Field measurements and numerical investigation on heat transfer characteristics and long-term performance of deep borehole heat exchangers

The coaxial deep borehole heat exchanger (DBHE) is widely applied in geothermal energy to extract heat from the ground for the building heating in the winter. The heat transfer characteristics and long-term performance of DBHE are of importance for the application. This work measured the heating season operation data of a middle-deep (2500 m) geothermal heating system from 2020 to 2021 in Xi'an, Shaanxi Province, China. And then a full-scale coaxial DBHE numerical model was developed based on the field measurement data. The influences of the material, the structure and the operation factors on the heat extraction were investigated for the DBHE. More -over, the long-term performance of DBHE was evaluated. The results show that DBHE has the promising heat extraction with an average power of 300 kW. The decrease of the inner pipe's thermal conductivity and the increase of the backfill material's thermal conductivity can improve the heat extraction power of DBHE. The intermittent operation strategy can increase the outlet-water temperature of DBHE by 23.55%. After a non-heating season, the ground temperature can recover. In the ten-year operation, DBHE has little effect on the temperature of the surrounding rock and soil, and the outlet-water temperature of DBHE only attenuated by 3.68%.