2011 3rd International Symposium & Exhibition in Sustainable Energy & Environment, 1-3 June 2011, Melaka, Malaysia
Thermal Comfort in Air-Conditioned Learning Environment Siti Rohana Mohd Yatim#1, Meor Ahmad Mustaqim Meor Mohd Zain#2, Fairus Muhamad Darus*3, Zitty Sarah Ismail*4 #
Faculty of Health Science, Universiti Teknologi MARA, 42300 Puncak Alam, Selangor, Malaysia 1
[email protected] 2
[email protected] * Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia 3
[email protected] 4
[email protected]
Abstract —This paper appraises the thermal comfort condition of in air conditioned learning environment of the new learning campus in Universiti Teknologi MARA, Puncak Alam. The physical measurement and survey questionnaires have been thoroughly measured among the occupants (235 respondents) in four air conditioned classrooms along with four lecture theatres. The result shows relative humidity and air velocity are within the recommended limit while operative temperature for both classrooms and lecture theatres were below recommended limit. The calculated Prediction Mean Vote (PMV) - Predicted Percentage Dissatisfied (PPD) indices computed merely 29% has shown dissatisfaction towards learning environments. In addition, computed Thermal Sensation Vote (TSV) for classroom divulges that people remarked the learning environment for classroom and lecture theatre to be ‘slightly cool’. However, the mean value of thermal comfort based on Predictive Mean Vote (PMV) for classroom was -0.05, which indicate ‘neutral’ condition, whereas PMV value for lecture theatres was -0.62 which is ‘slightly cool’. Therefore, those findings show calculated thermal states have not necessarily correlate to occupant’s desired thermal state. Keywords— Thermal comfort, PMV-PPD, TSV, classroom, lecture theatre.
I. INTRODUCTION The Thermal comfort is an important aspect in the establishment of comfort in a building. In Malaysia, education systems are very imperative though sometimes encountered transitions in the recent years. Many new buildings are being constructed or renovated in the programs of expansion of the campus. It also includes the installation of windows, wall, and ventilation systems such as an air conditioner in those buildings in order to provide thermal comfortable condition in classrooms, and in campus. The tropical climate in Malaysia is hot and humid with average temperatures between 23.7ºC to 31.3ºC throughout a day with the highest maximum recorded as 36.9ºC and the average relative humidity between 67% to 95% and this may have an adverse impact on occupant comfort indoor [1], [2]. There is increasing concern over the quality of the indoor environment as the standard of living improves in society. This is of paramount importance for buildings in the tropics where air-conditioning of the indoor environment is not uncommon [3].
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Thermal condition in learning environment has to be considered carefully not only because of the high occupant density in lecture theatres or classrooms but also the negative influences that an unsatisfactory thermal environment has on learning and performance [4], [5]. Thermal discomfort such as overheated or too cold classrooms can be associated to physical stress (thermal stress) and therefore be responsible for illnesses and poor performance of the students [6]. In fact, students have to spend lots of time listening and understanding lessons, remaining sitting at their desk without have a change to modifying and adjusting their activity level or the functioning parameters of the air-conditioned systems or to open/close the windows according to the thermal environment during the lesson time [5]. The objective of this paper is to evaluate the thermal comfort in classrooms and lecture theatres by applying objective measurement and subjective assessment. The predicted mean vote (PMV) index, predicted percentage dissatisfied (PPD) index and comfortable temperature are calculated and compared to the results from subjective assessment of thermal comfort in the air-conditioned learning environment. II. METHODOLOGY A. Sites This study was conducted in selected lecture theatres and classrooms in the building of Faculty of Health Sciences (FSK), Universiti Teknologi MARA (UiTM), Puncak Alam Campus. Four (4) classrooms (BK4, BK 31, BK 37 and BK 50) and four (4) lecture theatres (DK 3, DK 4, DK 5 and DK 6) were selected in this study. Those classrooms are located at FSK 6 block and comprised of twelve bulb of fluorescent lights and reflectors and also four units of split air-conditioning at the ceiling. The size of these classrooms is 74 m2. Meanwhile Lecture theatres were situated at FSK 7 block with a total area of 118 m2 each. Each lecture theatre has of twelve bulbs of fluorescent light and reflectors and also eight units of split air-conditioning at the ceiling. The size of the classrooms and lecture theatres are uniform and represents the learning environment in UiTM Puncak Alam campus. Figure 1 and 2 shows the layout plan of the classroom and the lecture theatre.
