1
The influence of air-conditioning managerial scheme in hybrid-ventilated
2
classrooms on students’ thermal perception
3
4
Chen-Peng Chena, Ruey-Lung Hwangb,*, Weiwei Liuc, Wen-Mei Shihb, and Shih-Yin
5
Changa
6
a
Department of Occupational Safety and Health, China Medical University, No. 91
7
Hsueh-Shih Road, Taichung 40402, Taiwan; chencp@mail.cmu.edu.tw (Chen-Peng
8
Chen)
9
b
10
11
12
Department of Architecture, National United University, 1 Lien-Da, Kung-Ching Li,
Miaoli 36003, Taiwan; rueylung@nuu.edu.tw (Ruey-Lung Hwang)
c
School of Energy Science and Engineering, Central South University, Changsha,
Hunan 410083, China; wliu@csu.edu.cn (Weiwei Liu)
13
14
15
* Corresponding author: Tel: +886-37-381643; fax: +886-37-354838
e-mail: rueylung@nuu.edu.tw
16
17
18
19
Short title for use as running head: AC managerial scheme on students’ thermal
perception
1
Keywords: Air-conditioning; Energy management; Hybrid ventilation; Indoor
2
environmental control; Thermal comfort
3
4
Abstract
5
Hybrid ventilation employing both natural ventilation and air-conditioning (AC) is
6
used in Taiwanese schools as a mechanism to maintain indoor environmental quality. A
7
field survey was conducted in two high schools in central Taiwan, one adopting a
8
centralized management for control of AC use while the other employing a user-
9
controlled approach, to investigate the influence of AC management scheme on thermal
10
perception as well as the behaviours of AC usage. As the results show, the AC usage was
11
significantly affected by the managerial scheme selected for AC control. When the AC
12
was in use, the mean operative temperature (top) in the classroom regulated under central
13
management was 2.9°C less than the value observed in user management. The
14
cumulative hours of AC operation in central management was three folds of the level
15
recorded in user management. Linear regression of thermal sensation vote against top
16
indicated a greater thermal sensitivity in the user management group to the shift of top.
17
These findings provide insights on the attributes of adaptive thermal comfort which may
18
facilitate design in hybrid ventilation that balances between energy-saving and thermal
19
comfort, particularly in areas of similar weather to that of Taiwan.
1
1
Introduction
2
3
In Taiwan, nowadays nearly all of the commercial buildings and over 80% of the
4
residential buildings are equipped with air-conditioning (AC) as a strategy to
5
accommodate the hot-and-humid local weather during summer time, and people are
6
accustomed to maintaining a thermally comfortable indoor environment by means of
7
mechanical cooling. In contrast, the school buildings constructed in Taiwan in decades
8
before the new millennium were typically designed to be climatically controlled by
9
means of natural ventilation. With the global warming and urban heat island effect
10
spreading, it became evident that the natural ventilation alone was not sufficient to
11
develop and maintain an indoor environment of adequate thermal comfort, especially in
12
the urban areas of hot-and-humid weather [1,2]. The issue of thermal comfort in the
13
school buildings was amplified in Taiwan by the concern of most parents on the indoor
14
environmental quality (IEQ) in the classroom, which was believed to play a pivotal role
15
in the performance of students in learning. In recent years, the newly developed or
16
retrofitted school buildings and classrooms have evolved to adopt the AC as a
17
mechanism of IEQ control to supplement the natural ventilation. However, this
18
movement inevitably presented a challenge to the ongoing effort in energy conservation
19
and development of sustainable environment, and an urgent task at these schools was to
2
1
examine the options available in energy management, specifically, for reducing the
2
energy consumption from AC use without an excessive cost of thermal comfort among
3
the indoor occupants.
