Analysis of the factors influencing the performance of medium-shallow borehole heat exchangers coupled with a ground source heat pump system

Abstract

Medium-shallow borehole heat exchangers (BHEs) offer high heat exchange efficiency and low initial investment, which is of great significance for achieving carbon emission reduction goals. Therefore, this paper establishes a heat transfer model for medium-shallow BHEs and uses this model to analyze the impact of six factors on the heat transfer capacity of BHEs, including geotechnical properties, backfill material properties, and borehole characteristics. Additionally, the factors influencing the performance of ground source heat pump systems were analyzed. The results indicate that higher soil thermal conductivity, soil specific heat capacity, and backfill material thermal conductivity can enhance the heat exchange capacity of BHEs. Specifically, when the soil thermal conductivity increases from 1.5 to 3.5 W/(m·°C), the heat extraction increases from 17 to 19.68 kW, an improvement of 15.76%, and the heat dissipation increases from 10 to 21.4 kW, an improvement of 114%. When the thermal conductivity of the backfill material increases from 0.5 to 2.5 W/(m·°C), the heat extraction increases from 13.66 to 20.13 kW, an improvement of 47.36%, and the heat dissipation increases from 11.28 to 19.57 kW, an improvement of 73.49%. The heat extraction capacity of the borehole is significantly affected by the borehole depth; when the depth increases from 150 to 550 m, the heat extraction increases from 6.26 to 36.93 kW, an improvement of 489%, and the heat dissipation increases from 8.85 to 19.91 kW, an improvement of 124%.