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 using existing structural piles, which is called "energy pile". In this study, six large-diameter cast-in-place energy piles that encase various types of heat exchange pipes (i.e., parallel U-types with 5 pairs, 8 pairs and 10 pairs, and coil-types with 200 mm pitches and 500 mm pitch, an S-type) were constructed in a test bed, and the constructability of each type of the energy piles was compared. In addition, 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) were performed to evaluate the 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 closed-loop vertical GHEX was normalized by the pile length (or borehole length) and the total length of heat exchange pipe. The result from the in-situ tests indicates that the longer heat exchange pipe was installed, the more the heat exchange efficiency per unit pile length occurs. However, in case of tight layouts of the heat exchange pipe, note that thermal interference will occur between adjacent pipe strands, and thus decrease heat exchange efficiency per unit pipe length. Also, it is concluded that the constructed cast-in-place energy pile can demonstrate enough capacity for heat exchange compared with the closed-loop vertical GHEX.