Influencing factors analysis and operation optimization for the long-term performance of medium-deep borehole heat exchanger coupled ground source heat pump system

Application of the medium-deep borehole heat exchanger (MDBHE) coupled ground source heat pump (GSHP) system on building heating has been gradually accepted by public and its sustainability becomes a key concern. In this work, a long-term transient heat transfer model for the MDBHE coupled GSHP system is proposed to investigate the effects of various influencing factors on the thermal extraction and energy efficiency of this system. Sensitive analysis highlights the significant effects of the specific heat transfer rate, rock-soil thermal conductivity, geothermal gradient, pipe depth and fluid velocity on the annual decline of the system performance during its long-term operation. Furthermore, the operation optimization for this system should be conducted based on its performance in quasi-steady state. An optimal fluid velocity exists to acquire the maximum specific heat transfer rate of MDBHE under a constant Seasonal Performance Factor of Heat Source (SPF₂). In order to guarantee an efficient thermal extraction, the comprehensive effects of both specific heat transfer rate and fluid velocity on the energy efficiency should be taken into account. For the benchmark MDBHE, the operation condition is optimized with the specific heat transfer rate of 142 W/m, and the fluid velocity of 0.7 m/s; the annual SPF₂ is always greater than or equal to 4.0 during the long-term operation, which ensures a sustainable and efficient operation.