Influence of coil pitch on thermal performance of coil-type cast-in-place energy piles

Recently, the energy pile has been introduced as a viable alternative to the conventional closed-loop vertical ground heat exchanger (GHEX). The energy pile contains a heat exchange pipe inside the pile foundation and allows a circulation of working fluid through the pipe inducing heat exchange with the ground formation. In this paper, a series of thermal performance tests was conducted on two coil-type cast-in-place energy piles with different coil pitches, 200 mm and 500 mm, to evaluate the effect of coil pitch on the heat exchange performance of energy piles. Thermal performance tests were carried out by applying intermittent (8 h operating-16 h pause) artificial cooling and heating loads with the aid of a constant-temperature water bath. The experimental result indicated that the heat exchange rate per pile length was not directly proportional to the installed pipe length because the tight coil pitch caused thermal interference between each pipe loop. In addition, a parametric study was conducted by computational fluid-dynamic (CFD) simulations to assess the effect of coil pitch and thermal interference on the thermal performance. The CFD model was verified by comparing the numerical simulation results with the field test data. Finally, an optimum coil pitch in the test bed condition was proposed considering the thermal performance and the economic feasibility of coil-type cast-in-place energy piles.