Relative constructability and thermal performance of cast-in-place concrete energy pile: Coil-type GHEX (ground heat exchanger)

Energy piles encase heat exchange pipes in a pile foundation to use geothermal energy. This paper evaluates the constructability and thermal performance of energy piles equipped in a large-diameter drilled shaft. Two energy piles were constructed by fabricating coil-type heat exchange pipes in cast-in-place concrete piles. The coil pitch was designed to be 200 mm and 500 mm, respectively. The constructability of each energy pile was evaluated in terms of time and ease for installing. In-situ TRTs were performed to compare the thermal performance of energy piles. The relative heat exchange efficiency for the coil pitch 200 mm is only 1.2 times greater than that of the coil pitch 500 mm energy pile, even though the coil pitch 200 mm energy pile encases the heat exchange pipe 2.4 times as longer as the coil pitch 500 mm. In addition, the result of TRTs was compared with two well-known analytical to estimate thermal properties of ground formation. A thermal performance test was carried out by applying artificial cooling operation to the energy piles, and indicates the heat exchange rate is not directly proportional to the pipe length because the tighter coil pitch configuration may cause thermal interference between the coil loops.