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The global energy demand has put forward new requirements for building cooling and heating. As a renewable energy, shallow geothermal resources have attracted much attention. Ground source heat pumps are an effective utilization form of geothermal resources, most of which use U-shaped borehole heat exchangers to exchange heat with rock and soil mass. Therefore, the thermal physical parameters of rock-soil and the stratum structure will affect the heat transfer performance of the borehole heat exchanger. In this paper, a sandbox test bench and a 3D single U numerical model are constructed for the main geological conditions in karst areas to explore the heat transfer law of borehole heat exchanger under special geological structure. The results indicate that: (1) The thermal conductivity of limestone is higher than that of dolomite and sandstone, and the limestone stratum has strong heat transfer capacity and the fastest heat recovery rate. In order to avoid the thermal accumulation caused by the long-term operation of the system, the recovery time of stratum temperature field shall be kept at 1:1 ratio to the operation time of the system; (2) The average heat exchange of the three layers is 22.6 W/m, which is better than that of the double layers 21.4 W/m and the single layer 18.4 W/m. The heat transfer capacity of layered strata is better than that of single-layer strata, and they have layered characteristics; (3) The heat transfer capacity of borehole heat exchanger increases with the increase in thermal conductivity along the vertical direction of the formation. Each increase of 0.4 W/(m·K) in the thermal conductivity of the lower rock-soil increases the heat exchange per unit well depth by about 6 W/m; (4) The calculation equation of heat transfer under multi-layer stratum structure is proposed by using the calculation method of comprehensive thermal conductivity.

Study on influence of geotechnical characteristics on heat transfer performance of borehole heat exchanger