There is *one* perpetual motion machine...
June 4, 2008 9:55 AM   Subscribe

How come there's no way to harness the rotation of the Earth as a power source? (although I guess wind power sort of does this...)
posted by Ziggurat to Science & Nature (23 answers total) 3 users marked this as a favorite
 
Wind isn't actually generated from the rotation of the Earth; it's generated from pressure differences in the Earth's atmosphere.

The only way I can imagine to harness energy from the rotation of the Earth would be to use a Hall Effect generator in the magnetic field. The Hall Effect says that you can get a potential difference by moving a conductor through a magnetic field - if you had a satellite in the right orbit, it could pass through the magnetic field as the Earth rotates underneath it and generate electricity.

Unfortunately, the amount of power generated this way is likely quite small. There's also the issue of getting it back to the surface of the Earth.
posted by backseatpilot at 10:08 AM on June 4, 2008


Well, on a large scale, if you take energy out of the Earth's rotation, you're going to start slowing it down (faster). That's sure to cause problems, right? The scale is way different, but if people started seeing it as free energy worthy of wasting, we'd eventually have way bigger problems than global warming.
posted by rikschell at 10:11 AM on June 4, 2008


They did some work with tethers and the earth's magnetic field, but that's all I know of.
posted by Lord_Pall at 10:13 AM on June 4, 2008


Space tether.
posted by Wet Spot at 10:14 AM on June 4, 2008


Tidal generators exist today, and the rotational energy of the earth contributes to that power.
posted by -harlequin- at 10:22 AM on June 4, 2008 [1 favorite]


Well, you can capture the energy from wind as it passes a windmill, from a river as it passes a dam, but from the earth as it passes what? It would have to be something that would stay still as the rotation of the earth pushes against it.
posted by winston at 10:23 AM on June 4, 2008


Plants have this figured out with the separate light and dark reactions of photosynthesis.
posted by Cool Papa Bell at 10:25 AM on June 4, 2008


The Space Elevator uses the Earth's inertia to some extent.

And as for "using up" this inertia, (I ≅ 0.4MR2). The days were I could hold my brain together long enough to work through the math to derive the energy content are long passed, but according to the internets the kinetic energy of the earth's rotation is 2.137 * 1029 Joules (Watt-sec). This is a very large number.

The site later goes on to assert that slowing the earth's daily rotation by 0.0007 per second would give us more energy than the US uses in a 1000 years, if we could harness that.
posted by tachikaze at 10:26 AM on June 4, 2008


How come there's no way to harness the energy of the Earth's rotation?

Well, of course there is. Anything that rotates, you can put some teeth on it, make it into a gear to drive something else mounted to another gear on a fixed axle. The problem here is that ... exactly where do you put the axle?
posted by adipocere at 10:33 AM on June 4, 2008


This is exactly what tidal power does. (Note: although gravitational forces cause the tidal bulges in the oceans, it is the rotation of the earth that causes the tides to move around the earch, and rise and fall in particular places, making tidal power possible).

In any case, it's not an infinite power source. Doing this slows down the earth's rotation. A rotating mass has a certain rotational energy, and conservation of energy means if you take energy from the earth's rotation, the earth's own rotational energy has to decrease, i.e., the earth slows down. Now, the amount being "withdrawn" from the earth's rotation is so little as to make pretty much no measurable difference in the earth's rotation, but it's not something that could be done forever.

The rotational energy of the earth is around 2.6*1029 J. Meanwhile, worldwide energy consumption is around 5*1020 J. Even if we took all of our energy needs from the earth's rotation, it would increase the length of a day by only about 0.0009 seconds per year.
posted by DevilsAdvocate at 10:33 AM on June 4, 2008


Now, the amount being "withdrawn" from the earth's rotation is so little as to make pretty much no measurable difference in the earth's rotation, but it's not something that could be done forever.

More to the point, it doesn't matter because orders of magnitude more energy is being withdrawn from the earth anyway and we can't stop it - continents thousands of miles long are dragged through the tidal bulges every day, withdrawing unthinkably massive amounts of energy from the earth, among other things creating massive currents in unthinkably vast masses of water. Depending on the generator location, it will take energy from earth, but most of it's energy will come from the ocean after the ocean has already taken that energy from earth. (Eg, positioned to take advantage of the faster currents through a strait, created by landmasses being dragged through the bulge)
posted by -harlequin- at 10:48 AM on June 4, 2008


Well, of course there is. Anything that rotates, you can put some teeth on it, make it into a gear to drive something else mounted to another gear on a fixed axle. The problem here is that ... exactly where do you put the axle?
posted by adipocere at 1:33 PM on June 4 [+] [!]

