Constructability and heat exchange efficiency of large diameter cast-in-place energy piles with various configurations of heat exchange pipe

A new ground heat exchanger (GHEX) assembled in a ground source heat pump (GSHP) system has been devised, which is equipped in a structural pile, and thus called an “energy pile”. In this study, six large-diameter cast-in-place energy piles that encased various types of heat exchange pipes (i.e., three parallel U-types with 5 pairs, 8 pairs and 10 pairs; two coil-types with 200 mm coil pitch and 500 mm coil pitch; and S-type) were constructed in a test bed, and the constructability of each type of the energy pile was compared. Additionally, a 30 m-deep closed-loop vertical GHEX was separately constructed to compare the thermal performance and economic feasibility with the energy piles. A series of in-situ thermal response tests (TRTs) was performed to evaluate the relative heat exchange efficiency of each energy pile according to the various heat exchange pipe configurations. The relative heat exchange efficiency obtained in the cast-in-place energy piles and the down-sized 30 m-deep closed-loop vertical GHEX was normalized by the pile length (or borehole length) and the total heat exchange pipe length. The result from the in-situ tests indicates that the longer the heat exchange pipe installed, the higher heat exchange efficiency per pile length. However, in the case of tight layouts of the heat exchange pipe, thermal interference occurs between adjacent pipe loops, which decreases heat exchange efficiency. Economic feasibility of the energy piles was evaluated, considering the material and installation costs for assembling the heat exchange pipe with consideration of the heat exchange efficiency. It is concluded that the coil-type cast-in-place energy pile can achieve sufficient capacity for heat exchange, compared with the closed-loop vertical GHEX.