Parameter optimization and ground temperature analysis of a solar-assisted medium-deep geothermal heating system under long-term operation

There has been growing interest in geothermal energy and solar energy combined heating systems with seasonal thermal energy storage. However, the long-term operational stability of such coupled systems remains challenging due to dynamic ground temperature variation around borehole heat exchangers (BHEs). This study develops a co-simulation model to evaluate a solar-assisted medium-deep geothermal heating system, analyzing 20-year thermal performance across 1500–2500 m BHE burial depths using a Shaanxi residential case. The results show that the introduction of solar thermal recharging leads to a temperature increase of 26 °C in the shallow ground layer, while the temperature decrease observed in the deep layer is significantly affected by the burial depth of the BHE. Meanwhile, the annual heat extraction decay rates reduce from 14.89 % to 2.62 % (1500 m) and 11.57 %–2.01 % (2500 m), demonstrating enhanced sustainability. Through multi-objective optimization of three key parameters—solar collector area, thermal storage tank volume and BHE burial depth—the optimized configuration achieves synergistic benefits, including an 9.02 % reduction in total energy consumption and a 20.75 % decrease lifecycle cost compared to conventional designs. The presented method can serve as a reference for the rational design of the medium-deep geothermal energy and solar energy coupled heating systems.