Back-analyses of in-situ thermal response test (TRT) for evaluating ground thermal conductivity

In this study, a series of computational fluid dynamics (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 high-density polyethylene pipe, borehole grouting and surrounding ground formation were modeled using FLUENT, a finite-volume method program, for analyzing the heat transfer process of the system. Two user-defined functions 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 numerical simulations provide verification for the in-situ thermal response test (TRT) results. The numerical analysis with the ground thermal conductivity of 4.0 W/m{dot operator}K yielded by the back-analysis was in better agreement with the in-situ TRT result than with the ground thermal conductivity of 3.0 W/m{dot operator}K. From the results of CFD back-analyses, the effective thermal conductivities estimated from both the in-situ TRT and numerical analysis are smaller than the ground thermal conductivity (=4.0 W/m{bullet operator}K) that is input in the numerical model because of the intrinsic limitation of the line source model that simplifies a borehole assemblage as an infinitely long line source in the homogeneous material. However, the discrepancy between the ground thermal conductivity and the effective thermal conductivity from the in-situ TRT decreases when borehole resistance decreases with a new three pipe-type heat exchanger leads to less thermal interference between the inlet and outlet pipes than the conventional U-loop type heat exchanger.