# Is there any mechanism to accelerate a flywheel with no max speed?

Hypothetical situation: I have a free source of energy, like a rotating drive, piston or moving water. I want to store this in a flywheel. The amount of energy I can store is limited by the max speed of the flywheel. But if I use, say, gears, the max speed is determined by the max gear ratio. Is there any mechanism with no max speed?

I was thinking if the energy source were a jet of air, I could use a turbine, and maybe change the angle of the blades as it sped up, but the speed would be limited when the speed of the air equalled the speed of the surface of the blades, no?

AFAIK, most flywheel systems use gear-less electric motors and the limitation is the strength of the material on how fast it can spin before it shatters, not how fast the electric motor can spin the flywheel.

I think you are right.

The most flexible coupling will be electromagnetic - you can vary the frequency of the external field to transfer energy to the flywheel from v. low to extremely high. Quite what the maximum is, I don’t know - some function of the collapsing magnetic field, and I expect efficiency to fall off. You also have electrical efficiency losses as you vary the frequency. However, you gain the ability to extract energy from the flywheel at any rate of rotation and without physical coupling.

Si

Not necessarily - or else sailing faster than the wind would not be possible.

Yes I know flywheels use electrical motors, I was wondering if this was possible mechanically?

With air you will be in trouble when the ends of the blades exceed the speed of sound - this isn’t a hard limit, but things will start to get weird at this point, and your efficiency will drop off.

A turbine with moving blade angles is really no different to a gearbox, indeed you are not that far from how an automatic gearbox works, albeit using a gas instead of a liquid.

Whatever system you use to spin the flywheel up, you need a symmetric system to get the energy out again efficiently. Normally you would want to use the input system in reverse.

As others have pointed out the physical material the spinning flywheel is composed of will be self (probably explosively) destructing way before the gearing mechanism or support spindles etc. get stressed to their breakdown points.

Just for laughs-

Just as the tips of aircraft propellers should not be allowed to exceed the speed of sound, the edge of any spinning disc **cannot **exceed the speed of light. Some quick (probably wrong) math later and:

The edge of a 6 inch disc would approach the speed of light at approximately 3,127,650,000 rpm give or take.

I doubt even any hypothetical bucky-ball-admantium-unobtanium disc could approach this speed. I’m not even going to consider that as it spun up the outer parts would approach c sooner than the inner parts and would appear to slow down in time whilst the inner would speed up- brain hurts…

I think it was the great engineer W.B. Yeats who once said, “Turning and turning in the widening gyre…Things fall apart; the centre cannot hold.”

Even a pretty tough material like the polycarbonate of CDs is not immune to g forces

True, but it’s not a problem, since in that span where you’re approaching the speed of light, you’ve got enough room for an unlimited amount of kinetic energy.

So use a rotating black hole instead; they can be spun up to near lightspeed. I’m not sure if that counts as a “flywheel” though.

I think the limiting factor for solid substances would be the radial tensile strength of the spinning flywheel as it continually has to keep those parts of itself near the rim of the disc in a circular orbit.

And it it has an electrical or magnetic charge on it, you can couple to it for acceleration and deceleration! Or am I understanding that wrong?

Well, what would happen as the edge of a spinning disc approached lightspeed?

Sunspace, you’re not understanding that wrong. Although putting a magnetic charge on a black hole does present some practical challenges…

Excellent.

This is not exactly right

The amount of energy you store in a flywheel is a product of both the center of mass in the flywheel and the speed of the flywheel, you determine roughly how much energy you want to store at a reasonable speed and then simply add enough weight to the flywheel to give you the energy figure.

Actually I was wondering if anyone had invented a gear set with a ratio 1 : infinity, or something similar, more than the practical aspects of it. So would a gas jet and a turbine have a maximum speed for the turbine? Maybe if the nozzle could be adjusted so the speed of the jet changed?

That leads to a theoretical possibility. Suppose you could design a flywheel that always moves at a constant speed? But you could add energy to the flywheel by increasing its mass. Double the mass at a constant speed and you’ve doubled the energy. And you could presumably withdraw energy by the same means - remove mass while keeping the rotation at a constant speed.

It’s obviously a theoretical system - I don’t know how you’d add and subtract mass from a constantly rotating object. But if you could build it, you’d avoid some of the problems of the variable speed flywheel. No light speed concerns anyway.

Even if you had a flywheel that could reach the speed of light without comming apart, the amount of energy needed to run it would be sucked out long before reaching that speed. I don’t think Niagra Falls would have nearly enough power to spin a small toyota flywheel anywhere near the speed of light.