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Year round experimental study on a constant temperature and humidity air-conditioning system driven by ground source heat pump
Energy 36 (2011) 1309e1318
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Year round experimental study on a constant temperature and humidity air-conditioning system driven by ground source heat pump X. Yu, R.Z. Wang*, X.Q. Zhai Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2010 Received in revised form 20 October 2010 Accepted 12 November 2010 Available online 22 December 2010
Numerous studies about the ground source heat pump building heating and cooling systems have been constructed in office building, hotel, residential building and school et al. However, few researches about the constant temperature and humidity air-conditioning system driven by ground-coupled heat pumps were carried out. In this paper, a constant temperature and humidity air-conditioning system driven by ground source heat pump was designed and constructed in an archives building in Shanghai, China. During the operation in the cooling mode, the heat extraction from the condenser of the heat pump was divided: part was rejected to the soil while another was used to reheat the air in AHUs. According to the experimental results, the indoor temperature and relative humidity fulfilled the “Archives Design Code”. In summer, the heat rejected to the soil was reduced by 32%, which was helpful for the earth energy conservation. The soil temperature increased only 0.5 C after the GSHP system operating for a year. The energy cost of the air-conditioning system was 56.1 kWh/m2. Compared with air source heat pump system and water cooled unit with boiler system, the operating cost of ground source heat pump was reduced by 55.8% and 48.4%, respectively. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Ground source heat pump Constant temperature and humidity system Experimental analysis Archives house
1. Introduction With a rapidly growing world population, and ever-increasing environmental concerns, sustainable energy technologies based on solar, wind and geothermal energy have become an issue of crucial importance for mankind [1,2]. Due to availability, economic and environmental issues it is expected that the worldwide use of oil, natural gas and coal is going to decline in the future and that geothermal energy will play an important role in the replacement of those fossil fuels. Shallow geothermal ground source heat pumps have had the most significant impact on direct use of geothermal energy. This is due, in part, to the ability of geothermal heat pumps to utilize groundwater or ground-coupled temperatures anywhere in the world [3,4]. Ground-coupled heat pumps are a highly efficient, renewable energy technology for space heating and cooling [5]. This technology has gained international attention as a means of energy conservation in the residential and commercial heating and air conditioning of indoor spaces [6].
* Corresponding author. Tel./fax: þ86 21 34206548. E-mail address: [email protected] (R.Z. Wang). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.11.013
Compared with other air conditioning systems, the groundcoupled heat pump systems have a more stable heat source. Due to the large heating capacity of soil, the variations of ambient air temperature are reflected only on the surface soil temperature, and their effects are reduced at deeper layers. Consequently, soil temperature is stabilized at the yearly average air temperature below the depth of about 10 m [7,8]. Ground-coupled heat pump projects contain various categories, such as office building, hotel, residential building, workshop building, school, villa house, hospital and emporium [9e16]. However, there are some buildings, for example the archives, which require constant indoor temperature and humidity all the time. In these building, few researches about the air-conditioning system driven by ground-coupled heat pumps were carried out. Furthermore, numerous studies about the ground source heat pump building heating and cooling systems have been presented [17e21]. Nevertheless, few experimental studies of the system for a year including spring and autumn were presented. In this paper, a constant temperature and humidity system driven by ground-coupled heat pump was designed and constructed in an archives building of Shanghai. The experimental results under a typical spring, summer, autumn and winter weather condition in Shanghai were analyzed. Furthermore, the investigation of the experiment in a year was presented.
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2.2. Design of the air-conditioning system
AHUs serve the archives houses in the east; ⅱ) western AHUs serve the archives houses in the west; ⅲ) offices fan-coil serves the office rooms; ⅳ) corridors fan-coil serves the corridors. The flow diagram of the system is shown in Fig. 2. In the cooling mode, the valves 1, 3, 9 and 11 which are depicted in Fig. 2 are closed, meanwhile the valves 2, 4, 5, 6, 7, 8, 10, 12, 13, 14, 15 and 16 are opened. Furthermore, the valves attached to the chilled water splitter and collector, which are 17, 18, 19, 20, 21, 22, 23 and 24, are opened. Meanwhile, the valves 25, 26, 29 and 30 are also opened for the purpose of reheating the air in AHUs. Thus, the evaporators of the heat pumps supply chilled water to the AHUs and the fan coil units. The water from the condensers was divided into two portions: one flow into the ground heat exchanger while the other into the heating coils in the AHUs. The air is at a very low temperature after the cooling coils because the cooling coils are used to dehumidify the air. Consequently, in order to fulfill the constant temperature and relative humidity, the air need to be reheated by the heating coils before it is sent into the archives houses. In the heating mode, one heat pump was operated in heating mode while the other was in cooling mode. With respect to the one in heating mode, the valves 2, 4, 5 and 7 which are depicted in Fig. 2 are closed, meanwhile the valves 1, 3, 6 and 8 are opened. On the other hand, with respect to the other heat pump in cooling mode, the valves 9, 11, 14 and 16 are closed while the valves 10, 12, 13 and 15 are opened. Furthermore, the valves attached to the hot water splitter and collector, which are 25, 26, 27, 28, 29, 30, 31 and 32, are opened. Meanwhile, the valves 17, 18, 21 and 22 are also opened for the purpose of dehumidifying the air in AHUs. Thus, the condenser of the heat pump in heating mode supplies hot water to the AHUs and the fan coil units. In addition, the air in the AHUs is all return air, which leads to that the air needs to be dehumidified sometimes. So the evaporator of the heat pump in cooling mode supplies chilled water to the AHUs to dehumidify the air.
The archives houses are air-conditioned by several Central Air Handling Units (AHUs). The offices and corridors are air-conditioned with fan-coil units. Based on the thermal characteristics and orientations of the rooms, four water loops are installed: ⅰ) eastern
2.2.1. Ground heat exchanger The ground heat exchanger consists of 280 vertical boreholes There is a single U-tube in each borehole. The primary circuit fluid is water without any anti-freezing agent. In order to promote the
Table 1 Basic design parameters of the building. Cooling design Temperature Cooling design Humidity Heating design Temperature Heating design Humidity Indoor design Temperature Indoor design Humidity
34 C 65% 4 C 75% 23 C 50%
2. Description of the archives building and the airconditioning system 2.1. Description of the archives building The archives building is located in Minhang district, Shanghai (30110 N, 121290 E). The basic design parameters are listed in Table 1. The building has one underground and four overground floors. The archives building covers a total air-conditioned area of 8000 m2, in which 6000 m2 is used to store archives. Fig. 1 shows the appearance of the building. The building stores the files of Minhang district of Shanghai. It is confirmed that the indoor environment is the most important factor for the conservation of the paper files. Therefore, the air conditioning system is very important to the files conservation and is necessary all- year-round. According to the “Archives Design Code” issued by China national archives, the indoor air temperature in archives house should be controlled between 14 C and 24 C and meanwhile, the relative humidity in archives house should be controlled between 45% and 60%. The fluctuation of indoor temperature and relative humidity in one day should not exceed 2 C and 5%, respectively.
