6 Air-Conditioning Load and Energy Consumption

By reading and using the thesis, the reader understands and agrees to the following terms:

1. The reader will abide by the rules and legal ordinances governing copyright regarding the use of the thesis.

2. The reader will use the thesis for the purpose of research or private study only and not for distribution or further reproduction or any other purpose.

3. The reader agrees to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.

IMPORTANT If you have reasons to believe that any materials in this thesis are deemed not suitable to be distributed in this form, or a copyright owner having difficulty with the material being included in our database, please contact [email protected] providing details. The Library will look into your claim and consider taking remedial action upon receipt of the written requests.

Pao Yue-kong Library, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

http://www.lib.polyu.edu.hk

THERMAL & INDOOR AIR QUALITY ENVIRONMENT

ON AIR-CONDITIONED BUSES

SHEK KA WING

Ph.D

The Hong Kong Polytechnic University

2010

The Hong Kong Polytechnic University

Department of Building Services Engineering

Thermal & Indoor Air Quality Environment

on Air-conditioned Buses

SHEK Ka Wing

A thesis submitted in partial fulfillment of the requirements for

the degree of Doctor of Philosophy

July 2009

Abstract

Air-conditioned buses have been serving Hong Kong over two decades. There are now approximately 5750; over 95% of the service fleets. Bus manufacturers and local operators are keen on the service quality improvement by modifying the air- conditioning designs, including system performance and reliability, parameter settings as well as energy effectiveness. However, complaints about poor air quality and thermal discomfort were received since the beginning of services. Such issues suggest less-than-satisfactory environments on these buses. Further study is necessary to enhance the in-bus commuting environment. Considering the in-bus air quality and thermal comfort environment, physical measurement and questionnaire survey were conducted to review the real scenario. Data collected from physical measurement provided clues to evaluate the dynamic effects from surrounding environment. The in-bus air quality varied when the buses travelled on different routes. The average in-bus CO concentration level was the highest on tunnel routes (4.4 ppm) followed by urban district routes (3.4 ppm). It was the lowest on rural routes (1.3 ppm). However, the I/O ratios on different routes were similar (between 2.1 and 3.4). Such variation caused by the change of roadway environment, like traffic density, surrounding building density as well as the outdoor air quality. Mechanical ventilation rate is another key to the in-bus air quality. The rate is fixed on present air-conditioned buses. Measurement result showed the mechanical -1 ventilation rate was 250 l · s on a stationary bus and it rose on a travelling bus. The -1 -1 -1 rate was 380 l · s when travelling at 30 km · hr and it reached 535 l · s when -1 traversing on a highway (at 65 km · hr ). It was equivalent to varying the outdoor air -1 -1 rate per person from 1.9 l · s to 4.1 l · s on a fully loaded bus. Since the bluff head generates aerodynamic drag on a travelling bus, pressure difference is induced across bus body surface. Thus ventilation rate varies with the bus travelling speed. Higher rate helps dilute in-bus air contaminants but it increases the risk of infiltrating concentrated air pollutants when travelling in congested area. Nevertheless, lower rate results as insufficient ventilation that causes air stuffiness and odour inside passenger compartment. Therefore the mechanical ventilation rate should not be fixed but adjusted depending on the roadway air quality. Empirical comfort models were developed by concluding the correlation between the

i physical parameters and the subjective sensation votes from passengers. They provide a convenient platform to quantify and identify the in-bus air quality and thermal comfort through the percentage of dissatisfaction. Moreover, the computation result from models can be applied in the air-conditioning system to optimize the system control. Real-time monitoring of the air quality and thermal comfort indicators are obtained by equipping sampling and data processing instruments on buses. The computation result of real-time data provides determinants for the control system to set an appropriate outdoor air intake rate and thermal comfort settings. The application can reduce the percentage of dissatisfaction level from 35% (present) to 8%. The in-bus environment can be controlled at the most comfortable condition along the journeys. Mixing-ventilation air distribution system was applied in the present passenger compartment. It aimed to provide a uniformly air distribution in the compartment. However, supply air was not well distributed due to tight space in the compartment. Thermally discomfort was caused due to temperature stratification and draft risk. Also, fresh air might not directly reach the passengers causing ineffective air contaminants dilution. Hence, personalized air supply and return system was proposed in new compartment in order to solve those faults. Diffusers were installed in front of each seat while return air grilles were located above each row for better air distribution. Also, computation result from the empirical comfort models was applied to improve the in-bus environment. Simulation result showed the new compartment minimized those thermal discomfort issues found in the present compartment. The new system improved the air distribution by increasing the ventilation efficiency (from 0.93 in present compartment) to 1.32. Moreover, the effectiveness of personalized air supply and return system was evaluated by means of thermal comfort, concentration distribution and particle transport. The tracks of coughed droplets expelled from an index person were simulated in the compartments. Simulation result showed that the level of influence caused by the particle dispersion depended on where the index person sat in the present compartment. Passengers sitting behind the index person were possibly influenced by the infectious droplets. Thus the number of influenced passengers reduced if the index person sitting closer to the rear section. However, the number was less in the new compartment, and which was similar for the index person sitting in different sections. The percentage of particles inhaled by other passengers maintained below 0.04%. It revealed the

ii personalized air supply and return system was effective in preventing the spread of infectious droplets in the whole compartment. The air-conditioning load and energy consumption were simulated to compare the system operating under different scenarios, including in present compartment, present compartment with applying new settings and new compartment. Set-point air temperature and mechanical ventilation strategy were adjusted depending on the computation result from empirical comfort models. Such adjustment was intended to improve the in-bus environment while satisfying a substantial majority of passengers. The application of new air-conditioning settings lowered the passengers’ dissatisfaction level. However, higher energy consumption in both heating and cooling was resulted in present compartment. The consumptions rose by 9.1 times and 49% respectively. The required heating and cooling capacities were increased by 3.1 times and 98% respectively. The present air-conditioning system might not be applicable to fulfil the settings adjustment. Applying the new air-conditioning settings in the new compartment, simulation result showed lower heating and cooling capacities were required. The required capacities were diminished by 55% and 36% respectively, as compared with that the new settings applied in present compartment. Also the energy consumptions in heating and cooling were reduced by 64% and 28% respectively. The results revealed the personalized air supply and return design helped to achieve energy effectiveness by increasing the ventilation efficiency. Also adjustment of air-conditioning settings improved the commuting environment while satisfying a substantial majority (above 80%) of passengers. Therefore the personalized air supply and return system and new air-conditioning settings were suggested on new buses. Finally three elements approach was developed. The three elements were the engine, passenger compartment and air-conditioning system. It assisted to improve the in-bus air quality and thermal comfort environment in the aspect of design, operation and maintenance on air-conditioned buses.

iii Acknowledgements

I would like to express my deepest gratitude to Prof. Daniel Chan W.T., the chief supervisor, for his guidance, support and encouragement over the past years. I would like to thank Dr. Chau C.K., the co-supervisor, for his encouragement, advice and support.

Thanks are extended to Dr. Tung C.W. for providing valuable research advice, Mr. Henry Mak and Dr. Emma Leung for providing guidance in making simulation and giving comments, Dr. Steven Ng and Mr. Ken Hui for providing technical advice.

Thanks are also given to Mr. Kenny Hung, Mr. Leung C.S. and Mr. Leung T.W. for providing professional technical assistance in instrumentation and measurement.

Finally, I want to express my gratitude to my parents and brothers for their fully support and love.

iv Table of Contents

Declaration Abstract i Acknowledgements iv Contents v List of figures ix List of tables xix Glossary terms xxi

1 Introduction 1 1.1 Objective of study 2 1.2 Structure of thesis 2

2 Background 5 2.1 Geography and climate in Hong Kong 5 2.2 Public transportation system in Hong Kong 6 2.3 Development of bus service in Hong Kong 11 2.3.1 Beginning of bus services 11 2.3.2 Birth of franchised routes 11 2.3.3 Double-deckers 12 2.3.4 Air-conditioned bus 13 2.3.5 Change of Bus Services 13 2.4 Design of bus 14 2.4.1 Engine 14 2.4.2 Passenger compartment 16 2.4.3 Air-conditioning system 19 2.5 Literature Review 22 2.5.1 Standards and guidelines concerned with indoor air quality 22 2.5.2 Literatures concerned with indoor air quality 25 2.5.3 Standards and guidelines concerned with thermal comfort 26 2.5.4 Literatures concerned with thermal comfort 29

3 Field study of in-bus air quality and thermal comfort environment 3.1 Introduction 35 3.2 Field study 36 3.2.1 Air quality 36 3.2.2 Thermal comfort 40 3.2.3 Measurement 41 3.2.4 Computational fluid dynamics 53 3.3 In-bus air quality condition 55 3.3.1 Summary of measurement data and distribution curves 55 3.3.2 Ventilation characteristics on a travelling bus 57 3.4 Outdoor air quality 63 3.4.1 Urban district routes 63 3.4.2 Sub-urban district routes 64 3.4.3 Rural district routes 65 3.4.4 Highway and tunnel routes 66 3.4.5 Effects of route natures to in-bus air quality 66 3.5 In-bus thermal comfort environment 69 3.6 Air quality inside compartment of other transportation modes 71 3.7 Conclusion 72

4 Comfort models 75 4.1 Introduction 75 4.2 Methodology 76 4.2.1 Air parameters measurement 76 4.2.2 Thermal comfort parameters measurement 77 4.2.3 Questionnaire survey concerning air quality issues 78 4.2.4 Questionnaire survey concerning thermal comfort issues 79 4.2.5 Questionnaire survey concerning combined comfort 80 4.3 In-bus air quality issues 80 4.3.1 Perception towards in-bus air quality 80 4.3.2 Sensation towards air staleness 82 4.3.3 Sensation towards air stuffiness 83

vi 4.3.4 Sensation towards odour 85 4.3.5 Sensation towards dusty air 86 4.3.6 Empirical model for in-bus air quality 87 4.4 In-bus thermal comfort issues 89 4.4.1 Perception towards in-bus thermal comfort 89 4.4.2 Sensation towards temperature 90 4.4.3 Sensation towards humidity 93 4.4.4 Sensation towards air movement 95 4.4.5 Empirical model for in-bus thermal comfort 97 4.5 In-bus combined comfort 98 4.6 Application of comfort models 100 4.6.1 Control strategy for air quality 101 4.6.2 Control strategy for thermal comfort 103 4.6.3 Control strategy for combined comfort 105 4.7 Conclusion 107

5 Design of new air distribution system 108 5.1 Introduction 108 5.2 Methodology 108 5.3 Validation of the numerical model 113 5.3.1 Validation of air temperature and air speed 116 5.3.2 Validation of ventilation by concentration distribution 121 5.4 Simulation result 124 5.4.1 Thermal comfort 125 5.4.2 Ventilation 144 5.4.3 Particle transport 154 5.4.4 Combined Comfort 171 5.5 Conclusion 174

vii 6 Air-conditioning load and energy consumption 175 6.1 Introduction 175 6.2 Methodology 175 6.3 Air-conditioning load and annual energy consumption 180 6.3.1 Present compartment 180 6.3.2 New air-conditioning settings applied in the present compartment 182 6.3.3 New settings applied in new compartment 185 6.4 Conclusion 191

7 Three elements approach for design and operation concerning with in-bus air quality and thermal comfort 193 7.1 Introduction 193 7.2 Three elements approach 194 7.2.1 Engine 194 7.2.2 Passenger compartment 196 7.2.3 Air-conditioning system 198 7.3 Conclusion 204

8 Conclusion 206

References 210

Appendix A: Calibration for sampling CO concentration level

Appendix B: Questionnaire

Appendix C: Governing equations for CFD model

viii List of figures

Figure Title Page

Figure 2.1 Geographic location of Hong Kong 5 Figure 2.2 Comparison of age distribution in 1992 and 2002 6 Figure 2.3 Percentage of daily mechanized trips by trip purpose 8 Figure 2.4 Hourly profiles of mechanized trips 8 Figure 2.5 Distribution of journey time 9 Figure 2.6 Daily passenger journeys by public transport modes 10 Figure 2.7 The first bus of China Motor Bus Company used in 1924 11 Figure 2.8 The first generation of double-decker used in Hong Kong 12 Figure 2.9 The engines of the new buses are all satisfied with the European Standard 13

Figure 2.10 Layout of a typical chassis design 15 Figure 2.11 Bus chassis including the engine, gearbox, axle, wheels, fuel tank, driving instrumentation panel, steering wheel and driver’s seat 15

Figure 2.12 Bus under assemble in the body builder factory 15 Figure 2.13 Half-cab design bus with the engine installed at the front 16 Figure 2.14 The engine compartment is sealed and completely isolated from the cabin 16

Figure 2.15 Air distribution duct and diffusers installed on the lower deck 17 Figure 2.16 Heater duct installed under the seats 17 Figure 2.17 Access plank is available for easy access of disabilities on new buses 18

Figure 2.18 Chenille seat covers are served for the purpose of luxury feeling on older buses 18

Figure 2.19 Leather seat covers are served for durability and hygiene on new buses 18

ix Figure 2.20 Handrails are available along the corridor and they are printed in sharp colour for safety purpose 19

Figure 2.21 The configuration of split type air-conditioning system on 19 Figure 2.22 The evaporators of the split type system installed on lower 20 Figure 2.23 The configuration of combined type air-conditioning system on bus 20

Figure 2.24 Return grilles of the evaporators of combined type system on the lower deck 21

Figure 2.25 The evaporator on the upper deck 21 Figure 2.26 Location of fresh air intake 21 Figure 2.27 A close-up of the door gap 22 Figure 2.28 The gaps are available at both the top and the bottom of the doors 22

Figure 2.29 ASHRAE thermal sensation scale 28 Figure 2.30 Distribution of the indoor climatic measurements on the ASHRAE Standard 55-1992 summer comfort charts 30

Figure 2.31 Distribution of the indoor climatic measurements on the ASHRAE Standard 55-1992 winter comfort charts 30

Figure 2.32 Combined physiological and psychological modelling 31 Figure 2.33 Statistics of thermal offenders 33

Figure 3.1 Literature review of CO2 prescribed level 37 Figure 3.2 Correlation of roadside air pollutants concentration levels 38

Figure 3.3 Correlation of in-bus CO2 and RSP concentration levels on air-conditioned buses 39

Figure 3.4 Routes of measurement journeys 41 Figure 3.5 The traffic was heavy at most of the daytime in the urban 46 Figure 3.6 There are plenty of residential buildings in sub-urban districts 46 Figure 3.7 Plenty of vegetation in rural districts 47 Figure 3.8 Traffic flow is high but fluent in the highway 47 Figure 3.9 Traffic density is very high in tunnel 48

x Figure 3.10 Location of monitoring stations 49 Figure 3.11 Views of bus model 53

Figure 3.12 Distribution curve of in-bus CO2 concentration measured on buses 56

Figure 3.13 Distribution curve of in-bus CO concentration measured on buses 56

Figure 3.14 Distribution curve of in-bus RSP concentration measured on buses 57

Figure 3.15 Profiles of ventilation rate and CO2 concentration obtained in a typical measurement journey 58

Figure 3.16 Illustration of air infiltration and exhaust for the passenger compartment 59

-1 Figure 3.17 Pressure induced around bus travelling at 30 km ⋅ hr 60

-1 Figure 3.18 Pressure induced around bus travelling at 50 km ⋅ hr 60

-1 Figure 3.19 Pressure induced around bus travelling at 70 km ⋅ hr 61 Figure 3.20 The induced pressure difference at various speeds 61 Figure 3.21 The ventilation rate at various speeds 62 Figure 3.22 In-bus CO concentration levels on different routes 67 Figure 3.23 In-bus RSP concentration levels on different routes 68 Figure 3.24 Distribution curve of in-bus air temperature measured on 70 Figure 3.25 Distribution curve of in-bus relative humidity measured on buses 70

Figure 3.26 Distribution curve of in-bus air speed measured on buses 71 Figure 4.1 Scale for sensation vote towards air quality 79 Figure 4.2 Scale for sensation vote towards particular thermal comfort issues 79

Figure 4.3 Scale for sensation vote towards thermal comfort 79 Figure 4.4 Scale for sensation vote towards in-bus combined comfort 80 Figure 4.5 Percentage of sensation vote towards air quality on air- conditioned buses 81

xi Figure 4.6 Percentage of sensation vote towards particular air quality issues on buses 81

Figure 4.7 Sensation vote of stale air against in-bus CO concentration level on buses 82

Figure 4.8 Percentage of dissatisfaction against sensation vote towards stale air on buses 83

Figure 4.9 Sensation vote of stuffy air against in-bus CO2 concentration level on buses 83

Figure 4.10 Percentage of dissatisfaction against sensation vote towards stuffy air on buses 84

Figure 4.11 Sensation vote of odour against in-bus CO2 concentration level on buses 85

Figure 4.12 Percentage of dissatisfaction against sensation vote towards odour on buses 86

Figure 4.13 Sensation vote of dusty air against in-bus RSP concentration level on buses 86

Figure 4.14 Percentage of dissatisfaction against sensation vote towards dusty air 87

Figure 4.15 Percentage of dissatisfaction against sensation vote towards in-bus air quality 89

Figure 4.16 Percentage of sensation vote towards thermal comfort on air- conditioned buses 89

Figure 4.17 Distribution of sensation vote towards particular thermal comfort issues on air-conditioned buses 90

Figure 4.18 Percentage of dissatisfaction against sensation vote towards temperature on buses 91

Figure 4.19 Sensation vote against in/out-bus air temperature difference on air-conditioned buses 91

Figure 4.20 Practical and preferred in-bus air temperature 92

xii Figure 4.21 Model of sensation vote towards temperature on air- conditioned buses 93

Figure 4.22 Percentage of dissatisfaction against sensation vote towards humidity on buses 94

Figure 4.23 Sensation vote against in-bus relative humidity on buses 95 Figure 4.24 Percentage of dissatisfaction against sensation vote towards air movement on buses 96

Figure 4.25 Sensation vote against in-bus air speed on buses 96 Figure 4.26 Percentage of dissatisfaction towards thermal comfort on 97 Figure 4.27 Percentage of sensation vote towards combined comfort level on air-conditioned buses 98

Figure 4.28 Combined comfort index on air-conditioned buses 99 Figure 4.29 Air-conditioning control scheme of outdoor air rate 102

Figure 4.30 Health effects for exposure under different CO2 concentration level 103

Figure 4.31 Air-conditioning control scheme for thermal comfort 104 Figure 4.32 Air-conditioning control scheme for combined comfort 106 Figure 5.1 Geometry of upper-deck passenger compartment with present air distribution system 109

Figure 5.2 Geometry of upper-deck passenger compartment with new air distribution system 110

Figure 5.3 Meshed geometry of the passenger compartment 111 Figure 5.4 Geometry of passenger located inside compartment 112 Figure 5.5 Sketch of test chamber (side) 114 Figure 5.6 Photo of test chamber (side) 114 Figure 5.7 Panel of air-conditioning control system 115 Figure 5.8 Photo of test chamber view from backward 115 Figure 5.9 Photo of test chamber (diffusers and return air grilles) 116 Figure 5.10 Photo of thermal manikin sitting inside the test chamber 117 Figure 5.11 Control panel of thermal manikin 117

xiii Figure 5.12 Photo of thermal manikin taken by thermal camera 118 Figure 5.13 Anemometers set 118 Figure 5.14 Anemometers installed at different vertical levels 119 Figure 5.15 Comparison of simulated and measured air temperature with supply air at 20oC, 21oC, 22oC and 23oC 119

Figure 5.16 Comparison of simulated and measured air speed with supply -1 -1 -1 -1 air at 1.2m ⋅ s , 1.4 m ⋅ s , 1.6 m ⋅ s and 1.8 m ⋅ s . 120 Figure 5.17 Index person was assumed to sit at seat 2a 121 Figure 5.18 Index person was assumed to sit at seat 2b 123 Figure 5.19 Distribution of air temperature at head level (1.1m) in present compartment 126

Figure 5.20 Distribution of air temperature at waist level (0.6m) in present compartment 127

Figure 5.21 Distribution of air temperature at feet level (0.1m) in present compartment 128

Figure 5.22 Air temperature stratification shown in horizontal contours in present compartment 129

Figure 5.23 Air temperature stratification shown in vertical contours in present compartment 130

Figure 5.24 Distribution of air speed at head level (1.1m) in present compartment 131

Figure 5.25 Air speed around passengers in present compartment 132 Figure 5.26 Sensation vote towards thermal comfort in present 133 Figure 5.27 Percentage of dissatisfaction towards thermal comfort in present compartment 134

Figure 5.28 Distribution of air temperature at head level (1.1m) in new compartment 136

Figure 5.29 Distribution of air temperature at waist level (0.6m) in new compartment 137

xiv Figure 5.30 Distribution of air temperature at feet level (0.1m) in new compartment 138

Figure 5.31 Air temperature stratification shown in horizontal contours in new compartment 139

Figure 5.32 Air temperature stratification shown in vertical contours in new compartment 139

Figure 5.33 Distribution of air speed at head level (1.1m) in new compartment 141

Figure 5.34 Sensation vote towards thermal comfort in new compartment 142 Figure 5.35 Percentage of dissatisfaction towards thermal comfort in new compartment 143

Figure 5.36 Air speed around passengers in new compartment 144

Figure 5.37 Distribution of CO2 concentration at breathing zone in present compartment 146

Figure 5.38 Distribution of CO2 concentration shown in vertical contours in present compartment 147

Figure 5.39 Sensation vote towards air quality in present compartment 148 Figure 5.40 Percentage of dissatisfaction towards air quality in present compartment 149

Figure 5.41 Distribution of CO2 concentration at breathing zone in new compartment 151

Figure 5.42 Sensation vote towards air quality in new compartment 152 Figure 5.43 Percentage of dissatisfaction towards air quality in new compartment 153

Figure 5.44 Distribution of CO2 concentration shown in vertical contours in new compartment 154

Figure 5.45 Air stream inside the upper-deck compartment with present air distribution system 155

Figure 5.46 Particle tracks of coughing in the front section of compartment (the 2nd row) 157

xv Figure 5.47 Close-up of particle tracks of coughing in the front section of compartment (the 2nd row) 158

Figure 5.48 Vectors of air stream in the front section of compartment (the 2nd row) 158

Figure 5.49 Particle tracks of coughing in the middle section of compartment (the 8th row) 160

Figure 5.50 Close-up of particle tracks of coughing in the middle section of compartment (the 8th row) 161

Figure 5.51 Vectors of air stream in the middle section of compartment (the 8th row) 161

Figure 5.52 Particle tracks of coughing in the rear section of compartment (the 13th row) 162

Figure 5.53 Close-up of particle tracks of coughing in the rear section of compartment (the 13th row) 162

Figure 5.54 Vectors of air stream in the rear section of compartment (the 13th row) 163

Figure 5.55 Air stream inside the compartment with new air distribution system 164

Figure 5.56 Particle tracks of coughing in the front section of compartment (the 2nd row) 165

Figure 5.57 Close-up of particle tracks of coughing in the front section of compartment (the 2nd row) 166

Figure 5.58 Vectors of air stream in the front section of compartment (the 2nd row) 166

Figure 5.59 Particle tracks of coughing in the middle section of compartment (the 8th row) 167

Figure 5.60 Close-up of particle tracks of coughing in the middle section of compartment (the 8th row) 168

Figure 5.61 Vectors of air stream in the middle section of compartment (the 8th row) 168

xvi Figure 5.62 Particle tracks of coughing in the rear section of compartment (the 13th row) 169

Figure 5.63 Close-up of particle tracks of coughing in the rear section of compartment (the 13th row) 170

Figure 5.64 Vectors of air stream in the rear section of compartment (the 13th row) 170

Figure 5.65 Percentage of dissatisfaction towards combined comfort in present compartment 172

Figure 5.66 Percentage of dissatisfaction towards combined comfort in new compartment 173

Figure 6.1 In-bus set-point temperature under ambient air temperature 178 Figure 6.2 Predicted passengers’ sensation vote towards temperature 178 Figure 6.3 Maximum required ventilation rate and percentage of dissatisfaction towards in-bus odour issue under various

upper-limit of CO2 concentration level thresholds 179

Figure 6.4 Monthly heating energy consumption and maximum heating load under present settings 181

Figure 6.5 Monthly cooling energy consumption and maximum cooling load under present settings 181

Figure 6.6 Monthly maximum heating load in present compartment with applying new air-conditioning settings 182

Figure 6.7 Monthly heating energy consumption in present compartment with applying new air-conditioning settings 183

Figure 6.8 Monthly maximum cooling load in present compartment with applying new air-conditioning settings 184

Figure 6.9 Monthly cooling energy consumption in present compartment with applying new air-conditioning settings 184

Figure 6.10 Monthly maximum heating load in new compartment 185 Figure 6.11 Monthly heating energy consumption in new compartment 186

xvii Figure 6.12 Annual heating energy consumption and required heating

capacity in new compartment under different CO2 thresholds levels of ventilation strategy 187

Figure 6.13 Monthly maximum cooling load in new compartment 187 Figure 6.14 Monthly cooling energy consumption in new compartment 188 Figure 6.15 Annual cooling energy consumption and required cooling

capacity in new compartment under different CO2 thresholds levels of ventilation strategy 189

Figure 6.16 Monthly air-conditioning energy consumption with applying new air-conditioning settings 189

Figure 7.1 Air-conditioning control scheme of outdoor air rate 201 Figure 7.2 Air-conditioning control scheme for thermal comfort 202 Figure 7.3 Air-conditioning control scheme for combined comfort 203

xviii List of tables

Table Title Page

Table 2.1 Distribution of boarding by transport mode 9 Table 2.2 Licensed bus fleet and average daily patronage 10 Table 2.3 The prescribed concentration levels of indoor air contaminants recommended by the local guidelines and the international standards 24

Table 2.4 Literatures concerning with in-bus air quality 26 Table 3.1 Brief description of measurement journeys 42 Table 3.2 Summary of background information of measurement 45 Table 3.3 Descriptions of the ambient monitoring stations 49 Table 3.4 Carbon dioxide generation rate against occupant activity 51 Table 3.5 Air quality monitor sensors specification 52 Table 3.6 Thermal comfort parameter monitor sensors specification 52 Table 3.7 Measurement result summary of in-bus air quality 55 Table 3.8 Summary of ventilation rate in measurement journeys 57 Table 3.9 Summary of the in-bus and ambient air pollutants concentration levels on urban district routes 63

Table 3.10 Summary of in-bus and ambient air pollutants concentration levels on sub-urban district routes 64

Table 3.11 Summary of the in-bus and ambient air pollutants concentration levels on rural district routes 65

Table 3.12 Summary of the in-bus air pollutants concentration levels on highway and tunnel routes 66

Table 3.13 Measurement result summary of in-bus thermal environment 69 Table 3.14 Summary of air sampling taken on other transport modes and bus stop 72

Table 5.1 Boundary conditions for discrete phase model 113

xix Table 5.2 Measurement of tracer gas distribution with index person at middle row seat a 122

Table 5.3 Simulation of tracer gas distribution with index person at middle row seat a 122

Table 5.4 Measurement of tracer gas distribution with index person at middle row seat b 124

Table 5.5 Simulation result of tracer gas distribution with index person at middle row seat b 124

Table 5.6 Summary of air temperature and air speed at different levels in present compartment 129

Table 5.7 Summary of air temperature and air speed at different levels in new compartment 139

Table 5.8 Summary of CO2 concentration at breathing zone 147 Table 5.9 Summary of the number of particles trapped in passengers’ noses 171

Table 6.1 Ambient air temperature and in-bus set-point temperature 178 Table 6.2 Air-conditioning load and energy consumption in present compartment with present settings 182

Table 6.3 Air-conditioning load and energy consumption in present compartment with applying new settings 183

Table 6.4 Air-conditioning load and energy consumption in new compartment 186

Table 6.5 Comparison of required capacity and annual energy consumption in air-conditioning under different scenarios 190

xx Glossary Terms

Abbreviation

AC …… Air-conditioned ASHRAE …… American Society of Heating, Refrigerating and Air-conditioning Engineers

ASTM …… American Society for Testing and Materials CFD …… Computational fluid dynamics clo …… Clo-value CMB …… China Motor Bus Ltd. CO …… Carbon monoxide

CO2 …… Carbon dioxide CTB …… Citybus Ltd. HKCSD …… Hong Kong Census & Statistics Department HKEPD …… Hong Kong Environmental Protection Department HKFOE …… Friends of the Earth (HK) HKO …… Hong Kong Observatory HKTD …… Hong Kong Transport Department hr …… Hour ISO …… International Organization for Standardization km …… Kilometre

-1 km · hr …… Kilometre per hour KMB …… Kowloon Motor Bus Company (1933) Ltd. kW …… Kilowatt

-1 l · s …… Litre per second LWB …… Long Win Bus Company Ltd. m …… Metre

-1 m · s …… Metre per second Max. …… Maximum

-3 mg · m …… Milligram per cubic metre min …… Minute

xxi Min. …… Minimum mm …… Millimetre MWh …… Megawatt hour NDIR …… Non-dispersive infrared NLB …… New Lantao Bus Company (1973) Ltd.

NO2 …… Nitrogen dioxide Non-AC …… Non-air-conditioned NWFB …… New World First Bus Services Ltd. oC …… Degree Celsius OSHA …… Occupational Safety & Health Administration pa …… Ambient pressure Pa …… Pascal PM10 …… Suspended particulate matters which have the nominal aerodynamic diameters of ten micrometres or less

PMV …… Predicted mean vote PN-PTF …… Practice Note for Managing Air Quality in Air-conditioned Public Transport Facilities - Buses

PPD …… Predicted percentage of dissatisfaction ppm …… Part per million RA …… Return air RSP …… Respirable suspended particulates SA …… Supply air

SF6 …… Sulphur hexafluoride

SO2 …… Sulphur dioxide TWA …… Time weighted average VOCs …… Volatile organic compounds WHO …… The World Health Organization yr …… Year

xxii Notations

% …… Percentage < …… Less than > …… Greater than ACH …… Air change per hour, [ hr-1 ] C …… Concentration

CCO …… Concentration of carbon monoxide, [ ppm ]

CCO2 …… Concentration of carbon dioxide, [ ppm ]

CNO2 …… Concentration of nitrogen dioxide, [ ppm ]

Co …… Outdoor air concentration -3 CRSP …… Concentration of respirable suspended particulates, [ mg · m ]

CSO2 …… Concentration of sulphur dioxide, [ ppm ] ε …… Dissipation rate of turbulent kinetic energy fcl …… Clothing area factor g …… Gravitational acceleration G …… Generation rate hc …… Convective heat transfer coefficient k …… Turbulent kinetic energy L …… Thermal load on body, [ W ] µ …… Molecular viscosity of fluid

-2 M …… Metabolic rate, [ W · m ]

PDAQ …… Percentage of dissatisfaction towards air quality, [ % ]

PDcombined …… Percentage of dissatisfaction towards combined comfort, [ % ]

PDd …… Percentage of dissatisfaction towards dusty air, [ % ]

PDf …… Percentage of dissatisfaction towards air stuffiness, [ % ]

PDo …… Percentage of dissatisfaction towards odour, [ % ]

PDs …… Percentage of dissatisfaction towards air staleness, [ % ]

PDt …… Percentage of dissatisfaction towards temperature comfort, [ % ]

PDTC …… Percentage of dissatisfaction towards thermal comfort, [ % ]

PDv …… Percentage of dissatisfaction towards air movement, [ % ]

xxiii PDw …… Percentage of dissatisfaction towards humidity, [ % ] -1 Q …… Ventilation rate, [ l · s ] ρ …… Air density

Si …… Momentum source component

Sij …… Strain tensor component t …… Time interval

o tcl …… Clothing surface temperature, [ C ] o ti …… Air temperature inside bus compartment, [ C ]

τij …… Stress tensor component o to …… Ambient air temperature, [ C ] o tr …… Radiant temperature, [ C ] o tset-pt …… Set-point temperature, [ C ] o tsk …… Skin temperature, [ C ] -1 u …… Bus travelling speed, [ km · hr ] ui …… Velocity component -1 v …… Air speed, [ m · s ] V …… Volume of space, [ m3 ]

VoteAQ …… Sensation vote towards air quality

Voted …… Sensation vote towards dusty air

Votef …… Sensation vote towards air stuffiness

Voteo …… Sensation vote towards odour

Votes …… Sensation vote towards air staleness

Votet …… Sensation vote towards temperature

VoteTC …… Sensation vote towards thermal comfort

Votev …… Sensation vote towards air movement

Votew …… Sensation vote towards humidity w …… Relative humidity, [ % ] W …… Work done, [ W ]

xxiv 1 Introduction

As in many other Asian cities, the vast majority of commuters in Hong Kong rely heavily on public transport. This diverse multi-modal transport system comprises of electrified railways, a mass transit railway, a light rapid transit, franchised buses, trams, public light buses, taxis, and ferry services provides transportation to all parts of the territory. Overall public transport serves around eleven millions passenger- trips per day. One consequence of increased affluence of Hong Kong over the past decade is that the vast majority of public transport is now air-conditioned. Bus is one of the most popular public transportation modes in Hong Kong owing to its significant characteristics of high carrying capacity, economic and convenient, meanwhile adapted to the specific local geometric environment. It shared 36% of local passenger journeys in 2008 (reaching 3.9 millions daily passenger journeys) (Transport Dept., 2008). Air-conditioned buses have been serving Hong Kong over two decades. There are now approximately 5750; over 95% of the service fleets. Commuting without an air-conditioner during the hot season may be rewarding for those on a diet, but for everyone else air-conditioner today is a necessity rather than a luxury. Whether it’s the heat or air pollution that bothers passengers, air-conditioning system is a “must” on new fleets. With increasing awareness the local population has become more concerned about health impacts of exposure to air pollution. Ambient air quality in Hong Kong suffers from pollutants dispersed from local transportation and industrial activity from cross- border sources. In the microclimates of its densely populated high-rise urban areas, ambient air quality is further compromised by emissions from road vehicles. The government has been active on various fronts to improve matters. This includes actions to promote the use of cleaner fuels and means to reduce vehicle exhaust emissions. Action has also included attention to air quality in transport facilities. Bus manufacturers and local operators are keen on the service quality improvement by modifying the bus designs, from concerning with the carrying capacity and power to commuting comfort. However complaints about poor air quality and thermal discomfort were ceaseless since the beginning of air-conditioning service. Such issues suggest less-than-satisfactory environments on buses. Hence this study is significant that explores and develops a protocol to improve the in-bus commuting environment.

