Ground energy balance for borehole heat exchangers: Vertical fluxes, groundwater and storage

Borehole heat exchangers (BHE) are the most frequent applications for extracting low-enthalpy geothermal energy. Their effect on shallow ground is commonly assessed by modeling the in-situ thermal conditions with little attention on the transient heat flux regime stimulated by BHEs. Here, we characterize these heat fluxes using analytical models. The approach is applied to a field site with long-term monitoring of the ground temperature development around a BHE. Our major findings are that advective transport shapes vertical heat fluxes and the power provided to the system from groundwater and from storage substantially varies over time. Examination of power sources reveals that during early operation phase, energy is extracted mainly from the storage. Then, local depletion enhances the vertical fluxes with the relative contribution from the bottom reaching a limit of 24% of the total power demand, whereas that from the ground surface becomes dominant for Fo > 0.13. Long-term energetic analysis, including the time after system shutdown, highlights that recovery may take much longer than the operation time. However, axial heat fluxes accelerate recovery and the ground surface then becomes even more dominant providing about two thirds of the power over the full life-cycle of the studied standard system.