Fig. 1. The appearance of building.
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Fig. 2. Flow diagram of the air-conditioning system.
the air and meanwhile, the heat extraction from the condenser was used to reheat the air in AHUs if necessary.
Table 2 Parameters of the ground heat exchanger. Distance between the boreholes (m) Deepness of the boreholes (m) Diameter of the boreholes (mm) U-tube material U-tube type U-tube conductivity (W/(m·k)) Shank spacing between the tubes (mm) Soil type Soil conductivity (W/(m·k))
4 80 160 PE DN32 0.46 80 sand clay 1.86
thermal conductivity between the U-tubes and the surrounding soil, a 1:9 bentonite/cement mixture was used. The parameters of the ground heat exchanger were shown in Table 2. 2.2.2. Ground source heat pump Two ground source heat pumps, which are screw machine with the type of Trane RTHDB2C2D2, were installed. Table 3 shows the main parameters of the heat pumps. The evaporator of heat pump supplied chilled water to the cooling coils in AHUs to dehumidify
2.2.3. Air handling units Fig. 3 shows the scheme of the AHU. The AHU consists of mixing section, filter section, cooling coil section, heating coil section, humidification section and fan section. The supply water temperature and the return water temperature of the cooling coil were 7 C and 12 C, respectively. For the heating coil, they were 45 C and 40 C, respectively.
2.3. Data acquisition and control system The whole system was controlled by a computer and operated automatically. The temperatures were recorded by temperature automatic recorders (0.5 C accuracy), which were fixed at the archives houses and some key points of the system. The data are recorded every 1 min. The flow rate of water in the ground heat exchanger, in the condensers and the evaporators of the heat pumps were measured by an ultrasonic flow meter (1e5% accuracy).
Table 3 The parameters of the heat pumps.
heat capacity (kW) Inlet/outlet temperature ( C) heat transfer area (m2) pressure drop (kPa) fluid type
evaporator
condenser
594 12e7 116.7 33.8 water/R134a horizontal
610 40e45 122.4 42.4 water/R134a horizontal
Fig. 3. The scheme of the AHU.
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Fig. 4. Variations of ambient temperature and relative humidity in a spring day.
Fig. 6. Variations of inlet and outlet temperatures of the evaporator of HP 2 in a spring day.
3. Experimental investigation of typical days 3.1. Experimental results in a typical spring day Fig. 4 shows the variations of the ambient temperature and relative humidity in a typical spring day. It was concluded that the average ambient temperature and relative humidity were 16.5 C and 76.6%, respectively. The two heat pumps are denoted as HP 1 and HP 2. In spring, HP 2 was operated in cooling mode while HP 1 in heating mode. With regard to HP 1 in the heating mode, Fig. 5 shows the variations of inlet and outlet temperatures of the condenser. It was concluded that the average inlet temperature of the condenser was 33.4 C and the average outlet temperature was 36.0 C. The flow rate of water was measured to be 60 m3/h. Hence, the average heating capacity was calculated to be 184 kW. The electric power of HP 1 was 60 kW. Therefore, the average coefficient of performance (COP) of the heat pump was 3.0. Since the electric power of the pumps and the fans kept constant with a total value of 62 kW, the average COP of the ground source heat pump (GSHP) system was 1.5. As for HP 2 in the cooling mode, Fig. 6 shows the variations of inlet and outlet temperatures of the evaporator. It was observed that the temperatures kept smooth in spring. It was because in most of the time in spring, the ambient temperature was higher than the indoor set temperature and the cooling was always necessary. Fig. 7 shows the variations of temperature and relative humidity in an archives house. It was shown that the average indoor temperature and the average indoor relative humidity were 18.9 C and 52.4%, respectively. The set temperature was 19 C and the set relative
Fig. 5. Variations of inlet and outlet temperatures of the condenser of HP 1 in a spring day.
Fig. 7. Variations of temperature and relative humidity in an archives house in a spring day.
humidity was 50%, which were depicted in Fig.7. It was concluded that the temperature and relative humidity in the archives house could be controlled in the fluctuation ranges of 19 0.5 C and 50% 5%, respectively. The indoor environment fulfilled the “Archives Design Code” issued by China national archives. 3.2. Experimental results in a typical summer day During the typical summer day, only one heat pump was operated to afford the cooling load. Fig. 8 shows the variations of the ambient temperature and relative humidity. It was concluded
Fig. 8. Variations of ambient temperature and relative humidity in a summer day.
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Fig. 9. Variations of inlet and outlet temperatures of the evaporator in a summer day.
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Fig. 11. Variations of the inlet and outlet temperatures of the condenser in a summer day.
that the average ambient temperature and relative humidity were 35 C and 53%, respectively. Fig. 9 shows the variations of inlet and outlet temperatures of the evaporator of the heat pump. The average outlet and inlet temperatures of the evaporator were calculated to be 8.1 C and 11.7 C, respectively. The flow rate of water was measured to be 112 m3/h. Consequently, the average cooling capacity was calculated to be 472 kW, as shown in Fig. 10. The electric power of the heat pump was 87 kW. Therefore, the average COP of the heat pump was 5.4. Since the electric power of the pumps and the fans kept constant with a total value of 70 kW, the average COP of the GSHP system was 3.0. The variations of the inlet and outlet temperatures of the condenser were shown in Fig. 11. It was observed that the outlet temperature fluctuated during the whole day. At about 9:00, the outlet temperature increased suddenly, which corresponded to the working time. The cooling load then increased which led to the increase of the heat extraction from the condenser. Similarly, at about 18:00, the air-conditionings of the office were turned off, which led to the decrease of the outlet temperature. It was concluded that the average outlet temperature was 34.3 C and the average inlet temperature was 32.4 C. The flow rate of water was measured to be 259 m3/h. Therefore, the average heat exchanged was calculated to be 572 kW. Fig. 12 shows the variations of the inlet and outlet temperatures of the ground heat exchanger. It was concluded that the heat rejected to the soil was 41.68 GJ for that whole day. According to Fig. 11, the heat extraction from the condenser was 51.19 GJ. As a result, the heat which was used to reheat the air in AHUs was 9.51 GJ. It was concluded that the heat rejected to the soil was
reduced due to the part of heat extraction from the condenser was used to reheat the air in AHUs. Fig. 13 shows the variations of temperature and relative humidity in an archives house. It was shown that the average indoor temperature and the average indoor relative humidity were 22.8 C and 46.6%, respectively. The set value of temperature was 23 C and the set value of relative humidity was 50%, which were depicted in Fig. 13. It was concluded that the temperature and relative humidity in the archives house could be controlled in the fluctuation ranges of 23 0.5 C and 50 5%, respectively. The
Fig. 10. Variation of the cooling capacity of the heat pump in a summer day.
