Evaluation on heat exchange efficiency of coaxial-type ground heat exchanger

The Ground Source Heat Pump (GSHP) system is the most effective way to reduce energy consumption in a building by utilizing geothermal energy for heating and cooling the building. In general, closed-loop vertical Ground Heat Exchangers (GHEXs) are most widely used in practice to induce heat exchange with the ground formation. However, the closed-loop vertical GHEXs demand relatively high initial construction costs including drilling boreholes. This construction costs of GHEXs can be reduced by improving the thermal performance of individual heat exchangers. In this paper, the coaxial-type GHEX has been considered to improve the thermal performance over the conventional U-type GHEXs because it possesses sufficient heat exchangeable area. Two 50-m-deep coaxial-type GHEXs with different pipe materials (i.e., high density polyethylene (HDPE) and stain less steel (STS)) were constructed. In addition, a closed-loop vertical GHEX was separately constructed in a test bed to compare the thermal performance with that of coaxial-type GHEXs. The effect of thermal conductivity of heat exchange pipes on the thermal performance of coaxial-type GHEXs was studied by performing a series of in-situ thermal performance Tests (TPTs).