4
Hybrid ventilation system is characterized as a system providing a thermally
5
comfortable internal environment by its alternative use of natural ventilation and
6
mechanical cooling. In hybrid ventilation, the natural ventilation operates as a primary
7
means of maintaining indoor thermal comfort, while the mechanical cooling functions as
8
an auxiliary to reduce the indoor temperature when an acceptable level of thermal
9
comfort level cannot be supported by natural ventilation alone [3]. The hybrid
10
ventilation presented a strategy of IEQ management that balanced the consumption of
11
energy used in building maintenance with the thermal comfort required by the indoor
12
occupants [4]. Its effectiveness as a mechanism of IEQ control was documented by the
13
International Energy Agency (IEA) Energy Conservation in Buildings and Community
14
Systems Programme in 2002 [5]: in the European countries a reduction in energy
15
expenditure for over 50% was observed in the office buildings where hybrid ventilation
16
was practiced.
17
As the indoor thermal comfort in relation to energy expenditure in hybrid-ventilated
18
buildings became a prominent area of research interests, a focal point had emerged on
19
developing models that described the patterns of using environmental control
3
1
mechanisms, including windows, fans and AC, by the occupants of hybrid-ventilated
2
buildings as a strategy of thermal adaptation [6-14]. A parallel emphasis was to analyze
3
the potential of hybrid ventilation on energy saving in balance with thermal comfort
4
when applied in different countries or areas of distinct climatic patterns. A few examples
5
include: the study on energy saving by hybrid ventilation in office buildings in the arid
6
climate conducted by Ezzeldin et al. [15], the evaluation of hybrid ventilation
7
performance in France by Cron and Inard [16], and the comparison of effectiveness of
8
natural, mechanical, and hybrid ventilation in Greece by Niachou et al. [17]. Studies of
9
similar aims were also pursued by Utzinger in the US [18], by Franks et al. in
10
Switzerland [19], by Mumovic et al. in England [20], by Haase and Amato in Hong
11
Kong [21], and by Ji et al. in southern China [22]. In areas of sub-tropical, warm-and-
12
humid weather similar to that of Taiwan, hybrid ventilation has been shown to facilitate
13
a reduction in energy expenditure in buildings of various designs and purposes [21,22].
14
With its attributes in occupant-initiated environmental control and energy saving, the
15
hybrid ventilation system gradually replaced the entirely AC-dependent ventilation and
16
became a significant means of promoting indoor thermal comfort at school in Taiwan.
17
The cost associated with AC use has been shown to be a factor dictating the choice
18
of strategy adopted in the management of indoor environmental control. Hwang et al.
19
[23] observed that in Taiwan the occupants of residential and office buildings would find
4
1
the thermal status of an indoor environment typically considered as hot thermally
2
acceptable when the occupants had to use AC at their own expense. Evaluating the
3
effectiveness of an interactive system allowing for a direct control of AC by the
4
occupants, Murakami et al. [24] also reported that the user request-commanded AC
5
operation could achieve 20% more energy saving compared to the level of energy
6
consumption when the AC was regulated by centralized control and maintained at a
7
constant 26°C. In Taiwan, as schools continued to adopt hybrid ventilation and phase out
8
a complete reliance of IEQ adjustment on AC, these schools also became aware of the
9
financial burden borne in AC use and the dominating effect of managerial scheme
10
selected in indoor ventilation on the actual usage of AC. For a majority of these schools,
11
the cost of AC usage is now shared by the users, i.e., the students, and two managerial
12
strategies have been implemented in the actual AC operation. The first strategy employs
13
a scheme of centralized management for AC usage (hereafter referred to as “central
14
management”), in which each student was charged a fixed amount of AC user fee and
15
the timing and intensity of actual AC use were determined by the school. Typically, the
16
school based its decision of AC use on the mean daytime ambient temperature projected
17
by the local weather service. In contrast, the second strategy adopted a fee-for-service
18
feature to allow for a direct control of AC by the students (“user management”). Using a
19
charge card pre-deposited by the students occupying the classroom, the students could
5
1
activate or end an AC operation at their will through a control device installed by the
2
school (Fig. 1); after each usage the users were informed of the expense generated from
3
the AC usage via the digital display shown on the device.
4
5
6
7
8
9
10
11
12
Fig. 1.
The electronic device installed by school for control of air-conditioning (AC)
13
in the classroom investigated in the study and the charge card (inserted into
14
the device on the right) pre-deposited by the occupants for activating AC. This
15
fee-for-service scheme of AC management was commonly practiced in high
16
schools in Taiwan.
