Influencing factors analysis for the long-term thermal performance of medium and deep U-type borehole heat exchanger system

The medium and deep U-type borehole heat exchanger (MDUBHE) system has recently received extensive attention for heating buildings. The MDUBHE system could operate with low energy efficiency or even shut down due to the improper design influencing factors. Therefore, studying the influencing factors is a great need to ensure the stable and efficient operation of MDUBHE system. In this paper, a transient heat transfer model for the MDUBHE system is proposed, and its accuracy is validated using experimental data. The impacts of influencing factors on the performance of MDUBHE system during long-term operations are quantified. The results show that the buried pipe depth and geothermal gradient significantly impact the MDUBHE system's performance, while the pipe specifications have less impact on it. Furthermore, after 15 years of operation, when the inlet water temperature increases from 4.5 °C to 24.5 °C, the heat extraction rate decreases by 43.4%, and the energy consumption decreases by 52.7%. The thermal conductivity of grout increases from 2.2 W/(m ∙ K) to 2.7 W/(m ∙ K), while the heat extraction rate increases by only 0.7%, which is not significant to improve the performance of MDUBHE. This study has significant guidance for the engineering practice of MDUBHE system.