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The Centrifugal Battery (flywheel as an energy storage device) has been around [a while]since at least 1857. This comment and geoffc's answer (batteries and Bearings) got me wondering what would the advantages and disadvantages of what in theory (with ceramic/magnetic bearings) could be a battery without end to its rechargeable life would be.

Or said another way; Assuming a Centrifugal Battery built with ceramic/magnetic bearings which would essentially be capable of endless (or 10,000 year) recharging capability. Would the low (Mars, Moon, etc) or zero gravity, make the Centrifugal Battery a better or worse option then it is on Earth. As Kinetic energy recovery systems these have been in use on Earth since the 1950's.

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  • $\begingroup$ The Centrifugal battery you link to is a gun: basically a variation on a trebuchet. How do you see this being used to store energy? $\endgroup$ – Hobbes Jun 11 '14 at 17:29
  • $\begingroup$ @Hobbes Not sure what there is to not get. Rotationally kinetic energy is just as viable regardless of it used to power a trebuchet, vehicle or uses an electrical motor to provide electricity for electrical devices.. $\endgroup$ – James Jenkins Jun 11 '14 at 17:34
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    $\begingroup$ @James: ah, you meant a flywheel. The 1857 link was talking about a battery in the sense of a gun battery, so the terminology threw me off, so to speak. $\endgroup$ – Hobbes Jun 11 '14 at 17:55
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    $\begingroup$ @self. : Rovers on Mars and Earth's Moon are neither always in sunlight, nor always in constant shadow. Satellites in geosynchronous orbit occasionally spend over an hour each day near the equinox passing through Earth's shadow. Low-altitude satellites enter Earth's shadow several times every day. $\endgroup$ – David Cary Jun 12 '14 at 2:16
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    $\begingroup$ "Centrifugal" really is the wrong name for this. It's a rotational momentum battery; the energy isn't being stored in the centrifugal/centripetal forces. But "motor/generator with flywheel" is probably a more useful description. $\endgroup$ – keshlam Jun 12 '14 at 5:21
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There are two issues I see with this idea:

  1. The overall mass of the system necessary to be able to store meaningful amounts of energy.
  2. The effect of such a device on the attitude control capabilities of a spacecraft.

If those can be solved, or at least mitigated, this might be a viable idea.

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  • $\begingroup$ On your item 2, would two counter rotating flywheels (with balanced input/output) be off little or no impact to the attitude control? RE Dynamics of counter-rotating flywheels $\endgroup$ – James Jenkins Jun 11 '14 at 18:29
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    $\begingroup$ If it works properly, potentially. Keeping them balanced is another issue entirely though. $\endgroup$ – Tristan Jun 11 '14 at 19:04
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    $\begingroup$ Use four of them same as a quadcopter. You could even use six of them and then you'd have complete ship attitude control without spending reaction mass. I wouldn't synch them mechanically, losses would be horrendous, but you might do it magnetically. Presumably there's something wrong with this idea or NASA would be using it. $\endgroup$ – Peter Wone Jun 12 '14 at 9:37
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    $\begingroup$ Two counterrotating flywheels exert a different amount of torque because they're not at the same distance from the center of gravity. You could compensate for that by adding/extracting energy at a different rate so the forces will balance out. @Peter, NASA already uses flywheels (called reaction wheels for this purpose) for attitude control. It works well, until you reach the maximum speed of the flywheel, then you have to unload the flywheels using the thrusters. Overall it uses less fuel than thrusters alone. Reaction wheels are among the most failure-prone items in a spacecraft though. $\endgroup$ – Hobbes Jun 12 '14 at 10:40
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You are correct that flywheels can be used for energy storage like batteries. As with all engineering trades, this comes down to many factors: mass, cost, efficiency, lifetime, reliability ...

Being in 0g means you don't have to support the weight of the flywheel on the bearings. That is an advantage, but not a huge one, compared to 1g. Having electromagnetic bearings will eliminate bearing wear as a failure mode, but there will be a complex electronic unit to handle suspension and power transfer. It will have some failure rate.

The fact that we see so many batteries in use indicates which way the trade seems to come out.

