Influence of the Thermal Response Test’s Data Range on the Interpretation of Results for Borehole Thermal Resistance

The increasing use of low-temperature geothermal systems for heating and cooling requires reliable determination of borehole heat exchanger (BHE) thermal parameters, which directly influence heat pump performance and system design. The thermal response test (TRT) is commonly used for this purpose, but its results may depend on the selected interpretation time interval. This study investigates the influence of the TRT interpretation time range on calculated effective thermal conductivity and borehole thermal resistance, and compares these parameters for different BHE configurations. Field tests were conducted at the AGH University of Krakow, Laboratory of Geoenergetics, on three BHEs: single U-tube, double U-tube and triple U-tube. Measurements were performed under identical flow rate and heating power conditions. The temperature data were interpreted using the classical infinite line source method, the point method and the constant thermal resistance method for selected characteristic time intervals. The results show that both effective thermal conductivity and borehole thermal resistance depend on the interpretation interval and exchanger construction. Across the three investigated configurations, average thermal resistance was lower for BHEs with a larger internal pipe surface area; however, the installations also differed in pipe geometry and construction, so this trend cannot be attributed to surface area alone. These findings emphasize the importance of selecting an appropriate TRT interpretation method and time range for accurate geothermal system design.