Capacity configuration and long-term performance of a deep borehole heat exchanger hybrid heating system with solar and gas assistance

The ongoing decarbonization of district heating is increasing interest in medium-deep geothermal systems. However, previous studies have focused on the heat extraction characteristics of the medium-deep borehole heat exchanger (MDBHE) and have neglected investigations of MDBHE-based hybrid heating systems, leaving the effects of subsystem capacity configurations on system performance unclear. This study aims to clarify how subsystem capacity configurations affect system operational performance and the subsurface temperature field. A novel MDBHE hybrid heating system assisted by a solar collector system and a gas boiler system was developed on a dynamic simulation platform. The results show that increasing the solar collector system capacity from 10% to 30% of peak load raises the MDBHE annual average inlet temperature from 16.11 °C to 20.77 °C, reduces energy consumption by 4.20 tce, and slightly improves system efficiency by 1.46%. CO₂ and N₂O emissions are reduced by 14.76 t and 0.11 kg, respectively. Increasing the ground source heat pump system capacity from 60% to 90% of peak load reduces MDBHE outlet temperature by 4.93%, nearly eliminates the energy consumption of the gas boiler system, and increases system energy efficiency by 76.75%. These findings provide valuable guidance for the design and coordinated operation of MDBHE hybrid heating systems.