11
$\begingroup$

The most common answer is "tidal forces" "wind" since all of these are by-products of the planet's Coriolis force.

Or there are rnd into generating energy from the earth's magnetic field, but that falls flat since allegedly nonuniform magnetic fields aren't good for generating energy.

I'm looking for a more direct way. If we strip down the problem: "Ok, we have this very massive spinny spinny rock." Let's attach a generator to the spinny spinny rock.

Let's say we don't care about the why, and the fact that the sun might be a way better power source, and building a Dyson swarm might be easier.

Just purely let's think about how would we do that.

(We don't care about the planet losing its angular momentum here.)

Question 2:

Is there a way to do this, but keep it on the planet's ground?

$\endgroup$
2
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Jul 22, 2022 at 21:07
  • $\begingroup$ If you are okay with supernatural abilities, then "vector manipulation" could do this. I don't remember perfectly, but I think that Earth's angular momentum is exactly (or close to) what Accelerator used near the end of season 1 of "Toaru Kagaku no Accelerator" anime (A Certain Scientific Accelerator, spin-off of A Certain Magical Index) $\endgroup$
    – Kusavil
    Feb 9, 2023 at 14:52

4 Answers 4

4
$\begingroup$

There are several methods of extracting energy from angular momentum of a planet. They're in use right now, on Earth, and all of them can be done on the ground.

Start with tide power -- this generates electricity with turbines spun by flowing water due to (mainly) Lunar induced tides. Tides also are behind the one-face visibility of the Moon, the internal heating of Jupiter's an Saturn's inner moons -- in other words, tides convert angular momentum into vertical stretching of material which then can be extracted as useful energy.

Next, wind power -- the wind patterns we know (dependable winds from a limited range of directions most of the year) are due to Coriolis effect bending the circulation cells that would otherwise run direct from poles to equator at ground level. Further, Coriolis increases the energy of those winds by converting Earth's angular momentum into angular momentum in the wind, which allows extracting energy from the linear velocity.

Finally, those wind patterns carry evaporated water to high ground; we extract energy from that water via hydroelectric generation. That energy is directly due to gravitational potential, but the water got up in the hills or mountains or high plains due to the Coriolis-drive winds, and the falling water also carries a little angular momentum, as mass moves closer to the axis of rotation.

I've seen proposed methods to extract energy without contact from extreme objects like micro black holes; many of those depend on the way a spinning hole drags spacetime around itself, but those aren't in current use and may never be practical (I certainly don't want a black hole based power plant on my planet).

$\endgroup$
3
  • $\begingroup$ This answer has some flaws too :p for a big part (not all of it), the ideas in your answer describe methods of harvesting energy that can also be performed when the planet has no angular momentum at all, just a big moon (like ours). When getting energy out of wind, the direction of the wind is irrelevant. $\endgroup$
    – Goodies
    Jul 21, 2022 at 23:46
  • $\begingroup$ If the Earth were tide locked to our Big Moon, however, there'd be no change in tides from which to extract energy. Jupiter's moons are so deep in the gravity well that the eccentricity of their orbits (plus a little from interaction with the other large moons) does the work. And as I noted, Coriolis adds energy to the winds over what would be there with simple convection cells. $\endgroup$
    – Zeiss Ikon
    Jul 22, 2022 at 11:06
  • 1
    $\begingroup$ Tides yes, wind NO. Coriolis forces are like the chain on a bicycle: they funnel energy from the sun (legs) into cyclones (wheels), but they aren't the actual energy source. Cyclones do not have a net effect of slowing Earth's rotation down, but tidal sloshing of our ocean does and quite a lot over geological time. $\endgroup$ Jul 23, 2022 at 8:24
3
$\begingroup$

Gyroscope

The idea of using a gyroscope to extract energy from the Earth's rotation has been around for decades.

A gyroscope (that is not aligned with the Earth's axis) in a frictionless environment will slowly wobble once over 24 hours. In theory, energy could be extracted from that wobble.

The problem is that gyroscopes lose energy to friction. You would need to design a gyroscope such that more energy could be extracted from the wobble than lost to friction.

If a super-low (or zero) friction system were feasible (e.g. superconductor levitation), then you could make an enormous gyroscope (perhaps several kilometres in diameter) to extract a modest amount of energy as it wobbles.

You would also need a system that could accelerate the gyroscope to full speed and then decelerate back to zero with near perfect efficiency. Thus you could reset the gyroscope after you have extracted the energy from the wobble each day.

$\endgroup$
2
  • $\begingroup$ Wouldn't you need to invest the energy that you want to extract in order to "reset the gyroscope"? $\endgroup$
    – Avun Jahei
    Jul 22, 2022 at 8:47
  • $\begingroup$ @AvunJahei The energy extracted will be very very slightly more than the energy invested on each reset, since it is extracting angular momentum from the precession. $\endgroup$ Jul 22, 2022 at 15:24
2
$\begingroup$

Note: when you extract tidal energy, it will reduce angular momentum of the moon, not the planet. Actually, a tidal energy plant will force the moon into a slightly lower orbit.

