Human perception of local air movement in the tropics

Study human perception of local air movement created by personalized air terminal device in the Tropics under short-term exposure, with focus on preference for air movement and local th

Chapter 1: Introduction

1.1 Background and Motivation

This study is about indoor environment and people Nowadays, people usually spend more

than 90% of their time in indoor environment (ASHRAE, 2003), thus indoor environment is

very important for human health, comfort, and even productivity (Wargocki et al, 1999; Tham,

2004) Ventilation and air-conditioning is commonly adopted to adjust and control the indoor

environment Being a top energy consumer in buildings, ventilation and air-conditioning

system is extremely important for energy conservation and sustainable development The

energy situation especially holds true for the Tropics A survey of commercial buildings in

Singapore (Lee, 2004) has shown that the ventilation and air-conditioning system can

consume more than 55% of overall energy in buildings

Mechanical Ventilation and air-conditioning plays a crucial role especially in achieving

comfortable conditions for work and living in the Tropics In Singapore as an example, the

diurnal temperature ranges from a minimum 23-26 °C to maximum of 31-34 °C, and the

relative humidity from around 90% in the early morning to around 60 % in the mid-afternoon

During prolonged heavy rain, relative humidity often reaches 100 % (Absoluteastronomy,

2005) This is probably why Lee Kuan Yew, Singapore’s former Prime Minister, and

currently Minister Mentor, called air-conditioning “the 20th century’s most important invention”!

Mixing ventilation (conventional air-conditioning) is widely adopted in public buildings in

the Tropics However, these buildings are usually operated to create conditions that are

overcooled (de Dear et al, 1991) Where such a centralized system is used, end users usually

cannot adjust the temperature and control their local environment In some places, jackets and

sweaters become essential officer wear in the Tropics The overcooling of buildings also means that there is potential for energy conservation with subsequent positive contribution to sustainability

The arguments for the present overcooling situation are usually the need of dehumidification, and that reheat is energy intensive and prohibited in some countries in the Tropics Under the overcooling situation, designers avoid having to deal with complaints about the place being too warm, and they argue that those who are too cold can just put on a jacket or something (Thestar, 2005)

The present overcooling situation of mixing ventilation does not follow human centered buildings design and is not consistent with the comfort, healthy and sustainable life style The high humidity in the Tropics should be taken care in a proper way, but not be tackled with price of sacrifice of comfort and with energy waste

Meanwhile, indoor air quality as an important issue for human health should be taken into consideration in addition to thermal comfort for indoor environment However, under mixing ventilation, fresh air is mixed with room air before it reaches occupants’ breathing zone As a result of the mixing, contaminant concentration is usually same in the whole space, and occupants usually breathe the mediocre quality mixed-air but not the high quality fresh-air

Personalized Ventilation (PV) is recently advocated for indoor environment to cater to the need of a paradigm shift from acceptable to excellent indoor environment (Fanger, 2001) Under a PV system, conditioned fresh air is supplied directly to the occupant’s breathing zone without mixing with recirculated air Occupants can control the PV air parameters such as air flow rate, i.e., velocity, direction or even temperature of the personalized air The individual control makes it possible to change from previous passive environmental adaptation (adding

clothing) to active environmental control, i.e., the air-conditioning process should not be human adapting to indoor environment but should be indoor environment providing optimal conditions to human Occupants’ satisfaction of indoor air quality and thermal comfort could then be greatly improved

Personalized ventilation also provides opportunities of energy conservation In PV, the outdoor air is efficiently dehumidified by cooling coil which needs to deal with the outdoor air only; the conditioned outdoor air is efficiently utilized by presenting it directly into the breathing zones thus minimizing cross contamination and mixing; the quantity of outdoor air needed is reduced as it is able to effectively achieve the desired dilution and freshness in the breathing zone Meanwhile, indoor ambient temperature can be maintained higher, with the thermal comfort requirements achieved via local cooling with localized air movement Hence energy efficiency may be achieved with better indoor air quality and occupant comfort and health

Most PV research is conducted in temperate climatic zones (Bauman et al, 1993; Tsuzuki et al, 1999; Kaczmarczyk et al, 2002a; Melikov et al, 2002; Zeng et al, 2002) In the studies, some

PV air terminal devices were developed and human responses were examined under PV system (The detail is introduced in Chapter 2) PV system should be examined for the tropical context to explore whether the local people will accept it, e.g., the acceptance of thermal comfort and inhaled air quality, and whether it has energy conservation potential in the scenario of tropical climate Those needs motivated the first study of human response to PV and energy saving in year 2002, which is described in Chapter 4

Due to the short distance supply of PV air, PV air flow may cause occupants’ local thermal discomfort, e.g., draft The draft guideline (ISO 7730, 1995), however, was developed under whole-body exposure to air movement and isothermal conditions (Fanger et al., 1988), and it

may not be applicable to human perception of air movement under local exposure and isothermal conditions under PV situation The situation necessitates further studies to examine human perception of local airflow Therefore following-up studies were initiated, which are described in Chapter 5 and 6

non-1.2 Research Objectives

This study aims to expand the knowledge of perception and acceptability of local air movement of people passively acclimatized by day-to-day life in the Tropics The objectives

of this thesis are described as follow:

a Evaluate the acceptability and energy saving potential of personalized ventilation system

in the Tropics

b Study human perception of local air movement created by personalized air terminal

device in the Tropics under short-term exposure, with focus on preference for air

movement and local thermal conditions

c Study human perception of local air movement created by personalized air terminal

device in the Tropics under long-term exposure, with focus on the influence of time of

occupation on people’s perception of air movement and local thermal conditions

1.3 Scope of Work

This study is in the field of human perception of local air movement in air-conditioned environment in the Tropics The scope of work and the structure of discussion in each chapter are described briefly as follows:

‰ Literature review This study is not independent to, but based on, previous research on

human perception of personalized ventilation (mainly about thermal comfort and indoor

air quality) and air movement (mainly about draft) Chapter 2 provides useful

information on experimental design and method adopted, and compares the results of those studies Particular emphasis is given to the studies carried out in the Tropics/hot humid climate, although unfortunately very few were focused on human perception of local air movement The research gaps are identified

