Thermal performance of novel cast-in-place energy piles equipped with multipurpose steel pipe heat exchangers (SPHXs)

A novel large-diameter cast-in-place energy pile equipped with steel pipe heat exchangers substitutes steel pipes for deformed rebars as not only main reinforcements but also heat exchangers. In this study, comprehensive field experiments and numerical analyses were conducted to evaluate the thermal performance of the novel energy pile. The two energy piles equipped with steel pipe heat exchangers were constructed in a test bed to perform in situ thermal performance tests. Then, a computational fluid dynamics model was developed and verified by comparing with the field test results. The developed numerical model was used for conducting a series of parametric studies with consideration of the relevant influential factors. The thermal conductivities of concrete and ground formations influence the thermal performance of the energy piles equipped with the steel pipe heat exchanger. In addition, the optimum flow rate considering the thermal performance and hydraulic power turned out to be 11.35 L/min for the 8 pairs of U-tube SPHX energy pile with 31.8-mm-diameter steel pipes. Note that the configurations of the steel pipe heat exchanger should be designed simultaneously considering the construction cost and thermal performance because an excessive number of steel pipes does not guarantee a proportional increase in the heat exchange amount of the steel pipe heat exchanger energy pile for the given borehole volume.