Fig. 13. Variations of temperature and relative humidity in an archives house in a summer day.
Fig. 12. Variations of the inlet and outlet temperatures of the ground heat exchanger in a summer day.
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Fig. 14. Variations of ambient temperature and relative humidity in an autumn day.
Fig. 16. Variation of the cooling capacity of the heat pump in an autumn day.
indoor environment satisfied the “Archives Design Code” issued by China national archives. 3.3. Experimental results in a typical autumn day The same as in the typical summer day, only one heat pump was operated during the typical autumn day. Fig. 14 shows the variations of the ambient temperature and relative humidity. It was concluded that the average ambient temperature and relative humidity were 22 C and 73.9%, respectively. Fig. 15 shows the variations of inlet and outlet temperatures of the evaporator of the heat pump. The average outlet and inlet temperatures of the evaporator were calculated to be 7.6 C and 8.4 C, respectively. The flow rate of water was measured to be 110 m3/h. Consequently, the average cooling capacity was calculated to be 100 kW, as shown in Fig. 16. The electric power of the heat pump was 40 kW. Therefore, the average COP of the heat pump was 2.5. Since the electric power of the pumps and the fans kept constant with a total value of 62 kW, the average COP of the GSHP system was 1.0. The variations of the inlet and outlet temperatures of the condenser were shown in Fig. 17. It was concluded that the average outlet temperature was 34.2 C and the average inlet temperature was 33.2 C. The flow rate of water was measured to be 125 m3/h. Therefore, the average heat exchanged was calculated to be 148 kW. Fig. 18 shows the variations of temperature and relative humidity in an archives house. It was shown that the average indoor temperature and the average indoor relative humidity were 22.2 C and 47.0%, respectively. The set value of temperature was 22 C and the set value of relative humidity was 50%, which
Fig. 15. Variations of inlet and outlet temperatures of the evaporator in an autumn day.
Fig. 17. Variations of the inlet and outlet temperatures of the condenser in an autumn day.
was depicted in Fig.18. It was concluded that the temperature and relative humidity in the archives house could be controlled in the fluctuation ranges of 22 0.5 C and 50 5%, respectively. The indoor environment satisfied the “Archives Design Code” issued by China national archives. 3.4. Experimental results in a typical winter day Fig. 19 shows the variations of the ambient temperature and relative humidity in a typical winter day. It was concluded that the average ambient temperature and relative humidity were 8.9 C and 69.5%, respectively.
Fig. 18. Variations of temperature and relative humidity in an archives house in an autumn day.
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Fig. 21. Variations of inlet and outlet temperatures of the evaporator of HP 2 in a winter day. Fig. 19. Variations of ambient temperature and relative humidity in a winter day.
The operation mode in winter was the same as that in spring. With regard to HP 1 in the heating mode, Fig. 20 shows the variations of inlet and outlet temperatures of the condenser. It was concluded that the average inlet temperature of the condenser was 38.8 C and the average outlet temperature was 43.2 C. The flow rate of water was measured to be 90 m3/h. Hence, the average heating capacity was calculated to be 463.7 kW. The electric power of HP 1 was 89 kW. Therefore, the average COP of the heat pump was 5.2. Since the electric power of the pumps and the fans kept constant with a total value of 70 kW, the average COP of the GSHP system was 2.9. As for HP 2 in the cooling mode, Fig. 21 shows the variations of inlet and outlet temperatures of the evaporator. It was observed that the temperatures fluctuated frequently. The HP 2 was used to dehumidify the air. As soon as the humidity of air reached the set value, HP 2 would be turned off. The HP 2 was turned on and turned off frequently, leading to the frequent change of the temperatures. Fig. 22 shows the variations of temperature and relative humidity in an archives house. It was shown that the average indoor temperature and the average indoor relative humidity were 22.8 C and 47.5%, respectively. The set temperature was 23 C and the set relative humidity was 50%, which was depicted in Fig.22. It was concluded that the temperature and relative humidity in the archives house could be controlled in the fluctuation ranges of
Fig. 22. Variations of temperature and relative humidity in an archives house in a winter day.
23 0.5 C and 50 5%, respectively. The indoor environment fulfilled the “Archives Design Code” issued by China national archives. 3.5. Analysis of the experimental results Table 4 concludes the experimental results in the four typical days. It is shown that in spring and autumn, the heating capacity and cooling capacity were low because of the low load. Consequently, the COP of the heat pump and the system were very low in spring and autumn. However, in summer and winter, the COP of the heat pump Table 4 Experimental results in the four typical days.
Fig. 20. Variations of inlet and outlet temperatures of the condenser of HP 1 in a winter day.
Average ambient temperature ( C) Average ambient relative humidity Operation mode Condenser capacity (kW) Evaporator capacity (kW) Compressor power capacity (kW) COPhp Circulation pumps and fans power capacity (kW) COPs Set indoor temperature ( C) Sverage indoor temperature ( C) Set indoor relative humidity Average indoor relative humidity
Spring
Summer
Autumn
Winter
16.5 76.6% heating 184 e 60 3.0 62.0
35 53.0% cooling e 472 87 5.4 70.0
22 73.9% cooling e 100 40 2.5 62.0
8.9 69.5% heating 463.7 e 89 5.2 70.0
1.5 19.0 18.9 50% 52.4%
3.0 23.0 22.8 50% 46.6%
1.0 22.0 22.2 50% 47.0%
2.9 23.0 22.8 50% 47.5%
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Fig. 23. Variation of temperature and relative humidity in a year.
Fig. 25. The heat transfer between ground heat exchanger and the soil in a year.
and the system were advantaged. For the requirement of the archives house, the system had to operate in the whole year even the performance of the system in spring and autumn were not satisfied. The results of the indoor environment figured that the temperature and relative humidity fulfilled the “Archives Design Code” according to the operation of the GSHP system. Meanwhile, the heat recovery was adopted in summer, which was helpful for the energy balance of soil during a year.
