# Harvesting energy from the earth rotation

by Low-Q
Tags: earth, energy, harvesting, rotation
P: 311
 Basically, the earth's rotation is too slow. You could, indeed, spin up a gyroscope with its axis orthogonal to that of the earth's. You'd then observe a rotation of that gyroscope relative to an earthbound observer - it would rotate once per day. Now, if you could figure out a way to efficiently extract energy from a device with a [relative] gyroscopic precession rotation rate of once per day whilst simultaneously feeding in energy enough to keep a big flywheel rotating at many rpm continuously, then maybe you could progress with the idea?
I dont know.If idea principally working and we would be able to obtain netto energy from Earth rotation then,I guess, the same idea could be extrapolated and we would be able to obtain energy from Earth movement around the Sun (this motion is much faster).Gyroscope not necessarily should be mechanical,I think.Maybe its possible to do something based on purelly electronic cycles?Then we may avoid some problems associated with strenghts of materials?
 P: 628 I think it is going to be worse than that. There is something about the physics of such an arrangement that will always mean the energy in is greater than the energy out, I'm afraid. You have to keep turning the flywheel to generate the rotation - it's a perpetual motion machine by another definition, and like all perpetual motion machines, you have to put energy in in a manner more efficient that taking it out.
Mentor
P: 12,003
 Quote by cmb I think it is going to be worse than that. There is something about the physics of such an arrangement that will always mean the energy in is greater than the energy out, I'm afraid. You have to keep turning the flywheel to generate the rotation - it's a perpetual motion machine by another definition, and like all perpetual motion machines, you have to put energy in in a manner more efficient that taking it out.
It's not a perpetual motion machine, it's just very inefficient due to the long rotation time of the Earth for what you are wanting to do. The energy you might be able to extract from it comes directly from slowing the Earth down slightly, so energy is conserved.
P: 628
 Quote by Drakkith The energy you might be able to extract from it comes directly from slowing the Earth down slightly
right, but you only slow down the Earth a little so long as the gyroscope is still rotating. Once you stop the gyroscope, you speed the Earth back up again. This is a bit like accelerating your car eastwards. naively, you have slowed the Earth down, so you must've generated some energy? Not really, because you have to stop the car sometime and you put exactly that energy back into the Earth.

So the physics says that you have to put [at least] the same energy in as you'd get out.
Mentor
P: 12,003
 Quote by cmb right, but you only slow down the Earth a little so long as the gyroscope is still rotating. Once you stop the gyroscope, you speed the Earth back up again.
What? I don't think this is correct. Once you extract the energy it is gone unless you intentionally give it back to the Earth.

 This is a bit like accelerating your car eastwards. naively, you have slowed the Earth down, so you must've generated some energy? Not really, because you have to stop the car sometime and you put exactly that energy back into the Earth.
No, the gyroscope would apply a torque against Earth's motion. Once stopped it would NOT apply a torque in the opposite direction. You would have to purposely input energy and reverse the gyroscope to make this happen.
P: 628
 Quote by Drakkith the gyroscope would apply a torque against Earth's motion. Once stopped it would NOT apply a torque in the opposite direction. You would have to purposely input energy and reverse the gyroscope to make this happen.
So, if you were to extract some energy out of the slow rotation of the gyroscope, then let the gyroscope spin down under its own friction to standstill, then after all that the Earth would be rotating slower that it was before? Right?

OK.... so to what was the angular momentum transferred, or does this defeat conservation of angular momentum?
Mentor
P: 12,003
 Quote by cmb So, if you were to extract some energy out of the slow rotation of the gyroscope, then let the gyroscope spin down under its own friction to standstill, then after all that the Earth would be rotating slower that it was before? Right?
Yes.

