[David Cooper (OP)]

There's a major difficulty with renewable energy in storing energy for use when when demand is high but the wind isn't blowing and the sun's either down or behind thick cloud. I've been wondering why this hasn't already been solved, because I've seen video of superconductors where something levitates over the superconductor and rotates - this leads me to think that the rotating part could be a flywheel inside a vacuum and be accelerated up to higher and higher speeds forever, the mass only going up a small amount for a vast amount of stored energy. Is this theoretically possible, or is there some barrier to making it a reality?

[alancalverd]

Centrifugal force will eventually disrupt any spinning body, so there's a limit to the stored energy in a flywheel.

The answer is to reduce demand (by making fewer babies) to the point where we can sustain a good standard of living through biofuels.

[jccc]

If the wheel is big or hard enough, it is possible.

[jccc]

Let's say a black hole traveling in space, eating up everything, it might just become a flywheel that has enough density to sustain the force and keep speeding up, to form singularity, to re bang.

[David Cooper (OP)]

If it spins too fast it will fly apart, so yes, it isn't infinite (unless it can hold itself together with gravity like a black hole, or a quark star). But, even though it might cost a fair bit of energy to keep a ten ton flywheel levitating, it could store a substantial amount of energy without any drag if the vacuum is perfect. Mechanisms would be needed to accelerate it and to draw power back from it, but it may be possible to do and could be the answer to our storage problems. In the event of a failure, the flywheel would vaporise on contact with the walls of the underground cave it's kept in and would create a much bigger, globular cavity, much like an underground nuclear weapons test but without the contamination.

(Oh, and the direction of spin would need to be lined up with the equator.)

[alancalverd]

Some interesting data on flywheel stored energy systems in Wikipedia, with a suggestion that the very best can achieve 120 Wh/kg, with commercial systems running at roughly a tenth of that capacity. This puts the flywheel in pretty much the same ballpark as a modern battery, but a lot more expensive.

The advantage of the flywheel is its ability to supply very large instantaneous loads, but since these are normally only encountered when starting other rotating machines, the intermediate electrical conversion is unnecessary: this feature is used for starting some aero engines as it's overall lighter than a high-torque electric starter motor.