Transient evaluation of a soil-borehole thermal energy storage system

Abstract

This study focuses on the simulation of transient ground temperatures in a field-scale soil-borehole thermal energy storage (SBTES) system in San Diego, California. The SBTES system consists of an array of thirteen 15 m-deep borehole heat exchangers installed in conglomerate bedrock at a spacing of approximately 1.5 m. Heat collected from solar thermal panels was injected into the SBTES system over a 4-month period, after which the subsurface was monitored during a 5-month ambient cooling period. The SBTES system is located in the vadose zone above the water table with relatively dry subsurface conditions, so a coupled heat transfer and water flow model was used to simulate the ground response using thermo-hydraulic constitutive relationships and parameters governing vapor diffusion and water phase change calibrated using soil collected from the site. The simulated ground temperatures from the model match well with measurements from thermistors installed at different radial locations and depths in the SBTES system and are greater than those simulated using a conduction-only model for saturated conditions. Significant overlap between the effects of the borehole heat exchangers was observed in terms of the ground temperature. Although the numerical simulations indicate that permanent decreases in degree of saturation and thermal conductivity occurred at the borehole heat exchanger locations, the zone of influence of these changes was relatively small for the particular site conditions.