[gregwebs]

Geography plus water plus land usage concerns make it rare. The western US has plenty of sites identified but little water to spare. There is one pumped hydro storage project in process in the entire USA and it is being challenged on environmental concerns. If we are going to make pumped hydro work these issues do need to be acknowledged.

[pydry]

The paucity of projects in the US is because about 50% of your electricity comes from natural gas. If it's very cheap (and it was) and you dont care about CO2 (& the US doesnt) it renders most kinds of storage fairly pointless.

Pretty much the only place without suitable geography in the US for pumped storage is Florida.

In Europe hydropower in the alps is already being used as a giant battery.