17
18
19
Here we presented a study that investigated the thermal perception of students and
their choice of using AC in two hybrid-ventilated classrooms, one adopting the central
6
1
management and the other the user management scheme. Through field survey and
2
simultaneous measurement of microclimatic variables, this study cross-analyzed the
3
thermal perception of the students under different management schemes against the
4
patterns of AC usage in relation to the thermal status in the classroom, with an ultimate
5
goal of realizing the influence of the strategy selected in AC management in the hybrid
6
ventilation on the behaviours of thermal adaptation of students as well as on the energy
7
utilization.
8
9
Materials and methods
10
11
The study was conducted in two high-school classrooms each located on a different
12
campus in central Taiwan. Both classrooms used a hybrid ventilation system for IEQ
13
control, in which the natural ventilation was achieved by cross ventilation of air flow
14
and facilitated by ceiling fans as needed. However, when the natural ventilation alone
15
was considered insufficient for adjustment of the thermal condition inside the classroom,
16
as would be the case in typical summer months in Taiwan, the AC unit equipped in the
17
room was used to cool down the room and re-establish thermal comfort of occupants.
18
One of the campuses used the central management approach for AC management,
19
whereas the other allowed the students to take entire control of the AC use through a
7
1
user management scheme. In the former case, the AC was available for use after mid
2
May (in 2011, the 16th of May) as a school policy, with a daily decision of actual use
3
made each morning by 8 am by the school’s Energy Management System (EMS)
4
according to the local daytime temperature projected by Taiwan’s Central Weather
5
Bureau. If a decision to use AC was made, the message was delivered to the students
6
through an electronic indicator installed in the classroom. The EMS did not
7
automatically run the AC until someone turned on the unit, and once the class ended, the
8
EMS system shut down the AC supply. In the case of user management, the school did
9
not have specific regulations in place to control the AC utilization. Rather, it was entirely
10
the students’ decision by vote to determine if the AC should be used. The two
11
classrooms selected for this study were comparable in their layout, size, and seating
12
arrangement. The AC operated in both locations was a direct expansion split AC unit.
13
In this study the field survey was conducted in classrooms in 2011 from the 16th of
14
May to the 30th of June, the last day of the spring semester, five days a week when the
15
school was in session. This period was selected as it was the time of the year in Taiwan
16
when the local weather gradually transitioned from cool spring to early summer, in
17
which the intermittent use of natural ventilation and mechanical ventilation (including
18
the AC) was considered sufficient to provide the ventilation needed for cooling the
19
building. During this period, as shown in Fig. 2, the ambient temperature in the second
8
1
half of May ranged from 20 to 30°C and while in June from 25 to 35°C. The relative
2
humidity was distributed between 50 and 90%. At the start of summer 2011 in July, there
3
would be days of the daytime ambient temperature exceeding 38°C, and the AC would
4
be heavily relied on for cooling the inside of the building. The subjects surveyed were
5
the students of an age of 15-16. The field study consisted of three components: the
6
continuous measurement of thermal condition in the classroom, a questionnaire-based
7
survey evaluating the perception of the students on the classroom’s thermal status, and
8
the on-site observation for the use of windows, ceiling fans, and AC in the adjustment of
9
indoor microclimate. To determine the indoor thermal condition, environmental
10
variables indicative of indoor thermal status including the air temperature, relative
11
humidity (RH), globe temperature, and wind speed were continuously monitored. The
12
air temperature, RH, and globe temperature were recorded using CENTER 314
13
Temperature/Humidity Datalogger (Center Technology Corp., Taipei, Taiwan) and the
14
wind speed using a hot-wired omni-directional DeltaOHM thermo-anemometer
15
HD2103.2 (DeltaOHM, Italy) in accordance with the ISO 7726:2001 requirements on
16
equipments for evaluating the status of thermal environment [25]. Five spots in the
17
classroom and one outside were selected as points of climatic monitoring.
18
19
Temperature (°C)
RH Range
Temp Range
Temp Mean
40
35
RH Mean
190
9
170
30
150
25
130
1
2
3
4
5
6
7
Fig. 2.
Distribution of ambient temperature and relative humidity (RH) in Taichung,
8
Taiwan from mid May to June in 2011.