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of thermal comfort they experience and the clothes they wore were also collected by this administrative questionnaire. The questionnaire of the thermal comfort judgments vote of respondents was based on the seven points psycho-physical thermal comfort scale of ASHRAE. C. Field measurement Measurements were carried out using QUESTemp° 36 where it was placed 1.1 m from the ground at the centre of the classrooms and lecture theatre. For lecture theatre, the measurements were taken periodically at the front, centre and back. Simultaneous measurements of air temperature (Ta), mean radiant temperature (Tmrt), air velocity (v) and relative humidity (Rh) were continuously made for 2 hours with 10 minutes interval because each class typically was of 2 hours in duration. Fig. 1: Floor plan of classroom
III. RESULT AND DISCUSSION A. Field measurement Measured values of thermal comfort parameters are tabulated in Table I. Operative temperature (Tn) is a weighted mean of the air temperature and the mean radiant temperature, the weights being in proportion to the convective and the radiant heat-transfer coefficients of the clothed human body. TABLE I OPERATIVE TEMPERATURE, RELATIVE HUMIDITY AND AIR VELOCITY IN CLASSROOM AND LECTURE THEATRE
Parameter
Classroom Mean Ranges ± SD 22.0 20.7 – 23.1 ± 1.1
Lecture Theatre Mean Ranges ± SD 21.2 19.3 – 24.1 ± 2.2
Relative Humidity (%)
70.8 ± 6.9
66.4 – 78.8
72.1 ± 7.9
61.3 – 79.0
Air Velocity (m/s)
0.20 ± 0.0
0.20
0.3 ± 0.1
0.2 – 0.4
Operative Temperature (oC)
Fig. 2: Floor plan of lecture theatre
B. Subjective assessment A questionnaire survey was conducted during the period of the physical measurements. Total respondents involved in this survey were 235 (62 male and 173 female) with average age of 18 to 27 age years old. The average numbers of students were 20 to 30 students and 30 to 50 students for classroom and lecture theatre respectively. The questionnaire used in this study was adopted from Cheong, et al. [3]. Prior to the survey, the subjects would have been seated at their chairs for approximately 30 min to 1 hour and they were briefed to answer the questionnaire booklet at least 30 minutes after they entered the class. The judgment of respondents about the level
Note: SD – Standard deviation
The mean operative temperature for classrooms was 22.0 C with the range between 20.7 oC – 23.1 oC. While, the mean operative temperature for lecture theatres were 21.2 oC with the range between 19.3oC – 23.7 oC. For the air-conditioned buildings, the ASHRAE Standards 55 [7] recommended range was 23oC – 26oC. In this study, the mean operative temperature for both classrooms and lecture theatre were no within the suggested ranges. The mean relative humidity for classrooms and lecture theatres was 70.8 % and 72.1 % with the range between 66.4 % – 78.8 % and 61.3 % - 79.0% respectively. Air velocity seems to be almost consistent in all classrooms. In the
o
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lecture theatres, the mean air velocity was 0.3 m/s with the range between 0.2 m/s – 0.4 m/s B. Subjective assessment Figure 3, 4 and 5 showed the frequency distribution of subjective response on temperature, humidity and air movement for both classroom and lecture theatre by occupants. Figure 3 shows that the respondents have voted bias towards the ‘slightly warm’ and ‘warm’ of the 7-point scale for classroom and lecture theatre respectively. There were 76% individuals claimed that temperature in classroom is ‘slightly warm’, ‘warm’ and ‘hot’, while 66% individuals responded similar vote for lecture theatre. The ASHRAE Standard 55 [7] specified that an acceptable thermal environment should have 80% of occupants voted for the central three categories (slightly cool, neutral, and slightly warm). In this study, only 60.0% and 64% voted within the central three categories showing that both learning environment were not in thermal acceptable conditions. These may due to different thermal sensations among occupants even in the same environment. These findings show not all occupants were satisfied with indoor thermal environment even though all surveyed locations were equipped with air-condition to provide comfortable learning environment. Figure 4 shows the frequency distribution of subjective response on humidity by occupants. Majority of the occupants agreed that the humidity level in their air-conditioned learning environment was at acceptable level, with 62% individuals at classroom and 48% at lecture theatre voted in just right segment. Figure 5 shows the frequency distribution of subjective response on air movement to be slightly skewed towards the left of the 7-point scale. Majority of the occupants felt the air movement in their air-conditioned learning environment was at acceptable level, with 48% individuals (classroom) and 52% individual (lecture theatre) voted in just right segment. The distribution of the subjective responses on overall thermal comfort is shown in Figure 6. The distribution is skewed towards the ‘comfortable’ regions on the 4-point scale. Half of the occupants felt comfortable with 54% (classroom) and 53% (lecture theatre) individuals voted in the comfortable segment. Meanwhile, about 2% occupants for both learning environment felt uncomfortable.
