Study on design and calculation method of borehole heat exchangers based on seasonal patterns of groundwater

The vertical closed-loop ground source heat pumps (GSHPs) utilize the ground as a heat sink. In areas where the groundwater is abundant with strong seasonal fluctuation, the flow rate and water table control the heat transfer rate of borehole heat exchangers (BHEs) and the thermal balance of the energy storage rock mass (ESRM). Current design focuses on the balance between rates of heat dissipation to and extraction from subsurface during operation seasons, but the energy carried by groundwater in other seasons is not utilized. This study proposes that thermal imbalance should be allowed during GSHP operation, which can be balanced off by appropriately utilizing the heat replenishment by groundwater in other seasons, achieving thermal balance year-round. A new approach to calculate BHE heat transfer rate, energy replenishment by groundwater, and annual energy budget of ESRM based on seasonal groundwater fluctuation was derived. The annual energy default and temperature deviation charts of ESRM were applied to predict whether and when the thermal balance would be restored. A case study of single heating condition showed that the thermal balance may be restored very soon after the heating period or in the flooding season 3–4 months later, depending on the groundwater pattern.