Role of considering non-constant soil thermal parameters in the simulation of geothermal heat storage systems in the vadose zone

This study focuses on understanding the role of thermal properties of soils (thermal conductivity and specific heat capacity) in the performance of geothermal heat exchanger arrays used to store heat in the vadose zone. Although the impacts of degree of saturation and temperature on the apparent thermal conductivity of soils has been widely studied, the same is not true for the volumetric heat capacity of soils. To investigate the role of the heat capacity, a three-dimensional (3D), transient finite element model was built in COMSOL to consider the representative field conditions as well as coupled heat transfer and water flow processes in the unsaturated soil within a soil-borehole thermal energy storage (SBTES) system. The numerical analyses were performed considering two cases: using constant thermal properties and using thermal properties that vary with changing degree of saturation. Two models were considered to predict volumetric heat capacity of different soils with changing degree of saturation and were compared with laboratory measurements. Results indicate that using constant thermal properties leads a difference in temperature distribution in the heat exchanger arrays. The proposed prediction equation in this study for volumetric heat capacity was more successful than the common prediction equation.