A numerical and experimental approach to the estimation of borehole thermal resistance in ground heat exchangers

This paper presents a numerical and experimental study on the evaluation of borehole thermal resistance with TRT (thermal response test) and TPT (thermal performance test) results observed in closed-loop vertical type boreholes with U and W type GHEs (ground heat exchangers). Field TRTs were carried out for 48 h on a closed-loop vertical type borehole, and an equivalent ground thermal conductivity was estimated using the infinite line source model. Closed-loop vertical type boreholes with U and W type GHEs and field ground conditions were numerically modeled using a three dimensional finite element method to estimate borehole thermal resistance and the TRT results were compared. Field TPTs were also conducted for 100 h continuously to calculate the heat exchange rate and borehole thermal resistance. The borehole thermal resistance values were compared with various analytical solutions, and the multipole and EQD (equivalent diameter) method produced results closer to those of the experimental and numerical analysis than the SF (shape factor) method.