 OK.... so to what was the angular momentum transferred, or does this defeat conservation of angular momentum?
To the gyroscope and then to whatever you are using to get power out of it.
P: 628
 Quote by Drakkith To the gyroscope and then to whatever you are using to get power out of it.
Let's say it fed some high-geared generator that charged up a battery. Now the generator has stopped too, and so the only difference before-to-after is that I've got a charged up battery and a slowed down Earth. Does this not defeat conservation of angular momentum?
 Mentor P: 12,003 I don't believe so. If it did, then someone should have pointed it out immediately. I recently tried out an example at the local Sci-Port where that had a wheel on an axle that you spun up with your hand. Then you sat in a chair that could turn and rotated the wheel and axle around perpendicular to it's rotational axis. This caused the chair and myself to turn one way. As the wheel spun down I did not return to my original direction.
P: 1,403
 Quote by Drakkith I don't believe so. If it did, then someone should have pointed it out immediately. I recently tried out an example at the local Sci-Port where that had a wheel on an axle that you spun up with your hand. Then you sat in a chair that could turn and rotated the wheel and axle around perpendicular to it's rotational axis. This caused the chair and myself to turn one way. As the wheel spun down I did not return to my original direction.
It does violate conservation of angular momentum, and is therefore impossible. A gyroscope that you let rotate freely won't slow down the earth, or produce any energy. The only way to slow down or speed up the earth with such a gyroscope, is to let it point more or less in the direction of the earths spin. The gyroscopy will have it's lowest energy state when it's spinning in the same direction of the earth, and you can't get any energy out of it anymore.
Mentor
P: 12,003
 Quote by willem2 It does violate conservation of angular momentum, and is therefore impossible. A gyroscope that you let rotate freely won't slow down the earth, or produce any energy. The only way to slow down or speed up the earth with such a gyroscope, is to let it point more or less in the direction of the earths spin. The gyroscopy will have it's lowest energy state when it's spinning in the same direction of the earth, and you can't get any energy out of it anymore.
Alright, I just re-read the thread again. I guess that it is impossible. Not sure why I was thinking it would work...perhaps I was remembering another thread.
P: 628
 Quote by willem2 The only way to slow down or speed up the earth with such a gyroscope, is to let it point more or less in the direction of the earths spin. The gyroscopy will have it's lowest energy state when it's spinning in the same direction of the earth, and you can't get any energy out of it anymore.
[EDITED OUT A SECTION HERE]... I don't think what I just wrote then was correct. I was discussing that a gyroscope with its axis perpendicular to the earth's axis would slow the earth down. I think that's not true, because it would have no component of angular momentum about the same aXIs as the earth, so the two would not interact in that way - is that correct??...

Gyroscopes' behaviour tends to defeat human's intuition (hence a load of 'anti-gravity' devices try to exploit gyroscopes, due to misconceptions on how they work). I suspect that this is, in part, because we have evolved with the 'lizard' parts of our brain never needing to intuitively comprehend gyroscopic motions, as nothing in the natural world behaves in such a way to need that intuitive comprehension.
 Homework Sci Advisor HW Helper Thanks P: 13,059 I'm guessing Low-Q is thinking of one of these: http://www.lhup.edu/~dsimanek/museum/advanced.htm BTW: I'm with LURCH.
 Sci Advisor P: 2,470 Angular momentum is still a conserved quantity. Meaning you cannot slow Earth's rotation down without either a) drastically altering it's moment of inertia or b) expelling mass. In particular, you cannot use a gyro to extract energy from Earth's rotation, because the torque would cause the gyro to tip, until its axis of rotation is aligned with Earth's.
Mentor
P: 15,201
 Quote by K^2 Angular momentum is still a conserved quantity. Meaning you cannot slow Earth's rotation down without either a) drastically altering it's moment of inertia or b) expelling mass. In particular, you cannot use a gyro to extract energy from Earth's rotation, because the torque would cause the gyro to tip, until its axis of rotation is aligned with Earth's.
Exactly. It's a perpetual motion machine.

Using a gyro or a flywheel as an energy storage device is okay. One of the many problems with renewable energy is that it is not available. Solar power needs to be stored during the daytime for use at nighttime, wind power needs to be stored when the wind blows for use when the winds aren't so strong. This isn't perpetual motion; it's just a storage device. And it's lossy, as are all real energy storage devices.

There is a way to harvest energy from the Earth's rotation without a) drastically altering it's moment of inertia or b) expelling mass. That way is to piggyback of an existing external torque on the Earth. This tidal station, for example, generates 254 MW of power.