Is pumped hydro really able to scale to a significant level where it can provide enough energy storage for a national (or even state/regional) grid? I was always under the impression that pumped hydro was indeed awesome, very cost-effective, etc. but could only be developed in places with very specific geographies. After all, you have to somehow place two reservoirs near each other with a significant elevation difference. Isn't this the limit that governs pumped hydro as a storage technology?