Energy pile groups for thermal energy storage in unsaturated soils

A coupled heat transfer and water flow model implemented in COMSOL validated against measurements from a tank scale test was applied to investigate the application of energy pile groups for thermal storage unsaturated soil layers. The novel focus investigation understanding long-term thermo-hydraulic response within group during injection at high temperature up 90 °C associated impacts on performance. Unsaturated layers are advantageous due enhanced convective with vapor diffusion favorable insulation properties lower conductivity soils surrounding system. Evolutions degree saturation having different hydraulic table depths were simulated five years operating piles inlet fluid temperatures 30 extraction. Transient fluctuations observed all simulated, but permanent decrease only layer greater air entry suction after several cycles heating cooling. While always between that dry saturated no groundwater flow, depth found control both rate total stored. When comparing performance borehole exchangers commonly used applications, approximately 1.2 times more effective extracting faster confirming their suitability storage.