This is a good one, how do you store excess energy for use when needed? I think their biggest flaw is they are using an open source to provide the water, which is subject to drought and evaporation. Something with a large cavern that is closed would seem much more efficient, and if it is a closed system, it should not have any losses that amount to much, thus being available in all situations. But this is a Galt like opportunity for some genius, they are exchanging electric for mechanical and back, the storage is based on the energy needed to pump back up. Something like a cascade with centrifugal pumps with a final stage lifting back might be better....Hnm..needs some math here..