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Small fan assisted air conditioner for thermal comfort and energy saving in Thailand
Energy Conversion and Management 49 (2008) 2499–2504
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Small fan assisted air conditioner for thermal comfort and energy saving in Thailand Surat Atthajariyakul *, Charoenporn Lertsatittanakorn Faculty of Engineering, Mahasarakham University, Khamriang, Kantaravichai, Mahasarakham 44150, Thailand
a r t i c l e
i n f o
Article history: Received 11 October 2007 Accepted 25 May 2008 Available online 18 July 2008 Keywords: Thermal comfort Local air movement Energy saving Fan assist
a b s t r a c t From the fact that Thai people have a tolerance to high air temperature and are accustomed to high air movement from electric fans in non-air conditioned space, this paper proposes the use of small fan assisted air conditioners for human thermal comfort and energy saving in Thailand. In the study, a total 15 students were tested in a 2.5 3.5 2.5 m3 test room equipped with a 12,000 Btu/h split type air conditioner. During the tests, the room air temperature was varied from 25, 26, 27 and 28 °C every 1 h. A small fan with 15 cm diameter was placed in front of each subject. In each hour, the small fan was varied to supply a small area with velocity from 0.2, 0.5, 1, 1.5 and 2 m/s. In each condition, the subjects were asked to vote for their thermal sensation. The results showed that the temperature set point could be increased up to 28 °C when a small fan was used to supply local air velocity from 0.5 to 2 m/s according to individual preference. This would reduce the electricity consumption of the air conditioning unit. According to the proposed method, this can save energy for office buildings in the commercial sector as high as 1959.51 GWh/year. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction At present, Thailand is faced with an energy price crisis because of the increasing costs of imported oil. Therefore, methods of energy saving are very important and urgently needed. In Thailand, year 2005, the commercial sector was the second largest sector consuming electricity by 37,863 GWh/year, which accounted for 33.42% [1]. Office buildings are one group in the commercial sector that is the highest electricity energy consumers [2]. Moreover, air conditioning systems demand the highest electrical energy in all office buildings. Now, almost all buildings trend to turn on air conditioners year-round. As a result, Thailand is looking for solutions to reduce the electricity demand in air conditioning systems for the commercial sector and also for other sectors using air conditioners. Basically, the purpose of the air conditioning system is to provide occupants thermal comfort, which is defined as ‘‘that condition of mind that expresses satisfaction with the thermal environment” [3]. The comfort zone is defined as the range of climatic conditions in which the majority of people would not feel thermal discomfort, either warm or cold. Human thermal comfort depends on six thermal ‘‘quantitative” variables: air temperature, air relative humidity, mean radiant temperature, air velocity, human activity and clothing insulation [4]. However, thermal comfort not only depends on those six variables but also on some ‘‘non quantitative” factors such as mental states, habits, education. * Corresponding author. Tel./fax: +66 43 754 316. E-mail address: [email protected] (S. Atthajariyakul). 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.05.028
Many studies confirm that human comfort preferences vary in different locations and long term experience in any climate [5–8]. This may result in a tolerance to higher temperatures of people in hot and humid climate when compared to people in colder regions. Therefore, Thai people seem to have a tolerance to high air temperature because Thailand is located in a hot and humid region. Air movement might be one of the most useful and least expensive methods to provide a comfortable indoor climate, and also, it can reduce energy consumption in hot regions. The impact of air movement on comfort sensation has been studied continuously [9,10]. However, these researches studied air movement for higher air temperatures than those of Thailand’s climate. Therefore, Khedari et al. [11] studied the potential of providing thermal comfort for Thai people by using commercial electric fans without air conditioners and develop a ventilation comfort chart for Thailand that could be used for designing ventilating systems of non-air conditioned buildings. The study recommended providing air velocity between 0.2 and 3 m/s for air temperature between 26 and 36 °C. However, the study was conducted in a naturally ventilated space. It is not suitable for office buildings where all spaces are air conditioned and leaking air is not allowed for energy saving. Yamtraipat et al. [12] presented the result of a large thermal comfort survey conducted in Thailand. The study recommended setting the temperature set point at 26 °C and 50–60% relative humidity with air velocity of 0.2 m/s as a comfort condition for energy saving. Some research studies [13–15] showed that local heating or cooling could improve subjects’ acceptability of the thermal environment. From the literature reviews, Thai people get used to using an electrical fan in the non-air conditioned space. However, there
S. Atthajariyakul, C. Lertsatittanakorn / Energy Conversion and Management 49 (2008) 2499–2504 3.5 m.
Chair
Small fan
Personal computer
Fig. 2. Room plan of the test room.
2. Field experiment Field measurements were performed in a room of size 2.5 3.5 2.5 m3. This room was located on the top floor of the Faculty of Engineering building in Mahasarakham University which is in the Northeastern part of Thailand. The climate of this region is hot and humid, where the air conditioner is very important, especially in the summer season. A split type air conditioner with capacity of 12,000 Btu/h was hung on the top of the left wall. There were three computers on a long table and three round chairs in the room. In the study, a total of 15 students of Mahasarakham University, comprised of 10 males and 5 females, were tested in this test room. They all wore normal cloths with thermal insulation value of 0.55– 0.6 clo under sedentary activity of 1 met. The students were separated into five groups of three. Each group was tested by sitting in front of the long table. The arrangements of table, small fans and test subjects are shown in Figs. 1 and 2. Three small electric fans with 15 cm diameter were placed on the table in front of each student at a distance of 30 cm from the students. Each group of students was tested in the test room for four hours for one experiment. During each experiment, the room air temperature was changed every one hour from 25 to 26, 27 and 28 °C. At every hour, while the temperature was kept constant, the small fan for each student was turned on and was gradually adjusted in speed by a speed controller to supply local air velocities from 0.2, 0.5, 1, 1.5 and 2 m/s every 10 min. However, at each hour, when the temperature set point is changed, it is necessary to wait about 10 min for the temperature to change grad-
30 cm.
