Thermal efficiency of energy piles in stratified soil under unbalanced operation

Shallow geothermal energy piles are novel and cost-effective heat exchangers used in ground source heat pump systems for heating and cooling buildings. Pile foundations are used to exchange heat with ground besides structural support. Ground thermal conductivity is a decisive design parameter in shallow geothermal applications. Ground homogeneity relying on depth-weighted averaging has been the common assumption in wide research around energy piles in recent years, with soil layering influence remaining mostly unexplored. This becomes particularly important under unbalanced thermal operation, due to thermal accumulation in the ground. To explore this influence, a 3D finite element numerical model is built to solve for the heat conduction-convection multi-physics of energy piles embedded in layered soil. Unbalanced thermal load regimes with different building cooling-to-heating ratios are adopted for long-term assessment, showcasing the effect of this controllable parameter, for soil profiles with different thermal conductivity distributions. Results underscore the necessity to account for the thermal properties’ spatial variability in layered soil and recommend depth-specific thermal conductivity testing under unbalanced thermal load conditions. The thermal performance of the energy pile system in the considered stratified soils is shown to differ from that of their equivalent depth-weighted homogeneous ground owing to the growing difference in the accumulated temperature over the operation life, leading to underpredict the actual thermal conductivity by up to 31.5% as the contrast between layers grows and the unbalanced cooling-to-heating ratios increase. Furthermore, the depth sequence of ground layers of different conductivities is found to be important in predicting the thermal performance of energy piles.