdoor climate, the subjective notion of comfort and preferred temperatures changes as a result of the availability of control, of different thermal experience and of resulting shifts in occupant perceptions or expectations. Such issues are particularly relevant with regard to naturally ventilated buildings.
By responding to the above questions, an alternative thermal comfort standard based on field measurements could account for contextual and perceptual factors absent in a laboratory setting. Research focusing on three primary modes of adaptation, namely physiological, behavioral and psychological, emerged to deal with the issues. Physiological adaptation (also known as acclimatization) refers to biological responses that result from prolonged exposure to characteristic and relatively extreme thermal conditions.
Behavioral adaptation refers to any conscious or unconscious action a person might make to alter their body’s thermal balance. The psychological adaptation refers to an altered perception of and reaction to physical conditions due to past experience and expectations. This research work led to the formation of the latest ASHRAE Standard 55 in 2004.
2.6
Adaptive thermal comfort model
Auliciems (1981) formulated an adaptive model of thermoregulation within which thermal preference was seen as the result of both physiological
responses to immediate indoor parameters (those measured by the indices) and expectations based on ‘climato-cultural’ determinants (past experiences). Figure 2.6 below shows the adaptive model by Auliciums.
Figure 2.6 The psycho-physiological model of thermal perception: the adaptive model (after Auliciems, 1981)
The adaptive model subsequently was investigated and verified by de Dear in Darwin (1985), Schiller and Auliciems in San Francisco Bay Area (1988), Busch in Bangkok (1990), de Dear, Leow and Ameen in Singapore (1991) and de Dear and Fountain in Townsville (1994). Much more discussion on the matter was carried out and it became evident that the notion of a constant or static optimum was no longer an acceptable hypothesis. In a major report to
ASHRAE, de Dear, Brager and Cooper (1997) exhaustively analysed all research reports from both naturally ventilated and mechanically controlled buildings, and concluded that while a mechanistic model of heat transfer may well describe the responses of people within closely controlled thermal environments like air-conditioned space, it is “… inapplicable to naturally ventilated premises because it only partially accounts for processes of thermal adaptation to indoor climate.”
Figure 2.7 shows the comparison of responses by people at the same location in a different environment, i.e. air conditioned buildings and naturally ventilated buildings. The observed results are even higher than the adaptive model predictions.
Figure 2.7 Neutralities predicted and compared with results of field experiments
