Research on single hole heat transfer power of ground source heat pump system

DOI: 10.19637/j.cnki.2305-7068.2018.01.008 Research on single hole heat transfer power of ground source heat pump system ZHAO Fang-hua * The Third Party of Hydrogeology and Engineering Geology, Hebei Provincial Bureau of Geology and Mineral Investigation, Hengshui 053000, China. Abstract: In this paper, the single hole heat transfer power of the ground source heat pump system in Hengshui is compared with data gained from thermal response test. The results show that maximum monitoring data of heat transfer power per meter in summer is 97.1% of the test data, and the average value accounts for 81.8%. The per meter heat power data through on-site thermal response test can provide references for designing engineering project and optimizing ground source heat pump system as these data do not vary greatly from the actual monitoring data. Keywords: Ground source heat pump; Monitoring; Heat transfer power; Thermal response test Introduction Shallow geothermal energy, which has been widely utilized mainly through developing ground source heat pump in China, is a new method to effectively save energy and reduce air pollution. At present, the design of such heat exchanger is mainly based on the data of field thermal response test. As the experimental conditions and actual conditions divide, if determining the number and length of buried pipes according to test data, heat transfer power cannot match with cooling load and thermal load, resulting in high costs and low efficiency in later stage. This paper will analyze the monitoring data of ground source heat pump (GSHP) system in specified buried pipes to evaluate single hole heat transfer power. In addition, suggestions on optimizing GSHP system are proposed via comparing monitoring data to the data of field thermal response test. 1 Project overview * ZHAO Fang-hua(1980-), female, Han nationality, bachelor s degree, engineer, mainly engaged in hydrogeology, engineering geology and other aspects. E-mail: 115383559@qq.com. 1.1 Physiographic condition The project is located in Hengshui region, central-eastern Hebei Province with a semi-arid continental monsoon climate typical in warm temperate zone. This region has four distinct seasons, whose temperature ranges from -23 to 42.7 with an average temperature of 13. The plain landscape is at a slant from southwest to northeast with an average elevation of 23-28 m. The study area belongs to the Fuyang River s alluvial-proluvial sub-region of physiographic region in Hebei plain. The Quaternary system is located between 450 m and 470 m and aquifer group can be defined as pore water from Quaternary loose rocks (DONG Qing-qing et al. 2014). 1.2 Project overview The project utilizes underground pipe systems to develop shallow geothermal energy. Heat pump systems are applied to cool and heat architectures including comprehensive office buildings with a construction area of 10 321 m 2, whose 1 st to 6 th floors are for commercial purpose and 7 th to 12 th floors for office work, or hotels covering an area of http://gwse.iheg.org.cn 65

2 280 m 2 (only available in winter). The total cooling area is 10 321 m 2 while heating area covers 12 601 m 2 (Table 1). Table 1 Heating and cooling loads for architecture design Types Comprehensive office building Hotel Total Cooling load(kw) 1 060 0 1 060 Thermal load(kw) 658 161 819 A combined GSHP system is adopted in the project. Heat exchange holes in buried pipes and cooling towers are used to cool the architectures in summer. While in winter, heat exchange holes in buried pipes are a main source to supply heating. Indoor terminal comprises fan coil units of central air conditioning fresh air system (LIU Jiang-tao et al. 2010). 1.3 GSHP system 95 single heat exchange holes in buried pipes are arranged in the project area. The depth of heat exchange holes is 140 m with bore diameter of φ150 mm. The ground heat exchanger adopts double U heat exchange tube, and the distance between every two heat exchange holes is 5 m (Fig. 1). Two climaveneta screw ground source heat pump units run in parallel. Unit 1 is connected with the heat exchange holes, which supply cooling and heating via temperature difference of source soil. Besides, unit 2 is connected to heat exchange holes and cooling tower, serving as a reserve set for unit 1. Apart from system debug, heat pump units in this paper all adopt underground heat transfer system within one-year monitoring period. Fig. 1 Layout of heat exchange holes included in GSHP system 1.4 Monitoring system The monitoring system mainly consists of flow monitoring, temperature monitoring and room temperature monitoring. Each monitoring device is connected to the data acquisition module through communication lines, and finally connected to the industrial personal computer (IPC) in the computer room. relevant software is used to receive and analyze the collected data, so as to monitor the overall operation of the heat pump system. Flow monitoring adopts electromagnetic flowmeter. Two sets of electromagnetic flowmeter are 66 arranged in computer room of GSHP units together with cold water source heat pump units, and two sets in user side with the monitoring frequency of once for every 10 minutes. Heat resistance temperature sensor is utilized to monitor temperature. Computer room of GSHP units, cold water source heat pump units together with ground tail water source heat pump is equipped with four sets of such sensors, and four sets on users water inlet manifold as well as outfall sewer with the same monitoring frequency with flow monitoring. Room temperature data gained regularly from infrared thermometers in room 1110 (in southern http://gwse.iheg.org.cn

