posted on May, 20 2008 @ 08:10 PM
It's quite simple. The tidal energy is taken from the earth's rotation. The earth has a tremendous amount of kinetic energy, as it has an
unbelievably huge mass. The amount of tidal energy used up yearly is but a tiny billionth of the rotational kinetic energy of the earth. Despite this,
though, if everything continues as normal, the earth will become tidally locked with the moon, just like the moon is tidally locked to the earth.
In the past couple billion years, the earth has slowed to about half it's original rotation speed.
Many bodies in the solar system have become tidally locked. (moons, mostly, but Pluto is locked to it's moon, and vice versa.) This is when the tidal
forces have used up all the rotational kinetic energy in an orbiting body, and it now permanently faces the thing it's orbiting with one face. This
happened to our moon long ago, and will eventually happen to the earth, unless it's destroyed first, or the moon drifts away.
It happened to the moon long ago, while the earth is still rotating at about half it's original speed because the earth is far, far more massive than
the moon. The moon is one fourth the diameter of the earth, and a bit less dense, which works out to just 0.0123 times the mass of the earth. (just
1/80 the mass of the earth!) (it has a full 1/6th the earth's gravity at it's surface because of it's much smaller radius)
If the earth wasn't rotating, there would be no tides; it would just be slightly deformed towards and away from the moon.
It ain't free or infinite energy any more than solar or wind or hydroelectric power is. It will simply last far longer than man will probably be
around. There's no reason not to use it; it's not going to slow the earth's rotation any faster (unlike some interesting proposals involving
gyroscopes at the equator).