This research focuses on understanding the key variables affecting the performance of soil-borehole thermal energy storage (SBTES) systems installed in the vadose zone. In particular, this study seeks to understand how the coupled thermal and hydraulic properties of unsaturated geomaterials and coupled heat transfer and water flow processes may be exploited to enhance heat injection and heat retention in an array of geothermal borehole heat exchangers. Specifically, heat injection into unsaturated geomaterials may be enhanced through convection of liquid water and water vapor as well as through latent heat energy transfer associated with water phase change. Further, heat retention in unsaturated geomaterials may be enhanced by the reduction in thermal conductivity associated with the thermally-induced drying expected during heat injection.

Multi-fidelity numerical models are employed in this study to consider the roles of different mechanisms of heat transfer during a cycle of heat injection and ambient cooling. A conduction-only model was found to be useful for evaluating scalability issues and for the geometric design of a full-scale SBTES system installed in Golden, Colorado, but required a low soil thermal conductivity to match collected field data. A numerical model the incorporates convection in the liquid and vapor phases as well as phase change was validated using the results from a full-scale SBTES system installed in an unsaturated rock layer in San Diego, California. The numerical model was calibrated using coupled thermo-hydraulic soil properties from element-scale experiments and parameters governing water vapor diffusion and phase change rates from tank-scale heat injection experiments. A comparison of the simulation results from this models with another model that does not incorporate vapor flow confirmed the significant effect of water vapor convection on the permanent drying of the subsurface during heat injection. The results from the validated numerical simulations and field experiments confirm the potential benefits of siting SBTES systems in the vadose zone. The validated numerical model considering convection in the vapor phase can be used in future studies to evaluate the evolution in the energy extraction efficiency over multiple heat injection and extraction cycles for SBTES systems in the vadose zone.