Thermal recovery characteristics of rock and soil around medium and deep U-type borehole heat exchanger

Medium and deep U-type borehole heat exchanger (MDUBHE) coupled with ground source heat pumps is a novel technique to extract geothermal energy from deep rock and soil. The thermal recovery characteristics of rock and soil are critical for the sustainable exploitation of deep geothermal energy. However, the thermal recovery characteristics of rock and soil around MDUBHE are unclear. In this study, a coupled heat transfer model considering multilayer properties is established to reveal the thermal recovery characteristics of rock and soil. The root mean square errors (RMSE) of this model are 1.8 degrees C and 0.3 degrees C compared with the experimental and CFD simulation data, respectively. The temperature distribution of rock and soil under short-term and long-term operations is investigated. The results show that the thermal recovery rate of deep rock and soil is over 91.2% after one heating-recovery cycle (1 year). The thermal decay rate of rock and soil after 15 years of operation is less than 9.3%. Furthermore, the thermal effect zone of MDUBHE increases faster early and slower later. The thermal effect zones of the descending pipe are 38.2 m, 74.3 m, 109.1 m, and 132.2 m in the 1st, 5th, 10th, and 15th years, respectively. This study could promote the stable utilization of deep geothermal energy with MDUBHE system.