Dual performance of novel steel pipe heat exchangers equipped in cast-in-place energy pile

Abstract

Pile foundations support superstructures by transferring structural loads into the ground, where rocks or hard ground formations exist. Energy piles additionally utilize geothermal energy for heating and cooling buildings via embedded heat exchange pipes. However, when constructing energy piles, additional budgets are required for materials and labor to install conventional high-density polyethylene (HDPE) heat exchange pipes. The applicability of cast-in-place energy piles equipped with novel steel pipe heat exchangers (SPHXs) is investigated herein as substitutes for deformed rebars to reinforce concrete. Energy piles encasing SPHXs are expected to fulfill multiple functions of supporting structural loads and operating as heat exchangers without requiring additional heat exchange pipes. Two cast-in-place energy piles encasing SPHXs of different diameters were constructed in a test bed. Then, pile load tests (i.e., dynamic load test and lateral load test) were conducted to estimate the bearing capacities of these energy piles. Consequently, both energy piles secured safety factors greater than 2.0, and the allowable lateral loads were sufficient, indicating acceptable structural capacities as pile foundations. In addition, a thermal performance test (TPT) was performed on the selected energy pile, and the result was compared with that of a conventional cast-in-place energy pile to assess the heat exchange capacity of the energy pile with SPHXs. The thermal performance of the cast-in-place energy pile considered herein was 30% greater than that of the conventional energy pile of similar scale, which shows the applicability of the proposed energy piles.