1. The lower lake still needs to be substantially above sea level and the local water table etc. so that it can actually be drained and so that it doesn't just fill up with local water. You can't do this by the hoover dam, since you'd also have to lower the river another few dozen meters. That's more challenging than it sounds, since you'd have to continue digging all along its path until you reached the place that the river naturally fell to that lower altitude.
2. Reservoirs are very large. The lower reservoir in this article is 7 million cubic meters[1]. The Hoover dam passes 289 million cubic meters of water daily, with around 78.3 million cubic meters of that generating electricity. The largest land vehicle in the world can excavate 700k cubic meters per day[2].
3. We do dig them out- the reservoir in the article was substantially enlarged. But it's still rare to find places you can actually do that.
4. The upper reservoir is a totally different question. It would take over 125 years to dig out a hole the size of Lake Mead for the Hoover Dam. It would take almost a million of the bombs used the Sedan nuclear test[3] to dig that hole.
I mean, when all you have is a hammer, and also your entire society is repressing massive post traumatic stress disorder, everything looks like a nail.
It'll definitely raise the cost, I acknowledge that what I'm talking about isn't going to be a small project. I just wouldn't necessarily assume that it would raise the cost so much as to make this infeasible. To be clear, it's also not like I know anything about civil or power engineering.
You would presumably be pumping air from one sphere to the other.
That air ends up compressing/decompressing, and therefore heating or getting cold. That dramatically lowers efficiency.
And on top of that, a football stadium sized sphere full of compressed air at the bottom of the ocean is going to need hold-down anchors at a scale the world has never seen...
> "The general idea is to have a closed vessel sitting on the seafloor. Surplus energy is then used to pump water out of this vessel, leaving the inside at a near-vacuum. When it’s desired to recover energy from the system, water can be allowed to flow back into the vessel under the pressure generated by the seawater above. As the vessel is filled, the water flowing in turns a turbine, generating electricity in just the same way as a traditional pumped hydro system."
2. Reservoirs are very large. The lower reservoir in this article is 7 million cubic meters[1]. The Hoover dam passes 289 million cubic meters of water daily, with around 78.3 million cubic meters of that generating electricity. The largest land vehicle in the world can excavate 700k cubic meters per day[2].
[1]: https://coflein.gov.uk/en/site/423380 [2]: https://en.wikipedia.org/wiki/Overburden_Conveyor_Bridge_F60
3. We do dig them out- the reservoir in the article was substantially enlarged. But it's still rare to find places you can actually do that.
4. The upper reservoir is a totally different question. It would take over 125 years to dig out a hole the size of Lake Mead for the Hoover Dam. It would take almost a million of the bombs used the Sedan nuclear test[3] to dig that hole.
[3]: https://en.wikipedia.org/wiki/Sedan_(nuclear_test)