On the resistance to heat flow across soil-structure interfaces

Abstract

A number of recent publications have suggested that in order to reproduce thermal testing of energy piles, a finite value for the geo-contact thermal resistance (geo-CTR) at the soil-structure interface needs to be introduced. There is currently no guidance as to what value the geo-CTR should have. The geo-CTR will have two potential impacts in terms of the use of energy geo-structures, (i) reducing heat exchange efficiency, and (ii) increasing temperature changes and associated mechanical impacts within the geo-structure. This article sets out a new experimental method for quantifying the geo-CTR. The proposed method is based on the imposition of a heat flux through the two solid materials that form the contact. Its novelty rests with the acknowledgement that heat loss is inevitable and that the geo-CTR can be more reliably defined based on heat flow measurements at the actual contact. This concept is demonstrated via numerical modelling of a generic test set-up, where the errors induced by not accounting for heat loss, the interpolation of temperatures to the contact and the presence of the heat flow sensor were assessed. Initial test results are then presented that demonstrate how the method works. These results suggest that for a dry medium sand, while the geo-CTR is sensitive to the soil density, it is small and the effect on heat transfer is also likely to be small. Further testing will explore the relative importance of a number of factors and in particular, the soil type, on the geo-CTR.