Heat transfer characteristics of energy piles considering temperature-dependent soil thermal conductivity

In energy pile operations, high inlet temperatures can lead to soil temperature increase and moisture movement within the soil pores, potentially altering soil thermal conductivity. Traditional models generally neglect this crucial aspect. In this study, three-dimensional numerical models considering temperature -dependent soil thermal conductivity were developed based on the vapor diffusion theory and validated against field test measurements. The models were subsequently adopted to comparatively analyze the thermal performance of the energy pile, the pile body temperature, and the surrounding soil temperature under single -pile conditions. Simultaneously, a sensitivity analysis of thermal parameters was conducted for grouped conditions, including group arrangements, pile positions, and operation modes. In the single -pile scenarios, the traditional model tended to underestimate the heat injection rate by nearly 11.6%. In pile group scenarios, the temperaturedependent soil thermal conductivity exhibited a delayed impact on the thermal accumulation. Additionally, the expedited thermal diffusion caused by the varying soil thermal conductivity accelerated the influence of thermal accumulation on the thermal performance of grouped piles, while intermittent operation modes mitigated the impact. In the energy geo-structure design, the thermal performance requires to be accurately evaluated, which calls for special attention to the temperature -dependent soil thermal conductivity.