Thermomechanical effects induced by energy piles operation in a small piled raft

In this work, a series of fully coupled three-dimensional thermomechanical finite-element analyses has been carried out to investigate the mechanical and thermal interaction effects induced in a small piled raft equipped with energy piles during the operation of an air-conditioning system based on ground source heat pumps (GSHPs). In particular, attention has focused on (1) the axial-load redistribution among the various piles of the raft as a result of differential thermal dilations occurring in the pile and the soil during the transient heat conduction process and (2) the thermal interaction effects that may affect the heat exchange process when multiple energy piles are placed at short distances within the same piled raft. The results of the numerical simulations, which are in qualitative agreement with the limited experimental observations from full-scale tests on energy piles currently available in the literature, show that significant (positive and negative) axial-load changes can be experienced by both thermally active and nonactive piles of the raft. The load redistribution effects among the piles of the raft reach their peak at a very early stage of the thermal conduction process, when the differences in temperature among active and nonactive piles are largest, and then reduce steadily with time up to steady-state conditions. For the thermal properties of the soils and the raft geometries considered in this study, the peaks in axial-load variations are observed within about 30 days after the beginning of the thermal stage. This time span is well inside the normal operation times of GSHP systems, of the order of a few months. As for the thermal efficiency of the system, the numerical results show that in the same time span the thermal interaction effects between the various heat exchangers in the raft are negligible; however, significant reduction in the specific heat flux to and from the soil can be observed for larger operation times.