Thermal performance evaluation of horizontal spiral coil-type ground heat exchangers

Geothermal energy has been widely used over the world instead of fossil fuel for environmental merits. A ground-source heat pump (GSHP) system utilizes a constant ground temperature to absorb heat during winter and to emit heat in summer. The GSHP system can be devieded into a closed type system and an open type system. The closed type system using a horizontal ground heat exchanger (HGHE) is relatively inexpensive and easy to install compared to the other types of systems. Among the types of HGHEs, since a horizontal spiral-coil type GHE has the advantageous arrangement for heat exchange between the heat exchanger and surrounding ground, it is the most effective type of the HGHE in terms of thermal performance. However, the thermal performance of a horizontal spiral-coil type GHE critically depends on its dimension which consists of the radius and pitch of a spiral, and thus it is important to accurately analyze how the radius and pitch affect the thermal performance of the horizontal spiral-coil type GHE. Therefore, this paper investigated the thermal performance of a horizontal spiral-coil type GHE according to its diverse shapes to suggest a beneficial design specification of the horizontal spiral-coil type GHE. In order to evaluate the thermal performance of the heat exchanger, a spiral-coil type HGHE was installed in a steel box of which the size was 5 m x 1 m x 1 m filled with dry Joomunjin standard sand, and a thermal response test (TRT) was conducted for 30 hours continuously. Besides, a numerical analysis was performed for its verification and a parametric study of the horizontal spiral-coil GHE using a numerical program which is based on a finite element method (FEM). As a result, in case that the pitch of the horizontal spiral-coil type GHE was more than 0.6 m, there was no more significant benefit in the thermal performance