Effects of stratified ground properties and borehole thermal capacity on the performance of coaxial borehole heat exchangers

The coaxial borehole heat exchanger (CBHE) is a crucial component of the mid-deep geothermal energy extraction process, which has been extensively utilized for building heating. However, the effects of soil properties at varying depths on the characteristics of the CBHE have seldom been investigated. In light of this, we employed a semi-analytical model to examine the impact of the thermal conductivity and thermal capacity of soil at different depths on the heat extraction capability of the CBHE. The findings demonstrate that the increasing thermal conductivity and thermal capacity of each soil layer is beneficial for elevating the outlet temperature and heat extraction of the CBHE, with a more pronounced effect observed in deeper soil layers. Furthermore, an increase in thermal conductivity and thermal capacity in the first layer results in a deeper reverse heat transfer, while similar increases in other layers lead to a reduction in this transfer depth. Additionally, the influence of the thermal capacities of the pipe walls and grout on the performance of the CBHE was examined using the constructed model. The results indicate that neglecting the thermal capacities of the pipes and grout will lead to an underestimation of the outlet fluid temperature of the CBHE. These research findings hold significant implications for the design and construction of CBHE systems tailored to specific geological conditions.