Numerical Modeling of a Soil-borehole Thermal Energy Storage System
Borehole thermal energy storage (BTES) in near-surface aquifers combined with solar thermal energy harvesting is a renewable energy system to garner and store energy for the heating of buildings. The first community-scale BTES system in North America was installed in 2007 in the Drake Landing Solar Community, about 35 miles south of Calgary, Canada. Data collected over a six year period indicate that this system can supply more than 90% of the winter heating energy needs for 52 houses in the community. One major challenge facing the BTES system technology is the relatively low efficiency of heat extraction. To better understand the physical processes of BTES and to improve BTES efficiency of heat extraction for future applications at larger scales, a three-dimensional transient coupled fluid flow and heat transfer model is established using TOUGH2. The time-dependent injection temperatures and fluid circulation rate measured over the six years of monitoring are used as model input. The simulations are calibrated using soil temperature times series measured at different locations. The simulated and measured temperatures agree well with an intrinsic permeability of 1.5x10-14 m2, thermal conductivity of 2.0 W/m°C, and a volumetric heat capacity of 2.3 MJ/m3°C. The calibrated model serves as the basis for a sensitivity analysis of soil thermal and hydrological parameters on BTES system heat extraction efficiency. Sensitivity analysis results suggest 1) BTES heat extraction efficiency increases with decreasing soil thermal conductivity; 2) BTES efficiency decreases with background groundwater flow; 3) BTES heat extraction efficiency decreases with convective heat losses associated with high soil permeability values; and 4) unsaturated soils show higher overall heat extraction efficiency due to convection onset at higher intrinsic permeability values and lower thermal conductivity.