[titzer]

Interesting! Given the small amount of power relative to the total amount in the tides (I've seen an estimate of 3.7TW), this doesn't make a big difference to the Earth/Moon gravitational system, but I'm curious how this will affect the Moon's orbit (yes, I know it's millimeters or meters). For example, the Moon's orbit is increasing by stealing rotational energy from the Earth--i.e. Earth rotation slows down, Moon speeds up and orbital distance increases. Does this friction in the tidal system reduce the energy transfer to the Moon and therefore preserve Earth's rotational energy, or just redirect that rotational energy into our power grid? I would guess it would have to be the latter...

[vikingerik]

I believe it would reduce the transfer to the Moon. The Moon's orbit increases by stealing rotational energy from the Earth -- this happens because Earth's rotation carries its tidal bulge ahead of the Moon angularly, so the bulge pulls the Moon forward. The tidal harness uses some of the motion of the water to do work, so it gets carried less farther ahead by Earth's rotation, so it pulls the Moon forward less.