Fig. 4: Distribution on subjective response on humidity
Fig. 5: Distribution on subjective response on air movement
Fig. 6: Distribution of subjective response on overall thermal comfort.
C. Predicted Mean Vote (PMV) and Predicted Percentage Dissatisfied (PPD) Table II shows mean vote of PMV, PPD and TSV for both classrooms and lecture theatres was -0.05 and -0.62; 7.96 and 20.7; -1.16 and -0.96 respectively.
Fig. 3: Distribution on subjective response on temperature
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TABLE III PREDICTED MEAN VOTE (PMV) AND PREDICTED PERCENTAGE DISSATISFIED (PPD)
PMV
PPD (%)
TSV
Mean
-0.05
7.96
-1.16
SD
0.37
4.26
0.90
Mean
-0.62
20.75
-0.96
SD
0.62
16.29
1.25
Classroom
Lecture theatre
Note: SD – Standard deviation
According to ISO 7730, the comfort range was taken to be the conditions when the PMV has the values between –1 and +1. The mean PMV for classroom was -0.05 and the mean PPD was 7.96. These indicate that the classroom is observed close to ‘neutral’ condition and that approximately 8% of the occupants are expected to express dissatisfaction with the classroom environment. While, the mean PMV and PPD for lecture theatre were -0.62 and 20.75 respectively. These indicate that the lecture theatre is observed close to ‘slightly cool’ condition and that approximately 21% of the occupants are expected to express dissatisfaction with the lecture theatre environment. Table II also shows the occupant’s perception on level of thermal comfort based on Thermal Sensation Vote (TSV). The mean TSV for classroom was -1.16 while for lecture theatre, the mean TSV was -0.96. These indicate the occupants experienced the slightly cool condition in both learning environment. However, the mean value of thermal comfort based on Predicted Mean Vote (PMV) for classroom was 0.05, which indicate the condition was ‘neutral’. While, PMV value for lecture theatre was -0.62, which indicated the condition was close to ‘slightly cool’. Only the lecture theatres show a close agreement when comparing with calculated values (PMV) to the subjective assessment (TSV). These findings show that calculated thermal states do not always correlate to occupant’s preferred thermal state. Previous studies also have concluded that there is a significant bias between PMV and the field result [8], [9], [10]. IV. CONCLUSIONS Providing comfortable learning environment is essential because it can affect teaching and learning process. Poor thermal comfort can be associated to thermal stress and may be responsible for illnesses and poor performance of the students. The main objective of this study is to evaluate the thermal comfort in classrooms and lecture theatres by applying objective measurement and subjective assessment. The result have shown that the operative temperature for both classroom and lecture theatre were not within the suggested ranges by ASHRAE standard [7].
The subjective assessment by occupants also indicated that they were not satisfied with indoor thermal condition for both learning environment. However, the majority of the respondents found that the relative humidity and air velocity that they have experienced to be acceptable. The study also showed that calculate thermal state does not correlate to the occupant’s preferred thermal state. The data and information obtained in this study were based on a short period of three week study and may not true to reflection of the sentiments of the entire student population. Therefore, it is suggested to extend this study with larger sample size and also to other classrooms and lecture theatres which were not covered in this study. ACKNOWLEDGMENT The authors would like to thank management of Faculty of Health Sciences, Universiti Teknologi MARA for the permission and support given. REFERENCES [1]
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