1.1 Objective of study

a) To explore the factors causing failure by reviewing the development of local air- conditioned bus service; b) To realize the scenario of air quality and thermal comfort environment inside air-conditioned bus compartments through physical parameters measurement and questionnaire survey; c) To evaluate the effects of ventilation characteristics and surrounding environment to the passengers’ air pollutant exposure level; d) To evaluate the commuting comfort by developing empirical comfort models concerning with air quality, thermal comfort and the combined comfort level on buses; e) To simulate the air quality and thermal comfort environment in present compartment, and propose a new air distribution system to minimize the passengers’ dissatisfaction level; f) To evaluate the effect of applying empirical comfort models to the air- conditioning load and energy consumption; g) To develop a protocol for design, maintenance and operation concerning with air quality and thermal comfort on air-conditioned buses.

1.2 Structure of thesis

The above objectives provide a theme for each chapter in this thesis. An outline of each chapter is summarized as follows: The geography, climate and public transportation development of Hong Kong are summarized in Chapter 2. These provide clues that the geographical and climatic environments are different from other countries. Even the demand of public transportation is much higher in Hong Kong. A completely imitation of foreign standards or guidelines may not suit the local situation. The review of bus design shows the high quality hardware is available but deficient in air-conditioning settings. Besides, relevant standards/guidelines and studies on indoor air quality and thermal comfort are reviewed. The standards/guidelines state prescribed levels of a variety of physical parameters concerning with the safety and health issues. Operators control

2 the bus air-conditioning system orientating towards conformance with the prescribed levels. Local studies showed that bus operators succeeded in doing so, since in-bus air pollutant concentration levels were found to be lower than the prescribed levels. However, complaints about air quality and thermal comfort issues inside air- conditioned bus compartments were ceaseless that suggested less-than-satisfactory environments on these buses.

Carbon monoxide (CO), carbon dioxide (CO2) and respirable suspended particulates (RSP) are selected as in-bus air quality indicators. Concentration level of CO reveals the roadway air quality; CO2 shows the ventilation sufficiency on bus; while RSP represents the particulates filtering effectiveness. Considering thermal comfort environment, four thermal comfort parameters are selected as indicators. They are ambient air temperature, in-bus air temperature, relative humidity and air speed. Chapter 3 gives a general review on air quality and thermal comfort environment on local buses. It concludes the data collected from physical parameters measurement and face-to-face questionnaire survey. The effects of outdoor air quality and mechanical ventilation rate to the in-bus air quality are studied. Mechanical ventilation is applied to improve the in-bus air quality by diluting in-bus air contaminants and bioeffluents. However, the quality is directly influenced by the dynamic roadway environment along the route. Low ventilation rate causes accumulation of in-bus air contaminants due to ventilation insufficiency. Nevertheless, high ventilation rate can effectively dilute the in-bus air contaminants but having a higher risk of introducing concentrated air pollutants that increasing the passengers’ air pollutants exposure level. Both outdoor air quality and ventilation rate are crucial factors to in-bus air quality. Appropriate air-conditioning setting should be applied in order to improve the quality. Realizing the effects, data collected from physical parameters measurement and subjective questionnaire survey are correlated in Chapter 4. Empirical comfort models are developed that help evaluate the sensation vote and percentage of dissatisfaction towards air quality, thermal comfort and the combined comfort. Besides, integrating the comfort models into the bus air-conditioning system, the computation result can be deterministic data to optimize the control. In Chapter 5, the air quality and thermal comfort environment in present compartment is simulated. Reviewing the factors of discomfort caused, a new air distribution system design is proposed and verified through computational fluid

3 dynamics program. Associated with the application of new air-conditioning settings, the new compartment helps improve thermal comfort and increase ventilation efficiency by providing personalized air supply and return system. Improving the in-bus environment, computation result from empirical comfort models are applied as air-conditioning settings in the new system. Chapter 6 compares the annual air-conditioning energy consumption and required capacities by applied the settings adjustment in present and new compartment. It shows the benefits provided by the personalized air supply and return system in the new compartment. Also, design and operation protocols are developed in Chapter 7. Three elements approach is suggested to further improve the in-bus air quality and thermal comfort in an effective way. Lastly, Chapter 8 provides the conclusions of the research.

4 2 Background 2.1 Geography and climate in Hong Kong Hong Kong is located on China's south coast, bordering Guangdong in the north (as illustrated in Figure 2.1). The territory consists of Hong Kong Island, Lantau Island, Kowloon Peninsula and the New Territories as well as some 260 other islands. Of the territory's 1,104 square kilometres, 75% is undeveloped as the terrain is mostly hilly to mountainous with steep slopes. Hong Kong's population increased sharply throughout the 1990s, reaching 6.99 million in 2006. About 95% of Hong Kong's population is of Chinese descent, the majority of which is Cantonese. Hong Kong is one of the most densely populated cities in the world, with an overall density of more than 6,200 people per square kilometre. Hong Kong's population has an extremely dense urban core, consisting of Kowloon and the north of Hong Kong Island. The rest of Hong Kong is relatively sparsely populated, with millions of residents scattered irregularly throughout the New Territories, south Hong Kong Island and Lantau Island.

Figure 2.1 Geographic location of Hong Kong (captured from HRW World ATLAS)

Hong Kong's climate is sub-tropical, tending towards temperate for nearly half the year. During November and December there are pleasant breezes, plenty of sunshine and comfortable temperatures. January and February are cloudy, with occasional cold fronts followed by dry northerly winds. It is common for temperatures to drop

5 below 10oC in urban areas. The lowest temperature recorded at the Observatory is 0oC, although sub-zero temperatures and frost occur at times on high ground and in the New Territories. March and April can also be very pleasant although there are occasional spells of high humidity. May to August is hot and humid with occasional showers and thunderstorms, particularly during the mornings. Afternoon temperatures often exceed 31oC whereas at night, temperatures generally remain around 26oC with high humidity. July to September is the months during which Hong Kong is most likely to be affected by tropical cyclones. If the tropical cyclones come closer to Hong Kong, winds will increase and rain can become heavy and widespread. The mean annual rainfall ranges from around 1300 to more than 3000 mm. About 80 percent of the rain falls between May and September. The wettest month is August, when rain occurs about four days out of seven and the average monthly rainfall at the Observatory is 391.4 mm. The driest month is January, when the monthly average is only 23.4 mm. (HKO, 2009)

2.2 Public transportation system in Hong Kong According to the data provided by Census & Statistics Department (HKCSD, 2002), the total number of households and population in Hong Kong were estimated to be 2.15 millions and 6.76 millions respectively, during the period of survey in 2002. And it reached 6.99 million in 2006. From the total population, there were estimated to be 48.8% of employed persons and 20.6% of full-time students.

Figure 2.2 Comparison of age distribution in 1992 and 2002 (captured from HKTD, 2002)

6 The age distribution of population is one of the key parameters that would have a major bearing on the trip-making characteristics. The territory-wide age distribution as derived from the Travel Characteristic Surveys 1992 and 2002 (HKTD, 1992 and 2002) are compared in Figure 2.2. It indicates that the peak age group of the population has shifted from 25-34 years old in 1992 to 35-44 years old in 2002. Among the Hong Kong residents, 85% made trips within the territory. 12.3 millions mechanized trips were estimated on an average weekday. The trips are conventionally categorized into 5 purposes, depending on their nature of place and activity performed at the trip destination. They are: • Home-based work: from home to usual workplace for work; • Home-based school: from home to school for lectures/lessons; • Home-based others: from home to places that are not usual workplace or school; such as home to/from shopping places, food premises, recreational places, etc.; • Non-home based: trips which do not start or end at home, nor between workplaces; such as from workplace to shopping or other social places; • Employers’ business: between workplaces for meetings, fieldwork or outdoor work duties.

The percentage of daily mechanized trips by trip purpose is illustrated in Figure 2.3. Among the trips made on weekdays, the home-based work trips accounted for the largest proportion (38%). It was followed by trips for home-based others purpose which accounted for 31%. The home-based work and home-based school trips formed the largest proportion (51%) of regular trips concentrated at peak hours. The number of work and school trips were found to have increased in line with the growth of the working and student population respectively, which provided significant traffic implication for transport planners.

Figure 2.3 Percentage of daily mechanized trips by trip purpose (captured from HKTD, 2002)

Figure 2.4 illustrates the profiles of mechanized trips made against different times of a day for various trip purposes. The overall peak hours were found to be 8 – 9 am and 6 – 7 pm, with a large proportion of home-based work trips. The two peak hours accounted for about 12% and 11% of the total daily trip respectively.

Figure 2.4 Hourly profiles of mechanized trips (captured from HKTD, 2002)

A different pattern was made by the home-based school trips with the morning peak hour occurring earlier between 7 and 8 am. The afternoon peak occurred from 3 to 5 pm. Non-home based trips concentrated during the periods of 12 noon – 2 pm and 5 – 7 pm, reflected the peak periods for lunch-time and after-work activities.

Figure 2.5 Distribution of journey time (captured from HKTD, 2002)

The survey done on journey time by public transport are illustrated in Figure 2.5 which showed that 44% of the trips by public transportation modes took half an hour or less to complete, 43% took more than half an hour to one hour, and the remaining 13% took more than one hour to complete. The mean journey time was estimated to be 43 minutes.

Table 2.1 Distribution of boarding by transport mode

Mode All purposes Home ↔ Work Home ↔ School

Franchised bus 33% 37% 31%

Railway 25% 30% 22%

Public light bus 12% 13% 12%

Private vehicle 11% 7% 3%

Special purpose bus 9% 6% 28%

Taxi 7% 3% 2%

Tram 2% 2% 1%

Ferry 1% 2% 1%

Table 2.1 illustrates the distribution of boarding by different mechanized transport modes. Franchised bus was the most popular transport mode which accounted for 33% of the total boarding for all trip purposes. Around 37% of boarding for home- based work trips were by franchised bus, while 31% for home-based school trips.

Figure 2.6 Daily passenger journeys by public transport modes

Franchised buses are the most efficient road-based passenger carrier. As a mass carrier, buses are more flexible than rail because their routes and service levels can be more readily adjusted to meet the changing demand. At present, there are 5 franchised bus companies that cover over 600 routes. They are Kowloon Motor Bus Company (1933) Ltd. (KMB), Citybus Ltd. (CTB), New World First Bus Services Ltd. (NWFB), New Lantao Bus Company (1973) Ltd. (NLB) and Long Win Bus Company Ltd. (LWB). As in 2008, they have a fleet size of totally 5,613 buses which carry about 3.9 million passenger journeys per day, representing 33% of the total public transport market. The figures of daily passenger journeys by franchised buses and other public transport modes are shown in Figure 2.6 and Table 2.2 respectively.

Table 2.2 Licensed Bus Fleet and Average Daily Patronage (as at April 2008)

Franchised Bus Major Service Licensed Average Daily Passenger Company Districts Fleet Size Journeys (million)

Kowloon, KMB 3858 2.74 New Territories

Hong Kong Island, CTB 838 0.57 Lantau Island

Hong Kong Island, NWFB 661 0.49 Kowloon

NLB Lantau Island 102 0.05

New Territories, LWB 154 0.08 Lantau Island

10 2.3 Development of bus service in Hong Kong The story of local bus service development may introduce a lot of ideas and view points towards the thermal comfort and air quality issues on buses.

2.3.1 Beginning of bus services In 1920, there was tram service on the north-shore of Hong Kong Island. However, there was none in Kowloon. Residents had to go everywhere on foot, unless those owning vehicles. Kowloon Motor Company was established in the following year. It provided bus service in Kowloon with several buses. In 1924, a group of Chinese bought a bus from the UK (as illustrated in Figure 2.7). They established the China Motor Bus Company and also provided service on the same route. Since motor vehicles were considered to be modern and attractive at that time, people were eager to ride. Hence, the business was profitable and gained more investors. Competition between bus companies was aroused. More and more small operators served the Kowloon peninsula and the Hong Kong Island.

Figure 2.7 The first bus of China Motor Bus Company used in 1924 (captured from google.com photo search)

2.3.2 Birth of franchised routes In the 30’s, Hong Kong Government implemented the Regional Franchise System for better management of bus services. Two regions were put up for bids: the Hong Kong Island and, the Kowloon peninsula and New Territories. The rule of game was one company serving a region, while part of the profit had to be paid as tax. In 1933, the regional franchise of the Hong Kong Island was awarded to the China Motor Bus Ltd. (CMB), while that of the Kowloon peninsula & New Territories was awarded to the Kowloon Motor Bus (1933) Ltd. (KMB).

11 2.3.3 Double-deckers During the Japanese occupation in the Second World War, the franchised bus companies were unable to operate since the Japanese army used up all the reserve petrol for military use. After the liberation of Hong Kong in 1945, CMB and KMB required to put in much repair work before a very limited number of buses could go back into operation. Even, operators had to convert large lorries into buses so as to satisfy passenger demand. 1949 was an important year for the local bus service history: the first double-decker landed Hong Kong. The usefulness of double-deckers in large carrying capacity was very apparent. Bus operators saw the great demand and brought in a large number of them in the 60’s. Then the half-cab design double-deckers (as illustrated in Figure 2.8) flourished and shouldered an important transport link for the urban area. Buses with front-engine and half-cab design had a lot of disadvantages, including low carrying capacity, transmission of heat into the passenger compartment and creation of much noise. Therefore, bus operators imported rear-engine buses in 1973. The characteristics of these buses were high carrying capacity with “3+2” seating design, silent passenger compartment and low floor design. Because of its success, “3+2” seating design became the trend.

Figure 2.8 The first generation of double-decker used in Hong Kong (captured from google.com photo search)

In 1979, powerful traction buses were introduced to replace the tasks of the old half- cab buses. In addition to the powerful engine output, the travelling time was much reduced. Thus, they were imported in a large numbers to run on the routes through the mountains.

12 Due to the rapid development of the community, population steeply increased. The demand for public transport steeply increased as well. 3-axle double-deckers were introduced which provided a higher carrying capacity in order to cope with the demand. The buses had long chassis of 12 metres and a high carrying capacity up to 170. Compared with the old fleets, the carrying capacity was increased by 45%. From then on, 3-axle double-deckers were the new trend for the companies’ purchases.

2.3.4 Air-conditioned bus In 1980, bus manufacturers delivered prototype of new buses with independently engined air-conditioning system. They were the first two double-decked air- conditioned buses in the world. Positive feedbacks were received from passengers. Nevertheless, the high operating costs forced the bus company to delay the idea of providing air-conditioning bus service. After the failure trial, bus companies tried the operation of double-deck air-conditioned bus service again in 1988. For the confirmation of its success in 1990, the two operators began to buy the air- conditioning buses for service.

2.3.5 Change of Bus Services For the reason of easy access to the disabilities, low-floor buses were introduced in 1996. The characteristics of low-floor and access plank for wheelchairs became the standard of new buses in Hong Kong. Instead of the convenient facilities, cabin facilities and space provided had been improved. The trips on buses became more comfortable.

Figure 2.9 The engines of the new buses are all satisfied with the European Standard (captured from google.com photo search)

13 Non-air-conditioned buses are being replaced in this decade. Meanwhile, operators began to concern with the environmental protection issue. New fleets are equipped with environmental friendly engines that meet the latest European Standard (as illustrated in Figure 2.9). For the pre-Euro buses, their engines are upgraded to meet the requirements. On one hand, bus operators equip catalytic converters on old buses to minimize black smoke emission. On the other hand, fleets are fuelled with “Ultra Low Sulphur Diesel” to reduce air pollutants exhaust. Actually, the operating strategies of the bus companies have been changed to focus on the service quality rather than the quantity.

2.4 Design of bus Designing a so called comfort space, the primary goal is to provide a pleasant environment for passengers staying inside. Basically this means the thermal comfort environment has to be satisfied. A person has pleasant thermal comfort if one feels that the temperature, humidity and air movement of the surrounding air, as well as the mean radiant temperature are optimal. In this case passengers staying in the compartment do not wish the surrounding air to be warmer, colder, and more or less humid. Furthermore, the in-bus air quality is taken into consideration. Good air quality inside the compartment can be maintained by introducing clean fresh air in a sufficient amount to dilute air contaminants and bioeffluents. Meanwhile, the infiltration of roadway vehicular exhaust should be avoided. Investigating the in-bus thermal comfort and air quality environment, the basic design of bus should be realized. The engineering structure can be divided into 3 major sections, i.e. engine, passenger compartment and air-conditioning system.

2.4.1 Engine In Hong Kong, there are strict controls over the overall length, width and height of buses. The maximum length, width and height of buses are 12m, 2.5m and 4.6m respectively. The chassis design shown in Figure 2.10 is a typical local bus. It is 12 meters long, and rear-engined.

Figure 2.10 Layout of a typical chassis design (captured from www.busonly.com)

The construction of buses starts with the chassis (as illustrated in Figure 2.11); following by fitting various parts of the power train, including engine, gearbox, axle and wheels. Afterwards it comes with the fuel tank, the driver’s instrumentation panel and of course the steering wheel. Two large fuel tanks (red) are located at the side of the middle, while the gearbox (blue) and the engine (green) are at the back. Figure 2.12 showed a bus under construction.

Figure 2.11 Bus chassis including the engine, gearbox, axle, wheels, fuel tank, driving panel, steering wheel and driver’s seat. (captured from google.com photo search)

Figure 2.12 Bus under assemble in the body builder factory (captured from google.com photo search)

15 In the past, most of the buses were front-engined with half-cab design (as illustrated in Figure 2.13). It had a lot of disadvantages, such as limitation in carrying capacity, transmission of heat into the passenger compartment, and creation of much noise. Nowadays, all buses have rear-engine design for better isolation of heat and noise while increasing the space inside the passenger compartment (as illustrated in Figure 2.14). Also, it can help lower the floor for easy access.

Figure 2.13 Half-cab design bus with the engine installed at the front (captured from google.com photo search)

Figure 2.14 The engine compartment is sealed and completely isolated from the cabin

2.4.2 Passenger compartment

Supply air diffusers Generally speaking two types of diffusers are installed on buses (as illustrated in Figure 2.15). One of the types is for the general air distribution throughout the passenger compartment, which can neither be closed nor adjusted. Another one is for

16 individual air supply. They are installed above the seats, and which the flow rate and direction can be adjusted for individual thermal preference.

Adjustable diffusers for individual comfort.

Non-adjustable diffusers for general purpose

Figure 2.15 Air distribution duct and diffusers installed on the lower deck (captured from google.com photo search)

Heater On the air-conditioned buses, heaters are equipped to achieve thermal comfort and dehumidification as well. They are covered by a duct installed on the sides of floor (as illustrated in Figure 2.16). It is because people may feel more comfortable with the temperature stratification of warmer at the feet level. However, this is integrated in the air-conditioning unit in order to provide larger space in the compartment.

Figure 2.16 Heater duct installed under the seats

Other facilities The low-floor design is a compulsory requirement as the bus companies purchases their new buses. Also access plank for the wheelchairs is available for easy access of the disabilities (as illustrated in Figure 2.17). Individual seats with leather covers are equipped for hygiene and comfort (as illustrated in Figure 2.18 and Figure 2.19). Adequate luminance is available for better sight of vision. Considering the safety

17 issues, handrails are painted in sharp colour and are available along the corridor (as illustrated in Figure 2.20); while non-slip material is equipped on floor and staircase.

Figure 2.17 Access plank for disabilities (captured from google.com photo search)

Figure 2.18 Chenille seat covers are served for the purpose of luxury feeling on older buses

Figure 2.19 Leather seat covers are served for durability and hygiene on new buses (captured from google.com photo search)

Figure 2.20 Handrails are available along corridor and they are printed in sharp colour for safety purpose. Lighting for adequate luminance. (captured from google.com photo search)

2.4.3 Air-conditioning system In the first generation of air-conditioned buses, “split-type unit” is equipped (as shown in Figure 2.21). The whole air-conditioning system components are placed at the rear, except the evaporators for lower deck (as illustrated in Figure 2.22) which are installed in the middle of the passenger compartment. The disadvantage of this design is that the evaporators are large that occupy space and lower the headroom above the seats. Also, sometimes, condensation water may drop through the louvers on humid days.

Components of the system: A: Upper saloon evaporators B: Air distribution ducts C: Control panel D: Control box E: Lower saloon evaporators F: Lower saloon air ducts G: Condenser air inlet H: Compressor for lower saloon I: Compressor for upper saloon J: Drive shaft K: Condenser for upper saloon L: Condenser for lower saloon

Figure 2.21 The configuration of split type air-conditioning system on bus. (captured from www.busonly.com)

Figure 2.22 The evaporators of the split type system installed on lower deck

Starting from the second generation, “combined-type unit” is used (as shown in Figure 2.23). The whole air-conditioning unit is placed at the rear, including the evaporators for the lower deck. Return air grilles are located in the back (as illustrated in Figure 2.24 and Figure 2.25). This design provides higher headroom in the passenger compartment and had better arrangement of the mechanical components.

Figure 2.23 The configuration of combined type air-conditioning system on bus. (captured from www.busonly.com)

Figure 2.24 Return grilles of the evaporators of combined type system on the lower deck

Figure 2.25 The evaporator on the upper deck

Mechanical outdoor air ventilation The ventilation is achieved through the outdoor air intakes located on the sides at the rear of buses as illustrated in Figure 2.26. Besides, gap under doors are left intended to raise the ventilation inside the passenger compartment (illustrated in Figure 2.27 and Figure 2.28).

Figure 2.26 Location of fresh air intake (captured from google.com photo search)

Figure 2.27 A close-up of the door gap

Figure 2.28 The gaps are available at both the top and the bottom of the doors

2.5 Literature Review

2.5.1 Standards and guidelines concerned with Indoor Air Quality The Hong Kong Environmental Protection Department (HKEPD) is a pioneer in launching the public awareness about indoor air quality. It adopts a set of air quality objectives as common benchmark in different premises. Practice Note for Managing the Air Quality in Public Transport Facilities (PN-PTF) was released in 2003 (HKEPD, 2003a). PN-PTF states an air quality guideline as well as practical recommendations about the management and implementation of the performance and monitoring processes on air-conditioned transportation modes and at stations. Two versions are published targeting for buses and railways. It is not intended to act as strict regulations but rather as recommendations for operators. It states that air- conditioned buses should be capable in isolating the outdoor air pollutants from

22 entering the passenger compartments. The practice note provides simplified and cost effective benchmark for the operators. The authority suggests selecting the carbon dioxide (CO2) concentration level as a surrogate indicator of in-bus air quality. It is because its concentration in an indoor environment is a good indicator of ventilation sufficiency. The prescribed in-bus CO2 concentration levels are 2500 ppm and 3500 ppm in TWA 1-hour for two comfort levels [Level 1 – CO2 < 2500 ppm, represents good air quality of a comfortable bus facility at which there is no health concern identified; Level 2 – CO2 < 3500 ppm, represents the air quality of a bus facility at which there is no health concerned identified]. However, for further consideration, other significant air pollutants, like carbon monoxide (CO) and respirable suspended particulates (RSP), should be concerned in the PN-PTF. These parameters correspond to the impacts from outdoor environment. No studies concerned with the ventilation rate on public transportation modes in Hong Kong. The study of ventilation characteristics on air-conditioned buses is crucial to quantify the main factors affecting the performance, through analysis of appropriate in-bus measurement. In the ASHRAE Handbook – HVAC Applications (ASHRAE, 2007), the issue of air-conditioned compartments on buses was discussed at length. It stated in-bus ventilation rate should be determined with special considerations, including length of occupancy, occupancy turnover, air infiltration, outside air quality, frequency and duration of door openings, personal preference, interior contamination sources and exterior pollutant sources. It suggested providing -1 an outdoor air ventilation rate between 2.5 and 5 l · s per passenger at nominal passenger load conditions. The magnitude of ventilation rate could reveal the performance in diluting and exhausting excessive human bioeffluents and air pollutants inside the passenger compartment. According to the outdoor air ventilation requirements stated in ASHRAE Standard 62 (ASHRAE, 2004), special considerations are required for determination of ventilation rate within vehicles, especially the outdoor air quality. Mechanical ventilation is applied to improve the in-bus air quality. However, the quality is directly influenced by the dynamic roadway environment along the route. The variation of ventilation and in-bus air pollutant concentration levels were studied in order to evaluate the impacts from the ventilation associated with surrounding environment.

23 Table 2.3 The prescribed concentration levels of indoor air contaminants recommended by the local guidelines and the international standards

CO2 CO RSP NO2 SO2 -3 (ppm) (ppm) (mg · m ) (ppm) (ppm) Local Guidelines (for indoor premises) GN for offices & public places - 1000 (8 hr) 8.7 (8 hr) 0.18 (8hr) 0.080 (8 hr) - Good class (HKEPD, 2003b) Control of air pollution in Public 26.2 (1 hr) 0.159 (1 hr) 0.305 (1 hr) Transport - - 100.4(5min) 0.957(5min) 0.381(5min) Interchanges (HKEPD, 1998) Labor Dept. 5000 (8 hr) 3 (8 hr) 2 (8 hr) 25 (8 hr) - (1998) 30000(15min) 5 (15 min) 5 (15 min) International Standards (for indoor premises) 10 (8 hr) 25 (1 hr) 0.02 (1 yr) 0.021 (1 yr) 0.008 (24hr) WHO (2005) - 50 (30min) 0.05 (24hr) 0.106 (1 hr) 0.191(10min) 90 (15min) Foreign Guidelines (for indoor premises) 0.25 (1 hr) Australia (1992) - 9 (8 hr) - - 0.50 (10min) 0.05 0.019 3500 11 (8 hr) Canada (1995) - (long-term) (long-term) (long-term) 25 (1 hr) 0.25 (1 hr) 0.38 (5 min) 1200 7 0.05 Finland (2003) - - (ceiling) (ceiling) (ceiling) 5000 (8 hr) 30 (8 hr) 5 (8 hr) Germany (2000) 4 (8 hr) 0.5 (8 hr) 10000 (1 hr) 60 (30 min) 10 (5 min) 10 (24 hr) 0.10(24hr) 0.04 (24 hr) Japan (1997) - 0.06 (24 hr) 20 (8 hr) 0.20 (1 hr) 0.1 (1 hr) Singapore(1996) 1000 (8 hr) 9 (8 hr) 0.15 (24hr) - - 5000 (8 hr) 30 (8 hr) 3 (8 hr) 2 (8 hr) UK (2005) - 15000(15min) 200 (15min) 5 (15 min) 5 (15 min) US OSHA 5000 (8 hr) 50 (8 hr) - 5 (ceiling) 5 (8 hr) Guidelines for public transportation modes

*Level 1 - PN-PTF - Buses 2500 (1 hr) & Railways - - - - *Level 2 - (HKEPD, 2003a) 3500 (1 hr)

*Normal: 2000 *Normal: 0.15 Korea (2006) *Rush hr: 3000 *Rush hr: 0.20 - - - – Buses (average for an (average for an operation) operation)

There are guidelines concerning with the indoor environment published by local authority and foreign countries, such as Australia, Canada, Finland, Germany, Japan, Korea, Singapore, United Kingdom and America. Details of prescribed levels are summarized in Table 2.3.

2.5.2 Literatures concerned with indoor air quality Local surveys concerning the in-bus air pollutants exposure have been reported. Chan et al. (1993, 1999 & 2001) studied the public transport commuters’ exposure affected by traffic air pollution. They showed that bus commuters exposed in the lowest CO concentration level among all roadway transport modes (including tram, car, taxi and mini-bus), and the in-bus CO concentration level ranged from 0.2 to 4.6 ppm. Wu (1990) reported that the CO concentration levels were 1.07 ppm (at roadside), 2.01 ppm (on lower-deck) and 1.25 ppm (on upper-deck) respectively; which the in-bus concentration levels were higher as compared to the roadside condition. Chan (2003) studied the indoor-outdoor relationships of carbon oxides on buses. It focused on the effect of passenger load to in-bus CO2 concentration level. However, the ventilation rate on buses was not involved. Chan et al. (2002a) reported -3 that the RSP concentration levels were 0.074 and 0.112 mg · m on air-conditioned and non-air-conditioned buses respectively when they were travelling in the urban districts. The difference of concentration showed the particulates filtering effectiveness on air-conditioned compartment. The literatures evidenced the local in-bus air quality conform to the prescribed levels stated in international standards, like WHO. However, complaints of poor in-bus air quality have been ceaseless (Oriented Daily, 1 Sept. 2005). It reveals the conformance is not equivalent to satisfying the passengers’ perception towards air quality. The operating practice neglects the passengers’ subjective sensation and satisfaction. It is no longer adequate to satisfy the passengers with healthy air only. Satisfaction and comfort must be emphasized that consideration of subjective perception towards the in-bus air quality is crucial to the service quality. Therefore, the comfort model concerning with in-bus air quality was studied. Overseas studies reported the in-bus CO concentration level was relatively higher in foreign countries. It was about 2.5 ppm in Paris (Dor et. al, 1995), 10 ppm in Athens (Duci et. al, 2003) and 30.4 ppm in Mexico (Fernandez-Bremauntz and Ashmore,

25 -3 1995). The in-bus RSP concentration level was 0.144 mg · m in Munich (Praml and -3 Schierl, 2000) and 0.300 mg · m in Manchester, UK (Gee and Raper, 1999). From the literatures, the in-bus air pollutants concentration levels (CO & RSP) measured in different cities were summarized (in Table 2.4). All of them reported the commuting exposure in the urban districts. Considering the conditions in Hong Kong, the in-bus air pollutants concentration levels were found to be lower. Comparing with those findings in other cities, most of them had higher air pollutants concentration levels. For example, the in-bus CO concentration level was as high as 30 ppm in Mexico. Thus, it could be said that the in-bus air quality in Hong Kong was better among other cities.