part of the architecture) and room 1101 (in northern part of the architecture) are counted through manual record. Temperature data is collected for two times per hour. Namely, the room temperature data is gained during the same working period of heat pumps. 2 Efficiency analysis 2.1 Room temperature The system terminal for monitoring room temperature targets at room 1101 on the southern part of office building and room 1110 on the northern part. In summer, the room temperature of two rooms is basically kept at 24-26, and maintains at 21-24 in winter. According to Code for design of heating ventilation and air conditioning (GB50019-2003), the preferential room temperature in civil architectures is 16-24 in winter and 22-28 in summer. So that the room temperature set in the paper meets the above requirements. 2.2 Efficiency calculation 2.2.1 Operating mode in summer Heat pump units adopt intermittent operation and all-day operation. In early summer and late summer, units run from 9:00 a.m. to 8:00 p.m. and are closed at other time. From the end of June to the middle of August, units open all day, cooling whole comprehensive experiment building from 7:00 a.m. to 10:00 p.m. while only for 1-6 floors from 10:00 p.m. to 7:00 a.m. the next day. The temperature of supply main on GSHP units ranges from 20.5 to 25.5, and the average temperature of return main is between 24.0 and 29.5, so that average temperature difference of supply and return water is 3.58 with an average flow of 83.1 m 3 /h (Fig. 2). Fig. 2 Variations of flow rate together with supply and return water temperature of heat pump units on the ground side during cooling period According to the monitoring data of the recycling capacity of GSHP units together with temperature of supply and return main, the total heat output of the heat exchange system during a cooling period can be calculated by equation (1) as follows: Q= q w ΔTc w ρ w ( t 1 -t 2 ) (1) In this equation, Q is total heat output measured (kj); q w is recycling capacity in terms of ground source (m 3 /h); ΔT is monitoring interval (h); ρ w is water density (kg/m 3 ); c w is specific heat capacity of water (kj/kg ); t 1 is the water temperature of supply main ( ); t 2 is the water temperature of return main ( ). The results show that the heat transferred from http://gwse.iheg.org.cn 67

GSHP system to gneiss totals 1 210.6 10 6 kj during cooling period in summer. Daytime heat output varies from 170 kw to 360 kw by changes of air temperature and operation time of heat pumps. Average output is 303.8 kw, which equals to transfer power of heat exchange pipes per meter ranging from 12.8 w to 27.1 w, with an average power of 22.84 w (CHEN Yan-hua et al. 2009; XU Wei et al. 2009; ZHAO Bing et al. 2012). 2.2.2 Operating mode in winter Apart from the Spring Festival holiday, heat pump units run round the clock with a total number of 1 898 hours in winter. The temperature of supply main on GSHP units ranges from 13.5 to 18.0, and the average temperature of return main is between 11.5 and 15.8, so that average temperature difference of supply and return water is 2.11 with an average flow of 93.01 m 3 /h (Fig. 3). According to the monitoring data of the recycling capacity of GSHP units together with temperature of supply and return main, the total heat supply volume of the heat exchange system during a heating period can be calculated. The results show that the heat transferred from gneiss to GSHP system totals 1 235.7 10 6 kj. Daytime heat supply fluctuates between 154 kw and 201 kw in response to changes of air temperature and operation time of heat pumps. Average output is 180.8 kw, which equals to transfer power of heat exchange pipes per meter varying from 11.6 w to 15.1 w, with an average power of 13.59 w (Table 2). Fig. 3 Variations of flow rate together with supply and return water temperature of heat pump units on the ground side during heating period Period Table 2 Heat transfer power per meter of heat transfer pipes Total heat transfer power (Kw) Single hole heat transfer power (Kw) Heat transfer power per meter of heat transfer pipes (w) Cooling period 303.8 3.20 22.84 Heating period 180.8 1.90 13.59 3 Analysis of field thermal response test 3.1 Test overview 68 The thermal response test is carried out in the early stage of the project. The bore diameter is 150 mm, and the depth of both pore-forming as well as pipe depth are all 140 m. Vertical buried pipes adopt DE32 double U PE pipes. Cement grout are http://gwse.iheg.org.cn