I'd think that the location for the axle is pretty obvious.

Where you put the bearings, however...
posted by IAmBroom at 11:00 AM on June 4, 2008


Everything on the earth rotates along with it. The only way to get energy out of the rotation is to find a place that doesn't rotate and set up a generator powered by that differential. Unfortunately, that non-rotating place has to hold steady while the earth rotates under it, which means that it has to be an immovable object, or at least have a mass that's a substantial fraction of the earth's mass.

There ain't no such thing as a free lunch.
posted by KRS at 11:14 AM on June 4, 2008


It's a strange way to think about it, but you could say that a Foucault Pendulum harnesses the Earth's rotational power to spin around its own axis.
posted by qxntpqbbbqxl at 11:26 AM on June 4, 2008


Power generation requires an energy differential. You need a place which is high energy and a place which is lower energy. The power generator then moves energy from the high energy place to the low energy place and harnesses some of it.

The rotating earth is your high energy place. What is your low energy place?

That's the problem. There's energy here, but no energy differential.
posted by Class Goat at 11:29 AM on June 4, 2008 [1 favorite]


The energy generated from an atmospheric Hall Effect machine would indeed be very small. My physics 116 class at CWRU made a series of homework questions out of this very concept. It was actually a really cool learning tool: we were asked not only about the energy generated, but the price needed to assemble such an absurdity.

The question may very well still be findable on the course's archive of old exams, if you're just that bored.
posted by spamguy at 11:30 AM on June 4, 2008


(Note: the above is slightly simplistic; combustion and fission are different models.)
posted by Class Goat at 11:30 AM on June 4, 2008


Sigh. I meant "my post above is slightly simplistic". I didn't know I'd hit with my disclaimer after SpamGuy.)
posted by Class Goat at 2:18 PM on June 4, 2008


The problem is creating the power differential. Sure the earth is turning, but you need something not turning at the same rate to attach to it.

As others have said, the ocean is one such thing, but it's all wet and sloppy making it difficult to get the energy out of it.

To more efficiently extract energy from earth's rotation we could attach a belt to the Moon and run it around the Earth's equator, attaching a generator in there somewhere. This would require a belt at least half a million kilometers long, and the attachment at the equator would create more problems than space elevators.

And because of the difference in mass, the moon would decelerate and eventually lead to (premature) falling out of orbit. That's a pretty large environmental impact.
posted by Ookseer at 4:12 PM on June 4, 2008


You absolutely can do this! Search for "gyrogenerator" or see this write-up (scroll down). Practicality is left as an exercise for the reader, though
posted by 0xFCAF at 4:15 PM on June 4, 2008


Several people have pointed out that tides extract energy from the rotation of the earth relative to the moon (and, to a lesser extent), the sun. Where's the energy differential there?

Currently the earth rotates once every 24 hours, while the moon rotates once every 29 days: the same as the earth-moon orbital period. This is an intermediate condition: it has less energy than a system (with the same total angular momentum) where the earth and the moon both rotate rapidly, but more than a system where the earth has a "moonward side" in the same way that the moon has an "earthward side." The energy converted into heat by tidal generators comes from the evolution of the earth-moon system towards that last state. Eventually all this energy will get extracted; then the same side of the earth will always face the same side of the moon and a day==month will last about 40*24 hours. I think the day has lengthened by about 10% in 100 million years.

In fact the moon hasn't quite finished wobbling yet; it undergoes libration.
posted by fantabulous timewaster at 4:46 PM on June 4, 2008


You could build a very (very) big electric generator that weighs a lot compared to the earth. Then put it out in space with a big stick attached to its driveshaft, such that once a year the earth smashes into the stack and rotates the driveshaft a few degrees. Then it's a simple matter of storing the energy generated to use over the next zillion years and burying the dead.
posted by jewzilla at 4:58 PM on June 4, 2008


Tesla used the earth itself, not just it's motion through space.
posted by doppleradar at 7:05 PM on June 4, 2008


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