Air conditioner
were no studies of the use of small fans as local air movement assisted air conditioner for human comfort and energy saving. The objective of this study is to study the effect of local air movement from a small electric fan used in an air conditioned space on human comfort and to propose a simple and practical method to reduce energy consumption of air conditioning systems in office buildings while thermal comfort is still maintained. The method of study on thermal comfort presented in this paper used a small number of subjects, but the result is in quite good agreement when compared to another study with a large number of subjects. It is expected that the proposed method can be applied to other commercial buildings as well as in residential buildings where the proposed method can be implemented. It is hoped that the proposed method will be the easiest way and least investment for electrical energy consumption reduction for all air conditioning systems.
2.5 m.
2500
Table 1 Thermal sensation scales Scale of vote
Meaning
3 2 1 0 1 2 3
Cold Cool Slightly cool Neutral Slightly warm Warm Hot
ually to the set point. The small area from the fan will be called ‘‘local air movement” in this study. It should be noted that the air velocity of 0.2 m/s was air movement induced by the air conditioner in the room without turning on the small fan. The direction of air flow from the fan was directed between the face and neck of the students. Though the distance of each small fan was near each subject, it was not annoying to the subject because of the small area of air supply. Moreover, Thai people were familiar with high air movement, as high as 3 m/s, which is supplied from a larger electric fan [11]. The students were asked to fill in the questionnaire with information about their gender, age, weight, height and types of cloths before the experiment. Then they had to vote for their thermal sensation by filling in the thermal sensation vote form every 10 min when the air velocity was gradually changed. The scale of sensation vote was selected based on the well known scales of Fanger [4] as shown in Table 1. During each experiment, the temperature and relative humidity in the test room and outdoors were measured and recorded every 10 s by small compact temperature/humidity data loggers (Testo model 175-H2). Its accuracy of temperature and humidity measurement was ±0.5 °C and ±3% RH, respectively. The mean radiant temperature was not measured in this study because it was assumed to be equal to the room air temperature. It was confirmed from Yamtraipat [12] that the mean radiant temperature is quite equal to room air temperature between 23 and 27 °C. After the field measurements were done, the analysis of the effect of local air velocity on thermal comfort was done, and the assessment of energy saving with changing room air temperature set point was analyzed. 3. Result and discussion 3.1. Characteristics of test subjects
Fig. 1. Position of subject and fan setting in the experiment.
The total numbers of test subjects were 15. They were all students with college age between 19 and 24 years. Ten and five
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students were male and female, respectively. Average height of male students was 170 cm, while that of females was 157 cm. Average weight of males was 64 kg, while that of females was 49 kg. The sizes of both males and females were in an average size though only 15 subjects were selected in this study. These average characteristics were almost the same as the study from Ref. [12] in which a total of 1520 volunteers of working age were taken for the survey of thermal comfort in Thailand. Table 2 shows the summary of physical characteristics compared between the results from this study and those from Ref. [12].
It can be seen that the average temperature of the outdoor air was about 35 °C during the afternoon until evening. However, it could be as hot as almost 40 °C in some days. For the room air temperature of the five days of the experiment, the room air temperature was first approximately 30–33 °C according to the outdoor air temperature before turning on the air conditioner. Then, it gradually decreased to the temperature set point at 25 °C, and then the temperature was increased to 26, 27 and 28 °C every hour according to the temperature set point. 3.3. Thermal sensation vote
3.2. Outdoor and indoor air conditions Outdoor air temperature and indoor air temperature for five days of the experiment were shown in Fig. 3a and b, respectively. Table 2 Physical characteristic of subjects compared with Ref. [12] Gender
Number
Average age (years old)
Average height (cm)
Average weight (kg)
Male Male [12]
10 620
23.3 32.3
170 168.9
64 64.7
Female Female [12]
5 900
19 32.3
157 157.7
49 51.4
a
The percentages of thermal sensation vote under room temperature, humidity and air velocity are shown in Table 3. The thermal sensation vote under air temperature at 25 °C and air velocity at 0.2 m/s was both 1 and 0 with almost equal percentage of almost 50%, whereas there was only 6% vote for slightly warm. The mean vote for this condition was 0.4, which means that the average sensation was slightly cool. This can indicate that the room air temperature at 25 °C for the normal air temperature set point of almost all office buildings in Thailand was slightly cool for Thai people, especially for people in the Northeastern part of Thailand. It can be seen from Table 3 that when the air velocity at the same air temperature was higher, the percent of sensation vote moved to the cool sensation vote side. When the room air temperature
45 40 35
Temperature (oC)
30 25 20
1st day
15
2nd day
10
3rd day 4th day
5
5th day 0 1: 00 PM
b
1:30 PM
2:00 PM
2:30 PM
3:00 PM Time
3:30 PM
4:00 PM
4:30 PM
5:00 PM
35
Temperature (oC)
30 25 20 1st day 15
2nd day
10
3rd day 4th day
5 0 1: 00 PM
5th day 1: 30 PM
2: 00 PM
2: 30 PM
3: 00 PM Time
3: 30 PM
4: 00 PM
Fig. 3. Air temperature of (a) outdoor air and (b) room air.
