To understand the influence of groundwater advection on the performance of borehole heat exchanger (BHE)

an unsteady state coupled heat transfer model for a BHE with single U-tube was established by combining the transient moving finite line heat source model in porous medium outside the borehole and the steady state model for the heat transfer inside the borehole. The coupled model was validated by the data obtained in a soil thermal response test. The influence of groundwater advection on outlet water temperature and dynamic soil temperature responses was discussed. The heat transfer efficiency of BHE and the heat transfer rate per unit borehole depth were taken as two indices to assess the heat transfer performance of BHE. The heat transfer performances of BHE varied significantly with the type of soil in which it was buried. Ignoring the influence of groundwater advection in sand gravel would lead to a deviation of heat transfer rate per unit borehole depth up to 41% in the ground loop design; groundwater advection improved the heat transfer of BHE and an increase in advection velocity might help to shorten the time for the surrounding soil to reach a stable temperature. It was recommended that the mass flow rate of the inlet fluid to the BHE be more than 0.4 kg·s-1

but within a certain upper limit; the influence of the rise in inlet fluid temperature on the heat transfer efficiency of BHE could be ignored. In addition

the effects of groundwater advection in different types of soil under typical hydrological conditions on the heat transfer efficiency of multiple BHEs connected in series were compared. Heat transfer efficiency was jointly determined by such factors as soil physical properties

ground water advection velocity and number of BHEs connected in series.