9
10
The perception of students toward the thermal condition in the classroom was
11
gauged as a rank in the thermal sensation, thermal preference, and thermal acceptability
12
of the occupants using a questionnaire including all three perception scales (Fig. 3). The
13
questionnaire survey was conducted twice a day, once in the morning around 10 am and
14
the other in the afternoon at about 2 pm when the classroom was in use. During the
15
course of the study, a total of 941 questionnaires were collected. The students were also
16
requested to record by the hour the use of AC so that the patterns of AC usage could be
17
explored. Fig. 4 exemplified a typical record for the use of environmental controls as
18
reported by students in the classroom from 8 am to 5 pm.
19
+3
Hot
+2
Warm
+1
Slightly warm
0
Neutral
+1
Acceptable
10
+1
0
Cooler
No change
1
2
3
4
5
6
7
Fig. 3.
Three scales of thermal perception employed in the questionnaire in this study
8
for evaluating thermal perception of the students toward indoor thermal
9
condition.
10
08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00
11
Classroom
Windows
12
Fans
AC
13
in use/opened
not in use/closed
14
15
Fig. 4.
Example of record examining use of environmental control mechanisms
16
including window-opening, ceiling fans, and air-conditioning (AC) in the
17
investigated classrooms.
18
19
Indoor thermal condition in relation to hybrid ventilation
11
1
2
As there were no spatial variations observed in the monitored microclimatic
3
variables in both of the evaluated classrooms, for each variable the average level
4
measured at the five sampling locations in the classroom was used to represent the
5
instantaneous thermal status of the classroom at the time of measurement.
6
Fig. 5 shows on a psychometric chart the distribution of the operative temperature
7
(top) inside the classrooms using different managerial schemes for AC control during the
8
investigated period. It also depicts how the AC was used as a part of hybrid ventilation
9
in the classrooms. The top and RH in the classroom under user management ranged
10
between 25 and 32°C and between 45 and 95%, respectively; while their counterparts in
11
the classroom regulated by central management were 25-31°C and 45-95%. Although
12
the upper/lower bounds of top corresponding to the usage of hybrid ventilation were
13
approximately the same in both schemes; the air temperature set to activate the AC and
14
the hours accumulated on the AC use were significantly different. During the course of
15
the study, the mean top measured in the classroom of user management was 29.3°C,
16
1.8°C higher than the mean top (27.5°C) in the classroom of central management. When
17
only the top measured during active AC use was compared, the mean top in the classroom
18
of user management was 30.3°C, 2.9°C higher than the level (27.4°C) observed in the
19
classroom of central management.
12
1
AC not in use
3
5
6
0.020
Relative Humidity (%)
4
0.025
AC in use
0.015
100%
80%
0.010
60%
40%
0.005
20%
20.0
7
25.0
t op (oC)
30.0
0.030
(b)
8
0.025
AC not in use
AC in use
Relative Humidity (%)
9
10
11
12
0.020
0.015
100%
80%
0.010
60%
40%
20%
20.0
0.005
25.0
30.0
o
0.000
35.0
t op ( C)
13
14
0.000
35.0
Moisture Content (kg/kg Dry Air)
2
Moisture Content (kg/kg Dry Air)
0.030
(a)
Fig. 5.
Air-conditioning (AC) use in hybrid ventilation in relation to status of thermal
15
condition as characterized by the operative temperature (top), moisture content,
16
and relative humidity in the classrooms exercising: (a) central management;
17
and (b) user management.
18
19
When the hours of cumulative AC use in the classrooms controlled by different AC
13
1
managerial schemes were compared to their respective total hours of hybrid ventilation,
2
the percentage of the AC use operated by central management was almost three folds of
3
the level reported by the AC use managed by the users (Fig. 6). These results suggested
4
that: when the occupants were in control of the ventilation mechanism, the fee-for-
5
service characteristic of the user management scheme was conducive to a lower AC use
6
compared to the energy utilization managed in a centralized manner, and the
7
8
AC Use in Hybrid Ventilation (%)
80
9
10
11
12
Central management
60
User management
40
60
20
37
34
21
0
13
ASHRAE Actual use
Standard 55
ASHRAE Actual use
Standard 55
14
15
Fig. 6.