4: 30 PM
5: 00 PM
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Table 3 Percentage of thermal sensation vote Temperature (°C)
RH (%)
Velocity (m/s)
Percent of sensation vote
Mean vote
Percent of 1 to 1 vote
PMV 3 25
26
27
28
45–75
60–80
65–80
70–80
0.2 0.5 1.0 1.5 2.0 0.2 0.5 1.0 1.5 2.0
2
1
0
1
48 13 13
6
40 73 88
46 87 47 27 6
2
3 0.4 0.9 1.3 1.7 2
100 100 60 27 12
53 66 67 20
40 7
13 40
7 27 33 67 60
0.3 0.2 0.3 0.9 1.4
100 100 100 87 60
6
0.2 0.5 1.0 1.5 2.0
74 60 53 60 27
13 27 13
13
13 34 40 67
0.4 0.13 0.13 0.4 0.8
87 100 100 100 100
0.2 0.5 1.0 1.5 2.0
33 53 60 33 40
47 27 20 20 6
20
20 20 47 47
0.9 0.5 0 0.3 0.5
80 100 100 100 94
6
was increased at the same air velocity, the percent of sensation vote tends to move to the hot sensation vote side. However, thermal comfort can be provided by increasing the air velocity to compensate for the higher temperature. When compared to results from previous research in Thailand of Yamtraipat et al. [12] under an air conditioning environment, the results are in good agreement. The results from Ref. [12] showed that the mean vote under air temperature at 25, 26 and 27 °C were 0.45, 0.13 and 0.52, respectively. The results from Ref. [12] were taken under the air velocity from the air conditioning system without external air movement. Therefore, the air velocity can be approximated at about 0.2 m/s. When compared to the present paper at air velocity of 0.2 m/s, the mean votes under air temperatures at 25, 26 and 27 °C were 0.4, 0.3 and 0.4, respectively. It can be seen that the results are in fairly good agreement. This indicates the accuracy of the methodology used in this study although the number of subjects used was really a small amount. To consider the conditions for thermal acceptability, comfort conditions would be defined from conditions that more than 80% of subjects vote for the three thermal sensations of slightly cool, neutral and slightly warm or value from 1 to 1. The reason for considering this range of thermal sensations is that each person is accustomed to different air conditions, and it is not easy to set a condition that can make every body feel ‘‘neutral”. It can be seen that more than 80% of subjects voted between 1 and 1 for air temperature at 25–28 °C and air velocity at 0.2 m/s. However, when the air velocity was increased, the percent of 1 to 1 votes for room air temperature at 25 and 26 °C was decreased to lower than 80%. Surprisingly, more than 80% of subjects voted from 1 to 1 at air temperatures of both 27 and 28 °C for all local air velocities. This can indicate that most Thai can accept temperature as high as 28 °C by using a small electric fan to supplement the air conditioner. Most people can adjust the local air velocity from 0.2 to 2 m/s according to their thermal preference. It is very interesting that the room temperature can be set as high as 28 °C with the fan assistance in the air conditioned room instead of setting the temperature at 25 °C as is the normal set point in most buildings. Consequently, it would help in reducing electricity consumption by air conditioners.
3.4. Thermal sensation equation The mean votes under room air temperatures shown in Table 3 were plotted under each local air velocity as shown in Fig. 4. The mean vote at each point was obtained by averaging the sensation vote at the same air temperature and local air velocity. It can be seen that when the air velocity increases, the neutral vote shifts to higher air temperature. Linear regression with the least square technique was applied to construct a thermal sensation equation. The obtained equation showed the relation between PMV (predicted mean vote) and air temperature and air velocity as shown in Eq. (1)
PMV ¼ 0:4444 T 0:7826 V 11:4157
ð1Þ
when T is room air temperature (°C) and V is local air velocity (m/s). From Eq. (1), the neutral temperature for each air velocity was calculated and shown in Table 4. It should be noted that the neutral temperature is the temperature for which the PMV is equal to zero. Table 4 shows that the neutral temperature increases when the local air velocity increases. 3.5. Estimation of energy saving from using small fan assisted air conditioner From this study, a small fan for each occupant was suggested to be used in all office buildings and residential buildings, including houses, in order to increase room air temperature for energy saving while still maintaining thermal comfort. Using a small fan will not annoy occupants because only a local area is supplied from the small fan. If a small fan is used to supply a local air velocity of 0.5 m/s, the room air temperature can be increased to 27 °C. Moreover, if the small fans are used to supply a local air velocity as high as 1–2 m/s, the room air temperature can be increased to 28 °C according to occupant preference. To estimate the energy saving from using a small fan to assist the air conditioning system, the study of Yamtraipat et al. [2] was reviewed first. It was found from Ref. [2] that most buildings in Thailand adjusted the indoor air temperature from 20 to 27 °C. The percentage of rooms with room temperature set point from 20 to 24 °C was 54.8%, while those of temperature set point at
S. Atthajariyakul, C. Lertsatittanakorn / Energy Conversion and Management 49 (2008) 2499–2504
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2
1
PMV
0
-1
0.2 m/s 0.2 m/s (predicted) 0.5 m/s 0.5 m/s (predicted) 1 m/s 1 m/s (predicted) 1.5 m/s 1.5 m/s (predicted) 2 m/s 2 m/s (predicted)
-2
-3 25
26
27
28
Temperature (oC) Fig. 4. Thermal sensation vs. room air temperature under various local air velocities.