Table 2.4 Literatures concerning with in-bus air quality

Non-Air-Conditioned Air-Conditioned Literatures City Buses Buses CO Chan et. al (1999) Hong Kong – 1.9 ppm

RSP (PM10) RSP (PM10) Chan et. al (2002a) Hong Kong -3 -3 – 0.112 mg · m – 0.074 mg · m

CO Wu (1990) Hong Kong – 2.01 ppm (lowerdeck) – 1.25 ppm (upperdeck)

CO Dor et. al(1995) Paris – 2.5 ppm

CO Duci et. al (2003) Athens – 10.4 ppm

Fernandez-Bremauntz CO Mexico & Ashmore (1995) – 30.4 ppm

RSP (PM10) Praml & Schierl (2000) Munich -3 – 0.144 mg · m

RSP (PM10) Gee & Raper (1999) Manchester -3 – 0.300 mg · m

2.5.3 Standards and guidelines concerned with Thermal Comfort

PN-PTF (HKEPD, 2003a) mentioned the in-bus thermal comfort environment in its appendix. It stated that the air temperature and relative humidity could be indicators of thermal comfort conditions on buses. The recommended air temperature range

26 from 20 to 28oC and relative humidity range from 40 to 70%, or comfortable ranges that achieving the passengers’ thermal preference. ASHRAE Standard 55 (ASHRAE, 2004) defined thermal comfort as “that condition of mind which expresses satisfaction with the thermal environment.” An important feature of this definition was the use of “the condition of mind” as opposed to “the condition of the body”. Psychological responses to an environment are determined by stimuli which affect all body senses. The occupant’s physiological response is determined essentially by the thermal exchange between the occupant and the environment. For an occupant to be satisfied with the thermal comfort environment, the physical parameters affecting a person’s thermal balance must be such that the heat generation was equal to the heat lost and that the person was in a zone of thermal neutrality. The predicted mean vote (PMV) by Fanger (1970), as presented in ASHRAE (ASHRAE, 2005) and International Standard ISO 7730 (ISO, 2006), was probably the index of thermal comfort most widely used for assessing moderate indoor thermal environments. Fanger used classical heat transfer theory and empirical studies to derive the general comfort equation which incorporate for environmental parameters (air velocity, mean radiant temperature, relative humidity and air temperature) and two personal parameters (clothing insulation and activity level). It predicted the expected comfort vote on the ASHRAE scale of subjective warmth ( hot Q warm Q slightly warm Q neutral Q slightly cool Q cool Q cold ). The equations for the predicted mean vote are as follows:

× −8 [()+ 4 − ()+ 4 ] M – W = 3.96 10 f cl tcl 273 tr 273 + − f cl hc (tcl ta ) + 3.05[]5.73 − 0.007()M −W − p a (2.1) + 0.42[]()M −W − 58.15 + ()− 0.0173M 5.87 pa + ()− 0.0014M 34 ta where

27 = − ()− tcl 35.7 0.0275 M W − ()− Rcl { M W − []− ()− − 3.05 5.73 0.007 M W pa (2.2) − []()− − − ()− 0.42 M W 58.15 0.0173M 5.87 pa − ()− 0.0014M 34 ta }

2.38()t − t 0.25 2.38()t − t 0.25 > 12.1 V h = cl a cl a (2.3) c  ()− 0.25 <  12.1 V 2.38 tcl ta 12.1 V + < 1.0 0.2I I cl 0.5clo f = cl (2.4) cl  + > 1.05 0.1I cl I cl 0.5clo

In the equations, Icl and fcl were functions of the type of clothing. M was the function of the type of activity. ta, pa and V were thermal environmental variables. Fanger related the predicted mean vote to the imbalance between the actual heat flow from the body in a given environment and the heat flow required for optimum comfort at the specified activity by the following equation:

()−0.036M PMV = [0.303e + 0.028]L (2.5)

The PMV index predicted the mean response of a large group of people according to the ASHRAE thermal sensation scale (as illustrated in Figure 2.29). Also Fanger related the predicted percent dissatisfied (PPD) to the PMV as follows:

[]−()0.03353PMV 4 +0.2179PMV 2 PPD = 100 − 95e (2.6)

-3 -2 -1 0 +1 +2 +3

cold cool slightly neutral slightly warm hot cool warm

Figure 2.29 ASHRAE thermal sensation scale

Fanger (1970) stated six major thermal comfort factors: air temperature, relative humidity, air movement, radiant temperature, clothing insulation and activity level. Many studies including Kaynakli & Kilic (2005), van der Kooi & Wan (1993), Olesen (1989) and Roy et al. (2003) investigated the thermal comfort in vehicles following these principal factors. But of these six, only the former three parameters can be controlled through air-conditioning.

28 Besides ISO (2006) stated the thermal comfort requirement as “-0.5 < PMV < 0.5”, non-uniformity from radiant temperature asymmetry is less than 5oC for horizontal asymmetry (heated ceiling) and less than 10oC for vertical asymmetry (cold window), vertical air temperature difference between feet and head less than 3oC and mean air velocity less than 0.15 m·s-1 (for air temperature < 23oC) or less than 0.25 m·s-1 (for air temperature > 23oC).

2.5.4 Literatures concerned with Thermal Comfort

Chan et al. (1998) has conducted a large-scale survey concerning with the occupants’ thermal comfort in offices in Hong Kong. It reported that the indoor mean air temperature was 21.8 oC and 20.6 oC in the summer and winter respectively. Among the measurement samples obtained in the offices, there were only 34.9% fell within the ASHRAE Standard 55 comfort zone in the summer, while 54.4% fell on the cool side of the comfort zone (shown in Figure 2.30). In winter, 68.1% of the samples fell within the winter comfort zone, while most of the rest (22.1%) fell in the region above the winter comfort zone (shown in Figure 2.31). The analysis showed that the preferred operative temperature in Hong Kong’s offices during summer was approximately 22.5 oC, which was a full degree cooler than the neutral temperature stated in ASHRAE. In the winter, the preferred operative temperature was approximately 20.8 oC, which was nearly a half-degree lower than the one in ASHRAE. Also, in the questionnaire survey conducted in the offices concerning with the occupant’s dressing, it was found that the clothing insulation of occupants in Hong Kong had a higher clo-value (30% and 60% higher in summer and winter respectively) than that found in Australia (ASHRAE, 1994). The average clothing insulations were 0.58 and 0.86 clo in Hong Kong, while they were 0.44 and 0.54 clo in Australia, in the summer and winter respectively. Thus, it gave evidence that office occupants in Hong Kong tended to expect a cooler office while used to wear more, since they had different culture in dressing code.

Figure 2.30 Distribution of the indoor climatic measurements on the ASHRAE Standard 55- 1992 summer comfort charts (captured from Chan et al., 1998)

Figure 2.31 Distribution of the indoor climatic measurements on the ASHRAE Standard 55- 1992 winter comfort charts (captured from Chan et al., 1998)

Guan (2003) stated that there were two general approaches applied in developing thermal comfort prediction models. The first approach is the “direct statistical correlation approach.” Environmental data and human thermal comfort responses to the environment were collected (as illustration in Figure 2.32). Statistical analyses were then performed to derive prediction equations. This approach usually provided accurate predictions as long as the resulting models are only applied to conditions

30 and vehicles that were similar to the test scenarios used to collect the data and as long as sample sizes were sufficiently large to ensure statistical reliability. In general, such requirements result in limited applicability of the statistical models. The lack of interpretability was another major drawback of the direct statistical correlation model. The models only predicted that people will respond in a certain way in certain conditions. They did not provide any logical reasons. Instead of a direct statistical correlation approach, the second approach was to model the physiological responses of the people to the environmental conditions and then related thermal sensations to these responses. Using this approach, the model would be based on physical processes that can be modelled using well-established laws, principles, and physical data. And the simple relationships between physiological variables (such as body temperature, heat gain, etc.) and subjective thermal sensations could be developed.

Direct statistical correlation approach

Collect physical environmental data Thermoregulation modeling software to obtain physiological responses in different conditions (ti, w, v) data (tsk, heat flux, sweat rate)

Conduct human subject testing at Combined physiological and the same conditions to obtain human sensation data approach for psychological thermal sensation data development of thermal sensation predictive model

Direct statistical correlation approach for development of thermal comfort sensation predictive model

Figure 2.32 Combined physiological and psychological modelling approach (Guan, 2003)

Fanger (2001) discussed the performance of future air-conditioning system that should enhance productivity, reduce sick building syndrome symptoms and be compatible with energy efficiency and sustainability. It suggested the provision of personalized supply system could fulfil individuals’ preferences.

31 Fountain et al. (1996) found the thermal comfort sensation and preferences were different due to individual culture associated with one’s expectation and adaptation. Hence application of a single temperature set-point may not be appropriate in public premises. Physiological assessments as well as psychological issues should be considered for thermal comfort. Besides, one’s thermal comfort varies relating to the thermal properties of clothing which changes with activity levels and subjective feelings (Hollies and Goldman, 1977). Humphreys and Nicol (1998) viewed thermal comfort as a dynamic system but not a fixed attribute. People are able to self-regulating the sensation. So Humphreys (1995) recommended designers should learn about the occupants’ daily routines, practices, habits, preferences, dressing codes and the zone nature before designing the thermal comfort environment. Adequate possibilities of air-conditioning setting adjustment should be provided so that occupants can adjust for individual thermal preferences. The “comfort-provision and discomfort-alleviation strategies” stated in Leaman and Bordass (1995) had similar viewpoints with Humphreys. There were seldom studies concerning with thermal comfort environment on local public transports. Shek & Chan (2008) and Chan & Shek (2009b) conducted parameters measurement and questionnaire survey about thermal comfort on local air-conditioned buses. They found the in-bus air temperature ranging from 17.1oC to 31.5oC, with the ambient condition between 10.0oC and 32.0oC. Meanwhile the relative humidity ranged from 37% to 95% and air speed ranged from 0.04 m·s-1 to 1.85 m·s-1 on buses. Zhang et al. (2007a) studied the thermal comfort environment in hot-humid climate of China. It reported that an individual’s perception towards thermal comfort was affected by behavioral, physiological and psychological factors. People preferred slightly cool environment in hot climates. Higher percentage of residential occupants accepted in air conditioned residences. Over 75% of occupants felt comfortable under the operative temperature between 22 and 27oC, while the preferred temperature was 22.8oC. Zhang et al. (2007b) considered the thermal comfort environment under air movement. The authors found the subjective perception affected by the combination of air temperature and air movement. Higher indoor air speed helps human body heat release in a hot environment. However, the draft risk appears when the percentage of dissatisfaction towards air movement reaches 15%.

32 Some local parties conducted surveys concerning with thermal comfort environment in indoor premises. Friends of the Earth (HK) launched a campaign 'Report Thermal Crimes' in the summer 2005, concerning with the overcooling premises in Hong Kong (HKFOE, 2005). There were 266 reports about thermal offenders received. The statistics of thermal offenders was illustrated in Figure 2.33. The thermal offenders distributed over various sectors. Public transportation sector was in the highest ranking which made up 24% of the total reports. Meanwhile, buses obtained the highest percentage of complaints in the sector, reaching 62%.

Figure 2.33 Statistics of thermal offenders (captured from HKFOE, 2005)

In the same year, another party received over 200 complaints on over-cooling in public premises in June 2005, while 60% complained about buses (The Standard, 27 June 2005). ‘Arctic condition’ was used to describe the in-bus thermal comfort environment. Also, complaints of hot and stuffy air on air-conditioned buses were found in the same summer (Oriental Daily, 1 September 2005). Similar activity was conducted again in 2006, buses kept the ‘championship’ (Apple Daily, 16 August 2006). Instead of the local survey, negative comments were found in the foreign touring guides about the over-cooling premises. Eyewitness Travel Top 10 (2004): Hong Kong gave such a tip-off: "A long-sleeved top is advisable for some of the arctic air- conditioning." Lonely Planet (1998) described the big chill inside the buses as this: "temperatures are set so low you may find your extremities turning blue." National Geographic Travellers Hong Kong (2002) remarked “In summer, air-conditioning in restaurants, bars, theatres, offices and on public transportation can be cool enough to warrant a light jacket.”

33 Air-conditioned double-deckers were introduced in 1988, and there are now approximately 5750; over 95% of the service fleets. Local bus service operators set the air temperature levels of air-conditioned passenger compartments at 19oC and 23oC in winter and summer respectively. However, numerous complaints about thermal discomfort suggest less-than-satisfactory environments on these buses. Improvement of the in-bus thermal comfort environment is the goal.

The implementation of improving air quality and thermal comfort on buses should be professional and effective. As the experience of Lam and Fong (2003), they suggested the promotion should be adopted by both the operators and the publics. Hence the campaign should include public education to promote public awareness, adopting a set of objectives as benchmark for evaluation and publishing professional practice notes for operators to improve the in-bus environment. Moreover, considering the ambient environmental impacts, the transport strategy must be emphasized (Lam, 2001).

34 3 Field study of in-bus air quality and thermal comfort environment

3.1 Introduction

In the beginning of air-conditioned bus service, either the operators or passengers focused on the coolness inside passenger compartments. Air freshness and ventilation were not put into serious consideration. These practical issues were neglected primarily due to the erroneous translation of air-conditioning as “cold-air” in Cantonese. Native speakers not only misunderstood the intended applications of air-conditioning systems, but also considered low air temperatures to be synonymous with better service. It leaded to numerous complaints about discomfort which suggested less-than-satisfactory environments on buses. Three major factors dominate the in-bus air quality, i.e. ventilation rate, outdoor air quality and particulates filtering effectiveness. Studies have been undertaken regarding in-cabin air quality and passengers’ air pollutant exposure on public transport facilities. Petersen and Allen, (1982); Kot and Lai, (1985); Flachsbart et al., (1987) and Luria et al., (1990) found correlation between CO concentration and traffic flow as well as vehicle travelling speed. CO detected inside passenger compartment enters from the out-bus environment if the passenger compartment is built isolated from the engine compartment (Duffy and Nelson, 1997; Gee and Raper, 1999). Sources of RSP can be fibres from the commuters’ clothes and luggage, and dust inside the passenger compartment. On the roadway environment, traffic volume is a major factor affecting the particulate matters concentration (Bevan et al., 1991; Lam et al., 1999; Praml and Schierl, 2000). Particulate matters enter the bus passenger compartments through outdoor air intake and door openings. As the bus air-conditioning system equipped with filters, in-bus RSP concentration level reveals the effectiveness of the particulate filtering. Thus carbon monoxide (CO) and respirable suspended particulates (RSP) are significant indicators of air pollutants exposure inside bus passenger compartment. Considering the thermal comfort environment on buses, passengers’ subjective perceptions should be focused. The significances are sensations towards temperature, humidity and air movement in the air-conditioned zone.

35 3.2 Field study

3.2.1 Air quality Air-conditioned buses are installed with fixed-type windows that in-bus air contaminants dilution is achieved through the mechanical ventilation system. Ventilation sufficiency is hence a key factor to the air contaminants dilution effectiveness inside the passenger compartment. Moreover, the quality of outdoor air introduced into the compartment directly affects passengers’ exposure level. Particulates filtering effectiveness of air-conditioning system sways the in-bus air quality too. Considering the acceptability of in-bus air quality, this study focuses on such issues in the air-conditioned compartment. The major air contaminants found inside the bus passenger compartments include the carbon monoxide (CO), carbon dioxide (CO2), nitrogen dioxide (NO2), sulphur dioxide (SO2), volatile organic compounds (VOCs) and respirable suspended particulates (RSP). Three of them were selected as the air quality indicators; they were carbon dioxide, carbon monoxide and respirable suspended particulates.

3.2.1.1 Ventilation sufficiency

Metabolic CO2, human odour and bioeffluents are the major air contaminants generated by passengers inside the compartments. These in-bus air contaminants do not have acute health effects under normal exposure. The literature review of CO2 prescribed level is illustrated in Figure 3.1. Its concentration varies depending on the air contaminant generation rate and the ventilation rate. It rises if ventilation is insufficient. Among the air contaminants existing inside the compartment, the concentration level of CO2 is the most easiest to be measured. CO2 is a colourless, odourless and non-flammable gas which is produced by metabolic processes. The average atmospheric concentration is about 385 ppm. However, the level may vary depending on the population density and ventilation in the space. An increase of level raises the rate and depth of breathing and blood acidity. People expose to a level over 50000 ppm would have health effects to the central nervous system, like headache and dizziness (MOSS Canada, 1995). Since it is widely accepted to reveal the issue (ASHRAE, 2007; ASTM, 2002; Fanger & Berg-Munch, 1983; HKEPD,

2003; Chan & Shek, 2009; Shek & Chan, 2008), in-bus CO2 concentration level is hence selected as the surrogate indicator of ventilation sufficiency on bus.

36 concentration level concentration 2 Human health effects and literatures of CO literatures effects and Human health Figure 3.1

37 3.2.1.2 Outdoor air quality Infiltration of air pollutants causes direct influence to the passengers’ exposure inside passenger compartment. A key factor is the roadway air quality along the routes (HKEPD, 2005). Hence the major automotive-related air pollutants are focused.

They are CO, NO2 and SO2, coming from the exhaust emission of vehicular engines. CO is a toxic gas since it would interfere with the oxygen transport capacity of blood. Sufferers might die if exposed in very high concentrations (HKEPD, 1997). Research studies show that long-term exposure to NO2 increases symptoms of bronchitis in asthmatic children (WHO, 2005). Reduced lung function growth is also linked to

NO2 at concentrations currently found in cities of Europe and North America.

Inhalation of low concentrated SO2 is negligible. It can only reach the gas-exchange region of the lungs after sorption onto fine particles; and the available particle surface is limited except when very large mass concentrations of fine particles are present (WHO, 2000). Concentration of CO was correlated with other automotive-related air pollutants

(NO2 and SO2) to evaluate whether it could be a representative indicator. Ambient air pollutants concentration data obtained at the roadside monitoring stations (from the year of 2000 to 2004) were correlated and plotted in Figure 3.2.

Figure 3.2 Correlation of roadside air pollutants concentration levels

38 By correlating the automotive-related air pollutants concentrations obtained in the roadside stations, the profiles of NO2 and SO2 level were found corresponding to the CO concentration level. Among these air pollutants, CO is the most toxic but stable one; and it is easier to be measured. Instead, VOCs are not considered since in-bus sources are found that lower the reliability of indication. CO is hence selected as the indicator for investigating the quality of outdoor air intake.

3.2.1.3 Particulates filtering effectiveness RSP refers to those suspended particulate matters which have the nominal aerodynamic diameters of ten micrometres or less (PM10). On the roadway environment, traffic volume is a major factor affecting the particulate matters concentration (Bevan et al., 1991; Lam et al., 1999; Praml and Schierl, 2000). The in-bus concentration level was selected as an indicator in revealing the effectiveness of the particulates filtering system. In-bus RSP could be the fibres from the commuters’ clothes and luggage, the dust inside the passenger compartment and the particles from outside environment. Exposure in high concentration can cause itching and irritation to the skin, eyes and the upper respiratory system and aggravation of existing respiratory or cardiovascular disease (HKEPD, 1997).

Figure 3.3 Correlation of in-bus CO2 and RSP concentration levels on air-conditioned buses

Concentrations of in-bus RSP and CO2 were correlated and plotted in Figure 3.3. It showed that RSP concentration descended with higher in-bus CO2 concentration. The negative gradient trend-line revealed that lower ventilation rate raised the particulates filtering effectiveness. It could be explained that a lower ventilation rate reduced the quantity of outdoor air intake; whilst larger proportion of in-bus air was recirculated inside the air-conditioned compartment. In-bus air recirculated and passed through the filtering system repeatedly, resulting as more particulate matters were filtered.

Therefore, RSP concentration reduced under higher in-bus CO2 concentration level or lower ventilation rate.

3.2.2 Thermal comfort Air temperature, relative humidity and air speed are essential thermal comfort settings of air-conditioning system. They are corresponding to the major sensations towards thermal comfort issues (Shek & Chan, 2008). Shek & Chan stated that it is inappropriate to fix these thermal comfort settings but adjust depending on the ambient condition for higher satisfaction.

3.2.2.1 Temperature Air temperature has the most direct effect on thermal comfort. The air temperature of indoor environment is influenced by factors such as the temperature control of the air-conditioning, external heat gain entering the compartment, the engines, and other heat sources such as lighting. Set-point temperature at which people feel comfortable depends on activity levels, dressing code, age, gender and natural body temperature. Such factors vary from individual to individual, and on seasonal conditions.

3.2.2.2 Humidity Humidity influences thermal comfort by affecting the human body’s ability to lose body heat through perspiration. In humid conditions, it is more difficult to lose heat. Such effect is therefore the same as raising the temperature and people feel “sticky”. Low relative humidity causes eyes, noses and throats to dry which may lead to discomfort, irritation and increased susceptibility to infection. Extremely low humidity can cause static electricity, which is uncomfortable for people.

40 3.2.2.3 Air movement Heat transfer by convection is usually caused by air movement within the space or by body movements (ASHRAE, 2005). Appropriate air movement increases the satisfaction towards thermal comfort. However, excessive air movement causes annoying feeling and undesired local cooling of human body.

3.2.3 Measurement

3.2.3.1 Measurement journeys The measurement activity commenced in January 2003 and was completed by August 2005. A total of 518 measurement journeys were conducted on 60 selected bus routes which travelled through Hong Kong. Among the journeys, 459 were conducted on air-conditioned buses while 59 were on non-air-conditioned buses. The major criterion for selecting the bus routes was that the traffic conditions and route natures must closely resemble the general local bus operation conditions. The route natures included urban, sub-urban and rural districts while some of the routes travelled through highways and tunnels. Figure 3.4 and Table 3.1 showed a brief description of measurement journeys.

Figure 3.4 Routes of measurement journeys

41 Table 3.1 Brief description of measurement journeys.

Serving *Distance *Trip Duration Route No. Fleets Route Nature Area (km) (min)

CTB-5 AC HK Urban 10.5 61

CTB-5B AC HK Urban 8.5 54

CTB-10 AC HK Urban 11.1 55

CTB-70 AC HK Urban & sub-urban 9.4 39

CTB-73 AC HK Sub-urban & rural 16.2 50

CTB-788 AC HK Urban & highway 14.8 28

Urban, sub-urban, CTB-973 AC HK & KLN 27.9 88 rural & tunnel

Urban, sub-urban, rural, CTB-A12 AC HK & LTI 51.8 94 highway & tunnel

Urban, sub-urban, rural, CTB-E23 AC KLN & LTI 47.1 90 highway & tunnel

AC & KMB-1 KLN Urban & sub-urban 9.1 51 non-AC

KMB-2E AC KLN Urban & sub-urban 9.0 58

AC & KMB-5 KLN Urban & sub-urban 8.4 48 non-AC

KMB-6 AC KLN Urban & sub-urban 7.7 44

KMB-6A AC KLN Urban & sub-urban 8.3 50

AC & KMB-6C KLN Urban & sub-urban 10.2 62 non-AC

KMB-8 AC KLN Urban & sub-urban 11.4 63

AC & KMB-11K KLN Urban & sub-urban 7.7 43 non-AC

KMB-28 AC KLN Urban & sub-urban 10.1 50

KMB-35A AC KLN Urban & sub-urban 15.1 70

AC & KMB-41A KLN Urban & sub-urban 16.4 70 non-AC

AC & KMB-42C KLN Urban, sub-urban & highway 26.1 70 non-AC

KMB-58X AC KLN & NT Urban, sub-urban & highway 31.7 65

KMB-64K AC NT Sub-urban & rural 22.6 66

42 Urban, sub-urban, KMB-68X AC KLN & NT 33.9 72 highway & tunnel

AC & Urban, sub-urban, KMB-70 KLN & NT 37.7 105 non-AC rural & tunnel

KMB-72 AC KLN & NT Urban & urban 24.7 69

AC & KMB-72A KLN & NT Sub-urban & rural 21.2 55 non-AC

KMB-73A non-AC NT Sub-urban & rural 31.1 85

KMB-74A Non-AC KLN & NT Sub-urban, rural & tunnel 30.1 80

KMB-76K AC NT Sub-urban & rural 24.6 67

KMB-80M AC KLN & NT Sub-urban & tunnel 11.1 34

AC & KMB-81C KLN & NT Urban, sub-urban & tunnel 22.6 75 non-AC

AC & KMB-81M KLN & NT Sub-urban & tunnel 8.5 26 non-AC

AC & KMB-82M KLN & NT Sub-urban & tunnel 12.6 41 non-AC

KMB-83K AC NT Sub-urban 10.8 44

Urban, sub-urban, KMB-87D AC KLN & NT 24.0 80 highway & tunnel

AC & KMB-88M KLN & NT Sub-urban & tunnel 6.9 23 non-AC

KMB-91 AC KLN & NT Sub-urban & rural 18.1 48

KMB-91M AC KLN & NT Sub-urban & rural 16.6 50

KMB-92 AC KLN & NT Sub-urban & rural 16.4 41

Urban, sub-urban, KMB-98D AC KLN & NT 20.3 63 highway & tunnel

KMB-101 AC HK & KLN Urban & tunnel 19.9 65

Urban, sub-urban, KMB-118 AC HK & KLN 21.1 75 highway & tunnel

KMB-215X AC KLN Urban, sub-urban & tunnel 16.0 58

Urban, sub-urban, rural, KMB-271 AC KLN & NT 26.9 70 highway & tunnel

Urban, sub-urban, KMB-281A AC KLN & NT 20.6 70 highway & tunnel

KMB-299 AC NT Sub-urban & rural 20.1 50

43 Urban, sub-urban, KMB-603 AC HK & KLN 17.1 47 highway & tunnel

Urban, sub-urban, KMB-692 AC HK & KLN 20.2 70 highway & tunnel

Urban, sub-urban, KMB-961 AC HK & NT 38.3 80 highway & tunnel

Urban, sub-urban, KMB-968 AC HK & NT 37.6 70 highway & tunnel

NLB-A35 AC LTI Sub-urban & rural 27.8 70

NWFB-2 AC HK Urban 12.9 55

NWFB-9 AC HK Sub-urban & rural 11.7 30

NWFB-14 AC HK Sub-urban & rural 13.1 35

NWFB-63 AC HK Sub-urban & rural 17.2 54

NWFB-104 AC HK & KLN Urban & tunnel 17.4 75

NWFB-111 AC HK & KLN Urban & tunnel 14.6 55

NWFB-720 AC HK Urban & highway 11.3 34

NWFB-971 AC HK & KLN Urban, sub-urban & tunnel 20.3 65

Remarks: CTB – Citybus Ltd. KMB – Kowloon Motor Bus Co. Bus company NLB – New Lantao Bus Co. NWFB – New World First Bus Ltd. AC – Air-conditioned bus Fleets Non-AC – Non-air-conditioned bus HK – Hong Kong Island KLN – Kowloon Serving area NT – New Territories LTI – Lantau Island * Routes information offered in bus company service homepages.

Over 95% of local franchised buses are air-conditioned (HKTD, 2007); meanwhile all old buses will be retired and replaced by the air-conditioned ones in years (HKTD, 2005). So, air-conditioned buses are focused in this study. Summarizing the background information, the ambient air temperature ranged from 9 to 33oC while the relative humidity ranged from 34 to 98% during the measurement.

44 The journeys lasted for 35 minutes averagely; with 17 passengers in the sampling saloon. Summary of the background information was shown in Table 3.2.

Table 3.2 Summary of background information of measurement journeys Ambient Condition Measurement Journey

Air Temperature Relative Humidity Duration No. of Passenger Minimum 9 oC 34 % 7 min 1 Average 25 oC 73 % 35 min 17 Maximum 33 oC 98 % 86 min 70 Median 27 oC 73 % 36 min 17 Standard 5.8 12.5 15.8 6 Deviation

3.2.3.2 Route natures The in-bus and outdoor air pollutant concentration levels were compared when buses travelling in different routes, i.e. the urban, sub-urban, rural, highways and tunnels. To evaluate the effect of outdoor air on in-bus air quality, outdoor data was compared with 5-minute-average values of in-bus air pollutant concentration levels at the time-point when the buses were travelling through the areas housing the monitoring stations.

Urban district routes In Hong Kong, about half of the bus routes can be defined as urban district routes. The buses run through the crowd urban area, where the roads are surrounded by high-rise buildings. The traffic density is very high and traffic congestion is a common occurrence (as illustrated in Figure 3.5). The buses have to run at a relatively lower speed. Since the traffic is heavy and the area is packed with high-rise buildings, the natural ventilation is obstructed. It reduces the dilution and dispersion of excessive vehicular exhaust that accumulates in the narrow street canyons. Hence, ambient air quality is usually poorer in urban districts, especially along busy roads. Also, as the buses run through the urban area, there are passengers taking on and off at all the stops. The variation of population flow in the cabin is high. The passengers may not stay on the bus for a long period. Usually they would not travel on the buses more than 30 minutes (HKTD, 2005). Plenty of outdoor air infiltrates into the passenger compartment through frequently door openings. The in-bus air quality

45 would be contaminated by the poor roadway condition. Moreover, both the infiltration of outdoor unconditioned air and variation of passenger load influence the steadiness of in-bus air quality and thermal comfort environment. Such condition reduces the recovering time that worsening the in-bus environment. Especially on hot days, passengers rush into the buses and expect that the air-conditioned buses can cool down their bodies immediately. It results as higher percentage of dissatisfaction.

Figure 3.5 The traffic was heavy at most of the daytime in the urban area. (captured from google.com photo search)

Sub-urban district routes Like urban district routes, sub-urban routes are common in Hong Kong. Usually, the routes run between areas consist of residential buildings and housing estates with community facilities (as illustrated in Figure 3.6). The density of buildings and population are comparatively lower than that of the urban areas. More open space is available between buildings for sufficient natural ventilation. Also the traffic density is medium during the day, and traffic congestion is rare. Ambient air quality tends to be good as these districts are distant from busy trunk roads. The main characteristic is that the passengers may take on the buses at one district and take off at the others. Thus, the population flow inside the buses changes in a manner of increasing and reducing gradually.

Figure 3.6 There are plenty of residential buildings in sub-urban districts. (captured from google.com photo search)

46 Rural district routes As the daily commercial and industrial activities are concentrated in the urban and suburban districts, the traffic density is always low in rural district. Almost people go to the rural for leisure at the weekends. The characteristic of these routes is that most of the routes travel along the green environment and there are only a few of short residential buildings along the roads (as illustrated in Figure 3.7). The outdoor air quality is much better than that of others since the low density of buildings, population and traffic flow. As the population density in the rural district is low, in addition most of the residents have their own cars, there are a few passengers travelling on these routes on weekdays.

Figure 3.7 Plenty of vegetation in rural districts (captured from upload.wikimedia.org)

Highway routes The highway routes provide express services between the residential areas and the urban areas. Buses run along the highways routes until there are several stops at both ends of destinations. Thus, there is no stop in the middle of routes and buses would run at a higher speed so as to minimize the duration of journeys (as illustrated in Figure 3.8). Usually, the express routes can shorten the duration of journeys by at least 20 minutes, as compared with the normal route services.

Figure 3.8 Traffic flow is high but fluent in the highway that buses run in a higher speed. (captured from google.com photo search)

47 Tunnel routes Due to the geographical condition, tunnels are built for convenient transportation between districts. The tunnel routes provide bus services between these districts. In Hong Kong there are 11 tunnels for vehicles. They are Cross Harbour Tunnel (Figure 3.9), Eastern Harbour Crossing, Western Harbour Crossing, Aberdeen Tunnel, Lion Rock Tunnel, Airport Tunnel, Tate’s Cairn Tunnel, Shing Mun Tunnel, Tseung Kwan O Tunnel, Cheung Tsing Tunnel and Tai Lam Tunnel. In the tunnels which connecting crowd areas, the traffic density is always very high that the mechanical ventilation is insufficient to ventilate or dilute the exhausted air from vehicles inside the tunnels. Thus, the in-cabin air quality may be contaminated as they travel along.

Figure 3.9 Traffic density is very high in tunnel (captured from upload.wikimedia.org)

3.2.3.3 Sampling Among the measurement journeys, 338 trips were conducted at 52 selected bus routes which travelled through areas near ambient monitoring stations in order to compare the indoor and outdoor air pollutant concentration levels. To study the effect of roadway environment to in-bus air quality, the air parameters were measured on the upper-deck compartment so as to minimize the direct influence of door openings on the lower-deck. Samplers were placed at the breathing level of passengers (about 1.1 metres from floor), but direct effect by passengers’ breath was avoided. During each journey, all relevant details were recorded to evaluate the effects on in-bus air quality. They included the time, location, meteorological condition (provided by the Hong Kong Observatory), traffic conditions and outdoor environment conditions.

Figure 3.10 Location of monitoring stations (captured from HKEPD homepage)

Table 3.3 Descriptions of the ambient monitoring stations

Monitoring Vertical Level Station Area Type Central/Western Residential area 18m (4th floor)

Roadside station in busy commercial area Central 4.5m surrounded by many tall buildings

Roadside station in busy commercial area Causeway Bay 2m surrounded by many tall buildings

Eastern Residential area 17.5m (4th floor)

Tsuen Wan Mixed residential / commercial / industrial area 17m (4th floor)

Kwai Chung Mixed residential / commercial / industrial area 13m (3rd floor)

Sham Shui Po Mixed residential / commercial area 17m (4th floor)

Roadside station in mixed residential / commercial Mong Kok 2m (1st floor) area surrounded by some moderately tall buildings

Kwun Tong Mixed residential / commercial / industrial area 25m (6th floor)

Tung Chung Residences surrounded by green area 21m (5th floor)

Yuen Long Residential area with fairly rapid development 25m (6th floor)

Shatin Residences surrounded by green area 21m (5th floor)

Tai Po Residences surrounded by green area 25m (6th floor)

Tap Mun Rural area on an outlying island 11m

49 Ambient air quality data was collected from the ambient monitoring stations of the Hong Kong Environmental Protection Department (HKEPD), 14 of which are located in different areas around Hong Kong. (*Tap Mun ambient station is not included since it is located on outlying island.) These stations collect aerographic data continuously and announce hourly levels of these parameters to the public through the department’s homepage. Figure 3.10 and Table 3.3 showed the locations and descriptions of the ambient monitoring stations. Also, 75 measurement journeys were performed on 26 selected bus routes, which travelled on rural district and highway routes. Buses travelling on these routes had less stops and ran at constant speeds. The data collected on these routes were suitable for analyzing the ventilation characteristics. Since the passenger compartments were occupied by passengers, it was inappropriate to measure the ventilation rate by spreading tracer gas sulphur hexafluoride (SF6). Thus, metabolic carbon dioxide

(CO2) concentration levels were selected as an indicator to evaluate ventilation rate. The profiles of exposure level and the number of passengers in the measurement journeys were simultaneously recorded. The number of passengers inside the passenger compartment was the main cause of metabolic CO2 generation. Meanwhile, the ambient CO2 concentration and the magnitude of ventilation rate affected the effectiveness of diluting the metabolic CO2 inside passenger compartments. The in- bus metabolic CO2 would accumulate if the ventilation rate was insufficient to dilute the exhalation from passengers. On the contrary, a higher ventilation rate could help effectively dilute the concentration of air contaminants on bus. The ventilation rate of the compartments was evaluated with the mass balance equation as follows.

