How does harnessing tidal energy affect the Moon's orbit?

Since tides are caused by the Moon, drawing energy from them should affect the Moon, right? I can’t figure out the mechanism and the effects on the moon.

Does interfering with tides affect the center of gravity, and hence cause “drag” on the moon? Does this make the moon slow down or move closer to earth? How concerned should we be about this (is tidal energy a non-renewable resource?)

IANAA, But this doesn’t sound right at all. If all the ocean water suddenly disappeared do you think the moon might fall out of orbit?

Rather than drawing energy from the Moon, tides rather impart orbital energy to the Moon. Because of this, the Moon moves (very, very slowly) away from the Earth each year. In the very long run, tides allow the Moon to draw off rotational energy from the Earth, slowing the Earth and lengthening our day. Someone with a better physics background than me will be along to explain it, I’m sure. We’re talking about billions, maybe tens of billions of years before the Moon will escape Earth’s orbit entirely.
Tidal power has absolutely no effect on the Moon’s orbit. We’re simply taking some of the energy that normally goes into sloshing the ocean around and harnessing it before allowing it to continue sloshing. This has about the same environmental impact that a wind turbine does in slowing down the wind.

Oh much less than a turbine has on the wind, I’d think. When the turbine spins energy is transferred to it from the wind which will slow the wind down at least marginally.

In the Earth Moon system, the Moon’s gravity tugs the oceans up to a higher level. The Earth does the same to the Moon (as does the Moon to Earth’s land mass), but rock doesn’t move nearly so much. But since the Earth is rotating faster than the Moon is revolving around the Earth, the tidal bulge moves ahead of a direct line between the Earths-Moon center line. The asymmetrical Earth now tugs a bit harder on the Moon to speed it up so energy is transferred from the Earth’s rotation to the Moon’s orbit.

What the tidal energy systems would do depends on how they extract the energy. If the water flows through turbines as the tide is going up, a little less of water will bunch up and the bulge won’t be so big and slightly less energy will be transferred to the moon but no energy will be transferred the other way. If they let the water flow in then extract the energy with turbines as it flows back out, then, the water might stay high a bit longer and transfer more energy to the Moon than now. I’d guess the most efficient way to do things would be to run turbines both as the water flows in and flows out, so I’d think there’d be little net effect on the energy transfer to the Moon from the Earth.

Note that the energy we’re getting from this is basically just wasted energy now – energy that goes into eroding the beach knocking down sand castles, etc.

Are you sure it will eventually escape Earth’s orbit?
What about slowing Earth’s rotation until the moon is in geo-synchronous orbit, and then there are no tides?

From http://en.wikipedia.org/wiki/Orbit_of_the_Moon

“The Moon is gradually receding from the Earth into a higher orbit, and calculations[9][10] suggest that this would continue for about fifty billion years. By that time, the Earth and Moon would become caught up in what is called a “spin–orbit resonance” or “tidal locking” in which the Moon will circle the Earth in about 47 days (currently 27 days), and both Moon and Earth would rotate around their axes in the same time, always facing each other with the same side. (This has already happened to the Moon – the same side always faces Earth. This is slowly happening to the Earth.) However, the slowdown of the Earth’s rotation is not occurring fast enough for the rotation to lengthen to a month before other effects change the situation: about 2.3 billion years from now, the increase of the Sun’s radiation will have caused the Earth’s oceans to vaporize[11], removing the bulk of the tidal friction and acceleration.”

So, if the sun wasn’t about to explode, they would eventually become tidally locked.

Hmm so the conclusions are:
[ul]
[li]Moon-Earth system isn’t really stable[/li][li]generating energy from tides has negligible effect on the system?[/li][/ul]

I was wondering, if it were a stable system, and we could generate energy from it, that would be a perpetual motion machine.

If by “stable system” you mean tidally locked, you would not be able to generate any energy as there would be no tides.

Wouldn’t there still be tides caused by the sun? (Albeit smaller.) I believe the biggest tides now are attributed to when the solar and lunar tides coincide.

Stability isn’t the issue. After all, a brick sitting on top of another brick is stable, but you could still get energy out of the system by pushing the top brick off and letting it fall.

Now, if you could extract energy from a system and leave it in exactly the same state as before, that would indeed be a perpetual motion system.

But for tidal energy, that’s not what we’re talking about. A negligible effect is not the same as no effect. Extracting tidal energy does affect the Earth-Moon system; it’s just that the amount of tidal energy extracted is so super tiny compared to the amount of energy in the Moon’s orbit (and Earth’s rotation), that it would take millions of years to be even be barely measurable.