Which energy storage technology is most suitable for long-duration energy storage (8+ hours) in a grid-scale application?

For long-duration energy storage (8+ hours) in grid-scale applications, pumped hydro storage (PHS) is generally the most suitable technology. Pumped hydro storage involves pumping water from a lower reservoir to a higher reservoir during periods of low electricity demand and then releasing the water back down through turbines to generate electricity during periods of high demand. This creates a closed-loop system for storing energy. The key advantage of PHS for long-duration storage is its scalability and cost-effectiveness at large capacities. PHS facilities can store vast amounts of energy, often measured in gigawatt-hours (GWh), making them capable of providing long-duration grid support. The technology is well-established with a long history of operation, providing a reliable and proven solution for grid-scale energy storage. The energy storage capacity of PHS is determined primarily by the size of the reservoirs and the elevation difference between them, meaning capacity can be increased relatively easily compared to other technologies. While the initial investment for constructing PHS facilities can be significant due to the need for suitable topography and environmental considerations, the operational costs are relatively low, and the lifespan of PHS facilities can extend to several decades, making them economically viable for long-term energy storage. Although other technologies like flow batteries and compressed air energy storage (CAES) are also capable of long-duration storage, PHS remains the most widely deployed and economically proven solution for grid-scale applications requiring extended storage durations.