V dC(t) = [ G + Q ( Co – C(t) ) ] dt (3.1)

where V = Volume of the ventilated space; C(t) = In-bus carbon dioxide concentration at time t; G = Generation rate of carbon dioxide by commuters; Q = Ventilation rate of the air-conditioning system;

Co = Outdoor carbon dioxide concentration, assume to be 400 ppm; t = Time interval.

50 Potter and Booth (1994) provided figures of respiratory output of metabolic CO2 for occupants working in different activities (as shown in Table 3.4). For the passengers on bus, it was assumed that the activity level was on average similar to light work, even though individual passengers were at various levels of activity before boarding on the bus. The metabolic CO2 generation rate was assumed to be 0.01 litre per second per person.

Table 3.4 Carbon dioxide generation rate against occupant activity level

Carbon dioxide Occupant activity level -1 generation rate (l · s )

Resting 0.004

Light work 0.006 – 0.012

Moderate work 0.013 – 0.019

Heavy work 0.020 – 0.026

Very heavy work 0.027 – 0.032

3.2.3.4 Instrumentation

In-bus CO and CO2 concentration levels were measured using Q-Trak (TSI Model 8551) portable IAQ monitor. It is an accredited instrument to analyze the indoor air quality in buildings (HKEPD, 2003b). DustTrak (TSI Model 8520) portable aerosol monitor was used to measure RSP. Typically, air samples are collected in Tedlar sample bags with portable sampling pumps (SKC Inc.) and analyzed by NDIR equipment to provide more accurate data. However, this only showed an average value in the sampling period but not a continuous real-time condition of the in-bus environment, concealing the dynamic changes in CO concentration taken in the measurement. Thus, a portable IAQ monitor was used. Q-Trak uses electro-chemical type sensor to measure CO and non-dispersive infrared sensor to measure CO2; while DustTrak is a real-time laser photometric instrumentation for the determination of aerosol mass concentrations, which is capable of measuring short-term exposure level and concentration profiles. The calibration curve of DustTrak stated in Tung et al. (1999) was applied in order to raise the accuracy. Also zero calibration process was done before each sampling to increase the data reliability. Detailed specifications about the monitoring sensors are summarized in Table 3.5. Since the

51 accuracy of sampling CO by Q-Trak is ± 3% or 3 ppm whichever is greater, a calibration study was performed to evaluate the accuracy of CO level which beyond that of the manufacturer’s specification. The calibration result was shown in Appendix A.

Table 3.5 Air quality monitor sensors specification

Sensor Type Range Accuracy Resolution

± 3% of reading or CO Electro-chemical 0 to 500 ppm 1 ppm 3 ppm whichever is greater

Non-dispersive ± 3% of reading or CO 0 to 6000 ppm o 1 ppm 2 infrared ± 50 ppm at 25 C

Laser 0.001 to -3 o -3 RSP -3 0.001 mg · m per C 0.001 mg · m photometric 100 mg · m

Air temperature and relative humidity were measured by using Q-Trak (TSI Model 8551) portable IAQ monitor. VelociCalc Plus Air Velocity Meter (TSI Model 8386) portable monitor was used to measure air speed. Q-Trak can record the real-time conditions of the in-bus environment continuously, which shows the dynamic changes in the measurement journeys. For the operation principles, thermistor and thin-film capacitive sensors are used for sampling air temperature and relative humidity respectively; and thermal sensor is used to measure air speed. The detailed specifications about the monitor sensors are summarized in Table 3.6.

Table 3.6 Thermal comfort parameter monitor sensors specification Sensor Type Range Accuracy Resolution

Temperature Thermistor 0 to 60oC ± 0.6oC 0.1oC Thin-film Humidity 5 to 95% RH ± 3% RH (at 25oC). 0.1 % RH capacitive

-1 Air Thermal -1 ± 3% of reading or ± 0.015 m · s , -1 0 to 50 m · s 0.01 m · s movement sensor whichever is greater

-1 The average travelling speed of buses is about 30 km · hr in the city. The data was logged continuously in 30-second-intervals. Hence 4 data points were obtained in every kilometre to evaluate the effect of dynamic outdoor environment, increasing the reliability of data recorded.

52 3.2.4 Computational fluid dynamics Studying the effect of bus travelling speed on induced ventilation rate, computational fluid dynamics (CFD) analysis was applied. The geometry of a doubled-deck bus body was modelled by using GAMBIT (version 2.4). Complete wire frame and face data were generated. The dimension of bus model was 12m (length) x 2.4m (width) x 4.4m (height) similar to that of a normal local double-decker (as illustrated in Figure 3.11); while the dimension of the wind tunnel was 200m (length) x 35m (width) x 35m (height). A half model was considered and the symmetric boundary conditions were applied to shorten the computation time.

Figure 3.11 Views of bus model

The finite volume CFD code FLUENT (version 6.3) was used. It is a popular program for fluid dynamics simulation since various mathematical models are supported. The provision makes easy converge of complicated matrix. Hence FLUENT was used in this study. Aerodynamic conditions were calculated by solving the three-dimensional incompressible steady-state Navier-Stokes equations along with the continuity and energy equations. The turbulent nature of the flow was modelled by the RNG-based

53 k-ε turbulence model with the standard wall function. As stated in Kim (2003), RNG k-ε model yields a relatively exact solution in separated flows and has excellent convergence. The application of turbulence models were as follows. ∂ Equation of continuity ()ρu = 0 (3.2) ∂ j x j ∂ ∂P Equation of momentum ()ρu u −τ = − + S (3.3) ∂ i j ij ∂ i x j xi Equation of turbulent kinetic energy

1 ∂ ∂  u ∂k  ()gρk  ρu k − eff  ∂ ∂  j σ ∂  g t u j  k x j 

2  ∂u  ∂u = µ ()P + P − ρε −  µ i + ρk  i (3.4) t B  t ∂  ∂ 3  xi  xi ∂u g 1 ∂ρ where µ = µ + µ ; P ≡ 2;S i P = − i . eff t ij ∂ B σ ρ ∂ x j h,t xi Equation of dissipation

1 ∂ ∂  u ∂ε  ()gρε +  ρu ε − eff  ∂ ∂  j σ ∂  g t u j  ε x j 

ε  2 ε  ∂u  ∂u  = C µ ()P + C P − C  µ i + ρk  i εl  t ε 3 B εl  t ∂  ∂  k  3 k  xi  xi 

2 η 3 ()−η η 2 ε ∂u Cµ 1 0 ρε − C ρ + C ρε i − (3.5) ε 2 ε 4 ∂ + βη 3 k xi 1 x

In the equations, ρ is the air density, ui is the velocity component, τij is the stress tensor component, Si is the momentum source component and, Sij is the strain tensor component. At the velocity-inlet of the wind tunnel, the discharge air flow rate was set at a -1 -1 constant wind speed from 2.78 m · s (10 km per hour) to 19.44 m · s (70 km per hour) facing the bus model. The pressure induced around the bus body was simulated when it travelled at various speeds.

54 3.3 In-bus air quality condition

3.3.1 Summary of measurement data and distribution curves Summarizing the air sampling result, the average in-bus CO concentration levels were 2.6 ppm on air-conditioned buses and 1.8 ppm on non-air-conditioned buses.

The average in-bus CO2 concentration levels were 1636 ppm and 592 ppm on air- conditioned and non-air-conditioned buses respectively. The average in-bus RSP -3 -3 concentration levels were 0.11 mg · m and 0.17 mg · m respectively. Statistical data of the in-bus air quality was shown in Table 3.7.

Table 3.7 Measurement result summary of in-bus air quality

Air-conditioned Buses Non Air-conditioned Buses

CO CO2 RSP CO CO2 RSP -3 -3 (ppm) (ppm) (mg · m ) (ppm) (ppm) (mg · m )

Minimum 0.0 467 0.01 0.0 350 0.01

Average 2.6 1636 0.11 1.8 592 0.17

Maximum 9.0 > 6000 0.51 18.0 1323 1.00

Median 2.6 1667 0.11 1.5 573 0.16

Standard 0.5 322 0.02 1.0 86 0.05 Deviation

(Mean / Max.)

TWA-5 min 2.6 / 8.3 1651 / >6000 0.11 / 0.49 1.9 / 6.0 596 / 1124 0.17 / 0.68

TWA-15 min 2.7 / 7.0 1696 / >6000 0.11 / 0.47 1.8 / 5.4 592 / 1066 0.17 / 0.42

TWA-30 min 2.8 / 6.0 1751 / 5761 0.11 / 0.45 2.1 / 4.6 622 / 916 0.16 / 0.38

TWA-60 min 3.3 / 5.1 2077 / 4713 0.08 / 0.24 2.5 / 2.8 669 / 741 0.13 / 0.20

Figure 3.12, Figure 3.13 and Figure 3.14 illustrated the distribution curves of in-bus

CO2, CO and RSP concentration respectively. Considering the in-bus CO2 concentration, the profile obtained on non-air-conditioned buses was left-shifted as compared with that of the air-conditioned buses. It can be explained as the application of natural ventilation. Outdoor air is infiltrated into the passenger compartment through window openings, which the air change rate increases with aerodynamic drag induced by travelling speed (Chan & Shek, 2007). Higher concentration level in air-conditioned compartment is caused by the fixed

55 mechanical ventilation rate as well as the number of passenger on buses. The application of natural ventilation gives a merit of higher air change rate on non-air- conditioned buses. However, it has the risk of uncontrolled air pollutants infiltration. The in-bus CO concentration level would be high when the bus travels along a congested route or inside tunnels. The measurement result showed a worst scenario that it was as high as 18 ppm. Further study was elaborated in section 3.4. Particulates filters are available on air-conditioned buses that minimize the in-bus RSP concentration level. Hence the in-bus condition varies depending on the particulates filtering effectiveness.

Figure 3.12 Distribution curve of in-bus CO2 concentration measured on buses

Figure 3.13 Distribution curve of in-bus CO concentration measured on buses

Figure 3.14 Distribution curve of in-bus RSP concentration measured on buses

3.3.2 Ventilation characteristics on a travelling bus The variation of ventilation rates along measurement journeys was evaluated. The profiles of CO2 concentration varied under a fixed number of passengers and no change of ventilation mode inside the passenger compartments. Calculating with mass balance equation, the ventilation rate could be evaluated by assuming a CO2 generation rate. -1 -1 It was found that the magnitude of ventilation rate ranged from 124 l · s to 810 l · s -1 with an average value of 400 l · s (details shown in Table 3.8). The ventilation rate varied along the measurement journeys under a fixed operation mode. This phenomenon revealed the effect of aerodynamic drag induced by travelling speed. The aerodynamic drag generated pressure difference across the bus body surface, which caused additional air exchange when travelling at different speeds.

Table 3.8 Summary of ventilation rate in measurement journeys

-1 Ventilation Rate (l · s ) on travelling bus Minimum 124 Average 400

Maximum 810 Median 407

Standard Derivation 161

57 Typical profile A typical profile obtained in a measurement journey was plotted in Figure 3.15. In -1 the beginning of the journey, the ventilation rate was lower than 150 l · s when the bus was either stationary or crawled in traffic congestion. When the bus stopped or travelled at very low speed, the induced infiltration was limited. But the in-bus CO2 concentration level stayed constant since there were a few passengers on board. The ventilation rate increased until it escaped from the traffic congestion and travelled at higher speed. The CO2 concentration level rose gradually due to an increased number of passengers. Owing to the limited travelling speed in urban district and the frequent stops for taking on new passengers, the induced ventilation rate maintained at the -1 levels between 250 and 350 l · s . At the moment, the growing profile of in-bus CO2 concentration level indicated an insufficient ventilation to dilute the metabolic bioeffluents. Until the bus traversed the highway at high speed, the ventilation rate -1 rose over 400 l · s . At this point, CO2 concentration levels exhibited a downward trend. The profile along the journey clearly illustrated the effect of travelling speed on the variation of induced ventilation rate.

Figure 3.15 Profiles of ventilation rate and CO2 concentration obtained in a typical measurement journey

58 3.3.2.1 Computational fluid dynamics analysis Mechanical ventilation is achieved through outdoor air dampers located at the rear side of the bus body above the engine (as shown in Figure 3.16). In aid of the computational fluid dynamics analysis, it evidences that the large bluff body generated aerodynamic drag whilst induced pressure difference around a travelling bus body. In-bus air discharged from door gaps at the front section of bus, as a potential pressure difference is developed across the outdoor air dampers and door gaps. A relatively higher pressure is induced at the outdoor air dampers.

Figure 3.16 Illustration of air infiltration and exhaust for the passenger compartment

Figure 3.17 illustrated the results of pressure difference induced around the bus body -1 when travelling at 30 km · hr . A pressure difference of 16 Pa was induced across the outdoor air intake and the front door gap. The pressure induced outdoor air infiltration from the intake, and in-bus air discharged through the front door gap. -1 -1 When the bus accelerated to higher speeds of 50 km · hr and 70 km · hr , the pressure difference was further increased to 53 Pa and 97 Pa respectively (as illustrated in Figure 3.18 and Figure 3.19). Figure 3.20 showed the simulation result of pressure difference induced around a bus travelling at various speed. The pressure difference increased at higher travelling speed. Since additional ventilation rate was induced under higher pressure difference, it revealed the ventilation rate would be ascending with travelling speed.

59 Pressure (Pa) (Pa) Pressure

-1 Figure 3.17 Pressure induced around bus travelling at 30 km · hr

Pressure (Pa) (Pa) Pressure

-1 Figure 3.18 Pressure induced around bus travelling at 50 km · hr

60 Pressure (Pa) (Pa) Pressure

-1 Figure 3.19 Pressure induced around bus travelling at 70 km · hr

Figure 3.20 The induced pressure difference at various speeds

3.3.2.2 Empirical model of ventilation characteristics Analysing the data obtained from the measurement activity, the induced ventilation rate and the bus travelling speed was correlated and plotted in Figure 3.21. In addition, an empirical model was concluded as follows.

Q = 4.39 u + 251.23 (3.6) ( R2 = 0.86, p < 0.05 for 0 ≤ u ≤ 75 ) -1 -1 where Q was the ventilation rate (l · s ) and u was the bus travelling speed (km · hr ).

Figure 3.21 The ventilation rate at various speeds

Induced ventilation rate could be estimated under different travelling speed by the -1 empirical model. For example, if a bus sustained a speed of 45 km · hr , the induced -1 ventilation rate inside the compartment would be about 450 l · s . Frequent bus stops and traffic junctions limit the bus travelling speed in local urban -1 areas, resulting in an average speed at about 30 km · hr . The induced ventilation rate -1 would be about 380 l · s . The carrying capacity of a double-decker is typically -1 around 130. Therefore, each passenger could obtain an outdoor air rate of 2.9 l · s on -1 a fully loaded bus. Furthermore, if the bus travelled on highway at 65 km · hr , the -1 -1 induced ventilation rate would rise to 535 l · s , which was equivalent to 4.1 l · s per -1 -1 passenger. Also, it would be about 250 l · s on a stationary bus, providing 1.9 l · s per passenger. According to the recommendations stated in ASHRAE (2007), the -1 mechanical ventilation system should provide 2.5 to 5 l · s outdoor air per passenger (i.e. air change rate at 13 to 27 times) in mass-transit vehicles. It reveals the present ventilation rate on a stationary bus may not conform to the prescribed level.

62 3.4 Outdoor air quality

3.4.1 Urban district routes Variation of air pollutant concentration levels were studied along different routes. The in-bus CO and RSP concentration levels when travelling in urban districts, as well as the simultaneous ambient air pollutant levels measured at roadside stations were summarized in Table 3.9. On air-conditioned buses, the average concentration -3 levels were 3.4 ppm and 0.12 mg · m for CO and RSP respectively. The CO levels were higher inside the passenger compartments where the average in/out ratio was 3.4. The ratio was 0.9 for RSP which meant the concentration level was lower inside the compartments.

Table 3.9 Summary of in-bus and ambient air pollutants concentration level on urban district routes

In-bus Concentration Outdoor Condition In/Out Ratio Level (Ambient Station) Urban District routes CO RSP CO RSP -3 -3 CO RSP (ppm) (mg · m ) (ppm) (mg · m )

Minimum 2.0 0.01 0.3 0.03 1.5 0.2

Average 3.4 0.12 1.0 0.08 3.4 0.9

Maximum 7.0 0.50 2.0 0.14 5.0 1.3

Median 3.4 0.07 0.9 0.07 3.3 1.0

Standard 0.4 0.01 0.4 0.03 - - Air-conditioned buses Air-conditioned deviation

Minimum 1.0 0.04 0.4 0.03 2.3 1.1

Average 2.8 0.19 1.3 0.11 3.4 1.5

Maximum 11.0 0.39 1.9 0.18 8.4 2.4

buses Median 2.7 0.20 1.4 0.11 2.9 1.4

Non-air-conditioned Non-air-conditioned Standard 0.9 0.04 0.4 0.05 - - deviation

On non-air-conditioned buses, the average CO concentration was lower. However, the maximum level was so high that reached 11 ppm. The average RSP level was -3 0.19 mg · m which was higher than that on air-conditioned fleets. The in-bus air

63 pollutants concentration level was higher on non-air-conditioned buses owing to the application of natural ventilation. Also, no particulates filters were installed. In-bus environment was directly influenced by the dynamic roadway condition. Vehicular exhaust might enter the compartment through windows. Worse scenario occurred when buses were trapped in traffic congestion.

3.4.2 Sub-urban district routes Table 3.10 summarized the in-bus air pollutant concentration level when travelling on sub-urban district routes. On air-conditioned buses, the average in-bus -3 concentration levels were 1.6 ppm and 0.14 mg · m , and the in/out ratios were 2.1 and 2.8 for CO and RSP respectively. The in-bus CO level was lower as compared with the condition on urban routes. Lower traffic density in sub-urban was a major factor that resulting as lower outdoor air pollutant concentration level.

Table 3.10 Summary of in-bus and ambient air pollutants concentration level on sub-urban district routes

In-bus Concentration Outdoor Condition In/Out Ratio Level (Ambient Station) Sub-urban district routes CO RSP CO RSP -3 -3 CO RSP (ppm) (mg · m ) (ppm) (mg · m )

Minimum 0.1 0.04 0.4 0.02 1.6 0.4

Average 1.6 0.14 0.7 0.06 2.1 2.8

Maximum 3.1 0.30 1.0 0.13 3.4 5.3

Median 1.5 0.15 0.7 0.05 2.2 3.4

Standard

Air-conditioned buses Air-conditioned 0.4 0.02 0.2 0.03 - - deviation

Minimum 0.1 0.03 0.4 0.02 1.3 1.2

Average 1.2 0.21 0.6 0.07 2.5 3.6

Maximum 5.0 0.60 1.0 0.12 3.9 6.1

buses Median 0.6 0.22 0.6 0.06 2.5 3.1

Non-air-conditioned Non-air-conditioned Standard 0.6 0.03 0.2 0.03 - - deviation

64 On non-air-conditioned buses, the average CO concentration level was lower due to fluent traffic flow. On a contrary, the RSP concentration level increased and was higher than that on urban routes. Sources inside the compartment are considerable factor since the ambient RSP concentration level was not high in the district. The sources could be fibres from passengers’ clothes, luggage or seat covers. Or it could be explained as the lower particulates filtering effectiveness on the fleets.

3.4.3 Rural district routes The passengers’ exposure level on rural district routes was summarized in Table 3.11. Lower in-bus air pollutant concentration levels were found. On air-conditioned buses, -3 the average in-bus levels were 1.3 ppm and 0.11 mg · m for CO and RSP -3 respectively; while they were 0.8 ppm and 0.11 mg · m on non-air-conditioned fleets. The lower in-bus CO concentration level could be explained as the better outdoor condition. However, the outdoor RSP concentration level was not further reduced associated with the traffic condition. The sources could be the soil from pavement flower bed which carried by vehicles’ aerodynamic drag.

Table 3.11 Summary of in-bus and ambient air pollutants concentration level on rural district routes

In-bus Concentration Outdoor Condition In/Out Ratio Level (Ambient Station) Rural district routes CO RSP CO RSP -3 -3 CO RSP (ppm) (mg · m ) (ppm) (mg · m )

Minimum 0.0 0.03 0.4 0.03 2.0 0.5

Average 1.3 0.11 0.5 0.09 2.4 1.1

Maximum 2.0 0.27 0.6 0.15 2.9 2.1

Median 1.3 0.09 0.5 0.08 2.3 1.1

Standard

Air-conditioned buses Air-conditioned 0.3 0.01 0.1 0.03 - - deviation

Minimum 0.0 0.05 0.4 0.03 2.0 0.6

Average 0.8 0.11 0.5 0.09 2.8 2.2

Maximum 2.0 0.35 0.6 0.13 4.0 10.5 buses Median 0.7 0.08 0.5 0.10 2.8 0.9

Non-air-conditioned Non-air-conditioned Standard 0.2 0.01 0.1 0.03 - - deviation

65 3.4.4 Highway and tunnel routes The passengers’ exposure levels on highway and tunnel district routes were summarized in Table 3.12. Passengers commuting on tunnel routes had higher air pollutants exposure level. On air-conditioned buses, the average CO concentration level was 2.8 ppm on highway routes, while it was 4.4 ppm on tunnel routes. The average level was 5.5 ppm on non-air-conditioned fleets when travelling through tunnel, and the maximum level was as high as 12 ppm. The average RSP -3 concentration levels were below 0.10 mg · m on air-conditioned fleets travelling on both routes. However, it was much higher inside non-air-conditioned compartment. It -3 reached the maximum of 1.00 mg · m when travelling through tunnel.

Table 3.12 Summary of in-bus air pollutants concentration level on highway and tunnel routes

Highway routes Tunnel routes In-bus Concentration Level CO RSP CO RSP -3 -3 (ppm) (mg · m ) (ppm) (mg · m )

Minimum 1.0 0.01 2.0 0.01

Average 2.8 0.04 4.4 0.07

Maximum 5.0 0.14 8.0 0.22

Median 3.0 0.04 4.0 0.07

Standard

Air-conditioned buses Air-conditioned 0.2 0.01 0.5 0.01 deviation

Minimum n/a n/a 2.0 0.06

Average n/a n/a 5.5 0.29

Maximum n/a n/a 12.0 1.00

buses Median n/a n/a 5.3 0.25

Non-air-conditioned Non-air-conditioned Standard n/a n/a 1.2 0.08 deviation

3.4.5 Effects of route natures to in-bus air quality Figure 3.22 and Figure 3.23 illustrated the in-bus CO and RSP concentration levels on different routes respectively. The in-bus CO concentration levels ascended with traffic density in different districts. The levels were the highest on tunnel routes,

66 followed by the urban and sub-urban district routes. As compared with the conditions in rural district routes, the in-bus CO concentration levels were 2.4 times and 1.6 times higher in the tunnel routes and urban routes respectively on air-conditioned buses. The differences rose to 5.9 times and 2.5 times respectively on non-air- conditioned buses. The RSP concentration level was comparatively steady inside air- conditioned compartment; whilst larger differences were found on non-air- conditioned fleets. The variation was caused by the influence from the dynamic roadway environment. Also, it revealed the particulates filtering effectiveness on air- conditioned buses.

Figure 3.22 In-bus CO concentration levels on different routes

Figure 3.23 In-bus RSP concentration levels on different routes

Higher in-bus air pollutant concentration levels would be obtained if the outdoor air quality was poor. Vehicles, including buses generate air pollutants. Road users are surrounded by air pollutants along the roadway environment. Outdoor air quality and mechanical ventilation affect the in-bus air quality. When traffic density is high, especially during traffic congestion, the roadway air pollutant concentration levels increase and disperse as street canyon effect. In such situation, outdoor air ventilation could neither dilute the in-bus air contaminants nor improve the in-bus air quality. Higher ventilation rate will actually result in higher air pollution exposure. On non- air-conditioned buses, the condition is even worse since the concentrated air pollutants enter the compartment directly through windows. The variation of in/out ratios obtained in different districts could be explained as the inconsistencies between roadside conditions and ambient conditions obtained at air quality monitoring stations. The purposes of stations affect the magnitude of ratios, since the distance from roads and the height of air sampling points are major factors. In urban districts, the monitoring stations are located at lower vertical level on the

68 roadside, and the data collected represents air pollutant concentrations along urban roadside areas. However, the stations in sub-urban and rural districts are located on buildings which may be distant from the routes. Furthermore, the stations are located on the 4th to 6th floors of buildings. The air pollutant concentration levels obtained from these stations are hence different from the roadside conditions in the districts.

3.5 In-bus thermal comfort environment

Summarizing the thermal comfort parameters sampling result, the in-bus air temperature ranged from 13.2 to 34.2oC on air-conditioned buses and 11.8 to 38.1oC on non-air-conditioned buses, while the ambient condition ranged from 9 to 33oC. The in-bus relative humidity ranged from 23.4 to 95.0% and 31.5 to 95.0% on air- conditioned and non-air-conditioned buses respectively, while the ambient condition -1 ranged from 34 to 98%. The in-bus air movement ranged from 0.04 to 1.85 m · s and -1 0.02 to 5.50 m · s on air-conditioned and non-air-conditioned buses respectively (Summary of measurement result shown in Table 3.13).

Table 3.13 Measurement result summary of in-bus thermal environment

Air-conditioned Buses Non Air-conditioned Buses

Air Relative Air Air Relative Air

Temperature Humidity Speed Temperature Humidity Speed

o -1 o -1 Minimum 13.2 C 23.4 % 0.04 m · s 11.8 C 31.5 % 0.02 m · s

o -1 o -1 Average 23.5 C 61.3 % 0.29 m · s 28.2 C 64.2 % 1.03 m · s

o -1 o -1 Maximum 34.2 C 95.0 % 1.85 m · s 38.1 C 95.0 % 5.50 m · s

o -1 o -1 Median 23.4 C 61.2 % 0.20 m · s 28.3 C 63.9 % 0.49 m · s

Standard 0.7 2.4 0.27 0.5 1.9 1.20 Deviation

Figure 3.24, Figure 3.25 and Figure 3.26 illustrated the distribution curves of in-bus air temperature, relative humidity and air speed respectively. Considering the in-bus air temperature, the result showed that almost air-conditioned compartments were controlled between 17oC and 29oC. The condition inside non-air-conditioned compartment varied depending on the ambient, which was as high as 38oC on the hot days. Also the air-conditioning system controls relative humidity on buses, which

69 mostly ranging between 40 and 80%. Similarly, the in-bus air speed is under control -1 (mostly below 0.35 m · s ). On non-air-conditioned buses, the air speed may be high -1 as 5.5 m · s when travelling at high speed; while air movement was not available on an idling bus. The variation creates thermal discomfort to the passengers.

Figure 3.24 Distribution curve of in-bus air temperature measured on buses

Figure 3.25 Distribution curve of in-bus relative humidity measured on buses

Figure 3.26 Distribution curve of in-bus air speed measured on buses

3.6 Air quality inside compartment of other transportation modes

Realizing the air quality environment on local buses, it is compared with the condition on other transportation modes. Measurement was conducted on minibuses, private car, trams, railways and ferries. Table 3.14 showed the summary of air sampling result. It was found that the in-cabin air quality was directly affected by the fleet type and the surrounding environment. Air-conditioned transportation modes had lower in-cabin CO and RSP concentration level, while the non-air-conditioned ones had lower in-cabin CO2 concentration levels. Among the air-conditioned transportation modes, railways had better in-cabin air quality. It is because railways are powered by electricity and the tracks are distant from busy roadway environment. The major pollutant source is hence minimized. CO concentration level was higher on ferries since emission exhaust was infiltrated into the cabin when berthed in pier. Minibuses had higher in-bus CO2 concentration level since there was no fresh air intake and air changed through door openings. For the non-air-conditioned transportation modes, the in-cabin air quality was obviously affected by the roadway environment. CO concentration level in private car reached the highest (26 ppm) when it was travelling through a vehicular tunnel; whilst it dropped to zero when travelling in rural area. The concentration of CO and RSP were relatively high on trams since they track along the busy roadways.

71 Besides, the air quality at bus stops was measured to study the passengers’ exposure level before taking on buses. Higher air pollutants exposure level was found when buses were stopping at the bus stops. The exhaust air emitted and influenced the surrounding area. The condition was worse at the semi-enclosed type bus terminus where ventilation was limited.

Table 3.14 Summary of air sampling taken on other transport modes and bus stop

-3 *Fleet No. of CO (ppm) CO2 (ppm) RSP (mg·m ) Modes type sample Min. / Mean / Max. Min. / Mean / Max. Min. / Mean / Max.

Mini-buses AC 4 1.0 / 2.9 / 5.0 609 / 1480 / 3635 0.03 / 0.03 / 0.04

Railways AC 3 0.0 / 0.4 / 2.0 624 / 1016 / 1552 0.02 / 0.05 / 0.14

Ferries AC 4 1.0 / 3.8 / 10.0 646 / 1441 / 2873 0.05 / 0.06 / 0.07

Private car Non-AC 6 0.0 / 5.1 / 26.0 420 / 647 / 1249 0.08 / 0.15 / 0.32

Trams Non-AC 2 2.0 / 3.0 / 6.0 519 / 630 / 840 0.06 / 0.15 / 0.27

Bus stops Non-AC 27 1.0 / 1.7 / 7.0 389 / 482 / 782 0.07 / 0.13 / 0.31

* AC: air-conditioned; Non-AC: non-air-conditioned.

3.7 Conclusion

The mechanical ventilation rate is a key factor to air contaminants dilution effectiveness. Sufficient rate should be applied to dilute the in-bus air contaminants and raise the passengers’ satisfaction towards air quality. However, the rate should be carefully determined to balance between the benefit of diluting in-bus air contaminants and the risk of outdoor air pollutants entering the passenger compartment.

Ventilation characteristics on buses were investigated. The in-bus CO2 concentration level varied depending on the number of passengers and the ventilation rate.

Assuming the in-bus metabolic CO2 generation rate, the variation of ventilation rate along the journeys was evaluated. The ventilation rate obtained in the measurement -1 -1 journeys ranged from 124 to 810 l · s with an average value of 400 l · s . Besides, computational simulation result showed that the aerodynamic drag increased the pressure difference around the bus body when travelling at higher speed. The increase of pressure difference induced between the positions of front door gap and

72 outdoor air damper. The correlation between ventilation rate and travelling speed was concluded. It showed the ventilation rate rising with travelling speed. The -1 normal mechanical ventilation rate was about 250 l · s on a stationary bus. It was -1 equivalent to providing 1.9 l · s per person of outdoor air ventilation rate at the maximum licensed passenger-carrying capacity, which did not reach the recommended rate stated in ASHRAE Standard. As frequent bus stops and traffic junctions limited the travelling speed in urban districts, buses would normally travel -1 -1 at around 30 km · hr . The ventilation rate was estimated to be about 380 l · s , -1 equivalent to 2.9 l · s per passenger on a fully loaded bus. If the bus travelled along a -1 -1 -1 highway at 65 km · hr , the ventilation rate would reach 535 l · s , providing 4.1 l · s per passenger. The variation of in-bus air pollutants concentration levels when travelling in different route natures was studied. The in-bus air pollutant concentration levels were higher when travelling on tunnel routes, followed by urban district routes. The pollutant concentration levels were the lowest on rural district routes. Traffic density is a major factor to such variation. Higher traffic density reveals vehicular exhaust emission increases. Also traffic congestion reduces the travelling speed of vehicles leading to operate in low gear or idling conditions. Air pollutants emitted from tail pipes stay in the roadway environment since they are not dispersed by aerodynamic drag induced by travelling vehicles. This results in the further accumulation of air pollutants in street canyons. In these conditions, outdoor air ventilation introduces concentrated air pollutants into the passenger compartments influencing the passengers’ exposure. Sufficient outdoor air ventilation rate should be applied to dilute the in-bus air contaminants and raise the passengers’ satisfaction towards air quality. However, an appropriate rate must be set in order to balance between the benefit of diluting in-bus air contaminants and the risk of outdoor air pollutants infiltration. In-bus air contaminants, including metabolic CO2 and body odour, come from passengers which would not influence one’s health under normal exposure level. Outdoor air pollutants from vehicular emission, like CO, generate acute health problems when in high concentrations. Hence, the influence of in-bus air contaminants is much lower as compared with that of the outdoor air pollutants. Considering the balance, minimizing the risk of air pollutants infiltration should be deemed more important than the dilution of in-bus air contaminants. Therefore the mechanical ventilation

73 rate should be considered depending on the outdoor environment along bus routes in order to control in-bus air quality. Such practice can help improve the in-bus air quality in a more effective and flexible manner. The strategy is elaborated in the following chapter.

