Evaluation of effective thermal conductivity of closed-loop vertical ground heat exchangers by performing in-situ thermal response tests

To investigate efficiency of closed-loop vertical ground heat exchangers, six boreholes were constructed to install closed-loop ground heat exchangers in a test bed in Wonju, South Korea, and a series of in-situ thermal response tests has been performed. This field investigation is designed to verify the effect of grout materials, additives and the configuration of circulation pipe on the overall performance of closed-loop vertical ground heat exchanging systems. The bentonite grout and the cement grout were chosen as grouting materials, and silica sand and graphite were considered as additives to the grout paste. Additionally, a new 3-hole pipe section invented to lessen thermal interference between the inlet and outlet pipes was compared with the conventional U-loop pipe section. From the experimental results, it is concluded that the effective thermal conductivity of the ground heat exchanger with cement grout is much higher than that of bentonite grout, and graphite is proved more efficient than silica for enhancing the effective thermal conductivity. Finally, the new 3-hole pipe section provides a better thermal performance of the ground heat exchanger than the conventional U-loop pipe section. In addition, a series of numerical analyses using FLUENT was performed in order to simulate the in-situ thermal response test in the selected boreholes and to compare with the field test results. From the comparison between the in-situ thermal response test results and numerical simulations, the average thermal conductivity of the ground formation in the construction site is back-calculated as approximately 4W/m·K. This value can be used in evaluating the long-term performance of the closed-loop vertical ground heat exchanger.