An integrated system combining MDBHE (multi-casing DBHE) and heat pump achieves heating and cooling for medium-deep geothermal energy utilization

Abstract

Due to its convenient application in utilizing the medium-deep geothermal energy, the deep borehole heat exchanger has received extensive attention. Here we initiated an integrated system by coupling the multi-casing deep borehole heat exchanger (MDBHE) and heat pump to realize heat extraction and heating in the whole well in winter, and heat storage and cooling in the well upper part in summer. In the MDBHE, an annular pipe was used to store heat. Through the analysis of the simulation results of the influence of several variables on the system performance, it is found that the increase of heat storage temperature, annular pipe length and formation thermal conductivity raised the heat storage rate of the system, but the geothermal gradient is opposite. Benefiting from heat storage, the heat extraction rates in winter is also improved. Meanwhile, the heat pump elevated the heating rate by 15.9% in winter, and the cooling rate of the heat pump were respectively 19.3%, 21.5% and 23.7% less than the heat storage rate of the MDBHE based on the heat storage temperature of 40, 45 and 50 °C. Therefore, the integrated system can simultaneously satisfy the requirements of summer cooling and winter heating and maintain the formation heat balance, so that the urban heat island effect is mitigated.