74 4 Comfort models

4.1 Introduction When air-conditioned buses were initially introduced, neither the service providers nor passengers concerned with the air freshness inside the passenger compartments. It was primarily due to the erroneous translation of air-conditioning as “cold-air” in Cantonese (Chan et al., 1998). Native speakers not only misunderstood the intended applications of air-conditioning systems, but also considered low air temperatures to be synonymous better service. Improving in-bus air quality is one of the goals. Insufficient outdoor air rate weakens the dilution of in-bus air contaminants and human bioeffluents, worsening the issues of air stuffiness and odour. However, high outdoor air rate would increase the risk of introducing concentrated vehicular exhaust from the busy roadway environment (Chan et al., 2002a). Local bus operators are used to set the outdoor air ventilation at a fixed rate to conform to the prescribed air quality level at design condition. Such practice is inappropriate to be applied on buses as the outdoor air quality is dynamic along the routes dependent on the surrounding environment and traffic density (Chan et al., 1991). Local surveys reported the in-bus air pollutant concentration levels were conformance with the prescribed levels in international standards, like WHO (2000). However, complaints of poor in-bus air quality have been ceaseless (Oriented Daily, 1 Sept. 2005). It reveals the conformance is not equivalent to satisfying the passengers’ perception towards air quality.

PN-PTF (HKEPD, 2003a) stated two prescribed in-bus CO2 concentraion levels (2500 ppm and 3500 ppm in TWA 1-hour) for comfort. However the corresponding satisfaction level to the indicator concentration is not realized. Improvement of the in-bus thermal comfort environment is another goal. PN-PTF states that air-conditioned buses should control the thermal comfort environment at the recommended air temperature ranging from 20 to 28oC and relative humidity from 40 to 70%. Besides, air quality managers of service operators can adjust the ranges taking into consideration the preference of passengers. The recommended ranges by PN-PTF lie within the comfort zone stated in ASHRAE Standard 55-2004 (ASHRAE, 2004). However, it is not equivalent to fulfilling the local passengers’

75 satisfaction. A variety of factors must be considered in order to achieve thermal comfort. Fanger (1970) stated six major thermal comfort factors: air temperature, relative humidity, air movement, radiant temperature, clothing insulation and activity level. But of these six, only the former three parameters can be controlled through air-conditioning. Local bus service operators set the air temperature levels of air-conditioned passenger compartments at 19oC and 23oC in winter and summer respectively. However, numerous complaints about thermal discomfort suggest less-than-satisfactory environments on these buses. Considering the in-bus air quality and thermal comfort environment, the present operating practice neglects the passengers’ subjective sensation and satisfaction. It is no longer adequate to satisfy the passengers with healthy air only. Satisfaction and comfort must be emphasized. Consideration of perception towards the in-bus air quality and thermal comfort is crucial to improve the service quality. The in-bus air quality, thermal comfort and combined comfort were studied. Empirical comfort models were developed by correlating the level of indicators with the subjective sensation votes towards particular in-bus air quality and thermal comfort issues. Instead of providing quantified description of dissatisfaction level towards the issues, they are practical and significant determinants for optimizing the air-conditioning control to provide a more comfortable commuting environment, while achieving energy effectiveness.

4.2 Methodology Questionnaire survey and physical parameters measurement were conducted on buses to collect subjective sensation responses and air parameters concentration levels concerning in-bus air quality and thermal comfort environment. There were a total of 563 passengers who participated in the survey, including 486 who completed the whole questionnaire. (58.4% completed, 9.3% incomplete and, 32.3% refused to participate.)

4.2.1 Air parameters measurement In each measurement trial, three air quality indicators were sampled, i.e. carbon monoxide (CO), carbon dioxide (CO2) and respirable suspended particulates (RSP).

76 CO is a toxic gas from vehicle exhaust fumes in the roadway environment. Exposure in high concentration would result in death. Concentration of CO was found corresponding to traffic flow as well as vehicle travelling speed. These reveal the in-bus CO concentration is a good indicator of air leakage in passenger compartment.

CO2 is one of the human respiratory products. Its concentration is commonly adopted as indicator in defining the ventilation sufficiency and odour (ASTM, 2002; Fanger & Berg-Munch, 1983). RSP refers to those suspended particulate matters which have the nominal aerodynamic diameters of ten micrometres or less. Traffic volume is a major factor affecting the particulate matters concentration on the roadway environment (Bevan et al., 1991; Lam et al., 1999; Praml and Schierl, 2000). Particulate matters enter the bus passenger compartments through outdoor air intake and door openings. In-bus RSP concentration reveals the effectiveness of particulate-filtering.

The air sampling and questionnaire survey were conducted on the upper-deck so as to minimize the dynamic influence from door openings. Air samplers were placed at the breathing level of passengers (about 1.1 metre from floor), but direct effect by passengers’ breath was avoided. The average travelling speed of buses is about 30 km/hr in the urban area (HKTD, 2005). The instruments were set to log the air parameter data continuously in 30-second-intervals. Hence 4 data points would be obtained per kilometre to record the change of in-bus air quality, while increasing the reliability of data record.

4.2.2 Thermal comfort parameters measurement In each measurement trial, three thermal comfort parameters were sampled, i.e. air temperature, relative humidity and air speed; while the radiant temperature, clothing insulation and metabolic rate were not included. This determination was made dependent upon the duration of face-to-face questionnaire survey and the portability of equipment. The passengers’ clothing insulation was not collected through questionnaire because it took longer for the respondents to complete the whole survey. It may increase the proportion of either incompletion or refusal to participate. To replace the parameter of clothing insulation, the ambient air temperature (data from the Hong Kong Observatory) was recorded to evaluate the effect of in/out-bus air temperature difference. It is assumed that passengers would wear dependent upon

77 the ambient condition in order to achieve the best thermal comfort. Once they entered the bus passenger compartments, the perception would be deflected dependent upon the in-bus thermal environment. The in/out-bus air temperature difference is significant to such deflection, replacing the parameter of clothing insulation.

The data is analyzed based on two categories. From the researcher’s observation, locals wear outer layer on the days with ambient temperature lower than 22oC which are defined as cold days, while the remainders are categorized as hot days. Also, it is found that most passengers do not like to take off their outer layers when they take on buses on cold days. They prefer to merely unzip or unbutton their jackets to adapt to the in-bus thermal comfort environment. On hot days, passengers wear short-sleeves clothes without outer layer, which expect thermally comfortable in-bus environment provided. It can be predicted that if the in/out-bus air temperature difference was too great, the passengers would feel uncomfortable with the over-heated or over-cooled in-bus environment. Therefore, both in-bus air temperature and in/out-bus temperature difference are influential towards the sensation of temperature. The radiant temperature was not included in the study because the equipment took long response time that the reading did not accurately reveal the transient conditions.

4.2.3 Questionnaire survey concerning air quality issues Face-to-face questionnaire survey was conducted concerning with the participants’ subjective perception and dissatisfaction level towards the simultaneous air quality issues, i.e. air staleness, air stuffiness, dusty air and odour inside passenger compartment. Air staleness inside passenger compartments represents air leakage or infiltration of vehicular exhaust from roadway environment. Air stuffiness is caused by insufficient fresh air ventilation resulting as accumulated in-bus air contaminants. Ineffective particulate filtration causes dusty air. The major source of in-bus odour is the metabolic bioeffluents from passengers. The participants’ subjective perception was described with 5-point-scale voting (as illustrated in Figure 4.1). The scale of “0” to “4” allowed the participants to express their feeling, with a vote of “0” representing feeling very comfortable with no sense of particular air quality issue, while the vote of “4” representing feeling extreme discomfort. The copy of

78 questionnaire was illustrated in Appendix B. Each subjective response was quantified and correlated with the concentration levels of air quality indicators at the time-point when conducting the questionnaire. Then the correlation between the air quality indicators and preferences could be evaluated.

Figure 4.1 Scale for sensation vote towards air quality

4.2.4 Questionnaire survey concerning thermal comfort issues Face-to-face questionnaire survey was conducted concerning with the participants’ subjective perception and dissatisfaction towards the simultaneous in-bus thermal comfort issues which described with the ASHRAE thermal sensation scale (7-point-scale voting as illustrated in Figure 4.2) (ASHRAE, 2004). They included the sensation votes towards simultaneous temperature, humidity and air movement inside the bus passenger compartment. The scale of -3 to 3 allowed passengers to express their feeling, with a vote “0” representing neutral-feeling on particular thermal comfort issue, while the vote of “-3” or “3” representing discomfort-feeling. Sensation vote towards in-bus thermal comfort was also collected using a 5-point-scale (as illustrated in Figure 4.3). “0” represented satisfactory comfort and a “4” represented feeling extreme discomfort.

Figure 4.2 Scale for sensation vote towards particular thermal comfort issues

Figure 4.3 Scale for sensation vote towards thermal comfort

79 Each subjective thermal response was quantified and correlated with the measured thermal comfort parameters at the time-point when conducting the questionnaire. Then the correlation between the preferences and parameters were evaluated.

4.2.5 Questionnaire survey concerning combined comfort Sensation votes towards combined comfort was collected with passengers using a 5-point-scale as illustrated in Figure 4.4. “0” represented satisfactory comfort and a “4” represented feeling extreme discomfort.

Figure 4.4 Scale for sensation vote towards in-bus combined comfort

Achieving a comfortable commuting environment, the corresponding air quality indicators concentration levels were evaluated in the basis of the passengers’ dissatisfaction level. As defined in ASHRAE Standard 62.1 (ASHRAE, 2007), an acceptable indoor air quality environment should have no known contaminants at harmful concentrations. Moreover, a substantial majority (80% or more) of the occupants do not express dissatisfaction concerning with the indoor air. Besides, ASHRAE Standard 55 (ASHRAE, 2004) stated a thermally comfortable zone should satisfy a substantial majority (at least 80%) of the occupants. Hence, the goal of dissatisfaction level should be set at 20% or lower for a comfortable commuting environment.

4.3 In-bus air quality issues Fixed-effects linear regression model was applied to the measurement data for determining the relationship between the percentages of dissatisfaction towards particular in-bus air quality issues and the passengers’ perceptions.

4.3.1 Perception towards in-bus air quality The questionnaire survey result showed that 68% of respondents were satisfied with the overall air quality environment on air-conditioned buses. 15% considered the

80 in-bus air quality to be very comfortable. A majority (73%) gave sensation votes of 1 and 2, equivalent to feeling comfortable and neutral respectively. 12% felt uncomfortable. The result concerning with the sensation vote towards overall air quality environment on buses was illustrated in Figure 4.5.

Figure 4.5 Percentage of sensation vote towards air quality on air-conditioned buses

Figure 4.6 Percentage of sensation vote towards particular air quality issues on buses

Figure 4.6 showed the result concerning with the sensation vote towards particular air quality issues on air-conditioned buses. 58 – 75% of respondents felt comfortable or very comfortable towards particular air quality issues. Meanwhile, 5 – 23% felt uncomfortable or very uncomfortable. These revealed a positive perception towards in-bus air quality on buses.

81 4.3.2 Sensation towards air staleness Natural ventilation is applied on non-air-conditioned buses; the ambient air accompanying the roadway air pollutants infiltrates into the passenger compartment (Chan et al., 2002b). On air-conditioned buses, roadway pollutants mix with the fresh air and enter the passenger compartment through the mechanical ventilation air intake. Passengers would be affected by the dynamic air quality from different out-bus environment along the routes. The issue of in-bus stale air is therefore more obvious when buses travel in the busy urban districts or inside tunnels as mentioned in previous chapter.

Figure 4.7 Sensation vote of stale air against in-bus CO concentration level on buses

Figure 4.7 illustrated the correlation between the in-bus CO concentration level and sensation vote towards air staleness. Higher air pollutants concentration level causes the in-bus air staleness and results as worse feeling. Thus the vote increases with higher concentration level. The regression line lay below the sensation vote of 2. It meant passengers felt average or comfortable towards the issue of stale air on air-conditioned buses. The correlation was concluded as follows.

2 Votes = 0.56 CCO ( R = 0.77, p < 0.05 ) (4.1) 2 PDs = 31.28 Votes – 1.50 ( R = 0.89, p < 0.05 ) (4.2)

for 0 ≤ PDs ≤ 100 and 0 ≤ Votes ≤ 4 where Vote s represented sensation vote towards air staleness, CCO was concentration of carbon monoxide and PDs was percentage of dissatisfaction towards air staleness.

82 Correlation between the percentage of dissatisfaction and the sensation vote towards in-bus air staleness was plotted in Figure 4.8. The percentage of dissatisfaction reached 100% when the sensation vote was 3.2. Satisfying a substantial majority (80% or more) of the passengers towards in-bus air staleness issue, the vote should not be exceeded by 0.7.

Figure 4.8 Percentage of dissatisfaction against sensation vote towards stale air on buses

4.3.3 Sensation towards air stuffiness At peak hours, buses are often fully packed of passengers. High occupancy load reduces the relative outdoor air rate per person, since mechanical ventilation is fixed on buses. Metabolic CO2 may not be diluted effectively causing air stuffiness, especially on long distance buses on highways (Chan, 2005). Hence, CO2 concentration is significant to reveal the air stuffiness issue.

Figure 4.9 Sensation vote of stuffy air against in-bus CO2 concentration level on buses

83 The correlation between sensation vote towards air stuffiness and the in-bus CO2 concentration level was illustrated in Figure 4.9. Passengers were less sensitive to stuffy air on air-conditioned buses. Passengers felt very uncomfortable when the CO2 concentration level was over 4400 ppm. The outdoor air rate was corresponding to less than 1 litre per second per person on a fully loaded bus. This explains the passengers’ delusion with the thermal comfort sensation and which dominates the combined comfort while lowering the effect of sensation towards air quality (Shek & Chan, 2008). Correlation model towards the issue of air stuffiness was concluded as follows.

2 Votef = 0.0010 CCO2 – 0.39 ( R = 0.82, p < 0.05 ) (4.3) 2 PDf = 43.95 Votef – 44.52 ( R = 0.90, p < 0.05 ) (4.4)

for 0 ≤ PDf ≤ 100 and 0 ≤ Votef ≤ 4 where Vote f represented sensation vote towards air stuffiness, CCO2 was concentration of carbon dioxide and PDf was percentage of dissatisfaction towards air stuffiness.

Figure 4.10 illustrated the percentage of dissatisfaction against the sensation vote towards air stuffiness on buses. The sensation votes must be lower than 1.5 in order to satisfy a substantial majority of passengers. This was equivalent to maintaining the in-bus CO2 concentration level below 1900 ppm (as shown in Figure 4.9).

Figure 4.10 Percentage of dissatisfaction against sensation vote towards stuffy air on buses

84 4.3.4 Sensation towards odour

People generate metabolic CO2 while producing odour-causing bioeffluents. The generation rate of bioeffluents depends on one’s activity level, similar to that of CO2.

Hence the CO2 concentration and odour intensity from bioeffluents exhibit a similar dependence on the occupancy load and outdoor air rate.

Figure 4.11 Sensation vote of odour against in-bus CO2 concentration level on buses

Figure 4.11 illustrated the correlation between sensation vote towards odour and the in-bus CO2 concentration level; and the percentage of dissatisfaction against the vote towards odour was shown in Figure 4.12. The respondents felt vey uncomfortable towards this issue when the CO2 concentration level rose beyond 4800 ppm in air-conditioned compartments. The percentage of dissatisfaction reached 100% when the sensation vote was exceeded by 3.0. In order to reduce the dissatisfaction level to be below 20%, the votes must be lower than 0.9. The following correlation model concluded the issue of odour.

2 Voteo = 0.0009 CCO2 – 0.37 ( R = 0.82, p < 0.05 ) (4.5) 2 PDo = 38.71 Voteo – 15.72 ( R = 0.89, p < 0.05 ) (4.6)

for 0 ≤ PDo ≤ 100 and 0 ≤ Voteo ≤ 4 where Vote o represented sensation vote towards odour, CCO2 was concentration of carbon dioxide and PDo was percentage of dissatisfaction towards odour.

The odour emission affected by diet, personal hygiene and habits. Hence people living in different countries or cities may have different of perception to body odour.

85 Berg-Munch et al. (1986) obtained the regression line with smaller slope. It found the percentage of dissatisfaction did not exceed 20% even if the indoor CO2 concentration level reached 2000 ppm. Also, according to ASTM (2002), the predicted dissatisfaction reached 60% in the occupied zone with 4800 ppm of CO2. These showed the locals are more sensitive to the odour.

Figure 4.12 Percentage of dissatisfaction against sensation vote towards odour on buses

4.3.5 Sensation towards dusty air Roadway transport is a major source of RSP emission. It contributed to 31% of total emission in 2006 in Hong Kong (HKEPD, 2006). The concentration of roadway RSP affects by the population of heavy duty vehicle (Junker et al., 2000) and the traffic density (Zhu et al., 2002). The roadway particles infiltrate into the passenger compartment through outdoor air ventilation and door openings (Chan et al., 2002a).

Figure 4.13 Sensation vote of dusty air against in-bus RSP concentration level on buses

86 Correlation between sensation vote towards dusty air and the in-bus RSP concentration level was concluded as follows and illustrated in Figure 4.13. The result of sensation votes ranged below “3”, which meant the respondents seldom experience the worst dusty air issue on local buses.

2 Voted = 11.62 CRSP ( R = 0.86, p < 0.05 ) (4.7) 2 PDd = 29.55 Voted – 11.17 ( R = 0.89, p < 0.05 ) (4.8)

for 0 ≤ PDd ≤ 100 and 0 ≤ Voted ≤ 4 where Vote d represented sensation vote towards dusty air, CRSP was concentration of respirable suspended particulates and PDd was percentage of dissatisfaction towards dusty air.

Figure 4.14 illustrated the percentage of dissatisfaction against the sensation vote towards in-bus dusty air. In order to minimize the dissatisfaction level towards in-bus dusty air issue to below 20%, the sensation vote must be lower than 1.1. It was -3 corresponding to the in-bus RSP concentration level at 0.09 mg · m inside air-conditioned compartments.

Figure 4.14 Percentage of dissatisfaction against sensation vote towards dusty air

4.3.6 Empirical model for in-bus air quality In previous sections, the sensation votes and percentage of dissatisfaction towards particular air quality issues were evaluated separately. Realizing the comfort level of overall air quality, empirical model was developed, which revealed the significance

87 of particular air quality issues. The sensation vote towards the overall air quality on air-conditioned buses was concluded as follows:

VoteAQ = 0.37 Votes + 0.30 Votef + 0.07 Voted + 0.21 Voteo + 0.15 ( R2=0.82, p<0.05 ) for 0 ≤ Vote ≤ 4 (4.9)

where Vote AQ represented sensation vote towards overall air quality.

The significance of particular votes can be revealed through the magnitudes of their coefficients. Thus, the sensation vote towards air staleness is in the highest ranking, following by that of the air stuffiness, indicating they are more significant among particular air quality issues. These two sectors contributed to about 70% of vote towards the overall air quality. The in-bus air contaminants concentration is affected by fixed ventilation rate, bus occupancy load as well as the roadway air quality. These factors vary the perception towards air staleness, stuffiness and odour, and raise the passengers’ consideration. The air-conditioning system is effective in particulates filtering in the compartment that minimizes the passengers’ concern towards the issue of dusty air. Hence the significance of vote towards dusty air is in the lowest ranking on air-conditioned buses. By the aid of the comfort model, the in-bus air quality environment could be quantified with a simple and clear indicator: the percentage of dissatisfaction. The percentage of dissatisfaction against sensation vote towards the overall air quality was concluded as follows and illustrated in Figure 4.15.

2 PDAQ = 21.87 VoteAQ + 1.34 ( R = 0.86, p < 0.05 )

for 0 ≤ PDAQ ≤ 100 and 0 ≤ VoteAQ ≤ 4 (4.10) where PDAQ was the percentage of dissatisfaction towards air quality.

Figure 4.15 Percentage of dissatisfaction against sensation vote towards in-bus air quality

4.4 In-bus thermal comfort issues The passengers’ subjective responses and dissatisfaction level collected from the questionnaire survey were quantified by correlating with the thermal comfort parameters levels.

4.4.1 Perception towards in-bus thermal comfort Figure 4.16 showed the thermal comfort survey result obtained on air-conditioned buses. It showed that 72% of respondents were satisfied with the thermal comfort environment. The majority (73%) gave sensation votes of 1 and 2, equivalent to feeling comfortable and average respectively. 12% considered the in-bus thermal environment to be very comfortable, while 13% felt uncomfortable.

Figure 4.16 Percentage of sensation vote towards thermal comfort on air-conditioned buses

Figure 4.17 Distribution of sensation vote towards particular thermal comfort issues on air-conditioned buses

The survey result concerning with sensation vote towards particular thermal comfort issues was illustrated in Figure 4.17. A majority (58 – 73%) of respondents felt very comfortable towards particular thermal comfort issues on air-conditioned buses. Less than 4% of vote was given to uncomfortable and very uncomfortable.

4.4.2 Sensation towards temperature The percentage of dissatisfaction against the sensation vote towards temperature was concluded and illustrated in Figure 4.18. The model was as follows:

2 2 PDt = 8.26 Votet + 2.31 Votet + 12.69 ( R = 0.69, p < 0.05)

for 0 ≤ PDt ≤ 100 and -3 ≤ Votet ≤ 3 (4.11)

where PDt was percentage of dissatisfaction towards temperature and Vote t represented sensation vote towards temperature.

Considering the thermal comfort environment inside air-conditioned passenger compartments, the sensation vote should range from -1.1 to 0.8 so as to satisfy a substantial majority of passengers. This was equivalent to the sensation response between slightly cool and slightly warm. The lowest dissatisfaction level (12.5%) was obtained when the sensation vote at -0.1. It meant passengers preferred a little bit cool on buses.

Figure 4.18 Percentage of dissatisfaction against sensation vote towards temperature on buses

Figure 4.19 Sensation vote against in/out-bus air temperature difference on air-conditioned buses

The profiles of sensation vote against in/out-bus air temperature difference were concluded and plotted in Figure 4.19. Two profiles were plotted representing the conditions on the cold and hot days. It was found that passengers preferred cooler air-conditioned passenger compartments on hot days, because the sensation vote of “0” was obtained at negative in/out-bus air temperature difference (-3.9oC). Since the

91 sensation vote between -1.1 and 0.8 was considered as thermally comfortable raised by the respondents, such condition could be achieved with negative in/out-bus air temperature difference. It meant the in-bus air temperature was preferred to be lower than the ambient condition to achieve thermally comfortable on hot days. On cold days, the sensation vote “0” was obtained with the temperature difference higher than 2.5oC. An optimum thermal comfort could be achieved with a temperature difference higher than -2.7oC.

Figure 4.20 Practical and preferred in-bus air temperature

The profiles of practical and preferred in-bus air temperature under different ambient condition were plotted in Figure 4.20. On cold days, passengers expected to have a warmer air-conditioned compartment. Hence, the regression line of preferred temperature tended to horizontal for ambient temperature below 13oC. It shows passengers like the compartments to be maintained at a constant temperature about 18.5oC on air-conditioned compartments when the ambient condition is cold. On the opposite, a cooler compartment is preferred on hot days. The regression line of preferred temperature became horizontal at higher ambient temperature, which showed the upper limit of the in-bus air temperature was about 26oC. The difference between profiles of practical and preferred temperature indicated fine-tuning of the in-bus thermal comfort settings was required for better commuting environment and lowering dissatisfaction level. The sensation vote was correlated with in-bus air temperature and ambient air

92 temperature. A comfortable in-bus air temperature should be designed depending on the ambient condition. The model was concluded in the following and illustrated in 3-dimensional in Figure 4.21.

3 2 2 Votet = [ 0.72 ti – 47.77 ti + 1101.89 ti – 0.51 to + 5.07 to – 8611.78 ] / 100 2 ( R = 0.75, p < 0.05 ) for -3 ≤ Votet ≤ 3 (4.12) where ti represented in-bus air temperature and to was ambient temperature.

Figure 4.21 Model of sensation vote towards temperature on air-conditioned buses

4.4.3 Sensation towards humidity Studying the sensation vote towards humidity, the in-bus relative humidity is concerned. Unlike the sensation towards temperature, the involvement of in/out-bus parameter difference is not required for relative humidity. It is because people do not take any action to adapt to the seasonal change of relative humidity in the ambient environment. Considering the percentage of dissatisfaction against sensation vote towards humidity, the correlation was concluded as follows and illustrated in Figure 4.22. 2 PDw = 7.62 Votew – 2.68 Votew + 14.00 (4.13) 2 R = 0.55, p < 0.05 for 0 ≤ PDw ≤ 100 and -3 ≤ Votew ≤ 3. where PDw was percentage of dissatisfaction towards humidity and Vote w represented sensation vote towards humidity.

Figure 4.22 Percentage of dissatisfaction against sensation vote towards humidity on buses

The lowest dissatisfaction level was obtained at the sensation vote of 0.18 on buses. Minimizing the dissatisfaction level to be below 20%, the sensation vote towards humidity should range between -1.1 and 0.7. Majority of the passengers were satisfied with the in-bus relative humidity ranging from 39 to 80% inside the air-conditioned compartments. The optimum level was 64% RH. The correlation of sensation vote towards humidity was concluded in the following and shown in Figure 4.23.

Votew = – 0.04 w + 2.79 (4.14) 2 R = 0.64, p < 0.05 for -3 ≤ Votew ≤ 3 where w represented the in-bus relative humidity.

The profile revealed the respondents were not sensitive to humidity as most of the votes ranged between -1 and 1 for the relative humidity varied from 40 to 85%. It means relative humidity does not influence the in-bus thermal comfort environment if it is controlled within the range.

Figure 4.23 Sensation vote against in-bus relative humidity on buses

4.4.4 Sensation towards air movement Studying the sensation towards air movement on buses, in-bus air speed was concerned. The in-bus air movement is not under control on non-air-conditioned buses. Heat release and air contaminants dilution depend on air change, affected by the aerodynamic drag induced by travelling (Chan & Shek, 2007). Silent and stuffy air would cause thermal discomfort on stationary bus (Madsen, 1984). In addition to the higher air temperature on non-air-conditioned buses, passengers prefer higher air speed to achieve heat release and ventilation. However, on air-conditioned bus, it facilitates constant air distribution and air movement to provide a steady and comfortable commuting environment. Passengers can adjust the supply air diffusers above the seats for individual preference. The maximum in-bus air speed was found -1 to be 1.85 m · s . The percentage of dissatisfaction towards in-bus air movement was shown in Figure 4.24 and the model was as follows.

2 PDv = 5.78 Votev + 1.12 Votev + 15.68 (4.15) 2 R = 0.78, p < 0.05 for 0 ≤ PDv ≤ 100 and -3 ≤ Votev ≤ 3 where PDv was percentage of dissatisfaction towards air movement and Vote v represented sensation vote towards air movement.

A substantial majority of passengers could be satisfied when the sensation vote ranged from -1.0 to 0.8. The lowest dissatisfaction level was 15.6% at the sensation

95 -1 vote of -0.1. Thus, the optimum in-bus air speed was 0.3 m · s on air-conditioned buses. The correlation of sensation vote towards air movement was concluded as follows and illustrated in Figure 4.25.

Votev = – 1.34 ln( v ) – 1.84 (4.16) 2 R = 0.62, p < 0.05 for -3 ≤ Votev ≤ 3 where v represented the in-bus air speed.

Figure 4.24 Percentage of dissatisfaction against sensation vote towards air movement on buses

Figure 4.25 Sensation vote against in-bus air speed on buses

96 4.4.5 Empirical model for in-bus thermal comfort In previous sections, the preferences of particular thermal comfort sensations were studied. However, the perception towards thermal comfort cannot be thoroughly evaluated by either of the particulars. For example, a high air speed would cause discomfort of draught risk even if a man stays in a zone with optimum air temperature. On the contrary, higher air speed can improve one’s thermal comfort in an over-warmed zone (ISO, 2005). The perception towards particular sensations and thermal comfort were combined and analyzed through multiple regressions. The model was concluded as follows and illustrated in Figure 4.26.

2 PDTC = 17.44 VoteTC ( R = 0.78, p < 0.05 ) (4.17) 2 VoteTC = | [ 0.91 Votet + 0.08 Votew + 0.50 Votev + 0.16 ] | ( R = 0.73, p < 0.05 ) (4.18)

for 0 ≤ PDTC ≤ 100, 0 ≤ VoteTC ≤ 4 and -3 ≤ Votet, w, v ≤ 3 where PDTC represented the percentage of dissatisfaction towards thermal comfort and Vote TC represented the sensation vote towards thermal comfort.

Figure 4.26 Percentage of dissatisfaction towards thermal comfort on buses

In the model, the magnitude of coefficients reveals the significance of particular sensations among thermal comfort. The coefficient of sensation vote towards temperature is larger than the sum of the remainders. Hence, the vote towards temperature is the most crucial factor that dominates the subjective thermal comfort sensation and dissatisfaction level. Meanwhile relative humidity and air speed act as

97 auxiliary factors to modulate the in-bus thermal comfort environment by varying the capability of convection effect.

4.5 In-bus combined comfort The survey results showed that 65% of respondents were satisfied with the combined comfort level on present air-conditioned buses. The majority (74%) gave sensation votes of “1” and “2”, equivalent to feeling comfortable and average respectively. 16% considered the in-bus environment to be very comfortable; while 9% felt uncomfortable. Detail was illustrated in Figure 4.27.

Figure 4.27 Percentage of sensation vote towards combined comfort level on air-conditioned buses

Evaluating the combined comfort level, it was more precise to weigh the sensations towards thermal comfort and air quality by reliable weighted proportions under the transient in-bus environment. Therefore, the combined comfort model was developed by correlating the combined comfort dissatisfaction level and the passengers’ sensation votes towards thermal comfort and air quality. The model was organized as follows:

Percentage of Dissatisfaction = a ⋅ VoteTC + b ⋅ Vote AQ + constant The combined comfort model helped determine the percentage of dissatisfaction with commuting comfort concerning with thermal comfort and air quality on buses. The empirical model was as follows. (Figure 4.28 illustrated the models.)

PDcombined = 7.47 VoteAQ + 14.67 VoteTC + 1.49 (4.19) 2 ( R = 0.65 for 0 ≤ PDcombined ≤ 100, 0 ≤ VoteAQ ≤ 4 and 0 ≤ VoteTC ≤ 4) where PDcombined represented the percentage of dissatisfaction towards combined comfort level.

Figure 4.28 Combined comfort index on air-conditioned buses

The dummies illustrated in Figure 4.28 indicated the equivalent percentage of dissatisfaction by different combinations of sensation votes; and the pattern revealed the domination of particular sensations. The domination indicated the importance and effect of the sensation vote towards the combined comfort index. The dummies tending to vertical represented a higher dominant position of thermal sensation; on the contrary, dummies tending to horizontal represented a higher dominant position of sensation towards in-bus air quality. Also the dominant position could be evaluated from the magnitudes of coefficients in the combined comfort models. The sensation vote towards in-bus thermal comfort was found to have higher dominant position in the combined comfort models. In Figure 4.28, the average sensation vote towards thermal comfort must be lower than 1.3 in order to be possibly obtaining the percentage of dissatisfaction level below 20%. Considering the combination, the upper-limit of sensation vote towards in-bus air quality was 2.5. It showed the shorter range of sensation vote towards thermal comfort was required to achieve 20% of dissatisfaction; while the requirement towards that of the air quality was looser. Therefore, if more effort was put to enhance the comfort level of the thermal sensation, a reduction in the percentage of dissatisfaction towards combined comfort level could be achieved more effectively.

99 4.6 Application of comfort models To evaluate the comfort level, both questionnaires and physical parameters sampling have to be conducted. This has proved an accurate and reliable method to collect the dissatisfaction levels and passengers’ sensation votes towards in-bus thermal comfort and air quality, as well as the combined comfort level. However, these surveying and sampling activities are often time-consuming with limited results. Due to the time required for data collection and analysis, the actual adjustment of air-conditioning settings for comfort would be delayed. Moreover, the outdoor air quality varies along the journey when the buses travel through different districts. Such a manpower-dependent approach is not effective in maintaining a comfortable in-bus environment faced with a dynamic outdoor environment. Therefore, it is not appropriate to set a fixed fresh air ventilation rate in order to control the in-bus air quality. Meanwhile, in-bus set-point temperature should be adjusted depending on ambient condition. To achieve a continuous comfortable in-bus environment, an effective approach must integrate the combined comfort model, as well as hardware and software systems with the air-conditioning system to effectively control the transient in-bus environment. Thermal and air parameters sampling and data logging instruments (hard-ware system) can be equipped for continuous real-time monitoring on buses. By processing and analyzing the data with the combined comfort model, the air-conditioning setting adjustment commands can be determined by a software system and delivered to the air-conditioning unit. It can then adjust the air-conditioning settings depending on real-time computation result along the bus journeys. Therefore, a comfortable in-bus air quality and thermal environment can be achieved and maintained. Besides this integrated approach facilitates adjusting air-conditioning settings and seeks to minimize the influence of air pollution from the roadway environment, as well as reduce manpower and running costs. By determining the levels of dissatisfaction about the combined comfort on a bus, a beneficial balance between energy effectiveness and raising passengers’ satisfaction level could be achieved.

