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I understand the concept of pumping water uphill into a reservoir as energy storage. And yes, a hydroelectric dam is what you use for this kind of energy storage.
The part I don't get, is where one uses a large dam like Hoover Dam for this. The thing's on a river. You can't pump more water upstream than you're letting flow downstream through the dam. The best you can do, is to pump all the water you let through back up. If you've got that much solar energy that you can do that, then you have more than enough to replace all of the electricity you get by letting any water through the dam. In fact, because of losses, you actually have more than enough energy.
Unless they're getting water from somewhere else, and the article doesn't say they're doing that, then this make absolutely no thermodynamic sense.
The important issue is does it make economic sense? This kind of thing is done near Yosemite at Shaver Reservoir. Water is pumped uphill during the night when demand is much lower and power much cheaper. It flows downhill during the day to generate power during the time of highest demand and most expensive power. Economically this is a win, largely because of the very high cost of power in the area.
Still doesn't cover the OP question, why not use less water when demand low. dunno about US hydro plants but visited some in NZ
medium sized plants they typically have 4 - 6 inlets, they can fully or jusr partially close any of these.
when consumer draw is low close the valves, and let the solar/wind run the consumer draw. rather then feed consumer draw from the dam and have wind/solar pump water uphill, (which as OP mentioned is simply way less efficient then slowing the water.)
- build huge fans to spin the windmills on sunny windless days?
- build powerful UV lights to make solar when the wind blows at night?
- why not charge drone batteries so they can fly up and seed rain clouds above dams.
combine all these ideas (incl the water pumps) and many more to create endless ways to consume excess power rather than just turn the damm plants down!
This internet thing is amazing! Letting people use it: worst idea ever!
Water up mountains' has been a preferred energy store for a long time, and very effective it is too.
Viz. the Dinorwic pumped storage scheme, with which I was intimately and rather dangerously associated - as a one time photogrammetrist I was 'volunteered' to go into the old mineshafts around the planned tunnel with a pair of calibrated survey cameras on a sledge and photograph long abandoned shafts that were deemed too dangerous to send a surveyor down, and subsequently pull the plates up on a stereo-comparator to see if there were any side shafts.
The this was in the mid seventies, and the Dinorwic scheme is still working successfully today.