Effects of underground parameters on the heat extraction performance of medium-depth coaxial borehole heat exchangers

The medium-depth coaxial borehole heat exchanger (MCBHE) is a clean-energy technology that can utilize geothermal energy at depths of 1,000-3,000 m for building heating. However, underground parameters significantly affect its heat extraction performance, and the underlying influence mechanisms of different parameters remain unclear. In this study, a three-dimensional CFD model of an MCBHE was established to investigate the effects of soil stratification, ground temperature gradients, groundwater seepage locations, and seepage velocities on its heat extraction performance. The results show that, under stratified soil conditions, the proportion of heat extracted from medium-depth soil layers increases, while the decrease in the inner-tube fluid temperature drop rate (ITFTDR) becomes slower. For the ground temperature gradient, an increase in the linear ground temperature gradient significantly improves the heat transfer efficiency between the MCBHE and the surrounding soil. Although an increase in the layered ground temperature gradient enhances the cumulative heat extraction amount (CHEA), it also leads to higher values of the heat loss rate of the inner tube (HLRIT) and ITFTDR. Under seepage conditions, for both single-layer and double-layer seepage, seepage closer to the lower part of the MCBHE results in a greater increase in CHEA and a lower ITFTDR. A higher uniform seepage velocity promotes temperature recovery of the soil surrounding the MCBHE. Moreover, a seepage pattern in which the velocity gradually increases with soil depth is beneficial for heat extraction from medium-depth soil layers, allowing the fluid in the lower part of the MCBHE to exchange heat more sufficiently with the surrounding soil.