GSHP system cannot run steadily. Consequently, how to treat the energy balance and make operation reasonable is very important and critical. Compared with the conventional operations, the operation mode in summer that part of the heat extraction from the condenser is used to reheat the air in AHUs could keep the earth energy more stable. Fig. 24 shows the heat rejected to the soil from June to October. The heat transfer was the heat rejected to the soil actually, while the heat transfer without recovery was the heat rejected to the soil if the part of the heat extraction from the condenser was not used to reheat the air but rejected to the soil. The total heat without recovery was 3896 GJ and the total actual heat was 2667 GJ, which presented that the heat recovery reduced the heat rejected to the soil by 32%. Fig. 25 shows the heat transfer between ground heat exchanger and the soil in a year. The positive value means the heat was rejected to the soil, while the negative value means the heat was
4. Experimental analysis of a whole year 4.1. The indoor environment The variation of temperature and relative humidity in an archives house for a year is shown in Fig. 23. It is observed that the temperature in April was lower than that in other months. The reason is that the indoor temperature was set to be 19 C in spring and 22 C in other seasons. Obviously, the temperature could be controlled in the required range. On the other hand, the relative humidity fluctuated in the range of 45e60% with a set value of 50%, which also accommodated the requirement. Nevertheless, there are some obvious fluctuations, like the relative humidity between November and December. It is because some people opened the door at that time and the indoor environment was influenced by the outdoor environment. On the whole, according to the operation of the GSHP, the indoor temperature and relative humidity fulfilled the “Archives Design Code” issued by China national archives. 4.2. Influence to the soil If the heat absorbed from the soil annually is not equal to the heat rejected to it, the earth energy cannot be balanced and the
Fig. 24. Heat rejected to the soil actually and without recovery from June to October.
Fig. 26. Temperature of soil before operated and after operated for a year.
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Table 5 The initial cost and operating cost.
Minhang archives (GSHP) Archives A (ASHP) Archives B (WC þ B)
Initial cost/Yuan/m2
Operating cost/Yuan/m2
379.4 274.1 229.0
46.3 104.7 89.7
4.3. The operating cost Fig. 27 shows the energy cost of the GSHP system. The total energy cost in a year was 449,168 kWh and the energy cost per square meter was 56.1 kWh/m2. Different from the energy cost in other office buildings, the energy cost of the air-conditioning system in the archives building accounted for about 70% of the total energy cost. Fig. 28 shows the operating cost of the GSHP system. The total operating cost in a year was 370,564 Yuan RMB (Chinese money). 4.4. Economic analysis
Fig. 27. Energy cost of the GSHP system in a year.
absorbed from the soil. The total heat rejected to the soil was 2861 GJ and the total heat absorbed from the soil was 2192 GJ, which indicated that the difference between the heat rejected to the soil and absorbed from the soil in a year was 23.3%. Hypothetically, if the heat recovery technology was not adopted the total heat rejected to the soil would be 4292 GJ and the difference to the heat absorbed from the soil would be 49%. It means that the heat recovery technology is helpful to the balance of the earth energy. Fig. 26 shows the temperature of the soil before the system was operated and after the system was operated for a year. It was observed from Fig. 26(a) that the initial temperature of the soil was about 18 C. In addition, from Fig. 26(b), it was concluded that the soil temperature was about 18.5 C after the GSHP system operated for a year. It is said that the soil temperature increased tinily by the system operated for a year. Unfortunately, although the heat rejected to the soil was reduced with heat recovery technology in a year, the earth energy still cannot totally be balanced. Other methods will be carried out for the problem in the future work.
Fig. 28. Operating cost of the GSHP system in a year.
Compared with the air source heat pump (ASHP) system and the water cooled unit with boiler system, ground source heat pump system has lower maintenance. Two other archives buildings in Shanghai, where the air source heat pump system and water cooled unit with boiler system were applied respectively, were tested and compared to the Minhang archives building. Table 5 shows the initial cost and the operating cost of the three archives buildings in a year. Compared with air source heat pump system, the GSHP system has a higher initial cost with the value of 105.3 Yuan/m2. However, the operating cost of the GSHP system was 58.4 Yuan/m2 lower than that of the ASHP system. Hypothetically, the area of the archives A is the same with that of Minhang archives. Then in a year, the initial cost of the GSHP system was 842,400 Yuan more than that of ASHP system and the operating cost of the GSHP system was 467,200 Yuan less than that of ASHP system. It was calculated that the operating cost was reduced by 55.8% and the payback time would be two years. Similarly, compared with archives B where the water cooled unit with boiler system was applied, the operating cost of Minhang archives was reduced by 48.4% and the payback time would be four years. 5. Conclusions A constant temperature and relative humidity system driven by ground source heat pump was developed for an archives building in Shanghai. Based upon the experimental results, the main conclusions are: (1) Under the typical summer weather condition of Shanghai, the average cooling capacity of the heat pump was 472 kW. The average COP of the heat pump and GSHP system were 5.4 and 3.0, respectively. For the typical winter weather condition, the average heating capacity of the heat pump was 463.7 kW. The average COP of the heat pump and GSHP system were 5.2 and 2.9, respectively. (2) Compared with the typical summer and winter weather condition, the COP of the heat pump was lower in the typical spring and autumn weather condition, which were 3.0 and 2.5, respectively. (3) The operation mode in summer that part of the heat extraction from the condenser is used to reheat the air in AHUs could reduce the heat rejected to the soil by 32%. It was helpful to the balance of the earth energy.
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(4) The initial temperature of soil was 18 C. After the GSHP system operating for a year, the temperature of soil increased to 18.5 C. The influence to the soil was existent but tiny. (5) During the whole year, the temperature was controlled within 19e24 C. Meanwhile, the relative humidity was controlled within 45e60%. The indoor environment met the “Archives Design Code” issued by China national archives. (6) The total energy cost in a year was 449,168 kWh and the energy cost per square meter was 56.1 kWh/m2. The total operating cost in a year was 370,564 Yuan RMB. (7) Compared with air source heat pump system applied in another archives building, the operating cost of ground source heat pump was reduced by 55.8% and the payback time would be two years; similarly, the reduction of the operating cost was 48.4% and the payback time would be four years compared with water cooled unit with boiler system. Acknowledgement This work was supported by the Commission of Science and Technology of Shanghai under the contract No.07DZ12021 and No.09160700200. The support from MOST China under the contract No. 2006BAA04B03 is appreciated. This publication is written with support from the research project CREATIV, financially supported by the Research Council of Norway (p.no.195182/S60) References [1] Doherty PS, Al-Huthaili S, Riffat SB, Abodahab N. Ground source heat pumpdescription and preliminary results of the eco house system. Applied Thermal Engineering 2004;24:2627e41. [2] Hepbasli A, Ozgener L. Development of geothermal energy utilization in Turkey: a review. Renewable and Sustainable Energy Reviews 2004;8:433e60. [3] Blum P, Campillo G, Munch W, Kolbel T. CO2 savings of ground source heat pump systems e A regional analysis. Renewable Energy 2010;35:122e7. [4] Lund JW, Freeston DH, Boyd TL. Direct application of geothermal energy: 2005 worldwide review. Geothermics 2005;34:691e727.
