Solar-assisted heating of a medium-deep U-type borehole heat exchanger system: Improved operational performance and mitigated subsurface thermal attenuation

Integrating medium-deep geothermal energy and solar thermal energy in buildings can enhance renewable energy utilization and coordinated multi-energy management while reducing carbon emissions. The long-term operation of medium-deep U-type borehole heat exchanger (MDUBHE) systems may lead to performance degradation and rock-soil temperature attenuation, especially in severe cold regions. Solar-assisted heating is an effective measure to mitigate these problems. However, design methods for such coupled systems remain underdeveloped, and the mechanisms by which solar assistance affects long-term system performance are still not fully understood. In this paper, a coupled simulation platform for a solar-assisted MDUBHE heating system is established, an operation control strategy for the coupled system is developed, the accuracy of the platform is validated, and the effects of solar-assisted measures are investigated. The results show that solar-assisted heating effectively alleviates the attenuation of MDUBHE water temperature, with the inlet and outlet water temperatures in the 15th year increasing by 18.7% and 11.1%, respectively. The operational efficiency of the coupled system is also improved after the integration of the solar collector system. Furthermore, solar-assisted heating reduces the thermal impact of the coupled system on the rock-soil and alleviates its temperature attenuation, with the thermal attenuation rates at depths of z = 1600 m to z = 2000 m reduced to 8.85%, 9.20%, 9.51%, 9.79%, and 10.03%, respectively. This study provides useful guidance for the design and implementation of solar-assisted MDUBHE systems.