Table 4 Neutral temperature for different local air velocity Air velocity (m/s)
Neutral Temperature (°C)
0.2 0.5 1.0 1.5 2.0
26.0 26.6 27.4 28.3 29.2
25, 26 and 27 °C accounted for 23.29%, 16.71% and 5.2%, respectively. From these data, it was interesting that most buildings set temperature below 24°C, more than 50%, while those with temperature set point at 26 and 27 °C are only about 20%. The study of Kongkiatumpai [16] found that the mean energy consumption reduction corresponding to a 1 °C increase of the set point was about 6.14%. This was also confirmed from the study of Ref. [17] that the energy saving when the temperature set point changed from 25 to 28°C was 17.5%, which was approximately 6% saving for 1 °C increase. Some research groups in Thailand [18,19] reported that office buildings were the highest electricity energy consumers in the commercial sector, which accounted for 43.53%. This value equaled an electricity consumption balance of 16481.8 GWh/year for the year 2005. The electrical energy used in air conditioning systems accounted for 59.09%. In this paper, a simple calculation of the energy saving for office buildings will be shown when the room air temperature is set at 28 °C and assisted with a small fan supplying local air movement. The energy saving will be calculated when the room air temperature was set at 28 °C, increasing from 24, 25, 26 and 27 °C. Changing the set point from 24 to 28 °C will increase energy saving by (28 °C 24 °C)0.0614 0.548 0.5909 16481.8 = 1310.77 GWh/year. When changing the set point from 25, 26 and 27 to 28 °C, the energy savings calculated by the same method shown above are 417.81, 199.84 and 31.09 GWh/year, respectively. Therefore, the total energy saving for office buildings when the temperature set point is changed to 28 °C is 1959.51 GWh/year. 4. Conclusion The technique of using small fans combined with an air conditioned space was proposed to increase the room air temperature set point. The technique was created from the idea that Thai people
can have a tolerance to higher air temperature, and also, Thai people are accustomed to high air movement from electric fans in nonair conditioned space. This method should be used in all air conditioned space in Thailand to help energy saving. The temperature set point was suggested to be increased to 28 °C and combined with a small fan to supply a local air velocity from 0.5 to 2 m/s according to individual preferences. This would decrease the electricity consumption of the air conditioning unit. The investment of buying the small fan and also the electricity consumption of fans is very small when compared to the energy consumed by an air conditioning system. According to the proposed method, this can save energy for office buildings in the commercial sector as high as 1959.51 GWh/year. However, the energy saving shown above was only for the office buildings in the commercial sector. The energy saving could be more than the value presented because all sectors using air conditioning systems can apply such proposed technique.
Acknowledgements The author would like to thank The Faculty of Engineering, Mahasarakham University for financial support for this research and also thank the senior students named Surachai Kamsai, Utane Pookrongjit and Naris Naowadee for data collecting. References [1] Department of Alternative Energy Development and Efficiency, Ministry of Energy. Electric Power in Thailand. Annual report, 2005. p. 26. [2] Yamtraipat N, Khedari J, Hirunlabh J, Kunchornrat J. Assessment of Thailand indoor set-point impact on energy consumption and environment. Energy Policy 2006;34:765–70. [3] ASHRAE. Thermal environmental conditions for human occupancy. ASRHAE standard 55-1992. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers; 1992. [4] Fanger PO. In: Thermal comfort analysis and application in environmental engineering. New York: McGraw-Hill; 1972. p. 244. [5] Dear De RJ, Leow KG, Foo SC. Thermal comfort in the humid tropics: field experiments in air-conditioned and naturally ventilated buildings in airconditioned and naturally ventilated buildings in Singapore. Int J Biometeorol 1991;34:259–65. [6] Mallick FH. Thermal comfort and building design in the tropical climates. Energy Buildings 1996;23:161–7. [7] Nicol F, Raja I, Allaudin A. Thermal comfort in Pakistan II-toward new indoor temperature standards. School of Architecture, Oxford Brookes University; 1997. [8] Tanabe S, Kimura K. Thermal comfort requirements under hot and humid conditions. In: Proceeding of ASHRAE far east conference, Singapore, 1987. p. 3–21.
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[9] Mc Intyre DA. Preferred air speeds for comfort in warm conditions. ASHRAE Trans 1976;84(II):264–77. [10] Tanabe S, Kimura K. Importance of air movement for thermal comfort under hot and humid conditions. In: ASHRAE F.E. conference on A.C. in hot climates, 1989. p. 95–103. [11] Khedari J, Yamtraipat N, Pratintong N, Hirunlabh J. Thailand ventilation comfort chart. Energy Buildings 2000;32:245–9. [12] Yamtraipat N, Khedari J, Hirunlabh J. Thermal comfort standards for air conditioned buildings in hot and humid Thailand considering additional factors of acclimatization and education level. Solar Energy 2005;78:504–17. [13] Williams BA, Shitzer A. Modular liquid-cooled helmet liner for thermal comfort. Aerospace Med 1974;45(9):1030–6. [14] Melikov AK, Arakelian RS, Halkjaer L. Spot cooling part 1: human responses to cooling with air jets. ASHRAE Trans 1994;100(2):476–99. [15] Bauman FS, Carter TG, Baughman AV. Field study of the impact of a desktop task/ambient conditioning system in office buildings. ASHRAE Trans 1998;104:1153–71.