100 4.6.1 Control strategy for air quality High mechanical ventilation rate increases outdoor air introduction to dilute in-bus contaminants; nevertheless it increases the risk of introducing stale air from the busy roadway environment at the same time. Low ventilation rate reduces the risk but it worsens the issues of air stuffiness and odour inside the passenger compartments. Hence, it is inappropriate to set a fixed outdoor air ventilation rate in order to control the in-bus air quality since the outdoor air quality varied along the journey when the buses travelled through different districts. The mechanical ventilation rate should be flexible and adjusted depending on roadway environment along the journeys. No outdoor air should be introduced if the roadway environment was poor with highly concentrated air pollutants. On the contrary, the rate should be increased afterwards if the roadway air quality was acceptable. Achieving a comfortable in-bus environment, it is an effective approach to integrate the comfort model into the air-conditioning control system. Air sampling and data logging instruments can be equipped on buses for real-time monitoring of out-bus

CO, in-bus CO2 and RSP concentration level. Processing and analyzing the data referring to the comfort model, commands for the air-conditioning settings adjustment is to be delivered to the control system. For example, the comfortable in-bus air quality is expected to be achieved with the percentage of dissatisfaction below 20%. Particular corresponding sensation votes and air quality indicators concentration levels can be calculated with the comfort model. Then the concentration levels become the set-points for determining the adjustment of outdoor air intake rate. The diagrammatic control scheme of outdoor air rate was illustrated in Figure 4.29. Out-bus CO concentration level is put in the highest priority for the determination since it will influence the passengers’ health if they expose in concentrated air pollutants. In-bus RSP is put in the second because it is less rigorous and ones would be influenced under long-term exposure. Also the particulates could be reduced under higher proportion of air recirculation on buses (as reported in previous chapter).

101

Monitoring out-bus CO concentration level

Monitoring in-bus RSP concentration level & CO2 concentration level

CCO ≤ 1.3 ppm Yes No

Shut outdoor CRSP air damper ≤ 0.09 mg m-3

Yes No

Reduce CCO2 outdoor air rate ≤ 1500 ppm Yes No

Reduce Increase outdoor air rate outdoor air rate

Figure 4.29 Air-conditioning control scheme of outdoor air rate

102

Literatures concerning with CO2 concentration health effects were plotted in Figure 4.30. It showed short-term exposure gave no noticeable symptoms (5500 ppm for 6 hours). Hence, in-bus CO2 concentration level is considered at the last as it gives light influence to the occupants. Applying the measurement data for computation, the percentage of dissatisfaction towards present in-bus air quality was 51%. It meant half of the passengers would feel dissatisfied. However, if the air-conditioning control system operated depending on the comfort model computation result, the dissatisfaction level could be reduced to below 20%.

Figure 4.30 Health effects for exposure under different CO2 concentration level

4.6.2 Control strategy for thermal comfort Sampling the ambient air temperature, comfortable in-bus air temperature can be evaluated dependent on the in/out-bus temperature difference. Then set-point temperature is reset in the air conditioning system. The in-bus air temperature and relative humidity should be monitored to assess the zone thermal comfort. In-bus relative humidity and air speed are adjusted to modulate the thermal comfort environment in order to achieve the highest satisfaction level. For example, higher air speed should be applied in the compartment when the in-bus air temperature was above the set point. The wind-chill effect minimizes the thermal discomfort caused

103 by under-cooling. The diagrammatic control scheme for thermal comfort was illustrated in Figure 4.31. Applying the measurement data for computation, the percentage of dissatisfaction towards present in-bus thermal comfort was 13%. However, if the air-conditioning control system operated depending on the comfort model computation result, the dissatisfaction level could be further decreased to 0.2%.

Monitor ambient air temperature ( to ) in-bus air temperature ( ti ) in-bus relative humidity ( w )

By comfort model computation, evaluate comfortable ti and adjust set-point temperature ( tset-pt )

w ≥ 80% No

Yes

Dehumidification No action

Calculate an appropriate air speed ( v ) by comfort model and adjust setting in system

Figure 4.31 Air-conditioning control scheme for thermal comfort

104

4.6.3 Control strategy for combined comfort To achieve a continuous comfortable in-bus environment, an effective approach must integrate the combined comfort model. Sampling the air quality and thermal comfort indicators, the percentage of dissatisfaction level towards combined comfort can be estimated. If it does not satisfy a substantial major (80%), there should be adjustment in both air quality and thermal comfort settings. Concerning with air quality, outdoor air rate should be increased if the outdoor air quality is acceptable. The increase of outdoor air rate can minimize the dissatisfaction level towards air stuffiness and odour issues. Concerning with thermal comfort, the in-bus air speed should be adjusted to compensate for the temperature difference to the proposed set-point. Figure 4.32 illustrated the diagrammatic control scheme for combined comfort. Applying the measurement data for computation, the percentage of dissatisfaction towards present combined comfort level was 30%. Since the computation result of real-time data provides determinants for the control system to set an appropriate outdoor air intake rate and thermal comfort settings, the dissatisfaction level can be diminished to below 8%.

105 Monitor air quality and thermal comfort indicators

Comfort models computation

PDcombined ≤ 20%

Yes No

AQ TC No action

CCO ≤ 1.3 ppm No ti ≤ tset-pt

Yes Yes

Reduce Increase CRSP air air ⋅ -3 No ≤ 0.09 mg m speed speed

Yes

Increase outdoor air rate

Figure 4.32 Air-conditioning control scheme for combined comfort

106 4.7 Conclusion The physical parameter levels and the passengers’ subjective perception towards in-bus air quality and thermal comfort issues were collected. Multiple regressions were applied to conclude the correlation between the effects of particular issues towards in-bus air quality and thermal comfort. Through correlating the data, the passengers’ sensation votes and dissatisfaction levels corresponding to the practical condition were realized. Empirical comfort models were developed which provided simple and clear indicators to quantify the comfort level of commuting environment on buses. In application, the empirical comfort models can be applied to optimize the air conditioning control system. With the physical parameters sampling and data processing instruments installed on buses, real-time monitoring of in-bus and roadway indicators can be conducted along the journeys. It provides real-time data for the control system to appropriately determine as well as adjusting the outdoor air intake rate and set point air temperature. This would be an effective tool to enhance the commuting environment while increasing the passengers’ satisfaction level as well as achieving energy effectiveness. The application of empirical comfort models leads to changes in the air-conditioned compartment. In the following chapter, computational fluid dynamics model was used to simulate the effects to the present in-bus environment. Also a new compartment was proposed to further improve the commuting environment.

107 5 Design of new air distribution system

5.1 Introduction

In the present passenger compartment, mixing-ventilation system was applied. Supply air was delivered to the compartment through general diffusers and top-up diffusers on the ceiling-mounted ductworks. Return air grilles were located at the back of compartment. (The rendered geometry was illustrated in Figure 5.1.) The present design aimed to provide a uniformly air distribution. However, supply air was not well distributed due the tight space in the compartment. It caused thermally discomfort because of temperature stratification and draft risk. Also, fresh air did not directly reach all passengers causing ineffective air contaminants dilution in the compartment. A new upper-deck compartment was proposed (as illustrated in Figure 5.2). Personalized air supply and return system was applied. Computation result from the empirical comfort models was used as air-conditioning settings to improve the in-bus environment. Slot-type diffusers were installed in front of each seat providing supply air to passenger. Also return air grilles were located above each row. This design aimed to improve the air distribution and ventilation efficiency while satisfying individual preferences. The conditions inside the present and new compartment were evaluated through computational fluid dynamics model. Geometries of the compartments were developed for simulation under different conditions. Also, an experimental chamber was built to validate the new design. The effectiveness of the new compartment was evaluated depending on the simulation result concerning with thermal comfort and ventilation issues.

5.2 Methodology

The finite volume CFD code FLUENT (version 6.3) was used to analyze air temperature distribution, air movement, dilution of metabolic CO2 and particle transport inside the upper-deck passenger compartment. An accompanying program, GAMBIT (version 2.4) was applied to develop the geometry. The governing equations were stated in Appendix C.

108 General diffuser

Top-up diffuser

Return air grille

Figure 5.1 Geometry of upper-deck passenger compartment with present air distribution system

109 Return air grille

Supply air diffuser

Figure 5.2 Geometry of upper-deck passenger compartment with new air distribution system

110 Two upper-deck passenger compartments were modelled by the 3D CFD mathematical model. The physical dimensions of the passenger compartment were similar to a typical 12-metre double-decker compartment: 12 m (length) x 2.4 m (wide) x 1.75 m (height). The geometry was illustrated in Figure 5.3. The spatial domain was discretized into tetrahedrons with over 190 thousands node points. Geometry 1 was developed according to the present air distribution design (as illustrated in Figure 5.1), while Geometry 2 was equipped with a new personalized air supply and return system (as illustrated in Figure 5.2) respectively.

Figure 5.3 Meshed geometry of the passenger compartment

In the geometry, all construction gaps were sealed. Supply air entered through diffusers and exhausted through return air grilles. It was assumed that the compartment was fully occupied with 59 passengers onboard who had the light -2 o activity level of 58 W · m and skin temperature at 34 C. The geometry of passenger was shown in Figure 5.4. Simulating the ventilation efficiency, steady exhalation at a -1 flow rate of 0.14 l · s was assumed for each passenger which contained a production -1 rate of CO2 at 0.01 l · s . Also, according to the practical measurement, the walls were set at a constant surface temperature of 32oC, the ceiling and windows at 38oC. -1 In the present compartment, the air speed at supply air diffusers was 3.5 m · s at o -1 16 C that the supply air rate was 23 l · s per person. In the new compartment, the

111 -1 o supply air speed was proposed as 0.9 m · s at 21 C. The supply air rate was equivalent to 18 l · s per person.

Figure 5.4 Geometry of passenger located inside compartment

Besides, particle transport was simulated in the compartments to evaluate the effectiveness of the personalized air supply and return system. An index person was assumed sitting inside the compartments. Droplets were expelled through coughing. The tracks of droplets were traced which revealed the influence to other passenger in the compartments. As stated in Morawska (2005) and Wells (1995), almost coughing droplets evaporated immediately to droplet nuclei in about 0.3 second. Xu (1998) and Jiang et al. (2003) indicated that about 95% of aerosols expelled during breathing, coughing and sneezing are less than 1 micro-meter in size. Complicated phenomena, like collision, splitting, and evaporation, actually occur during the dispersion processes of droplets. Since the purpose was to investigate the transport characteristics of droplets due to coughing, Zhu et al. (2006) suggested not taken the complicated phenomena into consideration. Non-evaporative nuclei were assumed to be with a diameter of 1 micro-metre. Thus the droplet diameter was set as 1 micro-meter and assumed as -3 -9 -1 liquid water with density of 1000 kg · m while discharge rate at 2.6 x 10 kg · s . In each simulation, 10000 hypothetical droplets were considered to determine the behaviour statistically. Zhu also found that the initial velocity of the coughed airflow -1 ranged widely between 6 and 22 m · s . To simulate a severe condition, an initial -1 speed of 22 m · s was applied. Meanwhile, other passengers were assumed to have

112 -1 steady inhalation at a flow rate of 0.14 l · s . The boundary conditions for the discrete phase model were tabulated in Table 5.1.

Table 5.1 Boundary conditions for discrete phase model Boundary Condition

Mouth of index person Wall-jet Noses of passengers Trap

Return air grilles Escape

Remarks: Wall-jet is defined on a face to discharge particles at a momentum flux by a defined direction and velocity. Trap is defined as both particle deposition and the trajectory calculations are terminated once the particles are in contact with the boundary. Escape is defined as particles that effectively vanish and are removed from the computation domain once in contact with the boundary.

5.3 Validation of the numerical model

Experiment was conducted in a test chamber specifically built to validate the model. Air temperature and air speed were measured to validate the thermal comfort section.

Also, sulphur hexafluoride (SF6) was spread in the chamber to validate the ventilation by concentration distribution in the compartment. Due to the limited space in the laboratory, the dimension of test chamber was 2.4 m in length, 2.4 m in width and 1.75 m in height. Three rows of seats were built in the test chamber. Sketch and photo of the test chamber were illustrated in Figure 5.5 and Figure 5.6. The air-conditioning was controlled by an AHU with programmable system as shown in Figure 5.7. Supply air was delivered to the floor plenum and discharged through slot-type diffusers in front of each seat (as illustrated in Figure 5.8 and Figure 5.9). The diffusers were sized as 0.5 m length and 0.05 m width. Adjustable dampers were equipped at each diffuser and return grille for air balancing.

113

Figure 5.5 Sketch of test chamber (side)

Return air grille

Slot-type supply air diffuser

Floor plenum for supply air

Figure 5.6 Photo of test chamber (side)

114

Figure 5.7 Panel of air-conditioning control system

Slots for equipping diffusers

Figure 5.8 Photo of test chamber view from backward (diffusers were not yet installed in this photo)

115 Return air grille

Slot-type supply air diffuser Figure 5.9 Photo of test chamber (diffusers and return air grilles)

5.3.1 Validation of air temperature and air speed

During the experiment, light bulbs and a thermal manikin were placed in the test chamber as heat sources. They were assumed to be passengers with metabolic rate at -2 light activity level (58 W · m ). The photo of thermal manikin and its control panel were illustrated in Figure 5.10 and Figure 5.11 respectively. Figure 5.12 showed the heat flux generated on the skin of thermal manikin taken by thermal camera. Temperature loggers and anemometers (as illustrated in Figure 5.13 and Figure 5.14) were installed at different vertical levels to measure the air temperature and air speed, including the head (1.1m), waist (0.6m) and feet (0.1m).

116

Figure 5.10 Photo of thermal manikin sitting inside the test chamber

Figure 5.11 Control panel of thermal manikin

117

Figure 5.12 Photo of thermal manikin taken by thermal camera

Anemometer set Diffusers

Figure 5.13 Anemometers set

118 Head level

Feet level

Waist level

Figure 5.14 Anemometers installed at different vertical levels

Figure 5.15 Comparison of simulated and measured air temperature with supply air at 20oC, 21oC, 22oC and 23oC.

119 The experiment was conducted under different supply air temperature and air speed in order to validate the result from simulation model. The supply air temperature was o o -1 adjusted from 20 C to 23 C while the air speed was increased from 1.2 m · s to 1.8 -1 m · s in the experiment. The data logging instrument recorded the air temperature and speed in an interval of 30-second. The time-averaged values were used to compare with the simulated results. The comparisons of simulated and measured air temperature were illustrated in Figure 5.15. Also the comparisons of simulated and measured result of air speed were illustrated in Figure 5.16. They proved that the simulated results showed a similar trend as the measured profiles and the values were close to the experiment result. These validated the numerical method produced creditable results.

-1 Figure 5.16 Comparison of simulated and measured air speed with supply air at 1.2m · s ,

-1 -1 -1 1.4m · s , 1.6m · s and 1.8m · s .

120 5.3.2 Validation of ventilation by concentration distribution In the experiment, thermal manikin was placed in the test chamber as an index person who exhaled inside the compartment. Tracer gas sulphur hexafluoride (SF6) -1 was discharged continuously through manikin’s mouth at a flow rate of 0.0005 l · s to play as exhalation of metabolic bioeffluents. Gas detectors were installed at breathing zones at other seats and at each return air grille to measure the gas concentration at different locations. The distribution indicated the spread of tracer gas and the effectiveness of personalized air supply and return system. The index person was assumed to sit in the middle row to evaluate the influence to surrounding passengers. Firstly, the experiment was conducted with the index person located at seat 2a as illustrated in Figure 5.17. The measurement result was summarized in Table 5.2. Higher tracer gas concentration was found at the nearest return air grille ‘Return-2L’, while the concentration at other seats was relatively low. Over 98% of the tracer gas exhausted through the return grille right above the index person. Only 1.7% was measured at breathing zone of other seats. It showed the tracer gas did not spread throughout the compartment.

Figure 5.17 Index person was assumed to sit at seat 2a

121 Table 5.2 Measurement result of tracer gas concentration distribution with index person at middle row seat a

Concentration of tracer gas (ppm)

Return L Seat a Seat b Seat d Seat e Return R

Row 1 0.04 0.03 0.04 0.03 0.03 0.03 Index Row 2 25.65 0.04 0.04 0.03 0.03 person Row 3 0.19 0.05 0.06 0.05 0.04 0.04

Table 5.3 Simulation result of tracer gas concentration distribution with index person at middle row seat a

Concentration of tracer gas (ppm)

Return L Seat a Seat b Seat d Seat e Return R

Row 7 0.06 0.07 0.11 0.01 0.00 0.01 Index Row 8 23.53 0.07 0.01 0.00 0.01 person Row 9 0.18 0.14 0.09 0.01 0.00 0.01

The simulation was conducted with the index person locating in the middle of the compartment, which was the 8th row to evaluate the influence to other passenger around him. Table 5.3 summarized the simulation result. Similarly higher concentration level was found at the return grille right above the index person, which discharged over 98% of tracer gas.

122

Figure 5.18 Index person was assumed to sit at seat 2b

Afterwards, the index person was shifted to seat b, which was nearby the corridor (as illustrated in Figure 5.18). The measurement result was summarized in Table 5.4. Higher tracer gas concentration was found at the nearest return air grille ‘Return-2L’ again. However concentration levels at the seats next to and behind the index person were obviously increased. Over 82% of the tracer gas exhausted through the return grille nearest to the index person, and 14% was discharged through the grille at the back. The simulation was summarized in Table 5.5. Similar to the measurement result, concentration level obtained at the seats next to and behind the source was increased. But most of the gas (75%) was still exhausted through the return grilles next to the index person. Hence the result validated the simulation of ventilation.

123

Table 5.4 Measurement result of tracer gas concentration distribution with index person at middle row seat b

Concentration of tracer gas (ppm)

Return L Seat a Seat b Seat d Seat e Return R

Row 1 0.11 0.23 0.40 0.21 0.25 0.28 Index Row 2 25.09 3.11 0.33 0.15 0.21 person Row 3 4.33 0.78 0.94 0.21 0.25 0.40

Table 5.5 Simulation result of tracer gas concentration distribution with index person at middle row seat b

Concentration of tracer gas (ppm)

Return L Seat a Seat b Seat d Seat e Return R

Row 7 0.05 0.14 0.27 0.23 0.09 0.40 Index Row 8 24.03 1.19 0.37 0.14 0.36 person Row 9 6.64 1.14 0.97 0.36 0.11 0.39

5.4 Simulation result

Two geometries were developed to simulate the condition inside the upper-deck passenger compartments under the present mixing-ventilation air-conditioning system and new personalized air supply and return system. Geometries of the present compartment and the new compartment were illustrated in Figure 5.1 and Figure 5.2 respectively. In the present compartment, supply air was delivered to the space through two ceiling-mounted ductworks at the sides and distributed by general diffusers and adjustable top-up diffusers. General diffusers were installed to provide supply air to condition the space of corridor. Adjustable top-up diffusers were located right above the seats at the window-side columns. Supply air temperature was 16oC and -1 discharged from the diffusers at 3.5 m · s . Return air grilles were located at the rear of compartment under the seats of the last row.

124 The new compartment was proposed to provide a personalized air supply and return system which improved the in-bus environment while satisfying individual preferences in the air-conditioned compartment. Supply air was discharged through adjustable slot-type diffusers installed in front of each seat. Every passenger (except the middle of the last row) had their own personal air supply for individual thermal preferences. The location of return air grilles were changed too. They were installed right above the seats at the window-side columns. The computation result from the empirical comfort models was considered in this new design. Since the diffusers were installed in front of passengers, the distance between diffusers and passengers’ bodies was reduced. Thus supply air temperature was expected to be increased o -1 associated with lower air speed. It was set at 21 C with air speed of 0.9 m · s . In the following, simulation results were compared concerning with the thermal comfort, zonal ventilation and particle transport issues in the compartments.

5.4.1 Thermal comfort

Present compartment

The distribution of air temperature at three different vertical levels (head, waist and feet) and air speed at head level were considered. Figure 5.19, Figure 5.20 and Figure 5.21 illustrated the distribution of air temperature at passengers’ head (1.1m), waist (0.6m) and feet (0.1m) levels respectively. The simulation result was summarized in Table 5.6. The average air temperature was 21.1oC at the head level, while it was 27.2oC at waist level and 25.9oC at feet level. It was cooler at head level since the cool supply air flowing downwards from top-up diffusers (as illustrated in Figure 5.22 and Figure 5.23). The temperature difference in vertical level caused discomfort due to temperature stratification (difference of over 3oC between head and feet level). Also, it was found to be cooler for the seats on window-side which was averagely 1.2oC lower than the seats next to corridor. It could be explained as the diffusers were installed on the window-side.

125

126

Figure 5.19 Distribution of air temperature at head level (1.1m) in present compartment

127

Figure 5.20 Distribution of air temperature at waist level (0.6m) in present compartment

128

Figure 5.21 Distribution of air temperature at feet level (0.1m) in present compartment

Table 5.6 Summary of air temperature and air speed at different levels in present compartment

o -1 Air temperature ( C) Air speed (m · s ) Head Waist Feet Head

Minimum 19.1 22.2 22.5 0.11 Average 21.1 27.2 25.9 0.44

Maximum 24.0 35.6 34.8 1.08

Median 21.1 26.8 25.5 0.35

Standard deviation 1.2 2.5 2.3 0.25

Figure 5.22 Air temperature stratification shown in horizontal contours in present compartment

129

Figure 5.23 Air temperature stratification shown in vertical contours in present compartment

-1 -1 The air speed at passengers’ head level ranged from 0.11 m · s to 1.08 m · s , and the -1 average value was 0.44 m · s (details shown in Table 5.6). Figure 5.24 showed the contour figure at passengers’ head level. The distribution showed that the air speed increased gradually with the air stream from the front to the rear. The speed of main -1 stream was 0.39 m · s (averagely) in the front section of compartment, while it -1 reached 0.55 m · s in the rear section. Also, passengers who were sitting by the window-side may feel uncomfortable due to draft risk. As illustrated in Figure 5.25, supply air surged to the passengers who were sitting right under the top-up diffusers. The average air speed was high as 0.63 -1 -1 m · s on the window-side columns, whilst it was 0.26 m · s on the columns next to the corridor. However, if the diffuser was shut to avoid discomfort of draft risk, thermal comfort could not be achieved since conditioned air did not reach the waist and feet level.

130

131

Figure 5.24 Distribution of air speed at head level (1.1m) in present compartment

Figure 5.25 Air speed around passengers in present compartment

As found in the simulation result, the present air distribution system did not well maintaining a thermally comfortable environment in the compartment. The sensation vote towards thermal comfort ranged from 0.1 to 2.5 (as illustrated in Figure 5.26), which was equivalent to feeling very comfortable to slightly uncomfortable. The average percentage of dissatisfaction level towards thermal comfort (illustrated in Figure 5.27) was 16.2%. It caused discomfort such as large temperature stratification and draft risk. The design of the present air distribution system was a main factor. Thus a new personalized air supply and return system was proposed to enhance the in-bus environment.

132

133

Figure 5.26 Sensation vote towards thermal comfort in present compartment

134

Figure 5.27 Percentage of dissatisfaction towards thermal comfort in present compartment

New compartment

In the geometry of new compartment, trials of supply air temperature and air speed were applied in order to create a more thermally comfortable environment rather than the present compartment. The supply air temperature was set at 21oC associated with -1 an air speed at 0.9 m · s . Distributions of air temperature at three different vertical levels (head, waist and feet) were illustrated in Figure 5.28, Figure 5.29 and Figure 5.30 respectively. The simulation result was summarized in Table 5.7. The average air temperature was 21.4oC at the head level, while it was 23.5oC at waist level and 22.6oC at feet level. The temperature in different vertical levels was closed, which the difference was within 2oC (as illustrated in Figure 5.31). The location of slot-type diffusers gave benefit in providing an evenly air flow on passenger’s upper body that reducing discomfort of temperature stratification (as illustrated in Figure 5.32). Also, the difference of zonal temperature level caused in present compartment was minimized by the installation of personal supply air diffusers.

135

136

Figure 5.28 Distribution of air temperature at head level (1.1m) in new compartment

137

Figure 5.29 Distribution of air temperature at waist level (0.6m) in new compartment

138

Figure 5.30 Distribution of air temperature at feet level (0.1m) in new compartment

Table 5.7 Summary of air temperature and air speed at different levels in new compartment

o -1 Air temperature ( C) Air speed (m · s ) Head Waist Feet Head

Minimum 21.3 22.3 21.8 0.13 Average 21.4 23.5 22.6 0.25

Maximum 21.9 24.0 23.7 0.37

Median 21.4 23.5 22.5 0.26

Standard deviation 0.1 0.4 0.5 0.05

Figure 5.31 Air temperature stratification shown in horizontal contours in new compartment

Figure 5.32 Air temperature stratification shown in vertical contours in new compartment

139 Figure 5.33 showed the horizontal contour figure at passengers’ head level. The -1 average air speed at passengers’ head level was 0.25 m · s (details shown in Table 5.7). This magnitude satisfied most passengers as found in previous chapter. The distribution of sensation vote and percentage of dissatisfaction towards thermal comfort in the new compartment were illustrated in Figure 5.34 and Figure 5.35 respectively. The average sensation vote and percentage of dissatisfaction level towards thermal comfort were further lowered to 0.2 and 3.0% respectively. In present compartment, higher air speed was set aiming to deliver the supply air efficiently. Nevertheless, it caused discomfort due to draft risk. In the new compartment, supply air coming from the slot-type diffuser in the front provided an evenly air distribution on passenger’s body as illustrated in Figure 5.36. Lower supply air speed was required that preventing the discomfort of draft risk. Moreover, higher supply air temperature (increased from 16oC to 21oC) associated with lower -1 -1 air speed (reduced from 3.5 m · s to 0.9 m · s ) was required in the new compartment due to the location of supply air diffusers. This personalized system distributed the supply air effectively to the passengers, whilst minimizing thermally discomfort caused.

140

141

Figure 5.33 Distribution of air speed at head level (1.1m) in new compartment

142

Figure 5.34 Sensation vote towards thermal comfort in new compartment

143

Figure 5.35 Percentage of dissatisfaction towards thermal comfort in new compartment

Figure 5.36 Air speed around passengers in new compartment

5.4.2 Ventilation Comparing the effect of air distribution system to ventilation efficiency, full fresh air -1 supply was considered in the simulation. The supply air speed was 3.5 m · s and 0.9 -1 m · s in the present and new compartment respectively; whilst the fresh air flow -1 -1 rates were 23 l · s per person and 18 l · s per person respectively. The distribution of CO2 concentration level at breathing zone was analyzed to determine the ventilation efficiency.

Present compartment

The distribution of CO2 concentration at breathing level in present compartment was illustrated in Figure 5.37. Mixing-ventilation system was applied in the present compartment that the zone air mixed and flowed backward to the return air grilles at the back. Figure 5.38 showed the concentration in a vertical contour in the middle of the compartment. The average concentration level was 4287 ppm at breathing zone in present compartment (details shown in Table 5.8). The distribution of sensation vote and percentage of dissatisfaction towards air quality were illustrated in Figure 5.39 and Figure 5.40. The sensation vote towards air quality ranged from 2.0 to 3.8, which was equivalent to feeling average to very uncomfortable. The average percentage of dissatisfaction level towards air quality was 59.9%.

144 Since the ventilation efficiency was found to be low as 0.93, the metabolic bioeffluents were not diluted effectively in the present compartment. The simulation result also showed that the concentration level was lower on the window-side columns (3700 ppm) since they were served with fresh air from supply air diffusers, while it was about 4900 ppm on the columns next to the corridor.

145

146

Figure 5.37 Distribution of CO2 concentration at breathing zone in present compartment

Figure 5.38 Distribution of CO2 concentration shown in vertical contours in present compartment

Table 5.8 Summary of CO2 concentration at breathing zone

CO2 concentration (ppm) at breathing zone Present New compartment compartment Minimum 2907 494 Average 4287 694

Maximum 6684 3290

Median 4265 639

Standard deviation 852 361

147

148

Figure 5.39 Sensation vote towards air quality in present compartment

149

Figure 5.40 Percentage of dissatisfaction towards air quality in present compartment

New compartment

Figure 5.41 illustrated the CO2 concentration distribution at breathing level in the new compartment. The simulation result showed that the average concentration was 694 ppm, which was much lower than the level found in the present compartment. Figure 5.42 showed the distribution of sensation vote towards air quality. It ranged from 0.4 to 1.7, which was equivalent to feeling very comfortable to average. Figure 5.43 showed the distribution of percentage of dissatisfaction level towards air quality. The average dissatisfaction was further reduced to 11.6% in the new compartment. The personalize air supply and return design raised the ventilation efficiency to 1.32 in the new compartment. Metabolic CO2 was effectively diluted by the system as illustrated in Figure 5.44. The bio-effluents exhaled from passengers were carried by the air stream. Associated buoyancy-driven convection plume, concentrated CO2 flowed upwards and exhausted through the return grilles above the seats immediately as illustrated in Figure 5.36. Thus the CO2 concentration maintained in a low level, despite the fresh air rate was lower in the new compartment. This revealed the advantages of the proposed personalized air supply and return system.

150

151

Figure 5.41 Distribution of CO2 concentration at breathing zone in new compartment

152

Figure 5.42 Sensation vote towards air quality in new compartment

153

Figure 5.43 Percentage of dissatisfaction towards air quality in new compartment

Figure 5.44 Distribution of CO2 concentration shown in vertical contours in new compartment

5.4.3 Particle transport

Studying the effectiveness of personalized air supply and return system, simulation of particle transport in the compartment was conducted. The study assumed all seats were occupied. An index person was assumed to sit at the front section (2nd row), middle section (8th row) and rear section (13th row) in each simulation. The index person was assumed to be a disease carrier. Infectious aerosols were expelled through his cough. The trajectory of aerosols was simulated as particle transport by discrete phase model. The index person coughed with discharging at a speed of 22 -1 m · s ; meanwhile other passengers were assumed to have steady inhalation through -1 nose at a flow rate of 0.14 l · s . In each simulation, 10000 hypothetical particles were considered to determine the behaviour statistically. The number of particles trapped in passengers’ noses was recorded. Also the simulated particle tracks indicated the possible influence by index person.

Present compartment The air flow pattern inside the present compartment was illustrated in Figure 5.45. Supply air was discharged from the ceiling-mounted ductworks. The air stream flowed towards the rear of compartment and exhausted through return air grilles under the seats of the last low.

154

155

Figure 5.45 Air stream inside the upper-deck compartment with present air distribution system

Index person in front section

Figure 5.46 illustrated possible particle tracks of coughing discharged from an index person sitting in the front section of the compartment (the 2nd row). A close-up of the index was illustrated in Figure 5.47. Figures showed the tracks dispersed throughout the front section and then flowed backward to the return air grilles. The space air stream carried the infectious droplets flowing from the front of compartment to the rear section. The simulation result showed that 39 particles of 10000 (0.39%) were trapped in other passengers’ noses. Hence if a disease carrier sat at the front section, the infectious droplets from coughing would influence the passengers sitting behind.

156

157

Figure 5.46 Particle tracks of coughing in the front section of compartment (2nd row)

Figure 5.47 Close-up of particle tracks of coughing in the front section of compartment (the 2nd row)

Figure 5.48 showed the zonal air stream in vector format. The vectors revealed the supply air stream flowed downward to the passenger’s body and changed the flow -1 direction. Although the momentum of cough was higher with the speed at 22 m · s , the speed decreased rapidly as leaving the mouth and the droplets followed the space air stream flow pattern to the rear section of compartment.

Figure 5.48 Vectors of air stream in the front section of compartment (2nd row)

158 Index person in middle section

Possible particle tracks of coughing discharged from an index person sitting in the middle section of compartment (the 8th row) were illustrated in Figure 5.49 and the close-up was illustrated in Figure 5.50. Figures showed the direction of coughed flow changed rapidly that the tracks dispersed in the zone and then flowed backward to the return air grilles. Figure 5.51 showed the vector format air stream in the zone. The supply air stream flowed downward from diffuser to the passenger’s body. The speed of coughed air flow decreased and its droplets associated with the space air stream flowing backward. The simulation result showed that 19 particles (0.19%) were trapped in other passengers’ noses. Hence if the disease carrier sat at the middle section, the passengers sitting behind him/her would be infected by the droplets from coughing. But passengers sitting in the front section were not influenced. Thus the number of particles trapped was reduced as compared with the index person sitting in front section.