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    $\begingroup$ Seeing batteries in use does not negate the potential application of better devices. Flight qualification counts towards a lot in the decision process. $\endgroup$ – ThePlanMan Jun 11 '14 at 22:08
  • $\begingroup$ Tidal energy systems use the moon as a huge flywheel on a gravity-balance bearing. Eventually it will fall to Earth as a result of tides removing its angular momentum. $\endgroup$ – Peter Wone Jun 12 '14 at 2:03
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    $\begingroup$ @PeterWone: the tidal forces are actually raising the moon's orbit as it steals angular momentum from the earth and slows the earth's rotation. See space.com/3373-earth-moon-destined-disintegrate.html $\endgroup$ – Ross Millikan Jun 12 '14 at 3:58
  • $\begingroup$ Um, ok certainly the literature (that piece and others I found) agrees with you, but now I'm rather puzzled as to why momentum is transferring to the moon. I'd expect dragging all that water around to brake both the planetary spin and the lunar orbit (obviously I thought so or I'd never have made my earlier comment). On the face of it, a lot of water is dragged back and forth dissipating energy as heat due to friction. In fact I think I'll pose this as a question in its own right. $\endgroup$ – Peter Wone Jun 12 '14 at 7:46
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    $\begingroup$ The big problem I see with this is somewhat technological: flywheel storage requires a lot of precisely-manufactured mass to be effective, transporting mass away from Earth is expensive, and mass that is available elsewhere isn't apt to be manufactured to the necessary tolerances. If equipment capable of doing the necessary manufacturing were transported to a place with the right kind of raw materials, that would seem like the combination necessary to make flywheel batteries practical. $\endgroup$ – supercat Jan 26 '17 at 16:46
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There are potential advantages to flywheels in space over their counterparts on Earth, but they don't have much to do with gravity. There are also advantages to flywheels over batteries, that could lead to future usage as energy storage in space.

  • Hard Vacuum: For flywheels to not loose energy over time they need to be able to rotate frictionlessly (or at least nearly so). This does not only apply to bearings, air resistance is also a major source of friction. All modern flywheels used in energy storage require a vacuum chamber. In space this won't be necessary. On planets with thin atmospheres, it can be lighter weight.
  • Non-Rotating (slow rotating) reference frame: Gravity is actually not the main force causing bearing friction, it is the torque applied as the rotation of the Earth forces the flywheel to spin and it resists. On bodies such as the moon that rotates 27 times slower then Earth, bearings can be simplified. This is even more so on free floating satellites.

Along with the cons mentioned by other posters, there are some pros over batteries.

  • High Discharge Rates: In the question you mention the need for energy storage when there is intermittent power and continuous usage, but the reverse, continuous power and intermittent usage, also requires energy storage. A number of future space technologies will require intermittent bursts of power a high levels, such as mass drivers.
  • ISRU: A flywheels rotor could easily be made from local regolith. Lower then optimal tensile strength would hurt mass/volume efficiency, but as long as the local environment is already hard vacuum, these inefficiencies don't really translate to any energy storage inefficiencies. Chemical batteries require specific elements which are not necessarily accessible.
  • Specific Energy: This is comparable to modern batteries on the ground, with no need for a vacuum chamber in space it should be better. If current research in high tensile strength carbon allotropes pans out, flywheels could see over an order of magnitude improvement in energy density. This would even bypass hydrolox in specific energy.
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    $\begingroup$ looks good, but refereces would make it better $\endgroup$ – user20636 Apr 18 '18 at 5:42
  • $\begingroup$ A flywheel rotor used for high rotational speed should be very strong, homogenous, well balanced statically and dynamically. A breaking flywheel will destroy a lot more than the flywheel and its case. Making a good flywheel from local regolith is not easy. You need to know the strength and density of the flywheel before you can make one. $\endgroup$ – Uwe Apr 18 '18 at 20:17
  • $\begingroup$ @Uwe an in situ flywheel would be for a stationary structure. It does not need to be space or mass efficient. Energy storage is linearly dependent on specific strength, but it is also linearly dependent on moment of inertia. Regolith is basically free just doubling the mass doubles the energy storage, there is no need to bring the flywheel anywhere near its breaking strength. The reason this can be done elsewhere, but not on Earth is all the advantages I mentioned in my answer. $\endgroup$ – Lex Apr 19 '18 at 0:07

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