Energy pile groups for thermal energy storage in unsaturated soils

Abstract

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