used to backfill space 50 m underground, while 0-50 m below the ground is backfilled by original substances and sandstone. Experiments of heat extraction conditions at the temperature of 30 and 35 are conducted respectively. 3.2 Analysis of test data The first step is to heat fluid in heat exchange pipes. Then the heated fluid will transfer heat through heat exchange pipes to surrounding soil with relatively low temperature. Heat transfer power per meter of heat exchange holes and comprehensive heat transfer coefficient can be calculated according to following equation 2 and equation 3. Q=cρqΔt/d (2) K=Q/(t-t 0 ) (3) In these equations, Q is heat transfer power per meter (W/m); q is circulation water flow (m 3 /h); c is specific heat capacity of water (kj/kg ); ρ is water density (kg/m 3 ); Δt is temperature difference between inlet and outlet water ( ); d is the depth of heat transfer holes (m); K is comprehensive heat transfer coefficient (W/m ); t is average water temperature of heat exchange pipes ( ); t 0 is average soil temperature ( ). The heating time lasts around 68.5 hours at temperature of 30. The inlet and outlet temperature will rise gradually during early period of the test. As heat exchange pipes transfer heat to the ground, the temperature of backfill substances and soil reach a balance with heat transfer power per meter of heat exchange holes being 42.1 W/m and comprehensive heat transfer coefficient of 4.01 W/(m ); when inlet temperature is set at 35, heating time lasts about 51 hours. The heat transfer power per meter of heat exchange holes being 61.4 W/m and comprehensive heat transfer coefficient of 4.07 W/(m ) (Table 3) after reaching a balance. Operating mode Table 3 Test data of heat exchange in this project Inlet temperature ( ) Outlet temperature ( ) Average flow (m 3 /h) Heat transfer power per meter of heat transfer pipes(w) comprehensive heat transfer coefficient (W/m ) Heat extraction 1 30 26.8 1.57 42.1 4.01 Heat extraction 2 35.3 30.9 1.56 61.4 4.07 According to the two experiments above, if conducting thermal response test with different flow and at different temperature, the figures of heat transfer power per meter of heat exchange holes divide while similar figures comprehensive heat transfer coefficient are gained. Through comparative analysis, when underground heat exchange pipes extract heat, comprehensive heat transfer coefficient can be regarded as a fixed figure. So that the average value of comprehensive heat transfer coefficient in two experiments can be used as a reference, to be precise, the value is 4.04 W/(m ). 4 Conclusions According to the thermal response test data, when average temperature of heat exchange holes in summer is 24.9, the heat transfer power of per meter heat exchange holes is 27.9 W/m. Comparing to actual monitoring data, maximum heat transfer power accounts for 97.1% of test data, and average heat transfer power ranks 81.8%. Conclusions can be drawn that the field test data is similar to monitoring data, so that design of engineering projects can be based on field test data. Considering the number and length of buried pipes, four factors should be taken into consideration as follows: (1) Limited by test equipment, the fluid flow in the heat exchange hole is generally about 1.5 m 3 /h during experiments, and the actual flow is 80-90 m 3 /h, which vary greatly from the experiment data. Under the condition of small flow, heat transfer to soil is more sufficient, and the temperature difference of inlet and outlet is relatively large, so the calculated figures of heat transfer power is relatively large. As a consequence, thermal response test should increase fluid flow in heat exchange holes to http://gwse.iheg.org.cn 69

minimize the error. (2) There is only one heat exchange hole during experiment, which is not affected by the variation of the surrounding geothermal field. If field test permits, the design interval of heat exchange pipes should be 5 m or above to avoid the situation that geothermal field reduce heat transfer efficiency. (3) According to the thermal response test data, when heat transfer power per meter is calculated under design situations, the average water temperature of heat exchange pipes should correspond to actual data to avoid mismatch between cooling load and thermal load which will result in high costs and low efficiency in later stage. (4) Backfill substances should take materials with higher heat conductivity to ensure adequate heat transfer (LI Shao-hua et al. 2015; MA Zui-liang et al. 2006; YAO Mu-shen et al. 2013). References CHEN Yan-hua, YU Zhong-yi, et al. 2009. Operation testing and analysis of groundsource heat pump systems in Wuhan area. Ventilating and Air Conditioning, 39(06): 12-17. DONG Qing-qing, ZHAO Ai-hua, et al. 2014. Monitoring report of shallow geothermal energy utilization project of typical buried pipes in Hengshui. Hengshui: The Third Party of Hydrogeology and Engineering Geology, Hebei Provincial Bureau of Geology and Mineral Investigation. LIU Jiang-tao, ZHANG Yong-shu, et al. 2010. Monitoring report of shallow geothermal energy utilization in Hebei Province by Heibei Geothermal Research Institute. Hengshui: The Third Party of Hydrogeology and Engineering Geology, Hebei Provincial Bureau of Geology and Mineral Investigation. LI Shao-hua, QIN Xiang-xi, et al. 2015. Analysis of influence factors for field heat response test of ground heat conduction coefficient. Ventilating and Air Conditioning, (12):49-52. MA Zui-liang, LV Yue. 2006. The design and application of ground source heat pump system. Beijing: China Machine Press. XU Wei, SUN Zhi-feng, et al. 2009. Interpretation of detection guide of the demonstration project of the application in renewable energy into building-inspaction program, inspection standard, assessment approach. Construction Science and Technology, (16):40-45. YAO Mu-shen, SUN Zhan-wen. 2013. Discussions on thermal response test process and related parameters. Refrigeration and Air-conditioning, (11):58-61. ZHAO Bing, ZHAN Wen-xian, ZHAO Ling. 2012. Analysis of the summer operation performance of a ground source heat pump system. Journal of Hebei University of Technology, 41(4):49-54. 70 http://gwse.iheg.org.cn