Percentage of actual air-conditioning (AC) use in hybrid ventilation in the
16
classrooms of AC control operated under user vs. central management and
17
projection of AC use based on ASHRAE Standard 55 thermal comfort criteria.
18
19
occupants were thermally more adaptive to the indoor thermal status. Fig. 6 also
14
1
illustrated the deviation of the hours during actual AC use from those projected by the
2
adaptive comfort model prescribed in the ASHRAE Standard 55-2010 [26]. Again,
3
under central management the actual AC use was much greater than the level required
4
for the occupants to attain thermal comfort in the classroom.
5
6
Usage of air-conditioning
7
8
The ambient temperature is recognized as a main stimulus for the occupants in an
9
indoor thermal zone to use environmental control mechanisms for regulating the thermal
10
status so to attain a desirable level of thermal comfort [27]. Fig. 7 shows the proportion
11
of AC usage in the overall hours of hybrid ventilation in relation to the outdoor
12
temperature in the classrooms controlled by the user vs. central management. In both
13
managerial scenarios, a sigmoidal relationship of increasing AC use tracking the
14
increase in the outdoor temperature was observed, indicating the existence of a threshold
15
in the local ambient temperature beyond which the AC would be relied upon as a major
16
mechanism of ventilation and thermal adaption. To describe in quantitative terms the
17
usage of AC as a strategy of thermal adaption, the stochastic model developed by Nicol
18
[27] and applied in various studies [8-11] characterizing the probability of indoor
19
id Ventilation (%)
100.0
User management (actual)
Central management (actual)
80.0
15
User management (forecasted)
Central management (forecasted)
60.0
1
2
3
4
5
6
7
Fig. 7.
The usage of air-conditioning (AC) as in percentage of overall hours of hybrid
8
ventilation in relation to the outdoor temperature for the classrooms controlled
9
by user vs. central management or in probability as forecasted following
10
Nicol’s stochastic model [27].
11
12
occupants using specific environmental controls was applied in this study to explore the
13
patterns of AC use by the students in the classroom.
14
In accordance with Nicol’s algorithm, the probability of the AC being switched on
15
(p) in relation the mean hourly outdoor temperature () could be defined by the
16
following Logit model:
17
18
 p( ) 
  a  b
log 
 1  p( ) 
or
p( ) 
16
exp( a  b)
1  exp( a  b)
(1)
1
2
where p() was the probability of the AC being used in response to the outdoor
3
temperature of , and a and b were constants. For the usage of AC in the classrooms
4
controlled by user vs. central management in this study, the Logit model as described in
5
Eq. (1) was re-defined to generate:
6
7
8
 p( ) 
User management: log 
  1.04    36.38 , R2 = 0.86
 1  p( ) 
(2)
 p( ) 
Central management: log 
  0.78    24.01 , R2 = 0.80
 1  p( ) 
(3)
9
10
As Fig. 7 demonstrated, the likelihood of AC being used in the classroom in
11
relation to the ambient temperature, as interpreted from Eqs. (2) and (3), closely tracked
12
the distribution of the AC usage from the on-site observation in the studied classrooms.
13
The AC would be used at all time in hybrid ventilation if the ambient temperature rose
14
to levels beyond 32 and 36°C in cases of central and user management, respectively. In
15
addition, the distribution frequency of AC use as influenced by the ambient temperature
16
exhibited a steeper slope in the classroom where the AC was user-controlled, suggesting
17
a greater sensitivity of students to the thermal stimulus under the user management
17
1
2
scheme.
As far as energy-saving in relation to thermal comfort was concerned; an aspect of
3
interest was the difference in the ambient temperature by which the AC would be
4
activated between these two managerial schemes. In the classroom where the users were
5
given the choice to determine the timing of AC activation, the AC was hardly used when
6
the outdoor temperature was below 30°C, which was 5.0°C higher than that was
7
observed in the classroom employing the central management. The lower usage of AC in
8
the user management group perhaps was associated with the fee-for-service feature of
9
the managerial scheme. The students in this case were constantly reminded by the
10
display of cost on the AC control panel installed in the classroom that the usage of AC
11
would be an expensive strategy of thermal adaption. As a result, the usage of AC was
12
largely reduced.