[16] Kongkiatumpai P. Study of impact of indoor set point temperature on energy consumption of air-conditioner and green-house gases emission. A special study report for the Degree of Master of Engineering, School of Energy Technology Program, King Mongkut’s University of Technology Thonburi, Thailand, 1999. p. 28. [17] Atthajariyakul S, Leephakpreeda T. Real-time determination of optimal indoorair condition for thermal comfort, air quality and efficient energy usage. Energy Buildings 2004;36:720–33. [18] Chirarattananon S, Taweekun J. A technical review of energy conservation programs for commercial and government buildings in Thailand. Energy Convers Manage 2003;44:743–62. [19] Yungchareon V, Limmeechokchai B. Energy estimation for commercial buildings in Thailand. In: Proceedings of the first international conference on sustainable energy and Green architecture, Bangkok, Thailand, 2003. p. 26–35.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Small fan assisted air conditioner for thermal comfort and energy saving in Thailand Surat Atthajariyakul *, Charoenporn Lertsatittanakorn Faculty of Engineering, Mahasarakham University, Khamriang, Kantaravichai, Mahasarakham 44150, Thailand
a r t i c l e
i n f o
Article history: Received 11 October 2007 Accepted 25 May 2008 Available online 18 July 2008 Keywords: Thermal comfort Local air movement Energy saving Fan assist
a b s t r a c t From the fact that Thai people have a tolerance to high air temperature and are accustomed to high air movement from electric fans in non-air conditioned space, this paper proposes the use of small fan assisted air conditioners for human thermal comfort and energy saving in Thailand. In the study, a total 15 students were tested in a 2.5 3.5 2.5 m3 test room equipped with a 12,000 Btu/h split type air conditioner. During the tests, the room air temperature was varied from 25, 26, 27 and 28 °C every 1 h. A small fan with 15 cm diameter was placed in front of each subject. In each hour, the small fan was varied to supply a small area with velocity from 0.2, 0.5, 1, 1.5 and 2 m/s. In each condition, the subjects were asked to vote for their thermal sensation. The results showed that the temperature set point could be increased up to 28 °C when a small fan was used to supply local air velocity from 0.5 to 2 m/s according to individual preference. This would reduce the electricity consumption of the air conditioning unit. According to the proposed method, this can save energy for office buildings in the commercial sector as high as 1959.51 GWh/year. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction At present, Thailand is faced with an energy price crisis because of the increasing costs of imported oil. Therefore, methods of energy saving are very important and urgently needed. In Thailand, year 2005, the commercial sector was the second largest sector consuming electricity by 37,863 GWh/year, which accounted for 33.42% [1]. Office buildings are one group in the commercial sector that is the highest electricity energy consumers [2]. Moreover, air conditioning systems demand the highest electrical energy in all office buildings. Now, almost all buildings trend to turn on air conditioners year-round. As a result, Thailand is looking for solutions to reduce the electricity demand in air conditioning systems for the commercial sector and also for other sectors using air conditioners. Basically, the purpose of the air conditioning system is to provide occupants thermal comfort, which is defined as ‘‘that condition of mind that expresses satisfaction with the thermal environment” [3]. The comfort zone is defined as the range of climatic conditions in which the majority of people would not feel thermal discomfort, either warm or cold. Human thermal comfort depends on six thermal ‘‘quantitative” variables: air temperature, air relative humidity, mean radiant temperature, air velocity, human activity and clothing insulation [4]. However, thermal comfort not only depends on those six variables but also on some ‘‘non quantitative” factors such as mental states, habits, education. * Corresponding author. Tel./fax: +66 43 754 316. E-mail address: [email protected] (S. Atthajariyakul). 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.05.028
Many studies confirm that human comfort preferences vary in different locations and long term experience in any climate [5–8]. This may result in a tolerance to higher temperatures of people in hot and humid climate when compared to people in colder regions. Therefore, Thai people seem to have a tolerance to high air temperature because Thailand is located in a hot and humid region. Air movement might be one of the most useful and least expensive methods to provide a comfortable indoor climate, and also, it can reduce energy consumption in hot regions. The impact of air movement on comfort sensation has been studied continuously [9,10]. However, these researches studied air movement for higher air temperatures than those of Thailand’s climate. Therefore, Khedari et al. [11] studied the potential of providing thermal comfort for Thai people by using commercial electric fans without air conditioners and develop a ventilation comfort chart for Thailand that could be used for designing ventilating systems of non-air conditioned buildings. The study recommended providing air velocity between 0.2 and 3 m/s for air temperature between 26 and 36 °C. However, the study was conducted in a naturally ventilated space. It is not suitable for office buildings where all spaces are air conditioned and leaking air is not allowed for energy saving. Yamtraipat et al. [12] presented the result of a large thermal comfort survey conducted in Thailand. The study recommended setting the temperature set point at 26 °C and 50–60% relative humidity with air velocity of 0.2 m/s as a comfort condition for energy saving. Some research studies [13–15] showed that local heating or cooling could improve subjects’ acceptability of the thermal environment. From the literature reviews, Thai people get used to using an electrical fan in the non-air conditioned space. However, there
S. Atthajariyakul, C. Lertsatittanakorn / Energy Conversion and Management 49 (2008) 2499–2504 3.5 m.
Chair
Small fan
Personal computer
Fig. 2. Room plan of the test room.
