A numerical model based on the finite difference method is developed for deep borehole heat exchangers used in medium-deep ground source heat pump systems, and the heat transfer characteristics under dynamic thermal load conditions are analyzed. Using this model along with actual meteorological data from Qingdao and validation via field experiments, the effect of fluid velocity, geothermal gradient, and geological parameters on the thermal performance of the deep borehole heat exchanger are investigated. The results indicate that while the inlet and outlet temperatures of the exchanger fluctuate in response to dynamic thermal loads, the outlet temperature exhibits greater stability. Although increasing fluid velocity enhances heat extraction, it also leads to higher heat loss caused by reverse heat transfer in shallow rock and soil layers. Higher geothermal gradients significantly improve heat extraction performance, and heat exchange efficiency is found to be greater in deeper regions compared to shallow zones.

geothermal energy  /  geothermal heat pumps  /  heat exchangers  /  numerical simulation  /  dynamic thermal load  /  heat flux per unit depth