Influence of Borehole Heat Exchanger (BHE) Array Cooling and Heating Load Ratio on the Ground Temperature Imbalance and the Heat Pump Efficiency

Borehole heat exchangers (BHEs) are the most common utilization of shallow geothermal resources. However, the ground temperature imbalance around the BHEs caused by the difference in cooling and heating loads is an important factor causing the low coefficient of performance (COP) of ground source heat pump (GSHP). In this paper, the ground temperature around the BHEs and the efficiency of the heat pump are taken as the research object, and the finite element method (FEM) verified by the thermal response test (TRT) is utilized to establish a 3D finite element model with the same inlet temperature. The research results show that the ground temperature in the center of the BHEs decreases from 16 ℃ to 5.3 ℃ after five years without cooling load, and when the cooling and heating load ratio is between 2/3 and 1, the ground temperature can maintain a considerable balance. When the load ratio increases from 0 to 1, the heating efficiency of the heat pump increases from 3.8 to 4.5. On the contrary, the increase of the cooling and heating load ratio is not beneficial to the improvement of the heat pump efficiency in the cooling season. When the load ratio increases from 0 to 1, the cooling efficiency of the heat pump decreases from 7.2 to 6.1. This paper can provide a reference for formulating the cooling and heating load of BHEs, as well as give guidance for the ground energy storage projects using BHEs.