The deep borehole heat exchanges (DBHEs) use a coaxial tube to obtain deep geothermal energy, avoiding the problem of groundwater corrosion and recharge, which have great application value. In this paper, a heat transfer model of a DBHE with a depth of 2000m and an outer diameter of 177.8mm is established by Fluent. The effects of various inlet temperature (5~20℃), inlet velocity (0.5~1m/s), thermal conductivity of rock (2.5~3.5W/(m·℃)) and geothermal gradient (0.035~0.045℃/m) on the heat transfer performance in a heating season are studied. The results show that the heat transfer performance of DBHEs decays with time. The rock temperature field is basically invariant outside the radius of 15m. Decreasing the inlet temperature, increasing the inlet velocity, increasing the thermal conductivity of the rock, and increasing the geothermal gradient can increase the heat extraction rate of DBHEs. Under the condition of constant geometric parameters, for every increase of 0.005℃/m in geothermal gradient, the heat extraction rate of the DBHE studied increases by an average of 22.76kW, and for every increase of 0.5W/(m·℃) in the thermal conductivity of the rock, it increases by an average of 26.44kW.
Keywords deep borehole heat exchanger, geothermal energy, numerical simulation, heat transfer