Field experiment on heat exchange performance of various coaxial-type ground heat exchangers considering construction conditions

The coaxial-type Ground Heat Exchanger (GHEX) possesses a concentric tube-in-tube configuration, which can provide a sufficient heat exchange area, and induces turbulent flow conditions. Therefore, the coaxial-type GHEX is expected to outperform the conventional U-type GHEX in terms of thermal performance. However, it is very important to design an optimal configuration (i.e., pipe length, the roughness of pipe wall and the shape of cross section) for the coaxial-type GHEX to generate turbulent flow inside the pipe and to achieve sufficient heat exchange area. In this paper, GHEXs of various construction conditions were considered, and the factors governing the thermal performance of coaxial-type GHEX were identified through field tests. Four 50-m-deep coaxial-type GHEXs were constructed in a test bed with different pipe materials, pipe diameters and grouting materials. In addition, a 50-m-deep closed-loop vertical GHEX was separately constructed to compare the thermal performance with the coaxial-type GHEXs. A series of in-situ thermal response test (TRT) and in-situ thermal performance test (TPT) was performed in the constructed coaxial-type GHEXs to investigate the effect of various construction conditions on thermal performance. As a result, the thermal performance of coaxial-type GHEXs is directly influenced by the thermal conductivity of the pipe and grouting material. The pipe diameter also influences the thermal performance of coaxial-type GHEX. Especially, it is noted that an optimal flow rate exists, which maximizes the thermal performance of the coaxial-type GHEX.