Parametric study on optimum coil pitch of coil-type energy pile: thermal performance experiment and CFD simulation

A coil-type energy pile encases a loop of heat exchange pipe in a cast-in-place concrete pile to exchange geothermal energy by circulating a working fluid through the pipe. Compared with the conventional U-pipe, the coil-type heat exchange pipe seems to maximize the available surface area for heat exchange in an energy pile with much longer pipe installation. In this paper, two cast-in-place energy piles were constructed, which encase the coil-type heat exchange pipe with different coil pitches of 200 mm and 500 mm, respectively. A series of thermal performance tests was carried out by applying intermittent (8 hours operating–16 hours pause) artificial cooling/heating operation to simulate a cooling/heating load for commercial buildings. The test result indicates the heat exchange amount is not directly proportional to the pipe length because the tight coil pitch may cause thermal interference between each loop of the heat exchange. A computational fluid-dynamic (CFD) model was developed in this paper and verified by simulating the thermal performance test. Along with the CFD model, a parametric study was conducted to assess the effect of coil pitch and thermal interference, and an optimum coil pitch configuration was proposed with consideration of both heat exchange efficiency and economic feasibility.