Reply #9: According to this source that's one-way [View All]

[wtmusic]

So round trip efficiency is 64%.

(figures from a site with animated GIF images taken with a grain of salt) :D

"Energy recovery depends on total volume of water and its height above the turbine

* need at least 100-meters this is a stringent limit on locations
* artificial lower reserviors can made via excavation can achieve higher energy density due to large vertical distance (up to 1000 feet!)
* facility does not impact free flowing stream
* sediment build-up at dam base is minimized
* Hydropower is 80% efficient (uphill or downhill). So to pump uphill and the get energy downhill, efficiency is 0.8x0.8 = 64%"

http://zebu.uoregon.edu/2001/ph162/l8.html