2. Field experiment Field measurements were performed in a room of size 2.5 3.5 2.5 m3. This room was located on the top floor of the Faculty of Engineering building in Mahasarakham University which is in the Northeastern part of Thailand. The climate of this region is hot and humid, where the air conditioner is very important, especially in the summer season. A split type air conditioner with capacity of 12,000 Btu/h was hung on the top of the left wall. There were three computers on a long table and three round chairs in the room. In the study, a total of 15 students of Mahasarakham University, comprised of 10 males and 5 females, were tested in this test room. They all wore normal cloths with thermal insulation value of 0.55– 0.6 clo under sedentary activity of 1 met. The students were separated into five groups of three. Each group was tested by sitting in front of the long table. The arrangements of table, small fans and test subjects are shown in Figs. 1 and 2. Three small electric fans with 15 cm diameter were placed on the table in front of each student at a distance of 30 cm from the students. Each group of students was tested in the test room for four hours for one experiment. During each experiment, the room air temperature was changed every one hour from 25 to 26, 27 and 28 °C. At every hour, while the temperature was kept constant, the small fan for each student was turned on and was gradually adjusted in speed by a speed controller to supply local air velocities from 0.2, 0.5, 1, 1.5 and 2 m/s every 10 min. However, at each hour, when the temperature set point is changed, it is necessary to wait about 10 min for the temperature to change grad-
30 cm.
Air conditioner
were no studies of the use of small fans as local air movement assisted air conditioner for human comfort and energy saving. The objective of this study is to study the effect of local air movement from a small electric fan used in an air conditioned space on human comfort and to propose a simple and practical method to reduce energy consumption of air conditioning systems in office buildings while thermal comfort is still maintained. The method of study on thermal comfort presented in this paper used a small number of subjects, but the result is in quite good agreement when compared to another study with a large number of subjects. It is expected that the proposed method can be applied to other commercial buildings as well as in residential buildings where the proposed method can be implemented. It is hoped that the proposed method will be the easiest way and least investment for electrical energy consumption reduction for all air conditioning systems.
2.5 m.
2500
Table 1 Thermal sensation scales Scale of vote
Meaning
3 2 1 0 1 2 3
Cold Cool Slightly cool Neutral Slightly warm Warm Hot
ually to the set point. The small area from the fan will be called ‘‘local air movement” in this study. It should be noted that the air velocity of 0.2 m/s was air movement induced by the air conditioner in the room without turning on the small fan. The direction of air flow from the fan was directed between the face and neck of the students. Though the distance of each small fan was near each subject, it was not annoying to the subject because of the small area of air supply. Moreover, Thai people were familiar with high air movement, as high as 3 m/s, which is supplied from a larger electric fan [11]. The students were asked to fill in the questionnaire with information about their gender, age, weight, height and types of cloths before the experiment. Then they had to vote for their thermal sensation by filling in the thermal sensation vote form every 10 min when the air velocity was gradually changed. The scale of sensation vote was selected based on the well known scales of Fanger [4] as shown in Table 1. During each experiment, the temperature and relative humidity in the test room and outdoors were measured and recorded every 10 s by small compact temperature/humidity data loggers (Testo model 175-H2). Its accuracy of temperature and humidity measurement was ±0.5 °C and ±3% RH, respectively. The mean radiant temperature was not measured in this study because it was assumed to be equal to the room air temperature. It was confirmed from Yamtraipat [12] that the mean radiant temperature is quite equal to room air temperature between 23 and 27 °C. After the field measurements were done, the analysis of the effect of local air velocity on thermal comfort was done, and the assessment of energy saving with changing room air temperature set point was analyzed. 3. Result and discussion 3.1. Characteristics of test subjects
Fig. 1. Position of subject and fan setting in the experiment.
The total numbers of test subjects were 15. They were all students with college age between 19 and 24 years. Ten and five
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students were male and female, respectively. Average height of male students was 170 cm, while that of females was 157 cm. Average weight of males was 64 kg, while that of females was 49 kg. The sizes of both males and females were in an average size though only 15 subjects were selected in this study. These average characteristics were almost the same as the study from Ref. [12] in which a total of 1520 volunteers of working age were taken for the survey of thermal comfort in Thailand. Table 2 shows the summary of physical characteristics compared between the results from this study and those from Ref. [12].
