Energy efficiency analysis for medium-deep coaxial borehole ground-source heat pump systems

The energy efficiency of the medium-deep coaxial borehole ground-source heat pump systems (MDGSHPs) is affected by multiple factors, and to ensure their efficient operation, it is particularly important to study their influencing factors. In this paper, a heat transfer model of MDGSHPs’ heat exchanger is established, and the accuracy of the model is verified by engineering-measured data. The influence of multiple factors on the energy efficiency of the system is quantified based on this model during one heating period. The results show that the inlet water temperature and the thermal conductivity of rock soil have an important impact on the energy efficiency of the system, while the thermal conductivity of the backfill material has little influence on it. Increasing the circulation flow rate will improve the heat extraction rate, but the energy efficiency of the system will decrease. When the circulation flow rate increases from 8.33 kg/s to 10.56 kg/s, the heat extraction rate increases by 4.95%, and the energy efficiency decreases by 1.23%. Increasing the outer pipe diameter and the thermal conductivity of rock soil and backfill material can improve the energy efficiency of the system, while it is not obvious to improve the energy efficiency after reaching a certain value. This study has reference significance for the long-term stable operation and energy efficiency improvement of MDGSHPs.