It is not just a concept, it represent the vast majority of grid energy storage today, we are talking about gigawatts. Hydro is the best, so good in fact that

It is not just a concept, it represent the vast majority of grid energy storage today, we are talking about gigawatts. Hydro is the best, so good in fact that most of the good spots are already taken, because as you said, it requires a very specific geography.