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Year round experimental study on a constant temperature and humidity air-conditioning system driven by ground source heat pump X. Yu, R.Z. Wang*, X.Q. Zhai Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2010 Received in revised form 20 October 2010 Accepted 12 November 2010 Available online 22 December 2010
Numerous studies about the ground source heat pump building heating and cooling systems have been constructed in office building, hotel, residential building and school et al. However, few researches about the constant temperature and humidity air-conditioning system driven by ground-coupled heat pumps were carried out. In this paper, a constant temperature and humidity air-conditioning system driven by ground source heat pump was designed and constructed in an archives building in Shanghai, China. During the operation in the cooling mode, the heat extraction from the condenser of the heat pump was divided: part was rejected to the soil while another was used to reheat the air in AHUs. According to the experimental results, the indoor temperature and relative humidity fulfilled the “Archives Design Code”. In summer, the heat rejected to the soil was reduced by 32%, which was helpful for the earth energy conservation. The soil temperature increased only 0.5 C after the GSHP system operating for a year. The energy cost of the air-conditioning system was 56.1 kWh/m2. Compared with air source heat pump system and water cooled unit with boiler system, the operating cost of ground source heat pump was reduced by 55.8% and 48.4%, respectively. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Ground source heat pump Constant temperature and humidity system Experimental analysis Archives house
1. Introduction With a rapidly growing world population, and ever-increasing environmental concerns, sustainable energy technologies based on solar, wind and geothermal energy have become an issue of crucial importance for mankind [1,2]. Due to availability, economic and environmental issues it is expected that the worldwide use of oil, natural gas and coal is going to decline in the future and that geothermal energy will play an important role in the replacement of those fossil fuels. Shallow geothermal ground source heat pumps have had the most significant impact on direct use of geothermal energy. This is due, in part, to the ability of geothermal heat pumps to utilize groundwater or ground-coupled temperatures anywhere in the world [3,4]. Ground-coupled heat pumps are a highly efficient, renewable energy technology for space heating and cooling [5]. This technology has gained international attention as a means of energy conservation in the residential and commercial heating and air conditioning of indoor spaces [6].
* Corresponding author. Tel./fax: þ86 21 34206548. E-mail address: [email protected] (R.Z. Wang). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.11.013
Compared with other air conditioning systems, the groundcoupled heat pump systems have a more stable heat source. Due to the large heating capacity of soil, the variations of ambient air temperature are reflected only on the surface soil temperature, and their effects are reduced at deeper layers. Consequently, soil temperature is stabilized at the yearly average air temperature below the depth of about 10 m [7,8]. Ground-coupled heat pump projects contain various categories, such as office building, hotel, residential building, workshop building, school, villa house, hospital and emporium [9e16]. However, there are some buildings, for example the archives, which require constant indoor temperature and humidity all the time. In these building, few researches about the air-conditioning system driven by ground-coupled heat pumps were carried out. Furthermore, numerous studies about the ground source heat pump building heating and cooling systems have been presented [17e21]. Nevertheless, few experimental studies of the system for a year including spring and autumn were presented. In this paper, a constant temperature and humidity system driven by ground-coupled heat pump was designed and constructed in an archives building of Shanghai. The experimental results under a typical spring, summer, autumn and winter weather condition in Shanghai were analyzed. Furthermore, the investigation of the experiment in a year was presented.
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2.2. Design of the air-conditioning system
AHUs serve the archives houses in the east; ⅱ) western AHUs serve the archives houses in the west; ⅲ) offices fan-coil serves the office rooms; ⅳ) corridors fan-coil serves the corridors. The flow diagram of the system is shown in Fig. 2. In the cooling mode, the valves 1, 3, 9 and 11 which are depicted in Fig. 2 are closed, meanwhile the valves 2, 4, 5, 6, 7, 8, 10, 12, 13, 14, 15 and 16 are opened. Furthermore, the valves attached to the chilled water splitter and collector, which are 17, 18, 19, 20, 21, 22, 23 and 24, are opened. Meanwhile, the valves 25, 26, 29 and 30 are also opened for the purpose of reheating the air in AHUs. Thus, the evaporators of the heat pumps supply chilled water to the AHUs and the fan coil units. The water from the condensers was divided into two portions: one flow into the ground heat exchanger while the other into the heating coils in the AHUs. The air is at a very low temperature after the cooling coils because the cooling coils are used to dehumidify the air. Consequently, in order to fulfill the constant temperature and relative humidity, the air need to be reheated by the heating coils before it is sent into the archives houses. In the heating mode, one heat pump was operated in heating mode while the other was in cooling mode. With respect to the one in heating mode, the valves 2, 4, 5 and 7 which are depicted in Fig. 2 are closed, meanwhile the valves 1, 3, 6 and 8 are opened. On the other hand, with respect to the other heat pump in cooling mode, the valves 9, 11, 14 and 16 are closed while the valves 10, 12, 13 and 15 are opened. Furthermore, the valves attached to the hot water splitter and collector, which are 25, 26, 27, 28, 29, 30, 31 and 32, are opened. Meanwhile, the valves 17, 18, 21 and 22 are also opened for the purpose of dehumidifying the air in AHUs. Thus, the condenser of the heat pump in heating mode supplies hot water to the AHUs and the fan coil units. In addition, the air in the AHUs is all return air, which leads to that the air needs to be dehumidified sometimes. So the evaporator of the heat pump in cooling mode supplies chilled water to the AHUs to dehumidify the air.
The archives houses are air-conditioned by several Central Air Handling Units (AHUs). The offices and corridors are air-conditioned with fan-coil units. Based on the thermal characteristics and orientations of the rooms, four water loops are installed: ⅰ) eastern
2.2.1. Ground heat exchanger The ground heat exchanger consists of 280 vertical boreholes There is a single U-tube in each borehole. The primary circuit fluid is water without any anti-freezing agent. In order to promote the
Table 1 Basic design parameters of the building. Cooling design Temperature Cooling design Humidity Heating design Temperature Heating design Humidity Indoor design Temperature Indoor design Humidity
34 C 65% 4 C 75% 23 C 50%
2. Description of the archives building and the airconditioning system 2.1. Description of the archives building The archives building is located in Minhang district, Shanghai (30110 N, 121290 E). The basic design parameters are listed in Table 1. The building has one underground and four overground floors. The archives building covers a total air-conditioned area of 8000 m2, in which 6000 m2 is used to store archives. Fig. 1 shows the appearance of the building. The building stores the files of Minhang district of Shanghai. It is confirmed that the indoor environment is the most important factor for the conservation of the paper files. Therefore, the air conditioning system is very important to the files conservation and is necessary all- year-round. According to the “Archives Design Code” issued by China national archives, the indoor air temperature in archives house should be controlled between 14 C and 24 C and meanwhile, the relative humidity in archives house should be controlled between 45% and 60%. The fluctuation of indoor temperature and relative humidity in one day should not exceed 2 C and 5%, respectively.