13
The inference presented above was also evidenced previously in Fig. 6, where the
14
actual hours of AC use as controlled by the users (21%) was approximately one-third of
15
the level observed when the AC was controlled via central management (60%) and much
16
lower than the level predicted using the ASHRAE Standard 55-2010 thermal comfort
17
criteria (34%). In comparison, the actual use of AC under central management was 23%
18
greater than the level projected following the ASHRAE criteria (37%) as required for
19
thermal adaption of the occupants. These findings clearly indicated that the mechanism
18
1
employed in management of AC usage played a significant role in how the students
2
reacted to the status of the indoor thermal environment. In the fee-for-service control,
3
the incentive from saving on the electricity cost resulted in an increase in the tolerance
4
of the occupants to the thermal stimulus indoors. In contrast, the greater exercise of
5
cooling by AC in the central management than the level considered adequate by the
6
ASHRAE criteria suggested that perhaps a higher threshold in the temperature for
7
activating AC use could be targeted in the current practice of central management.
8
9
Influence of air-conditioning management on thermal perception
10
11
Fig. 8 compared the distributions of thermal sensation vote (TSV) among the
12
students in the classrooms where the AC was controlled by different management
13
schemes. To better visualize the sensitivity of the thermal perception between different
14
groups in response to the shifting of indoor thermal status, the TSV were grouped by top
15
at an interval of 0.5°C, and then weighted by sample size and linearly regressed to the
16
top. The linear regression yielded the following models:
17
18
User management scheme:
TSV = 0.316 × top – 9.387, R2 = 0.75
19
19
(4)
1
3.0
User management (observed)
2
2.0
Central management (observed)
User management (regressed)
1.0
4
TSV
3
Central management (regressed)
0.0
-1.0
5
-2.0
6
-3.0
23.0
25.0
27.0
29.0
31.0
33.0
t op (oC)
7
8
Fig. 8.
Distributions of thermal sensation vote (TSV) against operative temperature
9
(top) as observed among indoor occupants in the classrooms operated under
10
user vs. central management of air-conditioning use (presented in dots) and
11
linear regression of the distributions (in lines).
12
13
Central management scheme: TSV = 0.163 × top – 4.643, R2 = 0.72
(5)
14
15
Between these two models, the thermal sensation of the students toward the
16
temperature was similar when the top was near the high end of the range measured in this
17
study (top ≥ 30°C), regardless of the managerial mechanism employed in the AC control.
18
However, the TSV between these managerial groups deviated from each other
19
increasingly as the top decreased. At most of the top values recorded, the students in the
20
1
central management group had a higher TSV, i.e., a warmer sensation, than the level
2
expressed by the students in the user management group. This difference increased
3
significantly as the top decreased. The slope in Eqs. (4) and (5) indicates the sensitivity
4
of the occupants in their thermal perception to the change in the environment. The
5
slope in the regression model for the user management group (0.316) was greater
6
than its counterpart for the central management group (0.163), suggesting that the
7
students in the user management group responded more sensitively to a shift in top.
8
The greater sensitivity among the students in the user management group was likely
9
associated with a heightened awareness of the fee-for-service feature in the AC use,
10
which constantly reminded the students to cut down on the AC use and thus on their
11
expense for energy consumption unless an extreme heat was encountered in the
12
classroom. In comparison, the students of the central management group were not
13
immediately concerned with the cost generated from the AC use, and they appeared to
14
be less sensitive with when and how the AC was used. In fact, the students in the central
15
management group were prepared for an indoor environment that was colder than they
16
would feel comfortable with as a result of AC use ― frequently they went to the class
17
with an additional shirt or vest as a personal approach to adapt to the cold environment.
18
These observations revealed the significance of the energy control mechanism in how
19
the users of an indoor environment would personally adapt to the thermal environment,
21
1
on both the psychological as well as on the behavioural level. The thermally neutral top
2
(TSV = 0) for the user and central management groups was 29.7 and 28.4°C,
3
respectively, with the thermal neutrality for the central management group being 1.3°C
4
lower than that of the user management group.
5
When the percentage of the thermal acceptability vote (TAV) cast by different
6
managerial groups was compared (Fig. 9), the proportion of the total TAV that identified
7
the indoor thermal condition as “acceptable” was found to be greater in the user
TAV in Acceptable Category (%)
8
9
10
11
12
13
80
60
40
User management (observed)
Central management (observed)
20
User management (regressed)
Central management (regressed)
0
23.0
25.0
27.0
29.0
31.0
33.0
top (oC)
14
15
100
Fig. 9.
Distributions of thermal acceptability vote (TAV) against operative
16
temperature (top) as observed among indoor occupants in the classrooms
17
operated under user vs. central management of air-conditioning use (presented
18
in dots) and linear regression of the distributions (in lines).
19
22
1
management group than in the central management group at most of the top measured in
2
the study. The rate of acceptance in the user management group was always over 80%
3
when the top was less than 30°C, whereas its counterpart in the central management
4
group was consistently below 80% in the range of top surveyed. When the top decreased,
5
the rate of acceptance for the thermal condition in the classroom where centralized AC
6
management was administered became increasingly lower than that in the classroom
7
where the AC was user-controlled.
8
9
Fig. 10 shows the percentage change in the thermal preference vote (TPV) cast by
the students in the category of “preferring change” in response to alteration of top. To
10
visualize the choice of the students in adjusting the thermal status, the “preferring
11
change” votes were further divided into the “preferring cooler” and “preferring warmer”
12
sub-groups. Within the range of the top observed in the study, at any point of the top
13
where votes of preferring change were available from both AC management groups, the
14
percentage of cooler preference for the central management was always greater than its
15
counterpart cast by the user management. However, it was also noteworthy that the
16
percentage of those who preferring a warmer environment in the central management
17
was overall greater than the level preferring warmer in the user management group when
18
the top was lower than the thermally neutral level, 28.4°C, determined for the central
19
management group. In fact, at temperature close to the commonly recognized room
23
1
100%
100
Cooler (user management)
TPV in Category of Change (%)
2
80%
80
3
Warmer (user management)
Cooler (central management)
Warmer (central management)
60%
60
4
40%
40
5
20%
20
6
0%0
23.0
25.0
27.0
29.0
31.0
33.0
top (oC)
7
8
9
Fig. 10.
Distributions of thermal preference vote (TPV) divided into groups of cooler
and warmer preference against operative temperature (top) as observed among
10
indoor occupants in the classrooms operated under user vs. central
11
management of air-conditioning use.
12
13
temperature, 25°C, the proportion of the students in the central management group
14
inclined for a warmer environment rose to over 20%, exceeding the ratio of the students
15
in the same group expressing a cooler preference. As the AC was more frequently used
16
in the central management scheme at top below thermal neutrality than it was in the user
17
management scheme (Fig. 5), the elevated warmer thermal preference witnessed among
18
the students in central management was likely attributed to the use of AC in the
19
classroom at a generally moderate top. In another word, in the case of central
24
1
management, the students might not choose to raise the temperature indoors or turn off
2
the AC even though they perceived the thermal environment as cold and preferred a
3
warmer condition. This interpretation was also supported by the relatively lower TAV in
4
the central management group than the level identified for the user management group
5
when the top approached the low end of the measured range (Fig. 9). The increase in the
6
difference of TAV between these groups at top lower than thermal neutrality was
7
probably, once again, rooted in the excess use of AC when the users in central
8
management were not financially concerned with proportionally sharing the AC cost. In
9
comparison, under user management the AC was used infrequently when the indoor
10
temperature was below the top that they considered as thermally neutral (29.7oC),
11
demonstrating the effectiveness of the fee-for-service strategy of AC control in
12
promoting energy conservation. These findings presented evidences that led us to
13
believe: for the many schools in Taiwan currently practicing a centralized management
14
of AC control, a more vigorous regulation on the conditions of AC use could be applied
15
to achieve better energy conservation without compromising the thermal comfort for the
16
students using the classroom.
