Thermal interaction between a group of geothermal piles in the presence of natural convection

Abstract

Adequate estimation of thermal efficiency for a group of simultaneously acting geothermal piles is an important factor that may affect sustainability and technical feasibility of pile-anchored energy harvesting technology. This paper presents a series of coupled finite element analyses (FEAs) of pile-soil heat transfer to explore the impact of natural convection on group thermal performance of geothermal piles. FEA results demonstrated that, in comparison to a purely conductive heat flow condition in the ground, the presence of natural convection may reduce pile thermal interaction within a large pile group up to 90 % and elevate average heat rejection capacity per unit pile length by around 25 %. Such positive influences were enhanced with increase in intrinsic soil permeability, inlet fluid temperature, and number of closely spaced geothermal piles in a group. Moreover, the mobilization of natural convection in soil surrounding geothermal piles resulted in reduction in average pile surface temperature as much as 9 °C in a 3 × 3 pile group and a steep temperature gradient along the length of a pile with a maximum difference between temperature near head and base of a pile reaching up to 14 °C. Such findings may further influence thermo-mechanical and settlement behaviour of a group of geothermal piles.