Continuum Model of Underground Thermal Energy Storage Applied to Efficiency Optimization

A major obstacle to extensive deployment of renewable energy sources is their seasonal intermittency. Borehole thermal energy storage (BTES) is an inexpensive technology that can mitigate this intermittency. Hot water is pumped from the center to the edge of an array of boreholes to heat it in the summer, and heat is discharged in winter by reversing the process. Mathematical modeling is key to efficiency improvement. I develop an approach to modeling BTES that is simpler than existing approaches that use either embedded symmetrized borehole temperature fields or explicit simulation of the full 3d geometry. This approach focuses on the average radial flow of water, and develops coupled 1d reaction-diffusion equations for the water and soil temperatures. These are supplemented by a time-dependent heat-transfer coefficient. With two adjustable parameters, the model fits four-year temperature data at 10-minute intervals from the Drake Landing Solar Community closely. I use the model to explore possible modifications to the charging and discharging strategies. A strategy in which heated water is discharged at variable distances can increase the efficiency by over 20% in some cases.