STUDIES ON THERMAL COMFORT AND ENERGY CONSUMPTION OF HVAC SYSTEM

Transcription

1 STUDIES ON THERMAL COMFORT AND ENERGY CONSUMPTION OF HVAC SYSTEM B Liu,*, N Zhu 2, RQ Zhang.Dep. Of Refrigeration, Tianjin University of Commerce, 334, lbtjcu@tjcu.edu.cn 2.Environment School, Tianjin University, 372 ABSTRACT There is a close relation between the thermal comfort and the energy consumption of HVAC system. Based on the analyzing of the relation between the thermal comfort and the energy consumption of HVAC system, the experiments of the thermal sensation of students were carried out at different relative humidity with different temperatures, respectively 25%, 5% and 75%. The effects of the temperature and the relative humidity on the thermal comfort were obtained after statistically analyzing experimental data. The results show that changing the relative humidity almost has no effect on the thermal comfort in the surveyed students. The results also show that changing the relative humidity has a greater effect on the energy consumption of HVAC system than that of changing temperature. So the energy saving of HVAC system and a suitable thermal environment can reach a good harmony. INDEX TERMS Thermal comfort, energy saving, survey, relative humidity, temperature INTRODUCTION Building as a large energy consumer, its energy saving has been recognized adequately. In the past, for the different idea of energy saving, the energy saving of buildings were performed by repressing the energy consumption, such as reducing fresh air, strengthening the seal of buildings, which resulted in the SBS (sick building symptom). This method has fallen behind for the requirement of a good indoor working environment. The outstanding results in thermal comfort were given by Fanger, which were the basis of ASHRAE Standard and ISO773. But this does not provide indications of the energy saving of a building. Richard Holz studied effects of air velocities, mean radiant temperature on the thermal comfort, and used DOE-2 to predict the building energy performance and the consequence on thermal comfort. He found humidity and air velocity play rather diminutive role (997). Yang.K.H studied the indoor air velocity on thermal comfort and the energy consumption, and found that the cooling can be reduced by a proper air velocity (997). C.A.Balaras found that by a good way, potential energy saving range at 5%~35% while improving and maintaining indoor environmental quality (3). Ying.P pointed out that at any case, a lower relative humidity will reduce the investment and the service cost of HVAC system and improve the thermal sensation (2). But in this paper, it will provide a students survey of thermal sensation in experimental chamber in order to know the effects of the temperature and the relative humidity on thermal sensation, and give some advices for energy saving of HVAC system. EXPERIMENTAL CONDITIONS OF THERMAL SENSATION SURVEY There are many factors affecting thermal comfort including the outside environment, dietetic habits, clothes, personal psychology and the indoor environment. At present, the attention is put on the effect of the indoor environment, such as the indoor air temperature, the relative humidity, the radiate temperature of walls, the air velocity, the organic compound in indoor air, the luminous intensity and the sound intensity. Among these factors, the indoor air temperature and the relative humidity are the most important, which are discussed at here. In order to get a good thermal comfort, the heat and mass balance between the body and the environment should be reached. Based on this, Fanger set up the famous steady model of the heat and mass transfer of the body, and obtained the relationship between the vote value of the thermal sensation and the heat load of the body through analyzing experimental data (97). He pointed out that there is a relationship between PPD (Predicted percentage of dissatisfied people) and (the predicted mean vote) as following: * Corresponding author lbtjcu@tjcu.edu.cn 48

2 4 2 PPD = 95 exp[ ( )] () According Fanger s views, the is classed into seven levels as Table. Table. seven levels of cold slightly cold cool neutral slightly warm warm hot Based on the seven levels theory, experimental votes of students were carried out. The experiments were carried out from April to May in a climate chamber. In each experiment, students were asked to fill a form, which listed different thermal sensations needed to be selected by students. The experimental conditions in chamber were shown in Fig.. The relative humidity is about 25%, 5% and 75%, respectively. Along the relative humidity, the temperature range is from to 28. Figure. the tested indoor environmental conditions in diagram of hydro-enthalpy There is a reason to do experiments in the range of relative humidity and temperature. From others experiences, they indicated that beyond these ranges, people would have a bad thermal sensation. Just in order to save time and money, above conditions were selected. During the experimental process, 2 students were asked to sit quietly and fill the form every 5 minutes according their sensations. Each experiment continued 2 hours. In order to consider the integer effect of the temperature and the relative humidity, ET (effective temperature) provided by A.Gagge was adopted (986). Table 2 shows the effective temperature of each experiment. Table 2. the effective temperature of each experiment No ET( ) No ET( ) In the chamber, 8 temperature-testing positions were set beside each student and on each wall of the chamber, shown by a temperature circuit meter. There was only one probe of relative humidity in the chamber. EXPERIEMENTAL RESULTS AND DISCUSSIONS Effects of temperature and relative humidity on Fig.2 shows the effects of temperature and relative humidity on 49

