Techno-economic study of subsurface pumped hydroelectric energy storage

The role of energy storage in maintaining system reliability and price stability becomes more important with an increasing fraction of renewable energy. While pumped hydroelectric energy storage remains the main energy storage technology in most countries, it is constrained in many regions due to limited land availability and topographical requirements. In this study, we propose a large-scale energy storage alternative that uses pressurized subsurface aquifers as a storage medium. The system stores energy by displacing fluid between non-communicating aquifers by pumping, and, thus, building a pressure difference between the aquifers. When needed, fluid from the over-pressurized aquifer is released to the under-pressurized aquifer through a turbine recovering stored energy. This study evaluates the techno-economic performance of the proposed storage system through operational simulation, cost analysis, and feasibility under time-varying electricity prices. Historical electricity price data are used to demonstrate system operation under realistic market conditions. The results are compared with the current dominant energy storage technologies, including battery storage and pumped hydroelectric storage, in terms of efficiency and cost. The study indicates that our proposed subsurface hydroelectric energy storage may not be comparable to battery storage in the case of short-duration storage; however, it shows the potential as a long-duration energy storage solution in regions where pumped hydroelectric storage is limited. In a grid with frequent short-term variability, it is advisable to combine the proposed system with battery storage, where batteries will provide the fast response to short-term changes, while the subsurface storage will balance the long-duration seasonal storage requirement.