Thermal resistance analysis of an energy pile and adjacent soil using radial temperature gradients

This paper examines the thermal resistances of energy piles and surrounding soils. A field-scale bored energy pile was installed through stiff sandy clay and dense sand and instrumented to measure temperatures on the external walls of the heat exchanger pipes, at the pile-soil interface, and in the soil. The radial temperature gradients between the pipes and the pile-soil interface, and the pile-soil interface and the soil, were used to evaluate the pile and soil thermal resistances, respectively. The thermal resistances were on the same order of magnitude with values of 0.053 mK/W, 0.072 mK/W, and 0.066 mK/W for the pile, stiff sandy clay, and dense sand due to similarities in their thermal properties. The analysis suggests that pile and soil thermal resistances are influenced by the pile dimensions, number of pipes, concrete cover, soil type and duration of heating. Hence, meticulous interpretation of thermal resistances considering these parameters should be conducted to understand heat transfer processes in energy piles accurately. Estimates of the pile thermal resistance from the equivalent diameter and thermal response test methods were found to be inconsistent with each other, highlighting the significance of considering steady state in-situ radial temperature gradients in designing energy pile systems.