‰ General methodology The work comprised a series of three related studies The general

research methodology is described for the series of experiments, which includes experimental design (facility and instrument, and measurement protocol) and data analysis (statistical analysis and analysis of relationship) The discussion of general

methodology is presented in Chapter 3 Other methods applied only in a specific study

will be introduced in separated chapters

‰ Human response to PV and energy saving Human response to PV was studied with a

group of 11 subjects in the Tropics, where there was individual control of the air terminal device Objective measurements of relevant indoor environmental & ventilation parameters in the vicinity of one human subject were conducted The information from these two parts of the experiment was analyzed to explore human perception when individual control of local air movement is made available Coil cooling load was estimated to examine the energy saving opportunities of the personalized ventilation

system adopted in this study The detailed data analysis is discussed in Chapter 4

‰ Human perception (short-term exposure) In Chapter 5, human perception of local air

movement under short-term exposure was studied using an intervention study of local air parameters, without subjects’ interference of the local air movement Subjective study is the key approach to reveal human perception to local air movement A group of 24 subjects took part in the experiments Each experimental intervention lasted 15 minutes to study the first impression of human perception Human preference of local air movement

and local thermal environment was the focus of data analysis A model to predict percentage of dissatisfied people was developed in this study as well

‰ Human perception (long-term exposure) Using the same experimental protocols of the

short-term exposure study, the human perception of a group of 24 subjects in the Tropics were studied under prolonged-stay of 90-minutes exposed to local air movement The impact of time of occupation on human perception is the focus of the study of long-term exposure It is observed from this study that thermal sensation, subjects’ preferred local air velocity and importance of reasons for air movement preference changed when time

elapsed The influence of time of occupation is discussed and summarized in Chapter 6

‰ Conclusion and Recommendation The objectives are reviewed and a summary of

significant findings is presented In particular, the contributions of the new understanding

of human perception are briefly discussed Lastly, some suggestions for further research

and the development of personalized ventilation system in the Tropics are given in

Chapter 7

Chapter 2: Literature Review

Three topics relevant to this study are discussed in detail in this literature review – personalized ventilation, human perception study of air movement and human perception study in the Tropics/hot humid climate The knowledge gaps are then summarized and hypotheses are proposed at the end of this chapter

2.1 Tropical Climate and Mechanical Ventilation

The tropical zone is defined as the zone between the Tropic of Cancer (latitude 23.5 °North) and the Tropic of Capricorn (latitude 23.5 °South) Occupying approximately forty percent of the land surface of the earth, the Tropics are the home to almost half of the world’s population The characteristics of tropical climate are abundant rainfall and high humidity associated with

a low diurnal temperature range and relatively high air temperature throughout the year

Singapore, for example, is an island state in Southeast Asia, at latitude 1°17'35"N longitude 103°51'20"E, situated on the southern tip of the Malay Peninsula, south of the state of Johore

in Peninsular Malaysia and north of the Indonesian islands of Riau (Absoluteastronomy, 2005)

Singapore's climate is tropical (“tropical rainforest climate”), with no distinct seasons Because of its geographical location and maritime exposure, its climate is characterized by uniform temperature and pressure, high humidity and abundant rainfall Temperature has a diurnal range of a minimum 23-26 °C and a maximum of 31-34 °C and relative humidity has

a diurnal range in the high 90% in the early morning to around 60 % in the mid-afternoon During prolonged heavy rain, relative humidity often reaches 100 % Singapore is influenced

by Northeast monsoon (wetter season) which lasts from October to March and Southwest monsoon (drier season) from May to September (Dutt and de Dear, 1991)

In general, tropical climate is characterised with uniformly high relative humidity and air temperature throughout the year

Given the feature of the high relative humidity and air temperature, it is natural that mechanical ventilation and air conditioning play a key role for the control of the indoor environment in the Tropics

Commercial office buildings usually adopt total-volume mechanical ventilation for conditioning in the Tropics The total-volume ventilation includes mixing ventilation and displacement ventilation Mixing ventilation aims to maintain uniform and constant indoor environments while displacement ventilation aims to maintain a temperature and contaminant gradient with the most acceptable region occurring in occupant zones However, such environments may not meet every individual’s thermal requirement due to the variations of the individual’s thermal preferences, clothing and heat load in the different places of the room Moreover, occupants usually cannot adjust local environments to meet their unique thermal requirement

air-On the other hand, it is obvious that under circumstances of both mixing ventilation and displacement ventilation, air inhaled by occupants is the mixture of fresh and ambient air (although fresh air at different levels of the two types of ventilation) Just as stated by Fanger (2001), in the supplied fresh air, say 10 L/s for a person, only 0.1 L/s, or 1% is eventually inhaled, and even the 1% being inhaled is polluted by bioeffluents from occupants or emissions from building materials etc Therefore, Fanger (2001) recommended that a small quantity of high-quality air be supplied directly to each individual rather than serving plenty

of mediocre air in the whole room Such “personalized air” should be clean, cool and dry (according to the findings of Fang et al, 1998a; 1998b), and supplied directly to the breathing zone

Personalized Ventilation (PV), also known as Task/Ambient Conditioning (Bauman et al, 1998), is such a ventilation method that provides the “personalized air” Under a PV system, conditioned fresh air is supplied directly to the occupant’s breathing zone without mixing with contaminated recirculated air The concept of PV has tremendous potential for enhancing the acceptability of ventilation, indoor air quality and thermal comfort in air-conditioned buildings Occupants can control the PV air parameters such as air flow rate/velocity, direction or even temperature In Chapter 2.2, personalized ventilation is reviewed in detail

2.2 Personalized Ventilation

2.2.1 Air Terminal Device and Air Flow

™ Air Terminal Device

Fundamentally, the present personalized ventilation differentiates from other ventilation approaches through its air supply parameters (usually fresh, clean, dry and cool air with low turbulence intensity) and supply position (close to breathing zones of occupants) The air terminal device plays key role in creating ‘high quality’ personalized air, and thus the design

of terminal device is important and some air terminal devices are reviewed

Localized ventilation has been applied in vehicle cabin (bus, car and aircraft) and theatre buildings for many years The localized ventilation usually only addresses thermal comfort, and air quality is usually not a concerned issue and therefore recirculated air is used in localized ventilation

Fanger (2001) advocated a paradigm shift to excellent indoor environment, and air terminal devices of PV have since been developed and studied for their contribution towards this goal Different to previous air terminal device used for localized ventilation, only fresh air is supplied by the PV air terminal devices Figure 2.2.1 shows a, the original prototype of PV air terminal device in Fanger (2001), and some air terminal devices, in which b, is called round

moveable panel (Bolashikov et al, 2003); c, five types of air terminal device- Movable Panel (MP), Computer Monitor Panel (CMP), Vertical Desk Grill (VDG), Horizontal Desk Grill (HDG), and Personal Environments Module (PEM) (Melikov et al, 2002); d, desk-edge-mounted task ventilation system (Faulkner et al, 2004); e, Headset-Incorporated Supply (Bolashikov et al, 2003) and f, microphone-like air supply nozzle (Zuo et al, 2002) Other PV air terminal devices, e.g., those of Akimoto et al (2003) and Levy (2002), are similar as those

in Figure 2.2.1c Hence, they are not included in the figure

Although the air terminal devices are of different appearances, shapes or positions relative to the occupants, the designs have some similar considerations, i.e., achieving high inhaled air quality by minimizing mixing between personalized air and ambient/exhaled air without causing much discomfort or inconvenience to occupants; with user-friendly control; being compatible with occupants’ movement; and being harmony with the surrounding environment

a Prototype of PV air terminal device b Round moveable panel

c Five types of air terminal devices (MP-Movable

Panel; CMP-Computer Monitor Panel;

VDG-Vertical Desk Grill; HDG-Horizontal Desk Grill;

PEM–Personal Environments Module)

d Desk-edge-mounted task ventilation system

e Headset-Incorporated Supply f Microphone-like air supply nozzle

Figure 2.2.1 Prototype and some PV air terminal devices (figure a from Fanger (2000), figure

b from Bolashikov et al (2003), figure c from Melikov (2004), figure d from Faulkner et al (2004), figure e from Bolashikov et al (2003), figure f from Zuo et al (2002))

™ Air Flow under PV Situation

Air flow under PV situation is usually very complex As depicted in Figure 2.2.2 a PV situation in office, there are at least five airflows interacting with each other around human body, i.e., free convection flow around the human body, personalized flow, respiration flow, ventilation flow and thermal flow (Melikov, 2004) All the flows may affect occupants’ inhaled air quality and comfort

Figure 2.2.2 Airflow interaction around human body: (1)—free convection flow, (2) —personalized flow, (3)—respiration flow, (4)– ventilation flow, (5)—thermal flow (Source: Melikov (2004))

Free convection flow around the human body is an upward free convection flow existing around the human body “This airflow is slow and laminar with a thin boundary layer at the lower parts of the body and becomes faster and turbulent with a thick boundary layer at the height of the head.” …and “in rooms, a large portion of the air that is inhaled by sedentary and standing persons is from the free convection flow.” (Melikov, 2004)

The personalized flow is typically a free jet and contains a core (potential core region) of almost unmixed clean air with constant velocity And it is suggested by Melikov (2004) that

the core be used when the location of an air terminal device is considered Only when the personalized airflow transverses the free convection flow can it most probably be inhaled

Respiration creates alternating inhalation and exhalation flows ‘The exhalation generates jets with relatively high velocity, 1 m/s and more, which can penetrate the free convection flow around the human body, effectively rejecting exhaled air from the flow or air that may subsequently be inhaled’ (Melikov, 2004) The design of personalized air should avoid mixing with the exhalation and avoid the exhalation be inhaled again

The ventilation flow may be from ambient system or natural ventilation The ventilation flow may be with different air temperatures, velocities or turbulence intensities In the experimental studies of personalized ventilation, the velocity of ventilation flow is usually controlled at a low level when it is near to occupants The detailed description of all the airflow can be found in Melikov (2004)

2.2.2 Performance of Personalized Ventilation

The performance of PV system has been extensively reported by both physical measurements and human response studies in recent years These studies provide some evidences that occupant satisfaction is improved with the use of PV, as compared to mixing ventilation The majority of work on PV system is conducted in laboratory settings

Studies demonstrate that PV system could accommodate different cooling loads and subjects perceive a better thermal environment with the cooling effect of the body (Bauman et al 1993; Bauman et al 1998; Arens et al 1998; Tsuzuki et al 1999; Melikov et al 2002; Kaczmarczyk et

al 2002a; Kaczmarczyk et al 2004)

PV system could accommodate different heat loads up to 446 W in one workstation (Bauman

et al 1993), and improve micro thermal satisfaction to “near very satisfied” in workstation (Bauman et al 1998) The cooling effect on the body by different types PV air terminal devices have been investigated by Tsuzuki et al (1999) and Melikov et al (2002) The research of Tsuzuki et al (1999) shows that the cooling effect is significant, which can lead whole-body (of thermal manikin) heat loss equivalent to room air temperature decrease of 9.0

°C to cool the manikin Melikov et al (2002) investigated the performance of five PV air terminal devices, i.e., Horizontal Desk Grill, Vertical Desk Grill, Personal Environmental Module, Computer Monitor Panel and Movable Panel It was found that the Vertical Desk Grill (VDG) was the best among the five air terminal devices and VDG provided greatest cooling of the manikin’s head (manikin-based equivalent temperature decreased by – 6.0 °C when PV air flow is 10 L/s) However, VDG also increased the amount of exhaled air in each inhalation in comparison with an indoor environment without PV Although in the experiments thermal comfort is obtained by exposing occupants to environments that are often thermally asymmetrical, with air movement and radiation directed onto some parts of the body and not on others, the subjective studies showed that subjects can maintain their whole body thermal neutrality (Kaczmarczyk et al 2002a) and the operation of the PV system did not cause thermal discomfort of the subjects (Faulkner et al, 2004)

Ventilation indices, e.g., ventilation effectiveness and personal exposure effectiveness (the definitions can be referred to, in Chapter 3), were also measured with PV system (Faulkner et

al 1999; Melikov et al 2002; Faulkner et al, 2004), and subjective experiments showed that

PV is perceived with a better air quality than mixing ventilation (Kaczmarczyk et al 2002b; Zeng et al 2002; Kaczmarczyk et al, 2004)

Faulkner et al (1999) compared the performance of two desk mounted task/ambient conditioning (TAC) systems in terms of the quality of the ventilation air at the breathing zone They used Air Change Effectiveness (ACE) and Pollutant Removal Efficiency (PRE) as

air-indicators to assess the ventilation condition in the breathing zone Their study found that a TAC system could provide ACE and PRE values considerably above unity, implying high ventilation efficiency It was also observed that both ACE and PRE depended strongly on the design of the TAC system Although a TAC system should maintain high values of ACE and PRE while enabling occupants to adjust their local thermal environment, adjustments made by occupants to achieve better thermal environment may have an adverse impact on the ACE and PRE values, e.g changing the supply air direction can reduce the ACE and PRE values Five years later, Faulkner et al (2004) investigated the ventilation effectiveness of a desk-edge-mounted task ventilation system that provided outdoor air The study concluded that an air change effectiveness of about 1.5 could be achieved with the task ventilation system, which represents a 50% increase in effective ventilation rate in the breathing zone

The research of Kaczmarczyk et al (2002a) showed that the best condition of perceived air quality, perception of freshness and intensity of Sick Building Syndrome (SBS) symptoms was when the PV system supplied outdoor air at 20 °C It was also observed that PV helped to decrease complaints of headache, and improved the ability to think clearly and to concentrate

In a similar subjective study, Zeng et al (2002) derived a maximum design flow rate of 20 L/s/person for PV systems The maximum preferred PV flow rate was based on an analysis of perceived air quality and draft ratings

Melikov et al (2003) investigated the impact of airflow interaction on inhaled air quality and transport of contaminants between occupants in rooms with personalized and total volume ventilation It was observed that the PV system supplying air against the face improved the ventilation efficiency in regard to the floor pollution up to 20 times and up to 13 times in regard to bioeffluents and exhaled air, compared to mixing or displacement ventilation alone Cermak and Melikov (2003) also explored the performance of PV system in a room with an underfloor air distribution system and concluded that the design of the PV air terminal devices and the interaction of personalized airflow and room airflow are important

considerations in order to achieve minimal transport of pollution between occupants Kaczmarczyk et al (2004) studied human response to PV and mixing ventilation systems and found that PV system decreased SBS symptoms and increased self-estimated performance compared to mixing systems

There are also a number of field studies reported on PV system Bauman et al (1998) reported

a field study on the application of desktop task/ambient air-conditioning system in office buildings using pre-post intervention with control group methodology The study involved three buildings with a total of 42 desktops provided with PV systems The study also involved other occupants who did not have the task air-conditioning system on their desktop, as a control group Intensive measurements and questionnaires survey were carried out in different periods over a total period of five months Six building assessment categories, i.e thermal quality, air quality, lighting quality, acoustical quality, spatial layout and office furnishings were used as the assessment parameters The result showed that the installation of task air-conditioning units increased overall occupant satisfaction Another field study (Kroner and Stark-Martin, 1994) compared occupants’ satisfaction and task performance before and after they moved to a new building equipped with Task/Ambient air distribution, and results indicated that both indicators improved following the move However, as indicated by Charles (2003), methodology limitation may undermine the strength of the result as the change to localized air distribution occurred in parallel with a building move, and differences between the two buildings could have confounded these results Obviously, more field studies

of PV are needed

The PV system could also offer an opportunity to reduce ventilation-related energy consumption (Niu,2003) The energy saving potential of a PV system has been attributed to its high ventilation efficiency (Faulkner et al.,1999) and the possibility of raising ambient air temperature in hot climates (Bauman et al, 1993) Seem and Braun (1992) investigated the impact of Personal Environment Control (PEC), a type of desktop module, on energy use by

computer simulation, and it is found that the PEC system could lead to a range between 7% saving and 15% penalty in lighting and HVAC electrical use PEC fans, electronics or radiant panel cause the major energy penalty The penalty, however, could be offset by only about a 0.08% annual increase in productivity associated with PEC The capacity of PV system to achieve a balance between energy conservation and providing optimal indoor environment quality still needs further study

2.2.3 Draft under Personalized Ventilation Scenario

Due to close supply of personalized air to the occupants, draft dissatisfaction may happen under PV scenario There have been numerous draft/air movement studies (The studies will

be reviewed in Chapter 2.3), however, the findings of previous draft/air movement studies may not be applicable to PV system, since they are usually conducted under isothermal conditions, while PV system often adopts non-isothermal conditions, i.e., temperature of personalized air is lower than that of ambient Furthermore, the PV users usually could control personalized air parameters or adjust PV air terminal device position according to their preference, while in most draft experiments, the occupants usually do not have the capacity to control airflow It is reasonable to speculate that individual adjustment can decrease draft risk in PV situation

Draft caused by personalized air movement is not often reported quantitatively up to this date There are some observations of air movement preference of human subjects

The study of Zeng et al (2002) showed that there are large differences in people’s preference for velocity and direction of personalized air - some people prefer rather high velocities while other people are very sensitive to air movement However, there is no quantitative data reported of the velocity preference Some subjects felt no draft even when the personalized air flow rate is increased to 20 L/s

The study of Yang et al (2002) showed that constant (not fluctuating) air movement is more preferred than that of fluctuating They investigated three periodically fluctuating airflows with frequencies of 0.1 Hz, 0.2 Hz, 0.3 Hz, and airflow with constant velocities provided by a

PV system Subjects can control mean air velocity and PV outlet position It is found that the air movement with a frequency of 0.2 Hz was the most preferred out of the three fluctuating air movements “The subjects selected a lower mean air velocity and felt more distracted when the air was fluctuating than when it was constant” (Yang et al 2002)

As individual control of personalized air may affect subjects’ perception of air movement, the individual control and optimum velocity were observed and measured by researchers Kaczmarczyk et al (2002b) observed individual control of airflow rate, direction of airflow and position of PV system outlet In the experiment, the subjects were allowed to continuously regulate both the positioning of PV air terminal device and personalized airflow rate The significant conclusions are that up to 96% subjects positioned air terminal devices at the front and more than 50% subjects placed air terminal devices at a distance of 30-40 cm to face The range of the preferred airflow rate is 3-15 L/s In another study of Kaczmarcyk (2003), the optimum velocities determined at subjects’ mostly selected local air movement ranged between 0.39 m/s and 0.48 m/s at room air temperature 23 °C; and between 0.42 m/s and 0.74 m/s at room air temperature 26 °C The personalized air supply was supplied at 20

°C The observations suggest that the subjects prefer air movement to a certain extent at their head regions

2.2.4 Knowledge Gap

PV has been widely studied for its air terminal device and performance However, the present

PV as a system is not sufficiently explored and understood, still far from the research goal articulated by Bauman et al (1998): a well-designed PV system should “take maximum

advantage of the potential improvements in thermal comfort, ventilation performance, indoor air quality, and occupant satisfaction and productivity while minimizing energy use and costs.” As pointed by Melikov (2004), “Research is needed in order to explore the potential of

PV and ensure its optimal performance.”