3 % 5% 75% Figure 2. curves of the effect of temperature and relative humidity Ta on Where Ta is the air temperature,. From Fig.2, it can be easily found that the effect of relative humidity on is less than that of temperature. If the temperature is 23, when relative humidity is 33%, 5% and 75%, is.95, -.83 and.65, respectively. While if the relative humidity 5%, when temperature is 23, 25 and 27.4, is.83, -.9 and.69, respectively. It indicates that the reducing or improving relative humidity of indoor air will affect the thermal environment slightly. This will be the basis of energy saving by improving the relative humidity of indoor air. Relations of PPD, and ET ET Figure 3. the curve of the relationship between ET and PPD(%) Figure 4. the curve of the relationship between and PPD 5

4 PPD (-3,-2) (3,2) Fig.3 shows the relationship between and ET, Fig.4 shows the relationship between and PPD, and Fig.5 shows the expected ET in the tested students. It can be found that the relationship between ET and is almost linearly, not like the relationship between the relative humidity and or the temperature and. Using the least square method, the following equation can be driven. = ET 8. 9 (2) The equation is different from Fanger s and just fitted with the tested students in experiments. If the equation is extended to a wide scope, we think it can be used to the working office with comfort HVAC system. If that the PPD of a thermal environment is % is acceptable, then is.85 and +.85 calculated by equation (). Submitting the values of.85 and +.85 into equation (2), the ET values of 23. and will be attained. Compared with the required temperature of summer in working office in Chinese Standard, it can be found that the upper limitation is almost the same, but the lower limitation is different. The experimental result of 23. is lower than that of the Standard of 24. This is resulted from the tested students. For young people, they would like to cooler environment. From Fig.4, we can found that when PPD is the lowest, is not neutral but a little cool. It is about.4. Submitting.4 into equation (2), ET can be solved and is In Fig.5, the expected ET can be found and is The two values are almost the same. It indicates that the tested students have a better thermal sensation in a cool environment than in a neutral environment. From the definition of thermal comfort, a better thermal sensation represents a better spirit, which means that the people will have a better thinking. This result is also proved by R.Kosonen (4). He found that when was.2, the productivity was the highest. Energy saving of HVAC system change rate(%) Figure 5. the expected ET in the tested students ET temperature( ) Figure 6. the curve of change rate of temperature change rate(%) relative humidity(%) Figure 7. the curve of change rate of relative humidity From the above discussions in 3., has a fewer relationship with the relative humidity than that of temperature. According this, we calculated the energy consumption of a typical room with a size of 3.6*4.5*3(m 3 ). The widow area is 3m 2, and the rate of ventilation of the fresh air is 2/hr. The designed temperature and relative 5

5 humidity of outdoor environment are 32.5 and 75%, respectively. The following two figures show the change rate of energy consumption of HVAC system compared with the energy consumption of HVAC system in an indoor environment with a temperature 24.7 and a relative humidity 5%. From Fig.6 and Fig.7, the effect of temperature and relative humidity on energy consumption is different. In this case, when the temperature and relative humidity of outdoor environment are kept at the same, compared with the energy consumption of HVAC system in an indoor environment with a temperature 24.7 and a relative humidity 5%, each temperature change of indoor air will result in 5% change of energy consumption of HVAC system; while each 5% relative humidity change of indoor air will result in % change of energy consumption of HVAC system. But as for their effect on PPM, they are opposite. It can be found in Fig.2 that each temperature change of indoor air will result in.32 change of ; each 5% relative humidity change of indoor air will result in.3 change of. So in the thermal comfort range, keeping a high relative humidity in the indoor environment will has a positive effect on energy saving of HVAC system. CONCLUSION From the above discussions, the following conclusions can be obtained: (). Among the tested students, the curve of and PPD is not symmetric. The lowest point of PPD is close to.4 of, not the point of neutral of. (2). The expected ET of the tested students is 24.5, not the neutral point of 25. (3). The relative humidity has a fewer effect on than that of the temperature, but has a greater effect on energy consumption of HVAC system than that of the temperature. In an indoor environment, keeping a high relative humidity in the indoor environment will has a positive effect on energy saving of HVAC system. REFERENCE Balaras CA., Dascalaki E., Gglia A. et al. 3. Energy conservation potential, HVAC installation and operational issues in Hellenic airports, Energy and Buildings. 35:5-. Fanger PO. 97. Thermal Comfort. Copenhagen: Danish Technical Press. Gagge A., Fobelets A. and Bergland L A standard predictive index of human response to the thermal environment, ASHRAE Transactions. 92(): Holz R., Howrigan A., Sllop R. et al.997. Effects of standard energy conserving measures on thermal comfort, Building and Environment. 32(): Kosonen R. and Tan F. 4. Assessment of productivity loss in air-conditioned buildings using index, Energy and Buildings. 36: Yang KH. and Su CH Approach to energy saving using the index, BLdg Environment. 32(): Ying P. 2. Effects of indoor air design conditions on first investment and operating cost of air conditioning system, Heating, ventilating & air conditioning. 32(2):