Thermal Performance Analysis and Multi-Factor Optimization of Middle-Deep Coaxial Borehole Heat Exchanger System for Low-Carbon Building Heating

Ground-source heat pumps with deep borehole heat exchangers can fully utilize deep geothermal energy, effectively reducing the consumption of non-renewable energy for building air conditioning and achieving energy conservation and emissions reduction goals. However, the middle-deep coaxial borehole heat exchange (MDBHE) development is insufficient, and there is currently a lack of definitive guidelines for system optimal design and operation. This paper firstly establishes an effective and efficient system model and examines nine important parameters related to the design and operation of the MDBHE using a single-factor analysis. Thereafter, we compare and analyze the impact of different parameters through an orthogonal experimentation method. The findings reveal that the three most significant factors are borehole depth, inlet temperature, and mass flow rate, in descending order of importance. In addition, in terms of operation mode, this paper makes a comparative analysis of the operation of the MDBHE in variable flow mode and constant flow mode. The results showed that the average energy consumption of the pump in the variable flow mode decreased by 9.6%, and the surrounding ground temperature recovered at a faster rate.