All you need is enough water to run the turbines for 12 hours.
Then when the sun comes up the VERY SAME water that just came through the turbines gets pumped back up using excess PV (or whatever) power.
Yes, but its not like you can turn Lake Mead and Hoover Dam into a perpetual motion machine that endlessly throws off more and more energy.
The fact is that energy will be
lost at each step of the water going around the cycle, i.e. it will take MORE energy to pump the water back up again then you will get by running the water back down through the turbines. Energy is lost to friction in the pipes, and energy is lost due to inefficiencies in the pumps that will be pumping the water back up hill and into the lake.
So if takes more energy to pump the water back up then you get from running the water down through the turbines, why not use that energy directly to power LA instead of wasting some of it pumping the water back up hill and using the energy from water going through the turbines to power LA. Why not just skip all the pumping and turbine turning and just use whatever energy source is doing all of that work?
The energy losses are not insignificant. Water turbines are about 85-90% efficient, and water pumps maybe about 80-85% efficient. There's a lot of energy lost to friction in pipelines---lets say 5%. Transmission lines also lose power, and are maybe 90% efficient. This means this project would waste about 30-40% of the energy created at lake Mead by the time it gets to LA. That doesn't seem like a good idea.
I'm just offended at the complexity and inefficiency and scale of this project, I guess. It seems to me that LA could save an amount of energy comparable to this project just by banning 6 cylinder cars, or painting all their roofs white, or banning swimming pools. Or, as I said earlier in this thread, a nuclear power plant built nearby in southern California could also provide baseline energy support at night when the sun don't shine.
Pumping ocean water from the California coast to Lake Mead in southern Nevada so Californians can continue to waste huge amounts of energy just seems like wretched excess to me.
Cheers!