Influence of moisture on heat transfer of ground heat exchangers in unsaturated soils

Abstract

The applications of ground heat exchangers (GHEs) in arid and semi-arid zones require knowledge about the effects of moisture on heat transfer in the ground. To evaluate the influence of moisture, this study integrates a finite line-source model, a normalized thermal conductivity model, and a saturation degree model. A normalized temperature difference, defined as a temperature-difference ratio (TDR) of unsaturated to saturated soils, is proposed to characterize the reduction in heat transfer of GHEs in unsaturated soils. The modeling results show that the effective thermal conductivity of soils increases with saturation degree in a nonlinear way, thus leading to the nonlinear effect of moisture on heat conduction of GHEs: the influence of moisture on heat transfer is more significant in low-moisture soils (Sl < 0.5) than that in high-moisture soils (Sl > 0.5). This result implies that the design length of borehole GHEs is highly sensitive to the degree of saturation in low-moisture soils.