159

160

Figure 5.49 Particle tracks of coughing in the middle section of compartment (the 8th row)

Figure 5.50 Close-up of particle tracks of coughing in the middle section of compartment (the 8th row)

Figure 5.51 Vectors of air stream in the middle section of compartment (the 8th row)

Index person in rear section

Possible particle tracks of coughing discharged from an index person sitting in the rear section of the compartment (13th row) were simulated and illustrated in Figure 5.52 and Figure 5.53. Since the speed of coughed flow reduced once leaving the mouth, the tracks dispersed and followed the space air stream flowing backward to the return air grilles. The vector format air flow pattern (illustrated in Figure 5.54)

161 showed a stronger air stream from the front carried the particles to the rear. Only 8 particles (0.08%) were trapped as shown in the simulation result. The number of passengers influenced by the infectious droplets from coughing was obviously reduced due to the location of index person. Only the three rows behind the disease carrier were influenced.

Figure 5.52 Particle tracks of coughing in the rear section of compartment (the 13th row)

Figure 5.53 Close-up of particle tracks of coughing in the rear section of compartment (the 13th row)

162

Figure 5.54 Vectors of air stream in the rear section of compartment (the 13th row)

New compartment

In the new compartment, every passenger had personal air supply diffusers in front of them. The return air grilles were installed right above the seats at the window-side columns. The new design is proposed to provide a personalized air supply and return system. The new air distribution system generated a different air flow pattern from the present system. Supply air discharged from the diffusers flowing towards the passenger and exhausted through the return grill right above the seat. The air stream inside the compartment was illustrated in Figure 5.55.

163

164

Figure 5.55 Air stream inside the compartment with new air distribution system

Index person in the front section

Figure 5.56 illustrated the particle tracks of coughing discharged from index person sitting in the front section of the compartment (the 2nd row). Close-up of the index person was illustrated in Figure 5.57. The tracks of cough discharged from the index person flowed forward until reaching the back of the former seat. The air speed reduced and the particles were carried by an upward-flowing air stream as illustrated in Figure 5.58. Then the stream exhausted through the return air grille. Simulation result found 4 trapped particles out of 10000 (0.04%) in the compartment. The figures showed that the tracks did not disperse throughout the zone but exhaust associated with the nearest air stream. Hence the possibility of influence from infectious droplets was minimized.

Figure 5.56 Particle tracks of coughing in the front section of compartment (the 2nd row)

165

Figure 5.57 Close-up of particle tracks of coughing in the front section of compartment (the 2nd row)

Figure 5.58 Vectors of air stream in the front section of compartment (the 2nd row)

Index person in the middle section

Figure 5.59 and Figure 5.60 illustrated the particle tracks of coughing discharged from an index sitting at the middle section of the compartment (the 8th row). Although the tracks were dispersed at his sitting area, they were guided by the air stream and exhausted through the nearest return air grilles. Figure 5.61 showed the zonal air stream. The simulation result found 4 particles trapped (0.04%) in other passengers’ noses, which was the same as if the index person sitting in the front section.

166

167

Figure 5.59 Particle tracks of coughing in the middle section of compartment (the 8th row)

Figure 5.60 Close-up of particle tracks of coughing in the middle section of compartment (the 8th row)

Figure 5.61 Vectors of air stream in the middle section of compartment (the 8th row)

Index person in the rear section

Figure 5.62 and Figure 5.63 illustrated the particle tracks of coughing discharged from an index sitting at the rear section of the compartment (the 13th row). The tracks were guided by the zonal air stream and exhaust through the return grille in the front as illustrated in Figure 5.64. The simulation result found 3 particles trapped (0.03%) if the index person sat on the 13th row.

168

169

Figure 5.62 Particle tracks of coughing in the rear section of compartment (the 13th row)

Figure 5.63 Close-up of particle tracks of coughing in the rear section of compartment (the 13th row)

Figure 5.64 Vectors of air stream in the rear section of compartment (the 13th row)

170 The number of particles trapped was summarized in Table 5.9. The simulation result revealed that the level of influence caused by particle dispersion depending on where the index person sat inside the present compartment. The number of particle trapped reduced for the index person sitting closer to the rear section. Passengers sitting behind the index person were possibly influenced by the infectious droplets. However, the number of particle trapped was less in the new compartment, and which was similar for the index person sitting in different sections. The percentage maintained below 0.04%. It revealed the personalized air supply and return system was effective in preventing the spread of infectious droplets in the whole compartment.

Table 5.9 Summary of the number of particles trapped in passengers’ noses

Number of particles trapped (out of 10000 particles expelled) Present compartment New compartment

# Location of index person:

Front section 44, 32, 38, 43 5, 5, 3, 4 (the 2nd row) Mean: 39 (0.39%) Mean: 4 (0.04%)

Middle section 15, 18, 22, 20 3, 4, 3, 4 (the 8th row) Mean: 19 (0.19%) Mean: 4 (0.04%)

Rear section 7, 8, 9, 9 4, 3, 1, 3 (the 13th row) Mean: 8 (0.08%) Mean: 3 (0.03%) # Trials were conducted for index person sitting on each seat in the row.

5.4.4 Combined Comfort

In the simulation, percentage of dissatisfaction towards combined comfort was evaluated in the compartments. The distribution contour figures in present and new compartments were illustrated in Figure 5.65 and Figure 5.66 respectively. In the present compartment, the dissatisfaction level towards combined comfort ranged between 22% and 53%, and the average was 35%. The commuting environment was further improved in the new compartment. The level ranged between 5% and 23%, with an average of 8% in the new compartment.

171

172

Figure 5.65 Percentage of dissatisfaction towards combined comfort in present compartment

173

Figure 5.66 Percentage of dissatisfaction towards combined comfort in new compartment

5.5 Conclusion

Mixing-ventilation system was applied in the present passenger compartment. Supply air was distributed through ceiling-mounted ductworks, whilst return air was collected at the back of passenger compartment. Simple design was the advantage of such air distribution system. Nevertheless, there were disadvantages including causing discomfort due to temperature stratification and air draft, and ineffective air contaminants dilution. Also there was a potential risk in spreading infectious droplets if there was a disease carrier onboard. Minimizing the influence, personalized air supply and return system design was proposed in a new compartment. Supply air diffusers were available in front of each seat; while return air grilles were located above each row of seats. Simulation result showed the new compartment minimized those thermally discomfort issues found in the present compartment. It reduced the air contaminants concentration level effectively under a lower fresh air rate. The personalized design raised the ventilation efficiency from 0.93 to 1.32. The average percentage of dissatisfaction towards combined comfort level was obviously reduced from 35% to 8%. Moreover, the result showed that the droplets expelled from index passenger were carried by the air stream and exhausted through the nearest return air grilles in the new system. It revealed the new system was applicable to minimize the spread of infectious airborne diseases inside the passenger compartment.

174 6 Air-conditioning load and energy consumption

6.1 Introduction

Integrating the empirical comfort models into the control system, the computation result was applied as air-conditioning settings. It facilitated in improving the commuting environment while minimizing the passengers’ dissatisfaction level.

Among the comfort indicators, in/out-bus air temperature and in-bus CO2 concentration level were corresponding dominants towards thermal comfort and air quality issues. They gave the most significant effects to the passengers’ perception. However, such adjustment of settings in present compartment involved an increase in ventilation rate and change of set-point temperature. It might lead to great increase in air-conditioning energy consumption. The new compartment was proposed with the application of personalized air supply and return design. The air-conditioning energy consumption in the new proposed compartment was not revealed in previous chapter. Evaluating the effects of setting adjustments to the air-conditioning load and annual energy consumption, simulation was performed under different scenarios, including the present condition, new settings applied in present compartment and new settings applied in new compartment. The simulation results were compared in order to evaluate the effects from different scenarios.

6.2 Methodology

The effects of changing air-conditioning settings and air distribution system to the energy consumption and load were evaluated with an air-conditioning load calculation program, HTB2. HTB2 is a computerized program used for air-conditioning load calculation in buildings. It calculates the hourly air-conditioning load depending on the explicitly specified supporting segments, including mechanical air change rate, occupancy load, lighting and appliances heat gain, enclosure structure and materials, space volume and orientation, set-point temperature, operation schedule, etc. (Alexander, 1997) Also, the calculation is based on a specified meteorological file input (a database containing the ambient air temperature, relative humidity and solar irradiance, etc). Steady state air conditioning load is assumed in the specified zone. However, transient state occurs on air-conditioned buses, which is frequently affected by

175 dynamic factors including travelling speed, door openings and roadway environment along the journey. Under this circumstance, the transient effects are not considered in this study. Also, the load calculation result was concluded by analyzing repeatedly with the geometry facing towards eight different orientations. Simulating a practical condition, meteorological database was developed referring to the local condition in the year 2005. The meteorological data of year 2005 was applied since it had the highest maximum and median of daily air temperature from the year of 1996 to 2005. Air-conditioning load calculation based on this meteorological record revealed the demand under the trend of global warming. Calculation was done to simulate the system operating under three scenarios: condition in present compartment, new settings applied in present compartment and new settings applied in new proposed compartment. The change of settings included set-point air temperature and mechanical ventilation strategy, which were adjusted depending on the computation result from empirical comfort models.

Condition in present compartment On present air-conditioned buses, mixing-ventilation system was applied which the ventilation efficiency was 0.93. The in-bus air temperature set-points are fixed at 19oC in the cold season and 23oC in the hot season; whilst the normal mechanical -1 ventilation rate was fixed at about 250 l · s as reported in previous chapter. However this ventilation rate was insufficient in a fully loaded compartment. The in-bus CO2 concentration would increase to 5700 ppm. In such condition, passengers might feel very uncomfortable towards the air stuffiness and odour issues. Thus the percentage of dissatisfaction towards in-bus air quality would be as high as 75%.

New settings applied in present compartment Applying the new air-conditioning settings, the in-bus set-point air temperature was adjusted hourly according to the computation result from empirical comfort models. Such adjustments were made depending on the ambient condition referring to the meteorological data. Also the mechanical ventilation rate was adjusted depending on the passenger load on bus. The in-bus CO2 concentration level would be controlled below 1500 ppm. The goal of adjustments was to satisfy the substantial majority of passengers towards in-bus air quality and thermal comfort.

176 New settings applied in new proposed compartment

In the new compartment, personalized air supply and return system was equipped. New air-conditioning settings were applied which adjusted as the computation result from empirical comfort models depending on the ambient condition and passenger load. As evaluated from the previous chapter, the air distribution system provided higher ventilation efficiency of 1.32. Hence the required outdoor air rate was lower than that required in present compartment. Air-conditioning load and energy consumption were expected to be lower, but maintaining the low dissatisfaction level.

Adjustment of set-point temperature

Table 6.1 summarized the ambient air temperature record in 2005, as well as the in- bus air temperature set-points under the present and new settings. The average ambient temperature was about 16.7oC and 28.2oC in the cold and hot seasons respectively. The lowest ambient temperature (6.6oC) was found on January; whilst it was the highest (35.0oC) on July. In the air-conditioning load calculation, the in-bus air temperature set-points were input. The present set-points were fixed at 2 constant temperatures and adjusted on May and November. The air temperature profiles were illustrated in Figure 6.1. The profiles showed that the system with present air temperature set-points fixed at constant temperature whilst the new temperature set- points varied depending on the ambient air temperature. Figure 6.2 illustrated the predicted passengers’ sensation vote towards temperature under the system with present and new air temperature set-points. The predicted sensation vote fluctuated between -3.00 to 1.08 in the present compartment. The profile revealed passengers might feel cold on April and May under present setting. For the in-bus environment applying new air temperature set-points, the predicted sensation vote kept steady at the level of -0.15. Such slightly cool environment was highly satisfied by the local passengers as suggested by the empirical comfort models. The percentage of dissatisfaction level towards thermal comfort was averagely 13% in present compartment. The level could be further reduced to as low as 0.2% if the new set- point was applied.

177 Table 6.1 Ambient air temperature and in-bus set-point temperature

Ambient New set-points Present set-points air temperature (varies depending on ambient)

o o Minimum 6.6 C o 18.5 C 19 C (in cold season) Average 23.3 oC 21.6 oC 23oC (in hot season) Maximum 35.0 oC 26.2 oC

Figure 6.1 In-bus set-point temperature under ambient air temperature

Figure 6.2 Predicted passengers’ sensation vote towards temperature

178 New mechanical ventilation strategy

Mechanical ventilation rate should not be fixed but varying dependent on the roadway environment as well as the in-bus metabolic CO2 generation rate. CO2 concentration level is a significant indicator concerning with the in-bus air quality issues. Figure 6.3 illustrated the maximum required ventilation rate to control the in- bus CO2 concentration level at a prescribed upper-limit threshold. Also the figure showed the predicted percentage of dissatisfaction towards odour (PDo) under various in-bus CO2 concentration level. On an air-conditioned bus with present -1 ventilation rate (250 l · s ), the in-bus CO2 concentration level might reach 5700 ppm when it was fully loaded. At this situation, the predicted percentage of dissatisfaction towards odour reaches 100%.

Figure 6.3 Maximum required ventilation rate and percentage of dissatisfaction towards in- bus odour issue under various upper-limit of CO2 concentration level thresholds

If the upper-limit CO2 concentration level threshold was set at 2500 ppm (level 1 of -1 HKEPD, 2003), the maximum required ventilation rate would be 630 l · s while the

PDo would be 58%; at 3500 ppm (level 2 of HKEPD, 2003), the maximum required -1 ventilation rate would be 430 l · s while the PDo would be 93%.

179 Satisfying a substantial majority of passengers, below 20% of PDo was intended to achieve. The upper-limit CO2 concentration level threshold should be set at 1500 -1 ppm while the maximum ventilation rate of 1210 l · s would be required. The change of mechanical ventilation rate increased both the required capacity of heating and cooling since the introduction of fresh air weighed both loads. The new ventilation strategy provided a flexible mechanical ventilation rate which adjusted depending on the in-bus CO2 concentration level. Hence, the rate might be increased in the peak hours when the passenger load was high. In opposite, it reduced in the off-peak hours.

6.3 Air-conditioning load and annual energy consumption

Air-conditioning load and energy consumption were evaluated under the following scenarios: • Present compartment with present air-conditioning settings (set-point temperature at 19oC (cold season) and 23oC (hot season); mechanical -1 ventilation fixed at 250 l · s ), • Applying new air-conditioning settings (computation result of empirical comfort models) in present compartment, • Applying new air-conditioning settings and personalized air supply and return system in new proposed compartment.

6.3.1 Present compartment

With the computational calculation, the air-conditioning load and energy consumption for heating and cooling in the present compartment were evaluated and plotted in Figure 6.4 and Figure 6.5 respectively. The maximum heating load was 8.8 kW found on January and the maximum cooling load was 21.3 kW found on July (details shown in Table 6.2). The maximum monthly energy consumption in heating was 0.4 MWh on January while no heating was required on the months from May to October. The maximum monthly energy consumption in cooling was 7.8 MWh on July. The annual energy consumptions were 1.2 MWh and 60.7 MWh in heating and cooling respectively.

180

Figure 6.4 Monthly heating energy consumption and maximum heating load under present settings

Figure 6.5 Monthly cooling energy consumption and maximum cooling load under present settings

181 Table 6.2 Air-conditioning load and energy consumption in present compartment with present settings

Air-conditioning Load (kW) Energy Consumption (MWh)

Heating Cooling Heating Cooling

Minimum 0.0 14.0 0.0 1. 9 (May – Oct) (Jan) (Apr – Nov) (Feb)

Maximum 8.8 21.3 0.4 7.8 (Jan) (Jul) (Jan) (Jul) Annual Total

Consumption 1.2 MWh 60.7 MWh

6.3.2 New air-conditioning settings applied in the present compartment New air-conditioning settings (including set-point temperature and mechanical ventilation strategy) were applied depending on the computation result from empirical comfort models. The change of settings minimized the passengers’ dissatisfaction level towards combined comfort from 35% to 8% in the present compartment. Figure 6.6 and Figure 6.7 illustrated the profiles of air-conditioning load and energy consumption in heating respectively. The required heating capacity rose by 3.1 times to 36.4 kW after applying new settings in the present compartment. Also, the annual heating energy consumption was greatly increased by 9.1 times to 12.1 MWh. Summary was shown in Table 6.3.

Figure 6.6 Monthly maximum heating load in present compartment with applying new air- conditioning settings

182

Figure 6.7 Monthly heating energy consumption in present compartment with applying new air-conditioning settings

Table 6.3 Air-conditioning load and energy consumption in present compartment with applying new settings

Air-conditioning Load (kW) Energy Consumption (MWh)

Heating Cooling Heating Cooling

Minimum 0.0 14.0 0.0 0.8 (May – Oct) (Jan) (May – Oct) (Jan)

Maximum 36.4 42.1 3.4 13.7 (Jan) (Jul) (Jan) (Jul)

Annual Total

Consumption 12.1 MWh 90.3 MWh

The profiles of maximum load and monthly energy consumption in cooling were plotted in Figure 6.8 and Figure 6.9 respectively. The required cooling capacity was 21.3 kW under present settings. However it became 42.1 kW (increased by 98%) after applying new settings. The cooling energy consumption was greatly rose in the hot season. The annual energy consumption surged to 90.3 MWh (increased by 49%).

183

Figure 6.8 Monthly maximum cooling load in present compartment with applying new air- conditioning settings

Figure 6.9 Monthly cooling energy consumption in present compartment with applying new air-conditioning settings

Mechanical ventilation rate was increased to lower the in-bus CO2 concentration level, whilst minimizing the dissatisfaction level towards air quality issue. The raise of rate increases the amount of outdoor air intake that enlarges the heat gain.

184 Moreover, since the passenger load is always higher in the day-time while the ambient air temperature is higher, the increase of ventilation rate weighed the cooling load. Hence, higher cooling capacity was required under the new ventilation strategy. Intending to improve the combined comfort level in present compartment, higher energy consumption in both heating and cooling was required. The total air- conditioning energy consumption rose from 61.9 MWh to 102.3 MWh (increased by 65%).

6.3.3 New settings applied in new compartment

In the new compartment, personalized air supply and return system was installed. Since the personalized system provided higher ventilation efficiency in the compartment, the air-conditioning energy consumption should be different as compared with the case in present compartment. Figure 6.10 and Figure 6.11 illustrated the profiles of air-conditioning load and energy consumption of heating in the new compartment. The required heating capacity became 16.2 kW, which was decreased by 55.6% as compared with that the new settings applied in present compartment. Meanwhile, the annual heating energy consumption was reduced by 64% to 4.4 MWh. Summary of result was shown in Table 6.4.

Figure 6.10 Monthly maximum heating load in new compartment

185

Figure 6.11 Monthly heating energy consumption in new compartment

Table 6.4 Air-conditioning load and energy consumption in new compartment

Air-conditioning Load (kW) Energy Consumption (MWh)

Heating Cooling Heating Cooling

Minimum 0.0 13.4 0.0 1.3 (May – Oct) (Jan) (May – Oct) (Feb)

Maximum 16.2 27.1 1.2 9.1 (Jan) (Jul) (Jan) (Jul)

Annual Total

Consumption 4.4 MWh 64.9 MWh

Figure 6.12 illustrated annual heating energy consumption and the required heating capacity under applying different CO2 thresholds level of ventilation strategy. Both profiles rose rapidly if the CO2 threshold level was further lowered. The annual consumption was 0.9 MWh for the system to control the CO2 threshold below 3500 ppm. It rose by 75% to control the threshold below 2500 ppm, and it surged by 3.8 times for the level controlled below 1500 ppm.

186

Figure 6.12 Annual heating energy consumption and required heating capacity in new compartment under different CO2 thresholds levels of ventilation strategy

Figure 6.13 Monthly maximum cooling load in new compartment

187

Figure 6.14 Monthly cooling energy consumption in new compartment

The profiles of maximum cooling load and monthly energy consumption in the new compartment were plotted in Figure 6.13 and Figure 6.14 respectively. The required cooling capacity was 27.1 kW in the new compartment. 35.6% of cooling capacity was reduced as compared with the same settings applied in the present compartment. The annual energy consumption in cooling was decreased to 64.9 MWh (reduced by 28.1%). From the calculation result, most of the reduction (92%) was concluded in the hot season (from May to October).

Figure 6.15 illustrated the annual energy consumption and required capacity of cooling in the new compartment. The profiles rose for lower CO2 threshold level that the annual consumption in cooling was increased by 29% if the CO2 threshold was reduced from 3500 ppm to 1500 ppm.

188

Figure 6.15 Annual cooling energy consumption and required cooling capacity in new compartment under different CO2 thresholds levels of ventilation strategy

Figure 6.16 Monthly air-conditioning energy consumption with applying new air-conditioning settings

189 As shown in Figure 6.16, the air-conditioning energy consumption was different in the compartments, even if the same air-conditioning settings were applied. It was caused by the difference of ventilation efficiency found in previous chapter. Higher total energy consumption was found in the hot season from May to October. The raise of cooling load leaded to the crest on July. The energy consumption in heating dropped to zero in these months; that the total air-conditioning consumption was completely contributed by the cooling sector.

Table 6.5 Comparison of required capacity and annual energy consumption in air- conditioning under different scenarios

Required capacity Annual energy consumption PD towards (kW) (MWh) combined comfort Heating Cooling Heating Cooling Total

Present settings applied in 8.8 21.3 1.2 60.7 61.9 35% present compartment

New settings applied in 36.4 42.1 12.1 90.3 102.4 8% present compartment

New settings applied in 16.2 27.1 4.4 64.9 69.3 8% new compartment

Table 6.5 summarized the total annual energy consumption in heating and cooling under different scenarios. The new air-conditioning settings leaded to an increase in the energy consumption. Meanwhile the percentage of dissatisfaction towards combined comfort was minimized. The new air-conditioning settings were applied intended to minimizing dissatisfaction level. The simulation result showed that energy consumption in heating and cooling rose by 9.1 times and 49% respectively for the new settings applied in present compartment. However the minimization of dissatisfaction level could not be achieved by simply paying for the additional energy consumption. The system capacity had to be considered as well. The present air-conditioning system might not be applicable to fulfil the settings adjustment, since the required heating and cooling capacities were increased by 3.1 times and 98% respectively. In the new compartment, personalized air supply and return system was proposed which provided higher ventilation efficiency. Such characteristic reduced the

190 required fresh air rate that lowering the air-conditioning load and energy consumption. The required heating and cooling capacities were diminished by 55% and 36% respectively, as compared with that the new settings applied in present compartment. Also the energy consumptions in heating and cooling were reduced by 64% and 28% respectively. Considering that hot coolant water from engine was applied as heat source of bus heating system, since no extra fuel was spent. Heating energy consumption could be neglected. Therefore the new air-conditioning settings applied in new compartment was effective in reducing the energy consumption by 28%. Moreover, the new compartment design should be adopted for the benefits of achieving both energy effectiveness and passengers’ satisfaction.

6.4 Conclusion

Calculation was done to simulate the air-conditioning load and energy consumption for the system operating under different scenarios: present condition, new settings applied in present compartment and new settings applied in new compartment. The change of settings included set-point temperature and mechanical ventilation strategy, which were adjusted depending on the computation result from empirical comfort models. The adjustment was intended to improve the in-bus environment while satisfying a substantial majority of passengers. The increase of ventilation rate improves the in-bus air quality, while causing increase of energy consumption in air-conditioning. The application of new air- conditioning settings in present compartment lowered the passengers’ dissatisfaction level from 35% to 8%. Meanwhile energy consumption in both heating and cooling rose by 9.1 times and 49% respectively. Moreover the required heating and cooling capacities were increased by 3.1 times and 98% respectively. The present air- conditioning system might not be applicable to fulfil the settings adjustment, owing to the increase of required capacities. In the new compartment, personalized air supply and return system was proposed which provided higher ventilation efficiency. It reduced the required fresh air rate, while lowering the air-conditioning load and energy consumption. Simulation result showed that the required heating and cooling capacities were diminished by 55% and 36% respectively in the new compartment, as compared with that the new settings

191 applied in present compartment. Also the energy consumptions in heating and cooling were reduced by 64% and 28% respectively. The comparison results revealed the benefits provided by the new compartment. The new air-conditioning settings improved the commuting environment while minimizing the passengers’ dissatisfaction level. Also the personalized air supply and return design helped achieve energy effectiveness by increasing the ventilation efficiency in the compartment. The calculation result would be a significant reference for the engineers to consider the enhancement of in-bus air quality and thermal comfort environment on air-conditioned buses.

192 7 Three elements approach for design and operation concerning with in-bus air quality and thermal comfort

7.1 Introduction

There is a wide range of expectation from the passengers with different background and characteristics, wearing different dressing codes, coming on and off the compartments at different locations along the routes, at different states of health and etc. There were 5750 registered franchised buses running in Hong Kong, which served 3.9 millions daily passenger trips. All effort to improve or enhance the in-bus environment should aim at satisfying the majority. However, such large number of buses and passenger trips impose great difficulties on the authority and the bus operators to monitor the satisfaction levels of air quality and thermal comfort on each bus. The mobility renders the in-bus air quality not only affected by the operation and maintenance of the buses, but also influenced by the outdoor environment where the bus traverses. The dilution of in-bus air contaminants is achieved by the outdoor air ventilation. Insufficient outdoor air ventilation rate would result in accumulation of air contaminants inside the passenger compartment, including human bioeffluents and odour. It rises the percentage of passengers’ dissatisfaction towards in-bus air stuffiness and odour issues. However, if the outdoor air quality was poor, the introduction of outdoor air would not improve the in-bus air quality but deteriorating by infiltrating concentrated air pollutants. Thus a great increase of the ventilation rate may not guarantee a good result. Ventilation rate and outdoor air quality are key factors to the in-bus air quality. Implementing the in-bus air quality and thermal comfort environment improvement, an accustomed but invalid approach is to adjust the air-conditioning settings that conforming to the guidelines. It is a shortcut to achieve the requirements in temporary. However, a synergistic normal practice requires the coordination between different systems which leads to a more effective and reliable operation to achieve a comfortable in-bus environment.

193 7.2 Three elements approach

Considerations of air quality and thermal comfort on buses focus on the design and operation under the ‘three elements approach’. The elements are: • the engine; • the passenger compartment; and • the air-conditioning system. Engine is the heart of vehicle; inappropriate operation and maintenance leads to a deteriorating in-bus environment, including short-circuiting of exhaust emission and excessive heat transfer to passenger compartment. Passenger compartment is the space for commuters, providing a healthy and comfortable commuting environment, and a protective shelter from exposing to the rigorous roadway environment. Its facilities are supported by the energy from engine; whilst the spatial condition is maintained by the air-conditioning system. Air-conditioning system is the environmental control unit for the passenger compartment, which maintains the air quality and thermal comfort in an acceptable condition. These elements are major sectors closely interactive in affecting the in-bus air quality and thermal comfort environment. Concluding the issues from the viewpoints of influences from both internal and external sources, as well as the control system, implementation of this approach helps to operate and improve the commuting environment in an effective way. Moreover it leads to achieve the goal of sustainability in design, operation and maintenance.

7.2.1 Engine

Engine is the prime mover, which supports all the energy consumption including the travelling power, air-conditioning, lighting and in-bus facilities. An engine maintained in good operating condition leads to a higher energy efficiency. On the contrary, inappropriate operation and maintenance leads to waste of energy but inadequate power output and excessive exhaust emission.

Design considerations

Vehicles with diesel drives generate exhaust emission. It is a potential source of pollutants entering the passenger compartment through short-circuiting of outdoor air

194 intake or leakage in passenger compartment. Chan et al. (1999) stated the locations of outdoor air intake and the engine exhaust were affected by the tight space in installing the engine and the air-conditioning system. A lower vertical level of the intake would result as higher air pollutants concentration levels inside the passenger compartment. Hence, a poorly located outdoor air intake and engine exhaust will increase the risk of infiltrating exhaust air into the compartment. Instantaneously windy conditions may induce short-circuiting as well. As reported in Chapter 3, the fluid dynamics simulation of air infiltration across the body surface of a traversing bus was shown. A relatively higher differential pressure was induced at the rear section of bus where located the outdoor air intake. The higher pressure increased the mechanical ventilation rate on a traversing bus. Nevertheless, bus exhaust tail-pipe is usually located at low level on the rear. It is vital that the location of the outdoor air intake point of the air-conditioning system should be located away from the exhaust, taking into account the possible wind pressures that may carry the exhaust plume close to the intake point. Thus outdoor air intake point should be at a high level on the bus body, to reduce the likelihood of taking in vehicular exhaust. In specifying the engine type, priority should be given to low pollution emission models, such as the designs of the latest EURO standard type engines. A low emission engine reduces both the outdoor air pollution and the potential pollutants infiltration into the passenger compartment. This practice should not be limited to the design of new buses; the systems on existing buses should also be upgraded or retrofitted to meet the exhaust emission standard. High temperature would be generated when the engine is operating. The engine compartment should be enclosed and isolated from passenger compartment. The comfort model developed in Chapter 4 showed that in-bus air temperature was the dominant sensation towards thermal comfort perception. Moreover the heat transferred to the passenger compartment increased the energy consumption in air- conditioning. Therefore effective heat release system and thermal insulations should be installed, in order to minimise either over-heat inside the engine compartment or the undesired heat transfer to the passenger compartment.

195 Operation and maintenance

Captains are encouraged to stop the engines when they are waiting or parking at the terminus, since the exhaust might disperse throughout the waiting area. The influence is serious especially inside the semi-closed terminus. They should not make “heavy-step” once they move the buses. These can reduce the pollution to the surrounding environment and in-bus air quality. The engine should be in efficient operating condition so engine maintenance is a matter for attention. The efficient operating condition could lead to a more energy efficient consumption while avoiding excessive pollutant emissions as well. Regular inspection concerning with the engine system should be conducted in order to maintain the quality of performance. The inspection includes exhaust emission, power transmission and fuel consumption. If the systems installed on existing buses could not operate in an efficient condition, they should be upgraded or retrofitted in order to meet the exhaust emission standard.

7.2.2 Passenger compartment

The compartment body accommodates the passengers and protects them from the external adverse environment. It provides a comfortable commuting environment by maintaining the facilities in a healthy and safe condition.

Design considerations

In the new compartment, facilities should be designed to match with personalized air supply and return system design as developed in Chapter 5. Adjustable slot-typed diffusers are installed in the front of seats for better air distribution. Also return air grilles are located at ceiling-mounted ductworks above the seats. Hence the ductwork system has to be rearranged. This design can raise the passengers’ satisfaction level by providing better air quality and adjustable individual preferred thermal comfort. It minimizes the influence of odour and spread of infectious airborne diseases inside the passenger compartment. Moreover, Chapter 6 evident the personalised system helps to reduce energy consumption by higher ventilation efficiency. The air tightness of passenger compartment is an important factor. As reported in Chapter 3, there was differential pressure induced across the bus body surface when

196 traversing at various speed. Any leakage in the passenger compartment could lead to uncontrolled air balancing, deteriorating air quality and thermal comfort environment. Moreover the leakage would interfere with the operation of strategic control system (developed in Chapter 4) that cannot properly maintain an acceptable in-bus condition. An air-tight in-bus environment is thoroughly under the control of the air- conditioning system by reducing undesirable infiltration, so that it can perform to its design intent. It should also isolate the interior from the exhaust of its own engine while reducing the heat and noise from the engines and the intruding traffic. Besides, materials used inside the compartment should be considered in order to reduce internal pollutant sources. Specifications of materials used should be conformed to relevant standards, in order to ensure that there would be no threat of health and safety to passengers. Materials with minimum emission of pollutants should be selected, such as paints with low VOCs content. The compartment should be designed to facilitate easy cleaning and to reduce niches that may encourage accumulation of dirt and growth of micro-organisms. Seat coverings should be durable and capable to resist soiling thereby reducing the opportunity for growth of bacteria and fungi, and have low VOCs emissions. All parts of the compartment should be adequately ventilated by the airflow induced by the ventilation system.

Operation and maintenance

The capacity of compartment determines the controllable condition by the installed air-conditioning system. Since the capacities of air-conditioning system and mechanical ventilation system were designed to serve the maximum registered number of passenger onboard, any overloading will jeopardize the in-bus air quality and thermal comfort. Therefore, the captains should prevent exceed of capacity on board. It is important that bus operators keep the passenger compartments under close scrutiny. All buses put into service must be in an acceptable condition. Cleaning regimes should be checked to ensure thorough removal of dirt and rubbish, avoidance of water or damp materials, and removal of stains, bacteria and mould growth, etc. A cleaning record should be posted at an obvious location inside each compartment to advise the cleaning regime adopted and its ventilation system.

