Impact of a Thermo-Hydraulic Insulation Layer on the Long-Term Response of Soil-Borehole Thermal Energy Storage Systems

This study focuses on the effect of including a surficial insulation layer above a soil-borehole thermal energy storage (SBTES) system installed in a low plasticity clay deposit in the vadose zone. SBTES systems function by injecting heat collected from solar thermal panels into an array of vertical boreholes containing closed-loop geothermal heat exchangers. The goal of placing an insulation layer on top of the soil layer is to retain as much heat as possible within the borehole array to increase the efficiency by preventing the heat loss from the system to the atmosphere. A two-dimensional (2D), transient finite element model was built in COMSOL to consider the coupled heat transfer and water flow processes in the unsaturated soil layer within the SBTES system. Results indicate that presence of an insulation layer leads to a lower upward heat loss from SBTES system, but it is not significant. The insulation was observed to play a more significant role when coupled heat transfer and water flow was considered than when heat transfer was due to conduction alone.