Performance evaluation of coaxial-type GHEX in GSHP system installed in Korean residential building

Abstract

A coaxial-type ground heat exchanger (GHEX) is a closed-loop GHEX with heat exchange pipes in a borehole, arranged in a concentric tube-in-tube configuration. A coaxial-type GHEX enlarges the heat exchange area and induces turbulent flow more actively than conventional closed-loop GHEXs owing to the annular space formed by outer and inner pipes. Therefore, it improves the thermal performance of conventional GHEXs. Herein, the thermal performance of the coaxial-type GHEX was evaluated, and then compared with that of the closed-loop vertical GHEX. The Ground source heat pump (GSHP) system connected with the coaxial-type GHEXs was designed with the ground loop heat exchanger design program (i.e., GLHE-Pro). In accordance with the design, the GSHP system was constructed in a building basement, with four coaxial-type GHEXs and a closed-loop vertical GHEX for comparison purpose. Then, a series of field experiments (i.e., thermal response tests (TRTs), thermal performance tests (TPTs), short-term heating COP tests, and energy saving tests) were conducted to compare the thermal performance of the coaxial-type GHEXs and the closed-loop vertical GHEX with or without the GSHP system. As a result, the coaxial-type GHEX had 20% and 6.7% better thermal performance in TRT and TPT, respectively, than the closed-loop vertical GHEX. In addition, geothermal energy showed more stable performance with the coaxial-type GHEX in the GSHP system, than with the closed-loop vertical GHEX.