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Yeah, the main problem with them is that if anything happens the "stored energy" is already kinetic energy and the flywheel can do some pretty serious damage.

The other problem is that they act like gyroscopes. Which means that depending on how you mount them, will either constantly exert a precessional force on your vehical, or really protest you making sharp turns at high speeds.

Fortunately, composite materials tend to disintegrate quickly to red-hot powder once broken, instead of large chunks of high-velocity shrapnel.[1]

There's also anything built in layers, where the outermost layer will shred, bleeding off massive amounts of energy in relatively low-mass pieces, much more easily contained. Often carbon fiber, but that's just the main material I've seen for this. Dangers associated with flywheels are becoming less and less of a problem, and will only continue to do so with more research.

That said, I'd still be interested in seeing how it reacts to a hard collision. I think that quote mostly refers to over-spinning.

[1] : http://en.wikipedia.org/wiki/Flywheel_energy_storage#Advanta...

The second one can be mitigated for the most part by using two flywheels that rotate in opposite directions.

Right. Except the smallest synchronization error between the two flywheels becomes disaster.

Why ? They don't need to be synchronized at all, as long as they're roughly spinning at the same speeds in opposite directions there is 0 effect. The only effects you'd be left with would be from the difference between the two rotational speeds. If you stop one and the other continues that's a different matter.

Besides, keeping two wheels synchronized is not the hardest thing to do, a car contains a lot of moving, synchronized parts.

And the first is a problem with anything that stores energy. Gas can explode. Batteries can explode. Etc.

I think an accident causes gasoline to burn more often than explode. Even when the tank explodes I don't think it would be as dangerous as a fly wheel shattering with shrapnel flying everywhere.

Flywheels running at high speed are not made of cast iron or other things that turn in to 'shrapnel', they're made of wound filaments of carbon fibre bound with resin, usually they are designed with failure in mind. The bigger problem is dealing with the conversion of the energy in the wheel in to heat.

If we have a flywheel falling on a movable object on the floor I imagine this movable object can attain quite high speeds.

@jacquesm Does the heat problem confine it to providing only bursts of energy?

I meant on destruction of a running wheel, not on normal use.

Except some things are more dangerous than others.

> the flywheel can do some pretty serious damage.

There is a solution to this: flywheels that use tightly wound metal tape instead of solid cores. Tape flywheels just unravel inside the casing safely when they break.

Actually, they use wire rather than tape. Powder has also been used and fails very safe but it helps if the material the rim is made of has intrinsic strength.

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