Effect of ground surface boundary conditions on thermal performance of ground source heat pump system with spiral ground heat exchanger

The spiral ground heat exchanger (GHE), characterized by its larger contact area with the surroundings, exhibits higher heat transfer efficiency compared to traditional U-shaped GHE. This study aimed to compare the effect of ground surface boundary conditions (BCs) on the thermal performance of ground source heat pump (GSHP) system with spiral GHE. Five commonly used BCs in the numerical models, namely adiabatic (BC1), constant temperature (BC2), time-varying temperature (BC3), convective heat transfer (BC4), and energy balance (BC5) boundary condition, were considered. A three-dimensional (3D) numerical model of spiral GHE considering unsaturated porous soil, time-varying flowrate control, and ambient environment disturbance was built and coupled with the heat pump model by COMSOL to analyze the thermal performance of GSHP system with various ground surface BCs. The impacts of borehole depth and ground thermal property on the ground surface BCs were further investigated. Results reveal that there is no definitive relationship between the surface temperature and time due to atmospheric thermal disturbances. Assigning adiabatic, constant temperature, or convective BC significantly reduces the accuracy of ground, grout, borewall, and fluid temperature, as well as the coefficient of performance (COP). As the BC changes from BC1 to BC2, BC4, and BC3, the main variation range of ΔCOP decreases by 17.21 %, 24.59 %, and 90.16 %. The errors caused by improper ground surface BC settings become more significant as the borehole depth (H) and ground porosity (φ) decrease. The main variation range of difference of mean fluid temperature increases by 5.74 %, 12.87 %, and 9.78 % as H reduces from 20 to 10 m and rises by 18.00 %, 5.97 %, and 14.81 % as φ reduces from 0.73 to 0.43. The energy balance BC or actual time-varying temperature BC calculated by energy balance equation is recommended for modeling the ground surface conditions. This paper underscored the significance of employing proper ground surface BC to accurately evaluate the thermal performance of GSHP system with spiral GHE.