197 The O&M team often overlooks the flushing of cleaning agents and fumigation. Fumigation helps to reduce insects in the compartments, however, its use and control has to be very carefully considered in terms of releasing the compartment to service after application of fumigants. Any residue at excessive concentration will impose a health risk to the passengers.

7.2.3 Air-conditioning system

The air-conditioning system intends to sustain an acceptable in-bus air quality and thermal comfort conditions. The systems are comprised of air-conditioning units with outdoor air intake capability, which the compressors and condensers for buses are mounted at the rear of the body. The installation is compact due to the space limitation imposed by design. Air distribution ductwork is integrated at the ceiling of passenger compartment. Due to the tight headroom, these air ducts are designed with extreme aspect ratios that limiting the delivery of supply air.

Design considerations

In present compartment, new air-conditioning settings should be applied in order to improve the in-bus environment. Referring to the comfort models developed in Chapter 4, the magnitude of air movement should be considered. High speed cold air jets would cause uncomfortable cold air drafts, especially to the passengers sitting right under the diffusers. Adjustable diffusers are recommended to be equipped for individual thermal preferences. Also the outlets should be arranged as even as possible to have a better air distribution and circulation effect. Referring to Chapter 3, the magnitude of mechanical ventilation rate affected the in- bus air contaminants dilution; however, the outdoor air quality varied along the routes depending on the roadway environment. Thereby, the capacity of mechanical ventilation system should be increased in order to provide sufficient fresh air and dilute the in-bus air contaminants. Meanwhile, on-off control of outdoor air damper should be available at the bus control panel. Captains can trigger the damper manually by determining the air freshness along the roadway environment. Nevertheless, it is strongly recommended to implement the strategic control system in order to improve the in-bus air quality and thermal comfort environment.

198 On new buses, personalized air supply and return system (as developed in Chapter 5) is suggested. Meanwhile, referring to the comfort models, sensors should be equipped to monitor the comfort indicators. At the outdoor air intake, sensors are equipped to monitor the outdoor air temperature and concentration of CO along the roadway environment. Inside the compartment, the parameters of air temperature, relative humidity, concentration of RSP and CO2 are monitored. As recommended in chapter 4, the real-time monitoring data is collected and processed through the empirical comfort models. The computation results can be applied as air- conditioning settings and control outdoor air damper to improve the commuting environment while minimizing the passengers’ dissatisfaction level. Data recording of sensors should be available in order to provide references to modify the control strategy. Also, alarm signal is sent to operator to remind the fault of air-conditioning settings. Outdoor air and recirculated air are filtered before entering the evaporator. The compactness of the air-conditioning system limits the choice of air filters to remove particulates. The bus operators should equip electrostatic filters on new buses to enhance the performance of the conventional particulate filters. The ductwork system should be designed with access for easy cleaning. For designs where the ductwork is integrally fixed to the passenger compartment, the outlets can be modified so that the duct can be cleaned with a high power vacuum cleaner. Linear air curtains at the doors are suggested. In principle, it has the advantage of reducing uncontrolled air exchange through door openings. Hence, the in-bus air quality is under better control, and it reduces energy loss in the exfiltration of cooled air through door openings.

Operation and maintenance

On new buses, the air-conditioning settings are automatically adjusted depending on the computation result from empirical comfort models. Captains do not have to pay attention to the adjustments. Nevertheless, captains, who drive buses with present air-conditioning systems, should be trained to determine the operation of outdoor air damper. When the bus is traversing highly polluted zones, during traffic congestion or inside tunnels, shutting the damper could stop infiltrating the highly concentrated air pollutants from the busy roadway environment.

199 According to the control scheme developed in chapter 4, the mechanical ventilation rate should be set depending on the outdoor concentration of CO and in-bus concentration of RSP and CO2. Out-bus CO concentration level is put in the highest priority for the determination since it will influence the passengers’ health if they expose in concentrated air pollutants. In-bus RSP is put in the second because it is less rigorous and ones would be influenced under long-term exposure. Also the particulates could be reduced by filtering system on buses. Literatures shows short- term exposure of higher CO2 gives no noticeable symptoms (5500 ppm for 6 hours).

And almost bus trips are less than 45 minutes. Hence, in-bus CO2 concentration level is considered at the lowest ranking. The strategic control scheme can be referring to Figure 7.1. Considering the thermal comfort issue, comfortable set-point temperature can be evaluated dependent on the in/out-bus temperature difference. In-bus relative humidity and air speed are adjusted to modulate the thermal comfort environment in order to achieve the highest satisfaction level. For example, higher air speed should be applied in the compartment when the in-bus air temperature was above the set point. The wind-chill effect minimizes the thermal discomfort caused by under- cooling. The strategic control scheme concerning with thermal comfort environment can be referred to Figure 7.2. Furthermore, integrating the combined comfort model in the air-conditioning system, the control scheme can be referring to Figure 7.3.

200 Monitoring out-bus CO concentration level

Monitoring in-bus RSP concentration level & CO2 concentration level

CCO ≤ 1.3 ppm Yes No

Shut outdoor CRSP air damper ≤ 0.09 mg m-3

Yes No

Reduce CCO2 outdoor air rate ≤ 1500 ppm Yes No

Reduce Increase outdoor air rate outdoor air rate

Figure 7.1 Air-conditioning control scheme of outdoor air rate (reproduced from Chapter 4)

201 Monitor ambient air temperature ( to ) in-bus air temperature ( ti ) in-bus relative humidity ( w )

By comfort model computation, evaluate comfortable ti and adjust set-point temperature ( tset-pt )

w ≥ 80% No

Yes

Dehumidification No action

Calculate an appropriate air speed ( v ) by comfort model and adjust setting in system

Figure 7.2 Air-conditioning control scheme for thermal comfort (reproduced from Chapter 4)

202 Monitor air quality and thermal comfort indicators

Comfort models computation

PDcombined ≤ 20%

Yes No

AQ TC No action

CCO ≤ 1.3 ppm No ti ≤ tset-pt

Yes Yes

Reduce Increase CRSP air air ⋅ -3 No ≤ 0.09 mg m speed speed

Yes

Increase outdoor air rate

Figure 7.3 Air-conditioning control scheme for combined comfort (reproduced from Chapter 4)

203 For the system maintenance, sensors equipped for strategic control system should be calibrated regularly so as to ensure the accuracy. Particulate filters should be cleaned and replaced to maintain the specified operating condition at all times. The cleaning and replacement of filters should be done when the pressure differential across the filter reaches the recommended values indicated by the filter manufacturer. Air ducts, cooling coils and drain pans should be brushed, vacuum cleaned and washed at least once a year. If vacuum cleaning of the ductwork is inadequate, the loose dust floating in the circulation air will affect the in-bus air quality. This dust may contain harmful material or spores of bacteria and fungi. A high suction power vacuum cleaner with HEPA filters should be used in order to avoid dust being returned to the compartment. After the cleaning procedure or fumigation is carried out, air ducts and cooling coils should be adequately flushed with outdoor air before the bus is returned to service.

7.3 Conclusion

Concerning with in-bus air quality and thermal comfort, the three elements are the essential and closely interactive factors. Implementation of the ‘three elements approach’ leads to achieve the goal of sustainability in all the stages of design, operation and maintenance. The engine should be maintained in a good operating condition that lead to a more energy efficient consumption. Excessive pollutant emissions could be minimized. It reduces the air pollution in the surrounding environment. Meanwhile, the risk of short-circuiting the exhaust to the fresh air intake could be avoided. In the new compartment, personalized air supply and return system is suggested. It provides a higher ventilation efficiency that reduces energy consumption in air- conditioning. Adjustable slot-typed diffusers are installed in the front of seats for better air distribution. Also return air grilles are located at ceiling-mounted ductworks above the seats. The materials used in the compartment have to be concerned to avoid pollution sources inside the passenger compartment. They should be selected for minimum emission of pollutants. Seat coverings should be resistant to soiling thereby reducing the opportunity for growth of bacteria and fungi. Cleaning regimes should be checked to ensure thorough removal of dirt and rubbish, bacteria

204 and mould growth. A cleaning record should be posted at an obvious location inside each compartment to advise the cleaning regime adopted. For the air-conditioning system, the strategic control schemes should be implemented. Sensors are equipped on buses for real-time monitoring and data processing. Settings are adjusted depending on the computation result from empirical comfort models. Then the control strategy can be optimized to improve the commuting environment whilst fulfilling the passengers and achieving energy efficient.

205 8 Conclusion

Air-conditioned buses have been serving Hong Kong over two decades. There are now approximately 5750; over 95% of the service fleets. Bus manufacturers and local operators are keen on the service quality improvement by modifying the air- conditioning designs, including system performance and reliability, parameter settings as well as energy effectiveness. However, complaints about poor air quality and thermal discomfort were received since the beginning of services. Such issues suggest less-than-satisfactory environments on these buses. Further study is necessary to enhance the in-bus commuting environment. The air quality and thermal comfort environment on air-conditioned buses were focused in this study. A real scenario of commuting environment was reviewed through conducting physical parameters measurement and questionnaire survey on buses which were in normal service. Data collected from the measurement included air temperature, relative humidity, air speed, and concentration levels of CO2, CO and RSP. Also, passengers’ subjective perception and dissatisfaction levels were collected through face-to-face questionnaire survey. The data provided clues to evaluate the dynamic effects from surrounding environment. The in-bus air quality varied when the buses travelled on different routes. Worst scenario was found on tunnel routes which followed by the urban district routes. The variation of in-bus air pollutant concentration level was caused by the change of roadway environment, like traffic density, surrounding building density as well as the roadway air quality. Hence better in-bus air quality was found on rural district routes where traffic density was low along the journey. Besides, mechanical ventilation rate is another key to the in-bus air quality. Higher rate helps dilute in-bus air contaminants but it increases the risk of infiltrating concentrated air pollutants when travelling in congested area. Nevertheless, lower rate results as insufficient ventilation that causes air stuffiness and odour inside passenger compartment. The mechanical ventilation rate should not be fixed but adjusted depending on the roadway air quality. Ventilation characteristics affect the rate too. Pressure difference is induced across bus body surface when a bus is travelling since the bluff head generates aerodynamic drag. The normal mechanical -1 ventilation rate was found to be 250 l · s on a stationary bus. The rate increases with -1 the bus travelling speed. Assuming a bus travels at around 30 km · hr in the urban

206 -1 -1 area, an induced rate was estimated to be about 380 l · s , equivalent to 2.9 l · s per passenger on a fully loaded bus. If the bus travels along a highway at a higher speed -1 -1 -1 of 65 km · hr , the ventilation rate would rise to 535 l · s , providing 4.1 l · s per passenger. Empirical comfort models were developed by concluding the correlation between the subjective sensation votes from passengers and the corresponding physical parameters. The models provide a convenient platform to quantify and identify the in-bus air quality and thermal comfort through the percentage of dissatisfaction. Also it helps evaluate the corresponding level of physical parameters in order to achieve a comfortable commuting environment. In application, the computation result of models can be optimal set-points for the air-conditioning system. Real-time monitoring of the air quality and thermal comfort indicators are obtained by equipping sampling instruments on buses. The data is processed by model computation and sent to the control system. The real-time signal is significant determinant to set an appropriate outdoor air intake rate and thermal comfort settings. Therefore the in-bus environment can be controlled at the most comfortable condition along the journeys through the application of empirical comfort models. In the present passenger compartment, mixing-ventilation air distribution system was applied. It aimed to provide a uniformly air distribution. However, supply air was not well distributed due the tight space in the compartment. Thermally discomfort was caused due to temperature stratification and draft risk. The average percentage of dissatisfaction towards thermal comfort was 16%. Owing to the design of air distribution system, the ventilation efficiency was low as 0.93. Fresh air did not reach the passengers directly causing ineffective air contaminants dilution, neither preventing spread of infectious aerosols in the compartment. The average percentage of dissatisfaction towards air quality was 60%. Personalized air supply and return system was proposed in new compartment. Computation result from the empirical comfort models was applied to improve the in-bus environment. Diffusers were installed in front of each seat while return air grilles were located above each row. Simulation result showed the new compartment minimized those thermal discomfort issues found in the present compartment. This design improved the air distribution while satisfying individual preferences. The ventilation efficiency was increased to 1.32 under applying the new air distribution system. The percentage of dissatisfaction towards thermal comfort and air quality

207 were diminished to 3% and 12% respectively. Meanwhile the average percentage of dissatisfaction towards combined comfort level was obviously reduced from 35% to 8%. Moreover, the result showed that the droplets expelled from index passenger were carried by the air stream and exhausted through the nearest return air grilles in the new system. It revealed the new system was applicable to minimize the spread of infectious airborne diseases inside the passenger compartment. The air-conditioning load and energy consumption were simulated to compare the system operating under different scenarios, including in condition present compartment, new settings applied in present compartment and new settings applied in new compartment. Air-conditioning settings including set-point air temperature and mechanical ventilation strategy were adjusted depending on the computation result from empirical comfort models. The adjustment was intended to improve the in-bus environment while satisfying a substantial majority of passengers. The application of new air-conditioning settings in present compartment lowered the passengers’ dissatisfaction level. However, higher energy consumption in both heating and cooling was resulted. They rose by 9.1 times and 49% respectively. Moreover the required heating and cooling capacities were increased by 3.1 times and 98% respectively. The present air-conditioning system might not be applicable to fulfil the settings adjustment. Simulation result showed that new air-conditioning settings applied in the new compartment required lower heating and cooling capacities as compared with that the settings applied in present compartment. The required capacities were diminished by 55% (in heating) and 36% (in cooling) respectively. Also the energy consumptions in heating and cooling were reduced by 64% and 28% respectively. The results revealed the benefits provided by the new compartment. The personalized air supply and return design helped to achieve energy effectiveness by increasing the ventilation efficiency. Also adjustment of air-conditioning settings improved the commuting environment while satisfying a substantial majority (above 80%) of passengers. The simulation result would be a significant reference for the operators and engineers to consider the enhancement of in-bus air quality and thermal comfort environment on air-conditioned buses. Finally, the three elements approach was developed concerning with the air quality and thermal comfort in the aspect of design, operation and maintenance on air- conditioned buses. The three elements were the engine, passenger compartment and

208 air-conditioning system. The approach concluded the issue from the viewpoints of influences from both internal and external sources, as well as the control system. It assisted to improve the in-bus air quality and thermal comfort environment in an effective way.

209 References

A reference note on occupational exposure limits for chemical substances in the work environment. Labour Department 1998. Hong Kong SAR.

Air quality goals for indoor air. National Health and Medical Research Council 1992. Australia.

Air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulphur dioxide. The World Health Organization 2005.

Alexander D.K. HTB2 – A model for the thermal environment of buildings in operation – User manual. Welsh School of Architecture, University of Wales College of Cardiff, Cardiff. 1997.

An overview on air quality and air pollution control in Hong Kong. The Hong Kong Environmental Protection Department, 2005.

Annual Transport Digest. The Hong Kong Transport Department 2007.

Apple Daily. Bus cabins are too cool. 16 August 2006.

ASHRAE. 2004. Thermal environmental conditions for human occupancy. ASHRAE Standard 55-2004. Atlanta: American Society of Heating, Refrigerating and Air- conditioning Engineers, Inc.

ASHRAE. 2005. Thermal Comfort. ASHRAE Handbook - Fundamentals. Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc.

Automobiles and Mass Transit. ASHRAE Handbook – HVAC Applications. Heating, ventilating, and air-conditioning systems and applications. American Society of Heating, Refrigerating and Air-conditioning Engineers, Atlanta, GA. 2007.

210 Berg-Munch B., Clausen G., Fanger P.O. Ventilation requirements for the control of body odour in spaces occupied by women. Environment International 1986; 12: 195-199.

Bevan M.A.J., Proctor C.J., Baker-Rogers J., Warren N.D. Exposure to carbon monoxide, respirable suspended particulates, and volatile organic compounds while commuting by bicycle. Environmental Science & Technology 1991; 25: 788-791.

Census Report 2002. Census & Statistics Department 2002. The Hong Kong Special Administrative Region Government.

Chan A.T. Commuter exposure and indoor-outdoor relationships of carbon oxides in buses in Hong Kong. Atmospheric Environment 2003; 37: 3809-3815.

Chan C.C., Ozkaynak H., Spengler J.D., Sheldon L. Driver exposure to volatile

organic compounds, CO, ozone and NO2 under different driving conditions. Environmental Science & Technology 1991; 25: 964-972.

Chan L.Y., Chan C.Y., Qin Y. The effect of commuting microenvironment on commuter exposures to vehicular emission in Hong Kong. Atmospheric Environment 1999; 33: 1777-1787.

Chan L.Y., Lau W.L., Chan C.Y., Lee S.C. Commuter exposure to particulate matter in public transportation modes in Hong Kong. Atmospheric Environment 2002a; 36: 3363-3373.

Chan L.Y., Lau W.L., Zou S.C., Cao Z.X., Lai S.C. Exposure level of carbon monoxide and respirable suspended particulate in public transportation modes while commuting in urban area of Guangzhou, China. Atmospheric Environment 2002b; 36: 5831-5840.

Chan L.Y., Liu Y.M. Carbon monoxide levels in popular passenger commuting modes traversing major commuting routes in Hong Kong. Atmospheric Environment 2001; 35: 2637-2646.

211

Chan L.Y., Wu W.Y. A study of bus commuter and pedestrian exposure to traffic air pollution in Hong Kong. Environment International 1993; 19: 121-132.

Chan M.Y. Commuters’ exposure to carbon monoxide and carbon dioxide in air- conditioned buses in Hong Kong. Indoor and Built Environment 2005; 14: 397- 403.

Chan W.T., Burnett J., de Dear R.J., Ng S.C.H. A large-scale survey of thermal comfort in office premises in Hong Kong. ASHRAE Transactions 1998; 104: 1172-1180.

Chan W.T., Shek K.W. Comfort model of air quality on buses in Hong Kong. Submitted to Indoor and Built Environment (2009a).

Chan W.T., Shek K.W. Comfort model of thermal comfort on buses in Hong Kong. Submitted to Indoor and Built Environment (2009b).

Chan W.T., Shek K.W. Evaluation of ventilation rate on air-conditioned buses travelling at various speeds. Submitted to Journal of Wind Engineering & Industrial Aerodynamics (2007).

Control of air pollution in public transport interchanges. The Hong Kong Environmental Protection Department, 1998.

de Dear R.J., Fountain M.E. Field experiments on occupant comfort and office thermal environments in a hot-humid climate. ASHRAE Transactions 1994; 100: 457-475.

Dor F., Le Moullec Y., Festy B. Exposure of city residents to carbon monoxide and monocyclic aromatic hydrocarbons during commuting trips in the Paris metropolitan area. Journal of the Air and Waste Management Association 1995; 45: 103-110.

212 Duci A., Chaloulakou A., Spyrellis N. Exposure to carbon monoxide in the Athens urban area during commuting. The Science of the Total Environment 2003; 309: 47-58.

Duffy B.L., Nelson A.F. Exposure to emission of 1,3-butadiene and benzene in the cabins of moving motor vehicles and buses in Sydney, Australia. Atmospheric Environment 1997; 31: 3877-3885.

Ebersole J.H. The new dimensions of submarine medicine. New England Journal of Medicine 1960; 262(24): 599-610.

Environmental quality standards. Ministry of the Environment 1997. Japan.

Ergonomics of the thermal environment – Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. Geneva: International Organization for Standardization (ISO 7730-2005).

Exposure guidelines for residential indoor air quality. Minister of Supply and Services Canada 1995.

Eyewitness Travel Top 10: Hong Kong 2004. DK Publisher.

Fanger P.O. Thermal comfort: analysis and application in environmental engineering. McGraw-Hill, New York. 1970.

Fanger P.O. Human requirements in future air-conditioned environments. International Journal of Refrigeration 2001; 24(2): 148-153.

Fanger P.O., Berg-Munch B. Ventilation and body odour. Conference on Management of atmospheres in tightly enclosed spaces. 1983; 45-50.

Fernandez-Bremauntz A.A., Ashmore M.R. Exposure of commuters to carbon monoxide in Mexico city – I. Measurement of in-vehicle concentrations. Atmospheric Environment 1995; 29: 525-532.

213 Flachsbart P.G., Howes J.E., Mack G.A. Rodes C.E. Carbon monoxide exposures of Washington commuters. Journal of Air Pollution Control Association 1987; 37: 135-142.

Flury F., Zernik F. Schadiche Gase. Berlin, Springer, 1931.

Fountain M., Brager G., de Dear R. Expectations of indoor climate control. Energy and Buildings 1996; 24(3): 179-182.

Gee I.L., Raper D.W. Commuter exposure to respirable particles inside buses and by bicycle. The Science of the Total Environment 1999; 235: 403-405.

Guan Y.Z. Investigation of human thermal comfort under highly transient conditions for automotive applications. ASHRAE Transactions 2003; 109:885-907.

Guidance Notes for the Management of Indoor Air Quality in Offices and Public Places. The Hong Kong Environmental Protection Department 2003b.

Guidelines for Air Quality. The World Health Organization, 2000.

Guidelines for Air Quality. The World Health Organization, 2005.

Hollies N.R.S., Goldman R.F. Clothing Comfort: Interaction of Thermal Ventilation, Construction and Assessment Factors. Ann Arbor Science Publisher Inc. 1977.

Homepage of the Hong Kong Observatory 2009. < http://www.hko.gov.hk > Hong Kong Observatory. The Hong Kong Special Administrative Region Government.

Hong Kong air pollution emission inventory. The Hong Kong Environmental Protection Department 2006.

Hong Kong air quality objectives. The Hong Kong Environmental Protection Department, 2001.

HRW World ATLAS. < http://go.hrw.com/atlas/norm_htm/hongkong.htm >

214 Humphreys M.A., Nicol F. Understanding the adaptive approach to thermal comfort. ASHRAE Transactions 1998; 104: 991-1004.

Humphreys, M.A. Thermal comfort temperatures and the habits of hobbits. Standards for thermal comfort: indoor air temperature standards for the 21st century. Nicol F., Humphreys M., Roaf S. and Sykes O. Taylor & Francis Publisher 1995.

Indoor climate and ventilation of buildings. Regulations and guidelines 2003. Ministry of the Environment. Finland.

Jiang Y., Li X.F., Zhao B. SARS and ventilation. In: Proceedings of the fourth international symposium on HVAC 2003; v.K04(1): 27-36.

Junker M., Kasper M., Roosli M., Camenzind M., Kunzli N., Monn Ch., Theis G., Braun-Fahrlander Ch. Airborne particle number profiles, particle mass distributions and particle-bound PAH concentrations within the city environment of Basel: an assessment as part of the BRISKA Project. Atmospheric Environment 2000; 34: 3171-3181.

Kaynakli O., Kilic M. An investigation of thermal comfort inside an automobile during the heating period. Applied Ergonomics 2005; 36: 301-312.

Kim M.H. Numerical study on the wake flow and rear-spoiler effect of a commercial bus body. SAE Transaction 2003; 2003-01-1253.

Kot S.C., Lai H.W. Statistical modelling of vehicular CO emission. Pollution in the urban environment. Polmet 1985: 239-244.

Kowloon Motor Bus Co. Ltd. homepage. < http://www.kmb.hk/english.php >

Lam A.K.L. An alternative transport strategy with due consideration of environmental impacts. Transport Planning, Demand Management, and Air Quality, 26-27 February 2001, Manila, Philippines.

215 Lam A.K.L., Fong J.W.Y. Striving for excellent indoor air quality – the Hong Kong experience. Healthy Buildings – Energy-efficient healthy buildings. Proceedings of ISIAQ 7th International Conference. Singapore.

Lam G.C.K., Leung D.Y.C., Niewiadomski M., Pang S.W., Lee A.W.F., Louie P.K.K. Street-level concentrations of nitrogen dioxide and suspended particulate matter in Hong Kong. Atmospheric Environment 1999; 33: 1-11.

Leaman A., Bordass B. Comfort and complexity: unmanageable bedfellows. Workplace comfort forum 1995. RIBA, London.

Liu J.J., Chan C.C., Jeng F.T. Predicting personal exposure levels to carbon monoxide in Taipei, based on actual CO measurements in microenvironments and a Monte Carlo simulation method. Atmospheric Environment 1994; 28: 2361-2368.

Lonely Planet: Hong Kong, 1998.

Luria M., Weisinger R., Peleg M. CO and NOx levels at the centre of city roads in Jerusalem. Atmospheric Environment 1990; 24B: 93-99.

Madsen T.L. Why low air velocities may cause thermal discomfort. Third international conference on indoor air quality and climate 1984; 5: 331-336.

Maximum concentrations at the workplace and biological tolerance values for working materials 2000, Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area, Federal Republic of Germany.

Morawska L. Droplet fate in indoor environments, or can we prevent the spread of infection? In: Proceedings of Indoor Air 2005, Beijing, China, pp.9-23.

New Lantau Bus Co. Ltd. homepage. < http://www.newlantaobus.com/main.html >

New World Group bus services homepage. < https://www.nwstbus.com.hk/home/default.aspx?intLangID=1 >

216 Olesen B.W. Measurement of thermal comfort in vehicles. Bruel & Kjaer Application Notes. 1989.

Oriental Daily. 2005. Complaints of hot and stuffy air in First Bus cabins. 1 September 2005.

Petersen W.B., Allen R. Carbon monoxide exposures to Los Angeles area commuters. Journal of Air Pollution Control Association 1982; 32: 826-833.

Potter I.N. and Booth W.B. CO2 controlled mechanical ventilation systems. BSRIA Technical Note TN 12/94.

Practice Note for Managing Air Quality in Air-conditioned Public Transport Facilities - Buses. The Hong Kong Environmental Protection Department, 2003a.

Practice Note on Control of Air Pollution in Car Parks. The Hong Kong Environmental Protection Department, 1996.

Praml G., Schierl R. Dust exposure in Munich public transportation: a comprehensive 4-year survey in buses and trams. International Archives of Occupational and Environmental Health 2000; 73: 209-214.

Public transportation indoor air quality management guideline. The Korean Ministry of Environment 2006.

Report thermal crimes. Friends of the Earth (HK) 2005.

Roy D., Petitjean P., Clodic D. Influence of various heat transfers on passenger thermal comfort. Advances in automotive climate control (SP-1733). SAE International 2003.

Saksena S., Prasad R.K., Shankar V.R. Daily exposure to air pollutants in indoor, outdoor and in-vehicle micro-environments: A pilot study in Delhi. Indoor and Built Environment 2007; 16: 39-46.

217 Schulte J.H. Sealed environments in relation to health and disease. Archives of Environmental Health 1964; 8: 438-452.

Shek K.W., Chan W.T. Combined comfort model of thermal comfort and air quality on buses in Hong Kong. Science of the Total Environment 2008; 389: 277-282.

Standard – 29 CFR. 1910 - Occupational Safety and Health Standards. Toxic and Hazardous Substance. Table Z-1 Limits for Air Contaminants. Occupational Safety & Health Administration, United States Department of Labor 2007.

Standard Code of Practice on Indoor Air Quality. Ministry of the Environment 1996. Singapore.

Standard guide for using indoor carbon dioxide concentrations to evaluate indoor air quality and ventilation. ASTM International 2002. Designation: D 6425-98.

The Hong Kong Environmental Protection Department homepage. < http://www.epd-asg.gov.hk/e/api/current/currentf.htm >

The Standard. Arctic conditions’ inside malls and buses blasted. 27 June 2005.

The Third Comprehensive Transport Study. The Hong Kong Transport Department 2005.

The year’s weather – 2005. The Hong Kong Observatory, 2005.

Thermal comfort. ASHRAE Handbook – Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA. 2005.

Thermal environmental conditions for human occupancy. ASHRAE Standard 55. American Society of Heating, Refrigerating and Air-conditioning Engineers, Atlanta, GA. 2004.

Transport Planning and Design Manual. Transport Department. The Hong Kong Special Administrative Region Government.

218 Travel Characteristics Survey 1992. Transport Department 1992. The Hong Kong Special Administrative Region Government.

Travel Characteristics Survey 2002. Transport Department 2002. The Hong Kong Special Administrative Region Government.

Travel wise information. National Geographic Traveller Hong Kong, 2002.

Tung T.C.W., Chao C.Y.H., Burnett J. A methodology to investigate the particulate penetration coefficient through building shell. Atmospheric Environment 1999; 33: 881-893.

US Department of Labor, Occupational Safety and Health Administration. Code of Federal Regulations, Title 29, Part 1910.1000-1910.1450.

van der Kooi J., Wan J.W. Evaluation of air quality and thermal comfort in a coach. International Journal of Vehicle Design 1993;14(5/6): 530-538.

Vellopoulou A.V., Ashmore M.R. Personal exposures to carbon monoxide in the city of Athens: I. Commuters’ exposures. Environment International 1998; 24: 713- 720.

Ventilation for acceptable indoor air quality. ASHRAE Standard 62.1, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA. 2004.

Ventilation for acceptable indoor air quality. ASHRAE Standard 62.1. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA. 2007.

Wells W.F. Airborne contagion and air infections. Harvard University Press, Cambridge, MA. 1995.

Workplace exposure limits. Health and Safety Executive 2005. UK.

219 Wu W.Y. Carbon monoxide impact study for bus passengers and on-route pedestrians. One day seminar on ground level air pollution in urban environment, the Chai Wan Technical Institute, Hong Kong. 1990.

Xu Z.L. The theory of air cleaning technology. Tongji University Press; 1998.

Zhang G.Q., Han J., Zhang Q., Zhang J.W., Liu J.L., Tian L.W., Zheng C., Hao J.H., Lin J.P., Liu Y.H., Moschandreas D.J. Field study on occupants’ thermal comfort and residential thermal environment in a hot-humid climate of China. Building and Environment 2007; 42: 4043-4050.

Zhang Q., Yuen H., Zhang G.Q., Deng T.F. Study of the impact of human motion on indoor thermal comfort. Journal of Hunan University (Natural Sciences) 2007; 34(8).

Zhang Z., Chen Q. Comparison of the Eulerian and Lagrangian methods for predicting particle transport in enclosed spaces. Atmospheric Environment 2007; 41: 5236-5248.

Zhu S.W., Kato S., Yang J.H. Study on transport characteristics of saliva droplets produced by coughing in a calm indoor environment. Building and Environment 2006; 41: 1691-1702.

Zhu Y., Hinds W.C., Kim S., Shen S., Sioutas C. Study of ultrafine particles near a major highway with heavy-duty diesel traffic. Atmospheric Environment 2002; 36: 4323-4335.

220 Appendix A.

Calibration study of sampling CO with Q-Trak

Regarding to the accuracy of CO measured by Q-Trak, calibration study of sampling CO was conducted with Q-Trak and SKC-Airbag with SKC-Airchek Sampler (Model 224-44XR) at the same time. The sampling time constant of Q-Trak was set at 5 seconds, while the logging interval was 30 seconds. SKC-Air bags (40L) were used and they were rinsed with nitrogen before used. Also, the Airchek Sampler was set to have a flow rate at 4.5 litres per minute. Air samples were taken in the campus of The Hong Kong Polytechnic University. The sampling duration was 6 minutes in order to collect adequate air sample for analysis. After the air sampling, airbags were delivered to the laboratory and analyzed by *Bruel & Kjaer Multi-gas Monitor (Type 1302). Each set of CO sampled by Q-Trak was averaged and compared with the results analyzed by B&K-1302. The result was illustrated in Figure A.1. The reading of CO by Q-Trak was slightly deviated from the B&K-1302 analysis result. The regression was found as follows:

( Q-Trak CO ) = 1.0332 ( B&K-1302 CO )

which meant the CO measured by Q-Trak might be slightly larger than the actual value by 3.3% in the environment where CO was under 3.5 ppm.

Figure A.1 Q-Trak and B&K-1302 multi-gas monitor correlation

A-1

Specifications and calibration data for the B&K multi-gas monitor (Type 1302) Accuracy: Zero drift: Typically = Detection threshold per 3 months Influence of temp: ± 10% of detection threshold / oC Influence of pressure: ± 0.5% of detection threshold / mbar Repeatability: 1% of measured value Range drift: ± 2.5% of measured value per 3 months Influence of temp: ± 0.3% of measured value / oC Influence of pressure: -0.01% of measured value / mbar Reference conditions: Measured at 20oC, 1013 mbar, and RH 60%. Calibration data for CO detection (B&K-1302) Detection limit 0.2 ppm Average zero level, (dry zero gas): 9.74 e-2 ppm Standard deviation, (dry zero gas): 7.69 e-2 ppm Average zero level, (wet zero gas): -2.60 e-2 ppm Standard deviation, (wet zero gas): 6.91 e-2 ppm Average gas concentration level: 50.6 ppm Standard deviation: 0.14 ppm

A-2 Appendix B.

Questionnaire (English version)

Q.1 At present, do you accept the COMBINED COMFORT LEVEL inside the cabin?

1 Unacceptable 0 Acceptable

Q.2 At present, how do you feel about the AIR QUALITY inside the cabin, is it acceptable?

0 Very good 1 Good 2 Neutral 3 Poor 4 Very poor