Effects of subzero temperature circulating fluids on the performance of Energy Micro-piles

Micro-piles are Italian-patented small-diameter (typically < 300 mm) drilled and grouted piles reinforced with a steel tube. Recently, the integration of heat exchangers into micro-piles has led to the development of Energy Micro-Piles (EMPs), which serve the dual purpose of providing both structural support and thermal energy for heating and cooling. This innovative solution expands the utility of micro-piles, offering a promising approach for sustainable energy applications in civil engineering. Maximising the temperature difference between the circulating fluid and the surrounding ground is crucial to optimise heat exchange. In winter, the fluid temperature of the heat exchangers can also drop below 0°C to enhance heat extraction. However, freezing of the soil at the EMP-soil interface must be avoided, as it may compromise foundation stability. For this reason, current guidelines conservatively recommend maintaining fluid temperatures above 0°C to prevent frost formation. In this study, a detailed threedimensional thermo-hydraulic model of the EMP-soil system is developed to investigate heat transfer and potential soil freezing due to subzero fluid temperatures. Simulations across different soil types reveal that fine-grained soils are more prone to freezing due to their lower thermal diffusivity, which causes cold to accumulate and limits thermal power extraction – an effect that is less pronounced in coarse-grained soils.