Spacecraft launch

When question #2 is allowed to be "no", a common use of angular momentum is launching spacecraft: there's a reason spacecraft carrying sats are launched from French Guyana. This place is on the equator, which results in maximum angular momentum. Less fuel is needed, because a lot of energy can be harvested.

Space elevator

Lifting mass out of Earth’s gravity well without using rockets. An extremely strong cable extends from Earth’s surface to the height of geostationary orbit. Competing forces of gravity at the lower end and outward centripetal acceleration at the farther end would keep the cable under tension and stationary over a single position on Earth. Mass will escape the surface much easier.

Walk west

Whenever you put a force to the ground in east-west direction, you will slow down the rotation of the planet.

$\endgroup$
4
  • $\begingroup$ In the Earth's case, however, the Moon is moving out because the drag of the Earth's tidal bulge pulls it forward in its orbit. In fact, it is the Earth's angular momentum that is supplying the power for tidal generation, not the Moon's. If the Moon were inside a 24 hour orbit, you'd be correct. A space elevator won't generate energy, just save a lot of it. And your third example is trivially silly. $\endgroup$
    – Zeiss Ikon
    Jul 21, 2022 at 18:28
  • $\begingroup$ @ZeissIkon I always supposed the moon is pulling the water up ? the moon needs not to be "dragged around" by earths's rotation, it has an orbit of its own. This is not about the moon's orbit and properties of the moon's orbit, it is about harvesting energy from planetary rotational momentum. Walk west is philosophy, not physics or relevant gain.. it is the easiest way to profit from the rotation of the planet. It would be in the micron range I suppose, but nevertheless.. $\endgroup$
    – Goodies
    Jul 21, 2022 at 23:51
  • 3
    $\begingroup$ If you walk west and then stop, the planet has exactly the same angular momentum as it did before you started walking, and you haven't extracted any energy. And if you walk west and never stop, you decrease the planet's angular momentum, but you don't extract any energy. $\endgroup$
    – Peter Shor
    Jul 22, 2022 at 0:30
  • 1
    $\begingroup$ @Goodies the tidal bulge always faces the Moon and "drags" across the Earth, like a brake, slowing Earth's rotation and transferring that lost angular momentum to the Moon's speed, raising its orbit. $\endgroup$
    – BMF
    Jul 22, 2022 at 0:30
2
$\begingroup$

In The Long Utopia (Pratchett & Baxter) there's a whole arc about an alien race speeding up the rotation of Earth (well, a version of Earth). The method used is a Planetary Spin Motor, as described by Freeman Dyson. It's a bit involved but ultimately it's transferring kinetic energy from a bunch of conductive masses thrown into the planet's magnetic field and applying it to the planet as rotation. Changing the direction of transfer - by altering the trajectory of the masses. This would extract rotational energy as velocity in your masses, which can then be converted to electrical energy by passing them through a series of coils that bleed away the kinetic energy.

Something similar could be achieved by tidal coupling rather than electromagnetic couple. The Moon is tidally coupled to the Earth and as a result the Earth's rotational energy is transferred to the Moon. The result is the Earth's rotation is slowing down and the Moon's momentum is being increased, raising its' orbit slowly. Very, very slowly.

Back to magnetic coupling, David Brin published a story called Tank Farm Dynamo which involved an orbiting double platform joined by tethers. As the station orbits the Earth it passes through the magnetic field, which induces current flow in the tethers between the two platforms. Here's an except from the story:

For an instant I saw the Earth not as a broad vague mass overhead, but as a spinning globe of rock, rushing air, and water, of molten core and invisible fields, reaching out to grapple with the tides that filled space. It was eerie. I could almost feel the Tank Farm, like a double-ended kite, coursing through those invisible fields, its tethers cutting the lines of force -- like the slowly turning bushings of a dynamo.

That was what young Emily Testa had compared it to. A dynamo. We could draw power from our motion if we ever had to -- buying electricity and paying for it in orbital momentum. It was a solution in search of a problem, for we already had all the power we needed.

As the POV character points out though, they had plenty of power. Turns out that what they really needed was to find a way to increase their orbit, not decrease it. You can always hook up solar sails or simply use thrusters to increase your momentum, then use the magnetic coupling to the Earth to turn that momentum back to electrical power. Each watt you draw comes directly from the rotational energy of the planet.

$\endgroup$
2
  • $\begingroup$ ... but indirectly from the energy you invested in the first place. $\endgroup$
    – ths
    Jul 22, 2022 at 13:15
  • $\begingroup$ @ths Yes, but you can use slingshots off other bodies (like the moon) and solar sails and the like to gain extra momentum, then push through the Earth's magnetosphere to convert to electrical power, transferring some of the momentum to Earth's rotation at the same time, either speeding or slowing the rotation. $\endgroup$
    – Corey
    Jul 24, 2022 at 8:19

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .