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

1

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

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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

3
may not be applicable to human perception of air movement under local exposure and non-
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.

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


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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


5
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.


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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).

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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 air-
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.

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.

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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

9
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.

























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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))

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 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

12
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).


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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 air-
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

14
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

15
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

16
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.

17

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

18
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,

19
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

20
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:
Tu =100•(V
sd
/V) (2.1)
Where, V
sd
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 (t
a
) and turbulence intensity (Tu), which are adopted in international standard
of ISO7730 (1995):
PD = 3.143(34- t
a
)( ν-0.05)
0.6223
+ 0.3696νTu(34- t
a
) ( ν-0.05)
0.6223
(2.2)

The model is valid for sedentary, thermally neutral persons dressed in normal clothing
with whole-body exposed to air movement.

As metabolic rate or clothing also affects human perception to air draft, the model is
further extended by Toftum (1994) to address the effects of work load and the extension

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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

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‘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.


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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

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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.

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