Evaluation of thermal efficiency in different types of horizontal ground heat exchangers

• We performed indoor TRT with horizontal slinky, spiral-coil and U-type GHEs installed in a steel box.
• We evaluated heat exchange rates from TRT results.
• Cost analysis with TRT results was conducted in order to evaluate optimal thermal efficiency of each type horizontal GHEs.

The utilization of geothermal energy is constantly increasing for economic and environmental advantages that this brings. Use of horizontal ground-heat exchangers (GHEs) can reduce installation cost and compromise between efficiency and cost. Among many kinds of horizontal GHEs, slinky and spiral-coil-type GHEs show higher thermal efficiency. This paper presents the results of experiments on the heat exchange rates of horizontal slinky, spiral-coil and U-type GHEs installed in a steel box (5 m × 1 m × 1 m). A commercial dry sand was used to fill the steel box, and thermal response tests (TRTs) were conducted for 30 h to evaluate heat-exchange rates according to various GHE-types. The U-type GHE showed the highest heat exchange rate per pipe length, about two and two and half times higher thermal efficiency than that for the horizontal slinky and spiral-coil-type GHEs, respectively. Furthermore, the heat exchange rates per pipe length with a relatively long pitch interval (pitch/diameter = 1) were 100–150% higher than those with a relatively short pitch interval (pitch/diameter = 0.2), in both spiral-coil and horizontal slinky-type GHEs. A cost-efficiency analysis was also performed, and it revealed that the U-type GHE was most economical under conditions of providing equivalent thermal performance.