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Comment by jaggederest | Hacker News Reader

It doesn't need to be fresh, you could dam a certain bay and put generator/pumps in the wall. Pump a cubic mile of water back and forth as needed, and you get tidal generation for free, too.

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Tidal generation using dams is a very bad idea. You're taking energy from and releasing energy to the system at times when it's most detrimental to the inertia of the system. Basically you're stealing inertia from the rotation if the earth, which would be ok, except you're amplifying the effects because of timing. You can think of it like swinging in a swing. The way you build up a huge amount of momentum is by shifting your weight at the peaks and troughs of the arc.

All hydropower does this, and changes the rotation of the earth. I believe the stated value was speeding the rotation up a few milliseconds per day due to the fact that so much water is held in the northern hemisphere in reservoirs.

The ultimate result is that the moon falls towards the earth a couple micrometers and everything is fine.

You can do the same thing with compressed gas as well, or really anything. Gas and fluid do make the most sense in terms of directly driving generator turbines though. The only issue with gas is thst you have to deal with thermal effects from expansion and compression.

Using compressed gas for energy storage has a lower cycle efficiency. Compressing heats up the gas. Heat has to be stored and used when decompressing, else a significant portion of energy is lost and the cycle efficiency is dragged into the low 50% range, or even less.

True, I did mention thermal effects, but it’s also something you can do in a place like Cali, Nevada, Riyadh, etc Without wasting precious water. Other options include giving up on the idea of these systems and going for giant Vanadium flow batteries or something similar.

Vanadium flow batteries are an extremely interesting concept. But with lower power densities, they seem to be more suitable for use in a local community, rather than individual homes or grid scale plants.

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