Coarse-grained model of underground thermal energy storage applied to efficiency optimization

Seasonal storage of thermal energy, by pumping heated water through a borehole array in the summer, and reversing the water flow to extract heat in the winter, can ameliorate some of the intermittency of renewable energy sources. Simulation can be a valuable tool in enhancing the efficiency of such storage systems. This paper develops a simple, efficient mathematical model of spatial temperature dynamics that focuses on the radial water flow in a cylindrical borehole array. The model calculates the time course of the temperature difference between outgoing and incoming water accurately, and allows new optimization strategies to be explored easily. A strategy based on discharging water heated by the array before it reaches the array center can increase the storage capacity by 25% for a system with a 20% smaller radius than the well-studied Drake Landing system. If the density of boreholes is also doubled, the improvement is 29%.