It can be seen that the average temperature of the outdoor air was about 35 °C during the afternoon until evening. However, it could be as hot as almost 40 °C in some days. For the room air temperature of the five days of the experiment, the room air temperature was first approximately 30–33 °C according to the outdoor air temperature before turning on the air conditioner. Then, it gradually decreased to the temperature set point at 25 °C, and then the temperature was increased to 26, 27 and 28 °C every hour according to the temperature set point. 3.3. Thermal sensation vote
3.2. Outdoor and indoor air conditions Outdoor air temperature and indoor air temperature for five days of the experiment were shown in Fig. 3a and b, respectively. Table 2 Physical characteristic of subjects compared with Ref. [12] Gender
Number
Average age (years old)
Average height (cm)
Average weight (kg)
Male Male [12]
10 620
23.3 32.3
170 168.9
64 64.7
Female Female [12]
5 900
19 32.3
157 157.7
49 51.4
a
The percentages of thermal sensation vote under room temperature, humidity and air velocity are shown in Table 3. The thermal sensation vote under air temperature at 25 °C and air velocity at 0.2 m/s was both 1 and 0 with almost equal percentage of almost 50%, whereas there was only 6% vote for slightly warm. The mean vote for this condition was 0.4, which means that the average sensation was slightly cool. This can indicate that the room air temperature at 25 °C for the normal air temperature set point of almost all office buildings in Thailand was slightly cool for Thai people, especially for people in the Northeastern part of Thailand. It can be seen from Table 3 that when the air velocity at the same air temperature was higher, the percent of sensation vote moved to the cool sensation vote side. When the room air temperature
45 40 35
Temperature (oC)
30 25 20
1st day
15
2nd day
10
3rd day 4th day
5
5th day 0 1: 00 PM
b
1:30 PM
2:00 PM
2:30 PM
3:00 PM Time
3:30 PM
4:00 PM
4:30 PM
5:00 PM
35
Temperature (oC)
30 25 20 1st day 15
2nd day
10
3rd day 4th day
5 0 1: 00 PM
5th day 1: 30 PM
2: 00 PM
2: 30 PM
3: 00 PM Time
3: 30 PM
4: 00 PM
Fig. 3. Air temperature of (a) outdoor air and (b) room air.
4: 30 PM
5: 00 PM
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Table 3 Percentage of thermal sensation vote Temperature (°C)
RH (%)
Velocity (m/s)
Percent of sensation vote
Mean vote
Percent of 1 to 1 vote
PMV 3 25
26
27
28
45–75
60–80
65–80
70–80
0.2 0.5 1.0 1.5 2.0 0.2 0.5 1.0 1.5 2.0
2
1
0
1
48 13 13
6
40 73 88
46 87 47 27 6
2
3 0.4 0.9 1.3 1.7 2
100 100 60 27 12
53 66 67 20
40 7
13 40
7 27 33 67 60
0.3 0.2 0.3 0.9 1.4
100 100 100 87 60
6
0.2 0.5 1.0 1.5 2.0
74 60 53 60 27
13 27 13
13
13 34 40 67
0.4 0.13 0.13 0.4 0.8
87 100 100 100 100
0.2 0.5 1.0 1.5 2.0
33 53 60 33 40
47 27 20 20 6
20
20 20 47 47
0.9 0.5 0 0.3 0.5
80 100 100 100 94
6
was increased at the same air velocity, the percent of sensation vote tends to move to the hot sensation vote side. However, thermal comfort can be provided by increasing the air velocity to compensate for the higher temperature. When compared to results from previous research in Thailand of Yamtraipat et al. [12] under an air conditioning environment, the results are in good agreement. The results from Ref. [12] showed that the mean vote under air temperature at 25, 26 and 27 °C were 0.45, 0.13 and 0.52, respectively. The results from Ref. [12] were taken under the air velocity from the air conditioning system without external air movement. Therefore, the air velocity can be approximated at about 0.2 m/s. When compared to the present paper at air velocity of 0.2 m/s, the mean votes under air temperatures at 25, 26 and 27 °C were 0.4, 0.3 and 0.4, respectively. It can be seen that the results are in fairly good agreement. This indicates the accuracy of the methodology used in this study although the number of subjects used was really a small amount. To consider the conditions for thermal acceptability, comfort conditions would be defined from conditions that more than 80% of subjects vote for the three thermal sensations of slightly cool, neutral and slightly warm or value from 1 to 1. The reason for considering this range of thermal sensations is that each person is accustomed to different air conditions, and it is not easy to set a condition that can make every body feel ‘‘neutral”. It can be seen that more than 80% of subjects voted between 1 and 1 for air temperature at 25–28 °C and air velocity at 0.2 m/s. However, when the air velocity was increased, the percent of 1 to 1 votes for room air temperature at 25 and 26 °C was decreased to lower than 80%. Surprisingly, more than 80% of subjects voted from 1 to 1 at air temperatures of both 27 and 28 °C for all local air velocities. This can indicate that most Thai can accept temperature as high as 28 °C by using a small electric fan to supplement the air conditioner. Most people can adjust the local air velocity from 0.2 to 2 m/s according to their thermal preference. It is very interesting that the room temperature can be set as high as 28 °C with the fan assistance in the air conditioned room instead of setting the temperature at 25 °C as is the normal set point in most buildings. Consequently, it would help in reducing electricity consumption by air conditioners.
