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I think that's the key right there: motion of the water. There's no free lunch here; it's not a battery.

Maybe a good application would be throwing the whole rig into the surf, or a salty tidal stream, where you get the induced current from lunar power.

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Your comment made me go on a total tangent considering whether the Moon with eventually fall on us if we harvest enough tidal energy due to gravitational drag. But interestingly enough, the opposite is true, tidal drag is actually moving the moon further away from us, and slowing down the Earth's spinning.

That didn't make sense to me, so I looked it up and found there's a good wikipedia article about it (which you probably read as well.) It's too much for me to explain poorly in an HN comment, so I'll just link it for anyone who's also scratching their head: https://en.wikipedia.org/wiki/Tidal_acceleration#Effects_of_...

The key point to me was that since earth is spinning about 28 times faster than the moon rotates around us, the drag force equalizes the frequencies by making earth spin slower, and the moon rotate faster, thus further away from us. Eventually they'd be in equilibrium, spinning in lockstep.

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