Evaluation of ground thermal conductivity by back-analyzing in-situ thermal response test (TRT)

In this study, a series of CFD numerical analyses was performed in order to evaluate the performance of six full-scale closed-loop vertical ground heat exchangers constructed in a test bed located in Wonju, South Korea. The circulation HDPE pipe, borehole and surrounding ground formation were modeled using FLUENT, a finite-volume method (FVM) program, for analyzing the heat transfer process of the system. Two user-defined functions (UDFs) accounting for the difference in the temperatures of the circulating inflow and outflow fluid and the variation of the surrounding ground temperature with depth were adopted in the FLUENT model. The relevant thermal properties of materials measured in laboratory were used in the numerical analyses to compare the thermal efficiency of various types of the heat exchangers installed in the test bed. The simulation results provide a verification for the in-situ thermal response test (TRT) resluts. The numerical analysis with the ground thermal conductivity of 4 W/mK yielded by the back-analysis was in better agreement with the in-situ thermal response test result than with the ground thermal conductivity of 3 W/mK. The results of the CFD numerical analyses and in-situ thermal response tests show that the effective thermal conductivities of the cement grouted borehole is higher than that of bentonite grouted. It was found that the graphite enhances the thermal conductivity of both cement and bentonite grouts. In addition, a new 3 pipe-type heat exchanger yields less thermal interference between the inlet and outlet pipes than the conventional U-loop type heat exchanger, which leads to better thermal performance.