3.4. Thermal sensation equation The mean votes under room air temperatures shown in Table 3 were plotted under each local air velocity as shown in Fig. 4. The mean vote at each point was obtained by averaging the sensation vote at the same air temperature and local air velocity. It can be seen that when the air velocity increases, the neutral vote shifts to higher air temperature. Linear regression with the least square technique was applied to construct a thermal sensation equation. The obtained equation showed the relation between PMV (predicted mean vote) and air temperature and air velocity as shown in Eq. (1)
PMV ¼ 0:4444 T 0:7826 V 11:4157
ð1Þ
when T is room air temperature (°C) and V is local air velocity (m/s). From Eq. (1), the neutral temperature for each air velocity was calculated and shown in Table 4. It should be noted that the neutral temperature is the temperature for which the PMV is equal to zero. Table 4 shows that the neutral temperature increases when the local air velocity increases. 3.5. Estimation of energy saving from using small fan assisted air conditioner From this study, a small fan for each occupant was suggested to be used in all office buildings and residential buildings, including houses, in order to increase room air temperature for energy saving while still maintaining thermal comfort. Using a small fan will not annoy occupants because only a local area is supplied from the small fan. If a small fan is used to supply a local air velocity of 0.5 m/s, the room air temperature can be increased to 27 °C. Moreover, if the small fans are used to supply a local air velocity as high as 1–2 m/s, the room air temperature can be increased to 28 °C according to occupant preference. To estimate the energy saving from using a small fan to assist the air conditioning system, the study of Yamtraipat et al. [2] was reviewed first. It was found from Ref. [2] that most buildings in Thailand adjusted the indoor air temperature from 20 to 27 °C. The percentage of rooms with room temperature set point from 20 to 24 °C was 54.8%, while those of temperature set point at
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2
1
PMV
0
-1
0.2 m/s 0.2 m/s (predicted) 0.5 m/s 0.5 m/s (predicted) 1 m/s 1 m/s (predicted) 1.5 m/s 1.5 m/s (predicted) 2 m/s 2 m/s (predicted)
-2
-3 25
26
27
28
Temperature (oC) Fig. 4. Thermal sensation vs. room air temperature under various local air velocities.
Table 4 Neutral temperature for different local air velocity Air velocity (m/s)
Neutral Temperature (°C)
0.2 0.5 1.0 1.5 2.0
26.0 26.6 27.4 28.3 29.2
25, 26 and 27 °C accounted for 23.29%, 16.71% and 5.2%, respectively. From these data, it was interesting that most buildings set temperature below 24°C, more than 50%, while those with temperature set point at 26 and 27 °C are only about 20%. The study of Kongkiatumpai [16] found that the mean energy consumption reduction corresponding to a 1 °C increase of the set point was about 6.14%. This was also confirmed from the study of Ref. [17] that the energy saving when the temperature set point changed from 25 to 28°C was 17.5%, which was approximately 6% saving for 1 °C increase. Some research groups in Thailand [18,19] reported that office buildings were the highest electricity energy consumers in the commercial sector, which accounted for 43.53%. This value equaled an electricity consumption balance of 16481.8 GWh/year for the year 2005. The electrical energy used in air conditioning systems accounted for 59.09%. In this paper, a simple calculation of the energy saving for office buildings will be shown when the room air temperature is set at 28 °C and assisted with a small fan supplying local air movement. The energy saving will be calculated when the room air temperature was set at 28 °C, increasing from 24, 25, 26 and 27 °C. Changing the set point from 24 to 28 °C will increase energy saving by (28 °C 24 °C)0.0614 0.548 0.5909 16481.8 = 1310.77 GWh/year. When changing the set point from 25, 26 and 27 to 28 °C, the energy savings calculated by the same method shown above are 417.81, 199.84 and 31.09 GWh/year, respectively. Therefore, the total energy saving for office buildings when the temperature set point is changed to 28 °C is 1959.51 GWh/year. 4. Conclusion The technique of using small fans combined with an air conditioned space was proposed to increase the room air temperature set point. The technique was created from the idea that Thai people
can have a tolerance to higher air temperature, and also, Thai people are accustomed to high air movement from electric fans in nonair conditioned space. This method should be used in all air conditioned space in Thailand to help energy saving. The temperature set point was suggested to be increased to 28 °C and combined with a small fan to supply a local air velocity from 0.5 to 2 m/s according to individual preferences. This would decrease the electricity consumption of the air conditioning unit. The investment of buying the small fan and also the electricity consumption of fans is very small when compared to the energy consumed by an air conditioning system. According to the proposed method, this can save energy for office buildings in the commercial sector as high as 1959.51 GWh/year. However, the energy saving shown above was only for the office buildings in the commercial sector. The energy saving could be more than the value presented because all sectors using air conditioning systems can apply such proposed technique.
Acknowledgements The author would like to thank The Faculty of Engineering, Mahasarakham University for financial support for this research and also thank the senior students named Surachai Kamsai, Utane Pookrongjit and Naris Naowadee for data collecting. References [1] Department of Alternative Energy Development and Efficiency, Ministry of Energy. Electric Power in Thailand. Annual report, 2005. p. 26. [2] Yamtraipat N, Khedari J, Hirunlabh J, Kunchornrat J. Assessment of Thailand indoor set-point impact on energy consumption and environment. Energy Policy 2006;34:765–70. [3] ASHRAE. Thermal environmental conditions for human occupancy. ASRHAE standard 55-1992. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers; 1992. [4] Fanger PO. In: Thermal comfort analysis and application in environmental engineering. New York: McGraw-Hill; 1972. p. 244. [5] Dear De RJ, Leow KG, Foo SC. Thermal comfort in the humid tropics: field experiments in air-conditioned and naturally ventilated buildings in airconditioned and naturally ventilated buildings in Singapore. Int J Biometeorol 1991;34:259–65. [6] Mallick FH. Thermal comfort and building design in the tropical climates. Energy Buildings 1996;23:161–7. [7] Nicol F, Raja I, Allaudin A. Thermal comfort in Pakistan II-toward new indoor temperature standards. School of Architecture, Oxford Brookes University; 1997. [8] Tanabe S, Kimura K. Thermal comfort requirements under hot and humid conditions. In: Proceeding of ASHRAE far east conference, Singapore, 1987. p. 3–21.
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