Tidal energy - drag on the moon?

[jschmidt]

I got to thinking about this recently, when thinking about various proposed "renewable energy" systems based on tidal energy capture.

Tidal energy is, basically, caused by the interaction of the kinetic energy of rotation of the earth/oceans, the kinetic energy of the moon's orbit, and the gravitational attraction (gravitational potential energy) of the earth/moon system.

Now, I realize the effect is probably pretty tiny, but as tidal energy is converted to other forms of energy (e.g. even without any man made energy extraction system, there will be natural conversion of tidal energy to heat as the water molecules interact with each other, and with the shoreline and the ocean bed), that energy has to be being subtracted from either or all of the Earth's rotational kinetic energy, the moon's orbital kinetic energy, or the gravitational potential energy of the earth/moon system (which would translate into a tiny decrease in the radius of the orbit - e.g. the moon slowly 'falls' into the earth).

So, the question - does tidal interaction with the Earth cause a gradual decrease in either the speed or height of the moon's orbit, or the rotation of the Earth (or all three), and has anyone tried to estimate how long such an effect will last before the moon can no longer orbit, and falls into the Earth (hopefully the timespan is longer than the 5 billion years before the Sun expands and consumes the earth)?

Would man-made energy extraction systems have any chance of accelerating the 'drag' on the moon from tidal effects?

 

[Janus]

Tidal drag actually increases the height of the Moon's orbit. It slows the Earth's rotation transferring angular momentum to the Moon. This increases the Moon's orbital energy while decreasing the kinetic energy of the Earth( However the vast majority o fthe energy lost by the Earth is lost as heat.

Tidal energy extraction would very slightly increase this effect, slowing the Earth more and increasing the Moon's distance. But we are talking about a miniscule increase.

 

[Drakkith]

Wouldn't any system to extract energy from the tides cause a slight suppression of the tidal drag effect by reducing the amount of water that is moved at once or by making it take longer to move the same distance?

 

[Janus]

Wouldn't any system to extract energy from the tides cause a slight suppression of the tidal drag effect by reducing the amount of water that is moved at once or by making it take longer to move the same distance?

No, the exact opposite. The friction between the tidal bulges and the rotating Earth pull the bulges out of line with the Moon. it is the gravitational attraction to these out of line bulges that pulls the Moon forward in its orbit, transferring energy to the Moon.

Tidal energy extraction would increase this friction, pulling the bulges even more out of line, causing them to pull forward on the moon even harder.

 

[Drakkith]

No, the exact opposite. The friction between the tidal bulges and the rotating Earth pull the bulges out of line with the Moon. it is the gravitational attraction to these out of line bulges that pulls the Moon forward in its orbit, transferring energy to the Moon.

Tidal energy extraction would increase this friction, pulling the bulges even more out of line, causing them to pull forward on the moon even harder.

Hrmm. I see what you are saying and don't disagree, but I thought that to get energy from the tides you effectively "suppressed" them. Ugh, I can picture it all in my head but I can't get it down in the post where it means anything.

 

[Lsos]

If Earth was perfectly plastic and deformable...perhaps made out of a superfluid, then the tidal bulges would always be in line with the moon, spinning along with the moon and not affecting it's orbit or spin at all.

The fact that the Earth is not a superfluid means that the tidal bulges will always drag slightly behind the moon, creating a torque. Attempting to extract energy from the bulges will make them drag even more.

I'm not sure if this is this correct...because I'm not entirely sure how tides work. But it kind of makes sense to me?

 

[Drakkith]

If I put a million buoys out in the ocean connected to generators on the ocean floor, am I increasing friction or simply reducing the height of the tides? In the latter case I picture it as keeping the water from bulging up as much, which I thought would reduce the effect of tidal acceleration on the moon. Am I just completely mistaken here?

 

[Lsos]

If I put a million buoys out in the ocean connected to generators on the ocean floor, am I increasing friction or simply reducing the height of the tides? In the latter case I picture it as keeping the water from bulging up as much, which I thought would reduce the effect of tidal acceleration on the moon. Am I just completely mistaken here?

I see what you're saying. Taking your logic further, if the Earth were made of a perfectly solid and stiff substance, would tides have no effect on the moon's orbit/ spin at all?

My guess is they wouldn't. This means I see a conflict in thoughts as to what is supposed to happen. Depending on whether we apply your logic or mine, we arrive at less or more effect