Effect of Tube-in-Tube Configuration on Thermal Performance of Coaxial-Type Ground Heat Exchanger

Among various types of ground heat exchangers (GHEXs), a coaxial-type GHEX exhibits outstanding thermal performance owing to the geometric characteristics of the concentric tube-in-tube configuration. In this study, a computational fluid dynamics (CFD) model was developed and verified using field test results for the coaxial-type GHEX constructed in a test bed. Parametric studies were conducted using the developed CFD model for various influential factors (i.e., flow rate of a circulating fluid, thermal conductivities of bentonite grout and inner pipe, and hydraulic diameter) to evaluate its thermal performance and energy efficient 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 coaxial-type GHEX should be composed of materials with 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