People in the Tropics may perceive PV systems differently compared with people from temperate climates due to differences in physiological acclimatization, clothing, behaviour, habituation and expectation Since there are very few PV studies in the Tropics, a study of PV systems under tropical conditions has significant implications in the application of PV systems in the Tropics Furthermore, in typical buildings in the Tropics, the air-conditioning systems maintain the indoor volume at relatively low temperatures in the vicinity of 23 °C (de Dear et al, 1991; Sekhar, 1995) A PV system can be envisaged as a system capable of achieving significant energy conservation due to the inherent possibility of maintaining the ambient space temperature higher while supplying the PV air at a preferred lower temperature This would be one of the hypotheses investigated in this study

Furthermore, the understanding of human perception of air movement under PV scenario is urgently needed for PV system design and application Unfortunately, the relation between human occupants and personalized air flow still remains unknown Melikov (2004) pointed out that “Human response to non-isothermal and locally applied airflow” (should be explored)

“Airflow temperature, velocity, size of the target area, etc should be parameters.”

In the following section 2.3, human perception studies on air movement are reviewed aiming

at providing background knowledge and hints for this study

2.3 Human Perception Study on Air movement

“Air movement – good or bad?”(Toftum, 2004) - a lot of researchers have tried to answer the question The question about the nuisance of a draft and the pleasure of a fresh breeze,

especially at breathing zone at facial part, has puzzled researchers for decades (e.g., from the study of Houghten et al, 1938) and there is still no clear understanding regarding how human perception develops from desirable cooling to uncomfortable draft, among the scientific community

Draft has been identified as one the most annoying factors in offices When people sense draft,

it often results in a demand for higher air temperatures in the room or for stopping ventilation system (ASHRAE 2003) Draft is usually defined as unwanted, local cooling of the body caused by air movement (ASHRAE 2003)

Both in field studies and in laboratory studies, it has been observed that large inter-individual differences exist – some occupants perceive air movement as pleasant while others feel unpleasant draft in the same environment As indicated by Toftum (2004), “the perception of air movement depends not only on the air velocity and other thermal environment parameters, but also on personal factors such as activity level, overall thermal sensation and clothing.” The human perception studies on air movement are therefore reviewed according the condition of overall environment as follows of moderate and cool, and warm and hot environments

2.3.1 Moderate and Cool Environments

Although there is no strict definition of the studies of moderate and cool environments, their temperatures usually are less than 26 °C, and draft is a frequent concern under these environments

Numerous experiments of human perception on air movement are conducted under indoor temperatures less than 26 °C (Fanger and Pedersen 1977; Fanger and Christensen 1986; Fanger et al 1988; Toftum 1994; Toftum and Nielsen 1996; Griefahn et al 2000; Griefahn

et al 2001; Griefahn et al 2002) The studies are commonly designed to explore people’s

perception of draft when whole-body is exposure to air movement under total-volume ventilation

Various indoor air factors affect human perception of air movement In addition to air velocity and air temperature, the effect of turbulence intensity on draft discomfort was identified in the study of Fanger et al (1988) Turbulence intensity is defined as:

Where, Vsd is the standard deviation of the velocity and V is the mean value of velocity

In the study of Fanger et al (1988), three turbulence air flow with intensities at low (Tu<12%), medium (20%<Tu<35%) and high level (Tu>55%) were tested with human subjects, and the results showed that higher turbulence intensity arouses higher percentage of dissatisfied The reason for the discomfort caused by high turbulence could

be the fluctuations of the skin temperature The mean level of the skin temperature will not change significantly during the fluctuations, but the rate of change of the skin temperature with time will be greater at high turbulence (Fanger et al 1988)

The effects of turbulence intensity was incorporated into a model that predicts the percentage of dissatisfied due to draft (PD) as a function of mean air velocity (v), air temperature (ta) and turbulence intensity (Tu), which are adopted in international standard

was later adjusted by Griefahn et al (2001) to account for effects of clothing and experimental procedure adopted in the two studies

Factors other than the above have also been identified by researchers Fanger and Pedersen (1977) showed that maximum discomfort was experienced at frequencies of air velocity between 0.3 and 0.5 Hz The effects of frequency were subsequently confirmed

by Zhou and Melikov (2002) and Zhou et al (2002) The effects of direction of air movement on human perception were studied by Zhou (1999) The study revealed that air flow direction has a significant impact on human perception of air movement Airflow from below results in the highest percentage of dissatisfied, whereas airflow from ceiling

to floor results in the least percentage of dissatisfied Airflow from behind, front and side caused a similar draft sensation The effects of time of occupation with air movement were also identified The draft-induced annoyance was found to be increased with time and reached a steady state after 40 to 50 minutes (Griefahn et al., 2001)

2.3.2 Warm and Hot Environments

The experiments under warm category are usually conducted at indoor temperatures higher than 26 °C (Mayer and Schwab 1988; Fountain et al 1994; Arens et al 1998; Xia et

al 2000) In these studies, high air velocity was usually tested and usually individual control of the air velocity adopted

It is known that air movement can offset increased temperatures Fanger et al (1974) and McIntyre (1978) showed that thermal comfort can be created by air movement around 0.8 m/s at air temperatures up to 28 °C Tanabe et al (1987) showed that air movement up to 1.6 m/s helps to achieve comfort at 31 °C The requirement of air speed to offset increased temperature can be found in ASHRAE (2003) Meanwhile, it should be noted that although air movement has the capacity of offsetting increased temperatures, the dominant preference is for a temperature in the comfort range, as Toftum (2004) indicated

‘Even though it has been proven possible to maintain thermal comfort and sensation by high air velocity at elevated temperatures, subjects’ dominant preference was for lower air velocity and a temperature in the comfort range.’ The claim is based on the findings of Toftum et al (2002)

Among the requirements in ASHRAE (2003) for air speed to offset increased temperature,

it also specifies that acceptance of increased air speed requires occupant control of the local air speed (ASHRAE, 2003) Occupant control helps to minimize draft risk The importance of individual control of the air velocity has been demonstrated in the study of Toftum et al (2002)

Meanwhile, various studies explored the relation between human preference and air parameters in warmer environments Fountain et al (1994) investigated human preference

to locally controlled air movement in warm isothermal condition The experiment included three different air supply terminal device: desk fan (FAN), floor-mounted diffuser (FMD), desk-mounted diffuser (DMD) The research developed a PS model to predict the percent of satisfied people as a function of air temperature and air movement

in warm conditions PS model recognized that people participated in shaping their environment, applicable when indoor temperature is from 25.5 –28.5 °C:

PS = 1.13Top 0.5 - 0.24Top + 2.7v 0.5 – 0.99v (2.3) Where, Top – Operative temperature (° C); v - Occupant preferred air velocity (m/s)

Xia et al (2000) did experiments to examine human response to air movement in warm isothermal condition The study revealed that in addition to draft discomfort due to cooling effect, air movement also causes annoying effect due to air pressure at higher velocities at higher temperature Furthermore, in contrast to the findings that turbulence may induce draft in moderate and cool environments, they found that turbulence can reduce discomfort in warm conditions due to its stronger cooling effects

In hot environments, non-isothermal air supply (usually spot cooling) is sometimes adopted Spot cooling is a kind of localized ventilation method investigated from early 1970s (Azer et al 1971, 1972, 1982a, 1982b, 1984; Ma and Qin, 1991; Melikov, et al., 1994a, 1994b) It is usually used in hot industrial settings (28 ° C or higher) where it is not economical to maintain a comfortable thermal environment by applying total volume

or stratified air conditioning In spot cooling system, a local individual jet of cool air is applied to reduce workers’ heat stress The cooling commonly concentrates on one part of the body, typically the head and upper torso (Melikov, et al., 1994a, 1994b) The studies usually showed that spot cooling improves the thermal conditions and increases the subjects’ acceptance of thermal environment However, draft discomfort or discomfort due to the pressure of air jet was reported at preferred air velocities Individual control can provide the subjects’ most acceptable thermal comfort conditions, however, the conditions are ‘achieved as a compromise between decreased warmth discomfort and increased discomfort due to the cooling or the pressure of the jet’ (Melikov et al., 1994a)

2.3.3 Knowledge Gap

The previous studies have explored people’s perception of draft/air movement in moderate and cool, and warm and hot environments The results of the studies under isothermal conditions (Fanger and Christensen, 1986; Fanger et al, 1988) have been well accepted by the international community while the study under non-isothermal conditions

is still at its preliminary stage How people perceive draft in terms of percentage dissatisfied remains unknown under non-isothermal conditions

The findings derived from isothermal studies (Fanger and Christensen, 1986; Fanger et al, 1988) may not be applicable to the non-isothermal conditions since the two different temperatures of ambient air and local air simultaneously affect the draft perception In order to improve the perceived inhaled air quality and to improve the cooling effect by

using personalized flow, it is beneficial to supply the personalized air several degrees cooler than the room air In this connection it is important to improve our understanding

of human perception to locally applied non-isothermal air movement This is especially important for the practical development of personalized ventilation systems with high performance in terms of occupants’ comfort and energy use

2.4 Human Perception Study in the Tropics / Hot Humid Climate

In addition to indoor air parameters, human perception of air movement may be affected

by different climatic zones The people living in a region with hot humid climate may perceive air movement differently from the people in temperate zone due to differences in physiological acclimatization, clothing, behaviour, habituation and expectation Therefore, the findings derived from the temperate zone may not be directly applicable to the hot and humid tropics

While there have been numerous researches on human perception of air movement in the temperate climate, there have been only a few air movement studies conducted in the Tropics/hot and humid climatic zones Two representative draft studies conducted in hot and humid climate are reviewed here One (Tanabe and Kimura, 1994) was conducted in hot and humid summer season in Japan, and the other (de Dear and Fountain, 1994) was conducted in hot and humid region of Australia’s tropical north, where the average temperature and maximum humidity are respectively 27 °C and 80% Both of these climatic conditions are similar to those of Singapore

Tanabe and Kimura (1994) examined effects of air movements on subjects’ thermal comfort in air-conditioned spaces Through the research, it was found that under hot and humid conditions only 10% of the subjects felt draft at a mean air velocity of 0.4m/s, which is a much higher velocity than the velocity of 0.15 m/s predicted by the model in ISO 7730 (1995) developed in temperate zones for the same level of draft discomfort

De Dear and Fountain (1994) reported similar findings as those of Tanabe and Kimura (1994) They performed a field investigation of indoor climates and occupant comfort in

12 air-conditioned office buildings The field study showed that draft or unwanted local cooling due to excessive air movement was much less a problem than insufficient levels

of air movement Although their thermal environments fell within ASHRAE Standard 55 summer comfort zone, most human subjects expressed dissatisfaction that they felt the air was too still The finding suggests that draft guidelines ISO 7730 may be inappropriate for hot humid climatic zones (de Dear and Fountain, 1994) Therefore, the draft guidelines in the standards may need to be re-examined in the Tropics/ hot humid climatic zones

Moreover, in four ASHRAE field studies conducted in different climatic regions (including the study of de Dear and Fountain, 1994) it is found that only few occupants complained of draft and preferred less air movement, while more people prefer no change/more air movement under the temperature range 22.5 °C to 23.5 °C The results are reproduced and shown in Table 2.4.1 The table shows that rather many occupants with thermal sensation of slightly cool preferred more air movement despite the fact that more air movement would make them feel even cooler In another air movement study (Toftum et al, 2002), stated preference for more air movement was verified when subjects was provided with more air movement under controlled conditions

Table 2.4.1 Air movement preference as observed in the four ASHRAE field studies (Schiller et al., 1988; de Dear & Fountain, 1994; Donini et al., 1997; Cena and de Dear, 1999) (Source: Toftum, 2004)

Thermal

sensation

Air velocity range (m/s)

Percent of occupants preferring

It should be noted that some data in Table 2.4.1 are not consistent and unexpected For example, when air velocity is in the range from 0.15 to 0.25 m/s, the percentage preferring less air movement is 8.4% under “slightly warm” condition, which is higher than that of 2% under ‘neutral’ condition The reason is unclear yet

2.5 Summary and Hypotheses

These studies conducted in temperate zones provide some evidences that occupant satisfaction is improved with the use of PV, as compared to mixing ventilation However, the performance of PV system has not tested yet in the Tropics Such a study of PV system under tropical conditions has significant implications in the application of PV systems in the Tropics

PV system has a potential to save energy as occupants’ thermal comfort can be achieved

by local cooling of personalized air under a higher ambient space temperature The energy saving potential is not often reported and it is suggested that the energy saving potential of PV system be investigated

Due to the fact that the personalized air is usually supplied to the occupants over a short distance, it is possible that draft adversely affects people’s comfort, particularly in the facial region This necessitates an exploration of human perception of local air movement

to facilitate the application of PV system The exploration may construct the relation between human perception and local air movement for both isothermal and especially non-isotherm conditions as the human perception under non-isotherm conditions is still relatively little known to date

The draft guideline in ISO 7730 may be inappropriate for the Tropics/ hot humid climatic zones, and therefore needs to be re-examined The human perception may be examined under short-term and long-term exposure to air movement

The following hypotheses are therefore proposed:

1 for the study under PV scenario in the Tropics

- PV system in conjunction with ceiling supply air distribution system enhances occupants' thermal comfort and perceived air quality in tropical buildings;

- PV system has a potential to save cooling energy consumption in tropical building designs

2 for the study of human perception under short-term exposure of local air movement in the Tropics

- The people in the Tropics may prefer some local air movement;

- Percentage dissatisfied due to local air movement may be related to multiple air parameters, for example, ambient temperature, local air temperature, velocity and turbulence intensity

3 for the study of human perception under long-term exposure of local air movement in the Tropics

- Time of occupation as a factor may affect human perception during prolonged exposure of local air movement and a steady state of perception may be reached; and

- At steady state, the people may still prefer some local air movement However, the optimum air velocity calculated at lowest percentage dissatisfied may decrease as time passed

Chapter 3: General Methodology

3.1 Introduction

The human perception of local air movement is explored in three studies in Chapter 4, 5, and

6, i.e., human response to personalized ventilation and energy saving, human perception under short-term exposure and long-term exposure This chapter introduces the general methods applied for all the three studies while the methods applied only in a specific study are introduced respectively in the experimental design in each chapter

3.2 Experimental Design

The study adopted a subject-centered intervention approach In addition to subjective study, objective measurements were conducted in Chapter 4 The objective studies help to explain the human perception observed in subjective studies

3.2.1 Facility and Instrument

™ Facility

The experiments of all three studies were conducted in a controlled Indoor Air Quality (IAQ) chamber, which was developed in the Department of Building at the National University of Singapore The IAQ chamber is served by two air conditioning systems, the primary and the secondary systems The primary system serving the chamber comprises of a fan coil unit (FCU) that delivers fresh air through the PV air terminal device The secondary system consists of an Air Handling Unit (AHU), which provides supplementary ambient cooling to the chamber

The fresh air supplied by the FCU is brought into the space through a main duct that terminates into a plenum box Six branch ducts originating from the plenum box enter the

chamber through openings in the wall at about 1.2 m height from the finished floor level The fresh air is then delivered to the occupant or the respondent through PV air terminal devices

The secondary ceiling supply system serves the space through two main diffusers, whose supply air flow rate controlled by a typical VAV box controller Two diagonally located return air grilles function during the course of the experiments and a completely ducted return route was used The space temperature was controlled using a space thermostat in the main supply duct and the PV air temperature was controlled individually using heaters in every branch duct The schematic layout of IAQ chamber and its air-conditioning system are shown

in Figure 3.2.1

Figure 3.2.1 Schematic layout of IAQ chamber and its air-conditioning system

The IAQ chamber is approximately 25 sq.m in floor area and has the dimension of 6.6m(L)

x 3.7m (W) x 2.6m(H) (Refer to Figure 3.2.2) This chamber was customized to the requirements of the experiment into a typical office environment with movable interior partitions dividing the whole chamber into six workstations Each workstation is about

1.6m(L) x 1.5m (W) All the workstations were provided with a personal computer and an Internet connection in order to engage the participants during the course of the experiments

The chamber was enclosed on three sides by an annular space, which minimizes external environmental interferences and also served as the airlock for the participants before they entered the chamber The fourth side of the chamber adjoins a control room, which is controlled at the same temperature as the chamber and serves as a “conditioning” room for the subjects The same total-volume mixing ventilation system also serves the control room

Figure 3.2.2 Layout plan of experimental chamber

PV Air Terminal Device

Light fittings are fluorescent tubes of 36 Watts that are typically used in offices The 2 x 6 numbers of fluorescent tubes are equally spaced out with prismatic covers The floor finishes used are vinyl tiles in light colour

As a follow-up study conducted by other researchers (Sun et al, 2006; Zhou et al, 2005), a breathing thermal manikin was used to simulate a human being, and it was measured in the same experimental chamber with the same PV air terminal device Relevant results from manikin study are referred or correlated to this study, hence the manikin information is briefly introduced here The manikin is shaped as an average size woman, with a height of 1.68m and weight of 20kg The face is also designed like a natural human in order to achieve a more

realistic air movement in the breathing zones The manikin was dressed in a clothing ensemble corresponding to approximately 0.7 clo

The body of the manikin is divided into 29 segments whereby each segment is supplied with its own measuring and control system The skin surface temperature of each body segment is controlled to be equal to that of a real person under thermal neutrality To quantify the cooling effect of the thermal environment, to which the manikin is exposed, the sensible heat loss from each body segments as well as the whole-body can be transformed into a parameter termed manikin-based equivalent temperature The manikin-based equivalent temperature is defined as the temperature of a uniform enclosure in which a thermal manikin with realistic skin surface temperatures would lose heat at the same rate as it would in the actual environment (Tanabe et al, 1994) The manikin-based equivalent temperature can be expressed as a function of the sensible heat loss from each body segment:

Where Teq is the manikin-based equivalent temperature, °C, 36.4 is the deep body temperature, °C, Qt is the sensible heat loss, W/m2, C is constant dependent on clothing, body posture, chamber characteristics and thermal resistance offset of the skin surface temperature control system, K.m2/W The C values of each body segment as well as whole-body were derived by calibration of the manikin in the experimental chamber

Figure 3.2.3 Breathing thermal manikin (left) and its control interface (right)

™ Instruments

The instruments used in measuring the objective data are the thermometers, temperature sensors, HOBO meters, omni-directional low velocity anemometers and INNOVA photoacoustic spectrometer multi-gas analyzer

Thermocouples are used to measure room ambient temperature Measurements are then logged through the data acquisition system at every thirty seconds interval and stored in a computer

The breathing zone temperatures of a human subject and manikin were typically measured using a temperature sensor, which was calibrated in ohms This temperature sensor was connected to a multi-meter to measure the temperature readings continuously in ohms, which through a calibration chart was later converted into relevant temperatures

Relative humidity (RH) was not controlled during the experiments due to the facility limitation The space RH was measured by HOBO meter at every 10 minutes interval

Omni-directional low velocity anemometers were placed at the occupied positions of each workstation to record air velocity and air temperature for the facial parts, with measurements taken at every 10 seconds interval

INNOVA photoacoustic spectrometer multi-gas analyzer was used to measure concentrations

of Carbon Dioxide (CO2), Carbon Monoxide (CO) and Total Volatile Organic Compound (TVOC) The analyzer is based on the Photo Acoustic Infra Red principle

The instruments and their accuracy are listed in Table 3.2.1, and the photos of the instruments are shown in Figure 3.2.4

Table 3.2.1 Instrumentations used in the study

Parameter Instrument Accuracy Room air temperature Thermocouple wire ± 0.2°C

Breathing temperature Temperature sensor ± 0.1 °C

Room air relative humidity HOBO meter ± 5%

Concentration of CO2, CO

and TVOC, SF6

Photoacoustic spectrometer multi-gas analyzer ± 2%

Local air velocity and

temperature

Omni-directional velocity sensor

Vel ± 0.02m/s Temp ±1% of readings

c Thermocouple measuring air temperature d Photoacoustic spectrometer multi-gas

analyzer

f Omni-directional velocity sensor

Figure 3.2.4 Instruments used in the study

3.2.2 Measurement Protocol

The ambient room temperature was monitored using thermocouples at 26 different locations

at heights of 0.1, 0.6, 1.1 and 1.7m Three sets are located evenly in the chamber room whilst the control room used one set PV air supply temperatures were monitored using a thermocouple placed in each PV duct as close as possible to the air terminal device Thermocouples were used to measure the surface temperature of the four surrounding walls, floor, ceiling and the return and supply diffusers as well Measurements were logged through the data acquisition system at every thirty seconds interval and stored in a computer

HOBO meters were used to record relative humidity in the chamber and control room, placed

at 1.1m height at the centre of both rooms

Personalized air flow was regulated and measured by using tracer gas of SF6 The potential indoor air pollutants, namely CO2, HCHO, TVOC and CO, were continuously monitored during the experiments using the multi-gas monitoring system The points of measurement were located at the centre of the chamber at three heights namely 0.1m, 1.3m, 2.5m

Ventilation effectiveness was measured by constant injection of tracer gas in the ceiling supply system, while the personalized air was free of tracer gas The Ventilation Effectiveness (εV ) of the PV system, is defined as

Where, C R is pollutant concentration in exhaust air,

C S is pollutant concentration in PV supply air,

C P is pollutant concentration in the inhalation zone

The definition shows that higher ventilation effectiveness means relatively lower pollutant concentration in the inhalation zone or better quality of the inhaled air

Another important index for assessment of inhaled air quality is personal exposure effectiveness (εor PEE) introduced by Melikov et al (2002), expressed as the percentage of personalized air in inhaled air It can be used to quantify the amount of local air in inhaled air and is defined as:

Where Ce is tracer gas concentration in the chamber (ppm), Cs is tracer gas concentration in

the local air supply (ppm), Ci is tracer gas concentration in the inhaled air (ppm) This index is equal to one if 100% of local air is inhaled and equal to zero if no local air is inhaled

3.3 Data Analysis

The analysis of the data comprises mostly on subjective responses The aim of data analysis is

to reveal the effects of air parameters on human perception and explore the relation between human perception and indoor air parameters

All subjective response data were collected from questionnaires in which the questions were answered by the human subjects during the experiments The questionnaires are described in detail in Chapter 4, 5 and 6 The subjective parameters such as thermal sensation, air movement related perceptions, thermal comfort, indoor air quality as well as percentage dissatisfied were analyzed The answers provided by the subjects were firstly translated into the respective values to be further analyzed Statistical analysis is adopted in Chapter 4, 5 and

6 and correlation analysis adopted for Chapter 5 and 6

3.3.1 Statistical Analysis

Descriptive statistic such as mean and percentages were applied for evaluating the subjective responses The normally distributed data are compared using ANNOVA, while those that are not normally distributed are compared using non-parametric test- Wilcoxon signed rank test

or Friedman Test (for more than two repeat measures) A criterion of p≤0.05 is applied in all situations to ascertain statistical significance

Chapter 4: Human Response to Personalized Ventilation and Energy Saving

4.1 Introduction and Objective

As introduced in Chapter 2, Personalized Ventilation (PV) concept has tremendous potential

in enhancing the acceptability of mechanical ventilation, indoor air quality and thermal comfort in air conditioned buildings by supplying fresh air directly to the occupant breathing zone without mixing with room air and recirculated air

While the PV concept has been evaluated in some details in temperate climates, there have been few similar studies in the Tropics This section illustrates a study on the performance of

a PV system in the Tropics The key objectives of this study are to:

a Study occupants’ perception of thermal comfort and indoor air quality in the context

of a PV system used in conjunction with a primary ceiling-supply (mixing volume) air-conditioning system;

b Estimate energy saving potential of PV systems in the Tropics As the Tropical

region climate requires year round cooling only, the use of PV to create an acceptable indoor environment while raising the indoor ambient temperature provides energy saving potential

The first objective is aimed at exploring the viability of a PV system in the Tropics, where conventional designs often employ the ceiling-supply air-conditioning system It is intended

to examine the response of occupants in a building when they are exposed to different environmental conditions, some of which employed with PV systems It is hypothesized that