Comparison of effective thermal conductivity in closed-loop vertical ground heat exchangers

Performing a series of in-situ thermal response tests, the effective thermal conductivity of six vertical closed-loop ground heat exchangers was experimentally evaluated and compared to each other, which were constructed in a test bed in Wonju, South Korea. To compare thermal efficiency of the ground heat exchangers in field, the six boreholes were constructed with different construction conditions: i.e., different grouting materials (cement vs. bentonite), different shape of heat exchange pipe-sections (conventional U-loop type vs. new 3 pipe-type), and different additives (silica sand vs. graphite). One observation borehole was installed in the middle of the test site to measure a subsurface temperature change during performing the in-situ thermal response test. From the test results, it can be shown that cement grouting has a higher effective thermal conductivity than that of bentonite grouting, and graphite better performs over silica sand as a thermally enhancing addictive. In addition, a new 3 pipe-type heat exchanger yields less thermal interference between the inlet and outlet pipe than the conventional U-loop type heat exchanger, which results in superior thermal performance.

► Cement grout provides higher effective thermal conductivity than bentonite grout with the same addictive and pipe type.
► An addition of graphite as a thermally enhancing additive can improve the effective thermal conductivity of the GHEX.
► The newly devised 3 pipe-type shows better thermal performance over the conventional U-loop pipe because it can significantly reduce thermal interference between the inlet and outlet pipe.