Performance evaluation of closed-loop vertical ground heat exchangers by conducting in-situ thermal response tests

The effective thermal conductivity of six vertical closed-loop ground heat exchangers (GHEXs), which were installed in a test bed located in Wonju, South Korea, has been experimentally evaluated by performing in-situ thermal response tests (TRTs). To compare the thermal efficiency of the GHEXs in field, various installation conditions are considered such as different grouting materials (cement vs. bentonite), different additives (silica sand vs. graphite) and shapes of the circulating pipe-section (conventional U-loop type vs. 3-pipe type). From the test results, it can be concluded that the cement grout has higher effective thermal conductivity than the bentonite grout by 7.4-10.1%, and the graphite outperforms the silica sand by 6.7-9.1% as a thermally-enhancing additive. In addition, the new 3-pipe type heat exchange pipe that yields less thermal interference between the inlet and outlet pipes shows better thermal performance over the conventional U-loop type heat exchange pipe by 14.1-14.5%. Based on the results from the in-situ thermal response tests, a series of cost analyses has been carried out to show the applicability of the cement grouting, the graphite additive, and the new 3-pipe type of heat exchange pipe section. For the same condition, the cement grouting can reduce the construction cost of GHEXs by around 40% in the given cost analysis scenario. In addition, an addition of graphite and use the new 3-pipe heat exchange pipe lead to about 8% and 6% cost reduction, respectively.