17
While the hybrid ventilation provides an alternative to the conventional
18
management of AC usage that is more conducive to energy conservation, as of current
19
its full operation as an AC control mechanism on campus may be subject to constraints
25
1
inherent in the architectural and interior designs of the building. For instance, limited by
2
land availability, many campuses located in the urban areas of Taiwan comprise of high-
3
rise buildings in which natural ventilation via the windows is not an option. In addition,
4
often time the power distribution in these buildings was initially fitted to central
5
management and the retrofitting for individualized control in each compartmentalized
6
classroom was considered cost-ineffective and thus discouraging to the management.
7
Furthermore, for schools located in areas enriched with business activities, the concern
8
of the IEQ inside the classroom being deteriorated by noise and air pollution outside
9
frequently renders the natural ventilation a less desired mechanism of IEQ maintenance.
10
Although the fee-for-service feature of the user management of AC as evidenced in
11
this study provides a strong incentive for energy conservation, for this mechanism to
12
properly work on a long-term basis, the student body using the classroom is best to have
13
a consistent makeup in its members so that voting on AC use can be conducted
14
constantly or daily. This may prove to be difficult if this approach is to be applied on the
15
campus of university, as the university students, even within the same academic
16
department, typically have diverse class schedules and thus the makeup of the students
17
using each classroom changes during the day. In addition, as observed in this study, the
18
mean top measured in the classroom of user management during active AC use was
19
30.3°C, 2.9°C higher than its counterpart (27.4°C) in the classroom where central AC
26
1
management was administered. While the fee-for-service control of AC usage appears to
2
enhance thermal tolerance of the students, a concern with IEQ may also arise as to the
3
hygienic conditions in a classroom of 30-40 people of a preference to circulating the air
4
mainly by natural ventilation and using fans even at high temperature in hot summer
5
days. This is an issue beyond the goal of our current study but warrants further
6
investigations.
7
8
Conclusion
9
10
This study evaluated the influences of the scheme selected in managing AC usage
11
in hybrid ventilation on the thermal perception as well as the corresponding behaviours
12
in AC use of the indoor occupants. The followings are a summary of key findings from
13
the study.
14
As the results show, the mechanism employed in managing the use of AC played a
15
significant role in how the students reacted to the variation in the indoor microclimate.
16
The patterns of AC use by the students were significantly influenced by the managerial
17
scheme selected in AC control. During the course of the study, the mean top in the
18
classroom where centralized AC management was administered was 27.5°C, 1.8°C
19
lower than the 29.3°C determined in the classroom managed under the fee-for-service
27
1
scheme. When the AC was in active use, the mean top in the classroom operated under
2
central management was 2.9°C less than the value measured in the user-controlled
3
classroom. The cumulative hours of AC use in the central AC management was
4
approximately three times the level observed in the user management.
5
The study findings also indicated a significant impact of the AC control scheme to
6
the thermal perception of the students toward the thermal status in the classroom.
7
Through linear regression of the TSV to the top, the students in the user management
8
group were found to be more thermally receptive to the shift in top than those in the
9
central management group, and the difference in thermal perception between these
10
groups was amplified when the top decreased to near the low end of the measured top.
11
The top corresponding to thermal neutrality in the user management group was 1.3°C
12
more than the level determined for the central management group. The user management
13
group was also more adaptive in terms of accepting the thermal condition in the
14
classroom than was the central management group.
15
Overall, both schemes of AC control as currently practiced in many Taiwanese
16
high schools and discussed in this study addressed the fundamental objective of applying
17
hybrid ventilation as a means of IEQ control ― reducing the energy consumption by
18
AC use while delivering an indoor environment of thermal comfort. However, based on
19
the findings reported here, we suggest that the schools currently employing a central
28
1
management of AC control in Taiwan should target a higher threshold in the air
2
temperature for activating AC use or consider switching to the user management scheme
3
so to achieve a better conservation of energy while still maintaining an adequate level of
4
thermal comfort for the students.
5
6
Acknowledgments
7
8
9
10
We offer our sincere appreciation for grant support from the National Science
Council of Taiwan under the project number NSC-102-2221-E-239 -028. All authors
contributed equally in the preparation of this manuscript.
11
12
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