Fig. 1. The appearance of building.
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Fig. 2. Flow diagram of the air-conditioning system.
the air and meanwhile, the heat extraction from the condenser was used to reheat the air in AHUs if necessary.
Table 2 Parameters of the ground heat exchanger. Distance between the boreholes (m) Deepness of the boreholes (m) Diameter of the boreholes (mm) U-tube material U-tube type U-tube conductivity (W/(m·k)) Shank spacing between the tubes (mm) Soil type Soil conductivity (W/(m·k))
4 80 160 PE DN32 0.46 80 sand clay 1.86
thermal conductivity between the U-tubes and the surrounding soil, a 1:9 bentonite/cement mixture was used. The parameters of the ground heat exchanger were shown in Table 2. 2.2.2. Ground source heat pump Two ground source heat pumps, which are screw machine with the type of Trane RTHDB2C2D2, were installed. Table 3 shows the main parameters of the heat pumps. The evaporator of heat pump supplied chilled water to the cooling coils in AHUs to dehumidify
2.2.3. Air handling units Fig. 3 shows the scheme of the AHU. The AHU consists of mixing section, filter section, cooling coil section, heating coil section, humidification section and fan section. The supply water temperature and the return water temperature of the cooling coil were 7 C and 12 C, respectively. For the heating coil, they were 45 C and 40 C, respectively.
2.3. Data acquisition and control system The whole system was controlled by a computer and operated automatically. The temperatures were recorded by temperature automatic recorders (0.5 C accuracy), which were fixed at the archives houses and some key points of the system. The data are recorded every 1 min. The flow rate of water in the ground heat exchanger, in the condensers and the evaporators of the heat pumps were measured by an ultrasonic flow meter (1e5% accuracy).
Table 3 The parameters of the heat pumps.
heat capacity (kW) Inlet/outlet temperature ( C) heat transfer area (m2) pressure drop (kPa) fluid type
evaporator
condenser
594 12e7 116.7 33.8 water/R134a horizontal
610 40e45 122.4 42.4 water/R134a horizontal
Fig. 3. The scheme of the AHU.
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Fig. 4. Variations of ambient temperature and relative humidity in a spring day.
Fig. 6. Variations of inlet and outlet temperatures of the evaporator of HP 2 in a spring day.
3. Experimental investigation of typical days 3.1. Experimental results in a typical spring day Fig. 4 shows the variations of the ambient temperature and relative humidity in a typical spring day. It was concluded that the average ambient temperature and relative humidity were 16.5 C and 76.6%, respectively. The two heat pumps are denoted as HP 1 and HP 2. In spring, HP 2 was operated in cooling mode while HP 1 in heating mode. With regard to HP 1 in the heating mode, Fig. 5 shows the variations of inlet and outlet temperatures of the condenser. It was concluded that the average inlet temperature of the condenser was 33.4 C and the average outlet temperature was 36.0 C. The flow rate of water was measured to be 60 m3/h. Hence, the average heating capacity was calculated to be 184 kW. The electric power of HP 1 was 60 kW. Therefore, the average coefficient of performance (COP) of the heat pump was 3.0. Since the electric power of the pumps and the fans kept constant with a total value of 62 kW, the average COP of the ground source heat pump (GSHP) system was 1.5. As for HP 2 in the cooling mode, Fig. 6 shows the variations of inlet and outlet temperatures of the evaporator. It was observed that the temperatures kept smooth in spring. It was because in most of the time in spring, the ambient temperature was higher than the indoor set temperature and the cooling was always necessary. Fig. 7 shows the variations of temperature and relative humidity in an archives house. It was shown that the average indoor temperature and the average indoor relative humidity were 18.9 C and 52.4%, respectively. The set temperature was 19 C and the set relative
Fig. 5. Variations of inlet and outlet temperatures of the condenser of HP 1 in a spring day.
Fig. 7. Variations of temperature and relative humidity in an archives house in a spring day.
humidity was 50%, which were depicted in Fig.7. It was concluded that the temperature and relative humidity in the archives house could be controlled in the fluctuation ranges of 19 0.5 C and 50% 5%, respectively. The indoor environment fulfilled the “Archives Design Code” issued by China national archives. 3.2. Experimental results in a typical summer day During the typical summer day, only one heat pump was operated to afford the cooling load. Fig. 8 shows the variations of the ambient temperature and relative humidity. It was concluded
Fig. 8. Variations of ambient temperature and relative humidity in a summer day.
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Fig. 9. Variations of inlet and outlet temperatures of the evaporator in a summer day.
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Fig. 11. Variations of the inlet and outlet temperatures of the condenser in a summer day.
that the average ambient temperature and relative humidity were 35 C and 53%, respectively. Fig. 9 shows the variations of inlet and outlet temperatures of the evaporator of the heat pump. The average outlet and inlet temperatures of the evaporator were calculated to be 8.1 C and 11.7 C, respectively. The flow rate of water was measured to be 112 m3/h. Consequently, the average cooling capacity was calculated to be 472 kW, as shown in Fig. 10. The electric power of the heat pump was 87 kW. Therefore, the average COP of the heat pump was 5.4. Since the electric power of the pumps and the fans kept constant with a total value of 70 kW, the average COP of the GSHP system was 3.0. The variations of the inlet and outlet temperatures of the condenser were shown in Fig. 11. It was observed that the outlet temperature fluctuated during the whole day. At about 9:00, the outlet temperature increased suddenly, which corresponded to the working time. The cooling load then increased which led to the increase of the heat extraction from the condenser. Similarly, at about 18:00, the air-conditionings of the office were turned off, which led to the decrease of the outlet temperature. It was concluded that the average outlet temperature was 34.3 C and the average inlet temperature was 32.4 C. The flow rate of water was measured to be 259 m3/h. Therefore, the average heat exchanged was calculated to be 572 kW. Fig. 12 shows the variations of the inlet and outlet temperatures of the ground heat exchanger. It was concluded that the heat rejected to the soil was 41.68 GJ for that whole day. According to Fig. 11, the heat extraction from the condenser was 51.19 GJ. As a result, the heat which was used to reheat the air in AHUs was 9.51 GJ. It was concluded that the heat rejected to the soil was
reduced due to the part of heat extraction from the condenser was used to reheat the air in AHUs. Fig. 13 shows the variations of temperature and relative humidity in an archives house. It was shown that the average indoor temperature and the average indoor relative humidity were 22.8 C and 46.6%, respectively. The set value of temperature was 23 C and the set value of relative humidity was 50%, which were depicted in Fig. 13. It was concluded that the temperature and relative humidity in the archives house could be controlled in the fluctuation ranges of 23 0.5 C and 50 5%, respectively. The
Fig. 10. Variation of the cooling capacity of the heat pump in a summer day.
