Abstract：[Objective] In the context of China's promotion of the development and utilization of renewable energy, ground source heat pump (GSHP) systems based on shallow geothermal energy has been extensively used. With increasing service life of GSHP systems, the risk increases if the operation and maintenance are not properly performed. The operation analysis and operation strategy optimization of existing large-scale GSHP systems are of great value in standardizing the design and operation management of GSHP systems. In this work, the GSHP system of a residential community in Beijing is used as a case study. The heating, ventilation and air conditioning (HVAC) system in the community is a compound system, which is composed of a GSHP system, a chiller, a cooling tower and a gas-fired hot water boiler. The GSHP system has been used for heating and cooling buildings since 2014 with no other equipment. According to its operational records from 2014 to 2020, the current operating characteristics and rules of the system are summarized. A 3-D finite element model (FEM) was developed to perform back analysis of the soil thermophysical parameters. A 2-D FEM was designed to analyze the heat transfer of the buried heat exchanger group. The characteristic parameters of the heating and cooling loads of the GSHP system were determined by comparing the 2-D numerical results with the operating data. Furthermore, using 2-D FEM, the changes of ground temperature in the borehole area, shutdown situation, and water supply/return temperature of the ground source end were predicted for the next 5 years for optimal operation strategies. Finally, the long-term stability of the system was assessed using these strategies. As an introduction, there are two GSHP subsystems in the residential community, one (#53) with 108 boreholes and the other (#54) with 304 boreholes. Each borehole is 120 m deep, 15—18 cm in diameter, and 3.6 m in spacing. The operational data involved the supply and return water temperature and flow velocity in the ground source. From 2014 to 2020, every year, heat extraction was higher than heat rejection. For #54, the average heat imbalance rate reached 16.2 % from 2017 to 2019. The ground temperature in the borehole area decreased from 14.78 ℃ to 13.00 ℃. For the #54 system, the results of the 2-D FEM analysis revealed the following. (1) Using current operational measures to continue for 5 years, it was observed that the temperature of the water supply would still be at a low level in winter, there was a risk of shutdown, and the heat extraction and output level would be further reduced. (2) Three possible operation strategies were predicted: (a) stop the operation of the GSHP system in one winter season; (b) increase the use of the GSHP system in summer (considering the two unbalance rates of 5 % and 10 %); and (c) guarantee a certain source of heat and use gas-fired boiler peak regulation. All three measures alleviate further decreases in ground temperature and enhance downtime. The heat imbalance rate is maintained at 5.0 %—8.3 % for (b) and (c). In this case, because of imbalanced heating and cooling loads, after the GSHP system had been running for several years, the ground temperature had decreased and energy efficiency of the system had decreased in winter. The operational strategy requires adjustments. In view of the change in ground temperature, shutdown situation, water supply temperature at the ground source, and heat extraction/rejection in the next 5 years, using the gas-fired boiler set in the composite system is the most appropriate operation strategy.
郭红仙, 王天麟, 程晓辉, 关文, 赵勇, 杨军, 李建民, 刘征. 地源热泵系统长期稳定性及运行策略案例研究[J]. 清华大学学报（自然科学版）, 2024, 64(5): 801-809.
GUO Hongxian, WANG Tianlin, CHENG Xiaohui, GUAN Wen, ZHAO Yong, YANG Jun, LI Jianmin, LIU Zheng. Case study of the long-term stability and strategy optimization of a ground source heat pump system. Journal of Tsinghua University(Science and Technology), 2024, 64(5): 801-809.
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