Coupled Thermal Conductivity Dryout Curve and Soil–Water Characteristic Curve in Modeling of Shallow Horizontal Geothermal Ground Loops

Shallow horizontal ground loops harness stored subsurface thermal energy and operate in conditions where the moisture and temperature of the surrounding soil vary spatially and temporally. The thermal conductivity of the soil is dependent on soil moisture and temperature and is a design parameter that greatly influences the size and performance of horizontal ground loops. However, soil thermal conductivity is often assumed to be constant and conservative estimates are used in the design of ground loops. Two fundamental constitutive relationships, the thermal conductivity dryout curve (TCDC) and the soil–water characteristic curve (SWCC), can be coupled and used to quantify transient moisture-dependent thermal behavior of soil. In this study, coupled TCDCs and SWCCs were utilized in two-dimensional models based on the finite-element method to predict moisture migration effects on transient hydraulic and thermal behavior of unsaturated soil surrounding geothermal exchange loops. Soil thermal conductivity predicted from using coupled TCDCs and SWCCs are compared to the conventional method of using a conservative value. Results suggest that employing coupled TCDCs and SWCCs can provide realistic and improved values of soil thermal conductivity for the design of horizontal ground loops.