Numerical Simulation of Long-Term Heat Extraction Performance of Deep Borehole Heat Exchanger Array with Variable Depth Arrangement

In this paper, a variable depth layout scheme is proposed for the heat extraction performance of deep borehole heat exchanger array in long-term operation. A three-dimensional unsteady heat transfer model is established by the OpenGeoSys platform, and the heat exchange process of five boreholes under different depth combinations is numerically simulated. The simulation period is 10 years, and four variable depth arrangements (groups B, C, D, and E) and an equal depth control group (group A) are set up to analyze the changes of inlet and outlet water temperature and subsurface temperature field. The results show that the variable depth layout can effectively improve the temperature field distribution and reduce the thermal interference between borehole array. After 10 years of operation, the maximum temperature difference between the subsurface temperature field reaches 1.35 °C, and the outlet water temperature increases by 8.25 °C at the highest, showing better thermal recovery ability and long-term stability. Symmetrical variable depth arrangement (such as group B or E) has good applicability in severe cold areas. The research provides a new idea for the vertical optimization design of underground borehole array, and has reference value for promoting the efficient and stable development of geothermal heating systems.