Numerical investigation on the long-term heating performance and sustainability analysis of medium-deep U-type borehole heat exchanger system

Exploiting low-carbon, clean, and stable medium-deep geothermal energy is critical to achieving clean and sustainable heating for buildings in northern China. The medium-deep U-type borehole heat exchanger (MDUBHE) system is a novel technology that has emerged recently for exploiting deep geothermal energy. However, previous studies mainly analyzed the heating characteristics of the MDUBHE system in a single building type (i.e., continuous operational conditions), and the long-term (15-year) heating performance under different operational conditions is still unclear. Moreover, the heating sustainability of the system in different regions has not been clarified. Therefore, to promote the application of the MDUBHE system, this paper conducts numerical simulations to analyze the system's long-term heating performance, thermal recovery characteristics of rock and soil, and the system's energy efficiency under different operational conditions and regions. The results show that the MDUBHE system has high heating sustainability under different working conditions. After 15 years of operation, the maximum decay rate of MDUBHE's outlet water temperature is less than 3.15 % under different operating conditions and less than 3.01 % in different regions. In addition, the maximum decay rate of total system heating capacity is less than 8.86 % under different operating conditions and less than 7.60 % in different regions. Furthermore, the intermittent operation of the system and the higher thermophysical properties of rock and soil can enhance the rock-soil's thermal recovery. The MDUBHE system can efficiently operate over the long term under different working conditions. The maximum decay rates of the system's energy efficiency are less than 1.86 % under different operating conditions and less than 1.60 % in different regions. The study could promote the application of the MDUBHE system in different regions.