Heat transfer analysis of deep coaxial geothermal heat exchangers: Insights into heat-flux distribution on borehole wall

Abstract

Deep geothermal heat exchangers (DGHEs) have emerged as a potential approach to exploiting geothermal energy as a stable high-temperature heat source for ground-coupled heat pumps. This study aimed to offer insights into the heat transfer of DGHEs by systematic numerical simulations. This study first created a finite-volume model (FVM) for the heat transfer of DGHEs, which uses the governing equations of circulating fluid inside the borehole as a time-varying boundary condition for the borehole wall. The FVM model was verified by comparison with several reported models. The heat transfer analysis emphasized the validity of the homogeneous-medium assumption about the ground and the time-varying characteristics of the heat flux on the borehole wall. The homogeneous-medium assumption seems to be only acceptable in cases of limited thermal conductivity differences between the ground layers (e.g., < 2.0 W/m·K). Not only the heat flux on the borehole wall varies linearly with borehole depth, but the slope and the interception of the line are linear functions of the logarithm of time.