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....