Fig. 13. Variations of temperature and relative humidity in an archives house in a summer day.
Fig. 12. Variations of the inlet and outlet temperatures of the ground heat exchanger in a summer day.
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Fig. 14. Variations of ambient temperature and relative humidity in an autumn day.
Fig. 16. Variation of the cooling capacity of the heat pump in an autumn day.
indoor environment satisfied the “Archives Design Code” issued by China national archives. 3.3. Experimental results in a typical autumn day The same as in the typical summer day, only one heat pump was operated during the typical autumn day. Fig. 14 shows the variations of the ambient temperature and relative humidity. It was concluded that the average ambient temperature and relative humidity were 22 C and 73.9%, respectively. Fig. 15 shows the variations of inlet and outlet temperatures of the evaporator of the heat pump. The average outlet and inlet temperatures of the evaporator were calculated to be 7.6 C and 8.4 C, respectively. The flow rate of water was measured to be 110 m3/h. Consequently, the average cooling capacity was calculated to be 100 kW, as shown in Fig. 16. The electric power of the heat pump was 40 kW. Therefore, the average COP of the heat pump was 2.5. Since the electric power of the pumps and the fans kept constant with a total value of 62 kW, the average COP of the GSHP system was 1.0. The variations of the inlet and outlet temperatures of the condenser were shown in Fig. 17. It was concluded that the average outlet temperature was 34.2 C and the average inlet temperature was 33.2 C. The flow rate of water was measured to be 125 m3/h. Therefore, the average heat exchanged was calculated to be 148 kW. Fig. 18 shows the variations of temperature and relative humidity in an archives house. It was shown that the average indoor temperature and the average indoor relative humidity were 22.2 C and 47.0%, respectively. The set value of temperature was 22 C and the set value of relative humidity was 50%, which
Fig. 15. Variations of inlet and outlet temperatures of the evaporator in an autumn day.
Fig. 17. Variations of the inlet and outlet temperatures of the condenser in an autumn day.
was depicted in Fig.18. It was concluded that the temperature and relative humidity in the archives house could be controlled in the fluctuation ranges of 22 0.5 C and 50 5%, respectively. The indoor environment satisfied the “Archives Design Code” issued by China national archives. 3.4. Experimental results in a typical winter day Fig. 19 shows the variations of the ambient temperature and relative humidity in a typical winter day. It was concluded that the average ambient temperature and relative humidity were 8.9 C and 69.5%, respectively.
Fig. 18. Variations of temperature and relative humidity in an archives house in an autumn day.
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Fig. 21. Variations of inlet and outlet temperatures of the evaporator of HP 2 in a winter day. Fig. 19. Variations of ambient temperature and relative humidity in a winter day.
The operation mode in winter was the same as that in spring. With regard to HP 1 in the heating mode, Fig. 20 shows the variations of inlet and outlet temperatures of the condenser. It was concluded that the average inlet temperature of the condenser was 38.8 C and the average outlet temperature was 43.2 C. The flow rate of water was measured to be 90 m3/h. Hence, the average heating capacity was calculated to be 463.7 kW. The electric power of HP 1 was 89 kW. Therefore, the average COP of the heat pump was 5.2. Since the electric power of the pumps and the fans kept constant with a total value of 70 kW, the average COP of the GSHP system was 2.9. As for HP 2 in the cooling mode, Fig. 21 shows the variations of inlet and outlet temperatures of the evaporator. It was observed that the temperatures fluctuated frequently. The HP 2 was used to dehumidify the air. As soon as the humidity of air reached the set value, HP 2 would be turned off. The HP 2 was turned on and turned off frequently, leading to the frequent change of the temperatures. Fig. 22 shows the variations of temperature and relative humidity in an archives house. It was shown that the average indoor temperature and the average indoor relative humidity were 22.8 C and 47.5%, respectively. The set temperature was 23 C and the set relative humidity was 50%, which was depicted in Fig.22. It was concluded that the temperature and relative humidity in the archives house could be controlled in the fluctuation ranges of
Fig. 22. Variations of temperature and relative humidity in an archives house in a winter day.
23 0.5 C and 50 5%, respectively. The indoor environment fulfilled the “Archives Design Code” issued by China national archives. 3.5. Analysis of the experimental results Table 4 concludes the experimental results in the four typical days. It is shown that in spring and autumn, the heating capacity and cooling capacity were low because of the low load. Consequently, the COP of the heat pump and the system were very low in spring and autumn. However, in summer and winter, the COP of the heat pump Table 4 Experimental results in the four typical days.
Fig. 20. Variations of inlet and outlet temperatures of the condenser of HP 1 in a winter day.
Average ambient temperature ( C) Average ambient relative humidity Operation mode Condenser capacity (kW) Evaporator capacity (kW) Compressor power capacity (kW) COPhp Circulation pumps and fans power capacity (kW) COPs Set indoor temperature ( C) Sverage indoor temperature ( C) Set indoor relative humidity Average indoor relative humidity
Spring
Summer
Autumn
Winter
16.5 76.6% heating 184 e 60 3.0 62.0
35 53.0% cooling e 472 87 5.4 70.0
22 73.9% cooling e 100 40 2.5 62.0
8.9 69.5% heating 463.7 e 89 5.2 70.0
1.5 19.0 18.9 50% 52.4%
3.0 23.0 22.8 50% 46.6%
1.0 22.0 22.2 50% 47.0%
2.9 23.0 22.8 50% 47.5%
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Fig. 23. Variation of temperature and relative humidity in a year.
Fig. 25. The heat transfer between ground heat exchanger and the soil in a year.
and the system were advantaged. For the requirement of the archives house, the system had to operate in the whole year even the performance of the system in spring and autumn were not satisfied. The results of the indoor environment figured that the temperature and relative humidity fulfilled the “Archives Design Code” according to the operation of the GSHP system. Meanwhile, the heat recovery was adopted in summer, which was helpful for the energy balance of soil during a year.
