Reliability and variability in thermal conductivity measurements: A comparison of CIT-TRT and conventional TRT in Japan

This study evaluates the effectiveness of the inverse heat exchange rate (1/q') method for estimating thermal conductivity of the ground using data from the constant inlet temperature thermal response test (CIT-TRT). In total seven TRTs including constant heat load TRT (conventional) and CIT-TRT were conducted in two different locations in Japan to compare the results of this method with those obtained from the Horner plot method and logarithmic graphical method for the conventional TRTs. The findings indicated that the estimated thermal conductivities using this method deviated by only 3.5% from those determined using the Horner plot method, confirming the applicability of this approach. In addition, the results from the CIT-TRT were compared with those obtained from the conventional TRT, which demonstrated the validity of the 1/q' method. Furthermore, the CIT-TRT was performed using two different inlet temperatures, 25°C and 35°C, to evaluate the effectiveness of this method under conditions with a limited heat load power. The results showed that the lower inlet temperature produced acceptable outcomes, though it was only 5°C above the initial ground temperature in the region. As the main achievement of this research, the 1/q′ method is shown to offer a practical and efficient approach for thermal conductivity estimation in CIT-TRTs, significantly reducing computational time while maintaining accuracy across varying inlet temperatures, GHE configurations (single or double U-tubes), and diverse geological conditions.