An average fluid temperature to estimate borehole thermal resistance of ground heat exchanger

Analysis of borehole thermal resistance is important for the standard sizing of ground heat exchanger (GHE). In this paper, a p-linear dimensionless average fluid temperature is proposed to estimate borehole thermal resistance. A p-linear dimensionless fluid temperature and p-linear dimensionless average fluid temperature are introduced, and the p-linear dimensionless fluid temperature is compared with theoretical dimensionless fluid temperature calculated by quasi-three-dimensional model for both single and double U-tubes. Results show that the p-linear dimensionless temperatures with parameters p → 0 and p = −1/2 are respectively in good agreement with the theoretical dimensionless fluid temperatures of single and double U-tubes. Therefore, the dimensionless logarithmic mean temperature for p → 0 and the dimensionless geometric mean temperature for p = −1/2 should respectively be adopted to reasonably estimate the thermal resistance of single and double U-tube boreholes.