> effective pumped hydro requires very specific locations (a sizeable river with sufficient flow; a comparably high drop, and space for a large reservoirs not o

> effective pumped hydro requires very specific locations (a sizeable river with sufficient flow; a comparably high drop, and space for a large reservoirs not only above the dam but also below the dam) which are quite rare I also thought that pumped hydro needed all that to work, until I found a study about potential locations for pumped hydro in my home state. First of all, you don't actually need a river with that much flow; you only need enough flow to replace the water lost through evaporation, and for the initial fill of either the lower or the upper reservoir. Second, both reservoirs can be completely separate, the lower reservoir doesn't need to be directly below the dam of the upper reservoir. Both reservoirs are connected by a tunnel, with the generator/pump being at some point within this tunnel. So what you actually need is a pair of areas with suitable geology for a reservoir, one being higher than the other, close enough that the tunnel isn't too long (that study had tunnels of over 4 km, so they don't have to be very close), and a river near one of them. > The best spots for it are already used; you can't simply take a normal hydro power dam location and use it effectively for pumped hydro. The best spots for normal hydro would be the ones where the river is near the upper reservoir; pumped hydro allows using spots where the river is only near the lower reservoir. That's the insight I was missing before I found that study. You could even use an already existing hydro power dam as the lower reservoir for pumped hydro, as long as there's a suitable spot for the upper reservoir nearby.