A novel study on influence of ground surface boundary conditions on thermal performance of vertical U-shaped ground heat exchanger

To better understand the influence of ground surface boundary conditions (GSBC) on the thermal performances of U-shaped ground heat exchangers (UGHE), this study proposes a numerical model with one energy balance boundary condition (EBBC) considering the spectrally selective properties of ground. This study comprehensively discusses the effects of operation and design parameters on the thermal responses of various GSBCs and UGHE. Additionally, the geographical performances of the EBBC are compared. The main results indicate that implementing EBBC in UGHE reduces prediction errors for water and borehole wall temperatures, with a maximum decrease of 1.70 °C and 1.78 °C, respectively. Regarding the average water temperature, the variation induced by different GSBCs can be reduced by 51.0% as the borehole depth increases from 50 to 125 m. Similarly, the difference induced by different GSBCs can be reduced by 22.7%, with ground thermal conductivity decreasing from 3.0 to 1.5 W/(m·K). Based on the geographical analysis, the convective GSBC can be imposed on UGHE for engineering approximation in only temperate and tropical regions. For other climate regions, the use of EBBC is recommended. Finally, the sensitivity analysis results indicate that borehole depth is the most important dependent variable, followed by ground conductivity and volumetric heat capacity.