Effect of tube-in-tube configuration on thermal performance of coaxial-type ground heat exchanger

Among various types of ground heat exchangers (GHEXs or GHEs), a coaxial-type GHEX exhibits outstanding thermal performance owing to the geometric characteristics of the concentric tube-in-tube configuration. In this study, the unique configuration of the coaxial-type GHEX was investigated through parametric studies, which provided the energy efficient configuration. A computational fluid dynamics (CFD) model was developed and verified by using the field test results for the coaxial-type GHEX constructed in a test bed. Then, the parametric studies were conducted using the developed CFD model for various influential factors (i.e., flow rate of a circulating fluid, hydraulic diameter, and thermal conductivities of inner pipe and bentonite grout), which were selected considering the tube-in-tube configuration. The thermal performance of the coaxial-type GHEX increased with an increase in the flow rate of a circulating fluid and the thermal conductivity of bentonite grout. In addition, the inner pipes of the coaxial-type GHEX should be composed of materials with a thermal conductivity lower than 0.025 W/(m⋅K) to relieve thermal interference between the outer and inner pipes. Notably, it was concluded that the coaxial-type GHEX should be designed to secure the sufficient heat exchangeable area and retention capacity of the circulating fluid.