GSHP system cannot run steadily. Consequently, how to treat the energy balance and make operation reasonable is very important and critical. Compared with the conventional operations, the operation mode in summer that part of the heat extraction from the condenser is used to reheat the air in AHUs could keep the earth energy more stable. Fig. 24 shows the heat rejected to the soil from June to October. The heat transfer was the heat rejected to the soil actually, while the heat transfer without recovery was the heat rejected to the soil if the part of the heat extraction from the condenser was not used to reheat the air but rejected to the soil. The total heat without recovery was 3896 GJ and the total actual heat was 2667 GJ, which presented that the heat recovery reduced the heat rejected to the soil by 32%. Fig. 25 shows the heat transfer between ground heat exchanger and the soil in a year. The positive value means the heat was rejected to the soil, while the negative value means the heat was
4. Experimental analysis of a whole year 4.1. The indoor environment The variation of temperature and relative humidity in an archives house for a year is shown in Fig. 23. It is observed that the temperature in April was lower than that in other months. The reason is that the indoor temperature was set to be 19 C in spring and 22 C in other seasons. Obviously, the temperature could be controlled in the required range. On the other hand, the relative humidity fluctuated in the range of 45e60% with a set value of 50%, which also accommodated the requirement. Nevertheless, there are some obvious fluctuations, like the relative humidity between November and December. It is because some people opened the door at that time and the indoor environment was influenced by the outdoor environment. On the whole, according to the operation of the GSHP, the indoor temperature and relative humidity fulfilled the “Archives Design Code” issued by China national archives. 4.2. Influence to the soil If the heat absorbed from the soil annually is not equal to the heat rejected to it, the earth energy cannot be balanced and the
Fig. 24. Heat rejected to the soil actually and without recovery from June to October.
Fig. 26. Temperature of soil before operated and after operated for a year.
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Table 5 The initial cost and operating cost.
Minhang archives (GSHP) Archives A (ASHP) Archives B (WC þ B)
Initial cost/Yuan/m2
Operating cost/Yuan/m2
379.4 274.1 229.0
46.3 104.7 89.7
4.3. The operating cost Fig. 27 shows the energy cost of the GSHP system. The total energy cost in a year was 449,168 kWh and the energy cost per square meter was 56.1 kWh/m2. Different from the energy cost in other office buildings, the energy cost of the air-conditioning system in the archives building accounted for about 70% of the total energy cost. Fig. 28 shows the operating cost of the GSHP system. The total operating cost in a year was 370,564 Yuan RMB (Chinese money). 4.4. Economic analysis
Fig. 27. Energy cost of the GSHP system in a year.
absorbed from the soil. The total heat rejected to the soil was 2861 GJ and the total heat absorbed from the soil was 2192 GJ, which indicated that the difference between the heat rejected to the soil and absorbed from the soil in a year was 23.3%. Hypothetically, if the heat recovery technology was not adopted the total heat rejected to the soil would be 4292 GJ and the difference to the heat absorbed from the soil would be 49%. It means that the heat recovery technology is helpful to the balance of the earth energy. Fig. 26 shows the temperature of the soil before the system was operated and after the system was operated for a year. It was observed from Fig. 26(a) that the initial temperature of the soil was about 18 C. In addition, from Fig. 26(b), it was concluded that the soil temperature was about 18.5 C after the GSHP system operated for a year. It is said that the soil temperature increased tinily by the system operated for a year. Unfortunately, although the heat rejected to the soil was reduced with heat recovery technology in a year, the earth energy still cannot totally be balanced. Other methods will be carried out for the problem in the future work.
Fig. 28. Operating cost of the GSHP system in a year.
Compared with the air source heat pump (ASHP) system and the water cooled unit with boiler system, ground source heat pump system has lower maintenance. Two other archives buildings in Shanghai, where the air source heat pump system and water cooled unit with boiler system were applied respectively, were tested and compared to the Minhang archives building. Table 5 shows the initial cost and the operating cost of the three archives buildings in a year. Compared with air source heat pump system, the GSHP system has a higher initial cost with the value of 105.3 Yuan/m2. However, the operating cost of the GSHP system was 58.4 Yuan/m2 lower than that of the ASHP system. Hypothetically, the area of the archives A is the same with that of Minhang archives. Then in a year, the initial cost of the GSHP system was 842,400 Yuan more than that of ASHP system and the operating cost of the GSHP system was 467,200 Yuan less than that of ASHP system. It was calculated that the operating cost was reduced by 55.8% and the payback time would be two years. Similarly, compared with archives B where the water cooled unit with boiler system was applied, the operating cost of Minhang archives was reduced by 48.4% and the payback time would be four years. 5. Conclusions A constant temperature and relative humidity system driven by ground source heat pump was developed for an archives building in Shanghai. Based upon the experimental results, the main conclusions are: (1) Under the typical summer weather condition of Shanghai, the average cooling capacity of the heat pump was 472 kW. The average COP of the heat pump and GSHP system were 5.4 and 3.0, respectively. For the typical winter weather condition, the average heating capacity of the heat pump was 463.7 kW. The average COP of the heat pump and GSHP system were 5.2 and 2.9, respectively. (2) Compared with the typical summer and winter weather condition, the COP of the heat pump was lower in the typical spring and autumn weather condition, which were 3.0 and 2.5, respectively. (3) The operation mode in summer that part of the heat extraction from the condenser is used to reheat the air in AHUs could reduce the heat rejected to the soil by 32%. It was helpful to the balance of the earth energy.
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(4) The initial temperature of soil was 18 C. After the GSHP system operating for a year, the temperature of soil increased to 18.5 C. The influence to the soil was existent but tiny. (5) During the whole year, the temperature was controlled within 19e24 C. Meanwhile, the relative humidity was controlled within 45e60%. The indoor environment met the “Archives Design Code” issued by China national archives. (6) The total energy cost in a year was 449,168 kWh and the energy cost per square meter was 56.1 kWh/m2. The total operating cost in a year was 370,564 Yuan RMB. (7) Compared with air source heat pump system applied in another archives building, the operating cost of ground source heat pump was reduced by 55.8% and the payback time would be two years; similarly, the reduction of the operating cost was 48.4% and the payback time would be four years compared with water cooled unit with boiler system. Acknowledgement This work was supported by the Commission of Science and Technology of Shanghai under the contract No.07DZ12021 and No.09160700200. The support from MOST China under the contract No. 2006BAA04B03 is appreciated. This publication is written with support from the research project CREATIV, financially supported by the Research Council of Norway (p.no.195182/S60) References [1] Doherty PS, Al-Huthaili S, Riffat SB, Abodahab N. Ground source heat pumpdescription and preliminary results of the eco house system. Applied Thermal Engineering 2004;24:2627e41. [2] Hepbasli A, Ozgener L. Development of geothermal energy utilization in Turkey: a review. Renewable and Sustainable Energy Reviews 2004;8:433e60. [3] Blum P, Campillo G, Munch W, Kolbel T. CO2 savings of ground source heat pump systems e A regional analysis. Renewable Energy 2010;35:122e7. [4] Lund JW, Freeston DH, Boyd TL. Direct application of geothermal energy: 2005 worldwide review. Geothermics 2005;34:691e727.
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