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[waterlesscloud]

A lot of hydroelectric in the US pumps the water back up at night (when demand is low) so that they can use it to generate electricity the next day.

Just south of the Bay Area, you can see such a facility on the road between the 101 and the 5, at Pacheco Pass.

http://en.wikipedia.org/wiki/San_Luis_Reservoir

See also : http://en.wikipedia.org/wiki/Pumped_storage