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A modern flywheel rotor is suspended in a vacuum by magnetic bearings. This means that nothing touches the rotor as it spins. When time comes that we need to recover that stored kinetic energy, how do we recover it?

I imagine a clutch making contact with the rotor, but wouldn't that give quite a jolt, slowing down the rotor considerably?

Are there other ways of recovering that stored energy?

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up vote 4 down vote accepted

We recover the energy in a maglev flywheel in the same way we almost always convert mechanical energy to electrical energy: with a 3 phase electric power generator/motor, also called an alternator, with the rotor on the same shaft or otherwise integrated with the flywheel.

In cars with a combined starter/generator, pumped-storage hydroelectric dams, maglev flywheels, etc., 3-phase electric power goes into the coils of the stator, and gets converted to mechanical torque to spin the rotor. Later, the mechanical energy of the same spinning rotor pushes a magnetic field around that pushes electrons through those same coils of the stator, converting mechanical energy to electrical energy that exits out the wires of the generator.

Such 3-phase motor/generator electric machines don't need anything to touch the rotor. (Unlike some DC motors/generators that require a "brush" to touch the rotor). In practice, most of them do have something that touches the rotor -- a long input shaft to carry mechanical energy into the generator, a long output shaft to carry mechanical energy out of the generator, various bearings to keep the rotor more or less centered end-to-end and radially, or some combination.

What makes magnetic levitated flywheel energy storage a little special is that nothing actually does touch the rotor. Some of the coils surrounding the rotor act like the coils of a 3 phase electric machines. Those coils convert electric energy to mechanical energy to spin up the rotor in motor mode. The same coils later convert the mechanical energy of the rotor back to electrical energy, slowing down the rotor in generator mode. Other coils surrounding the rotor are used as a magnetic bearing to levitate the flywheel.

A typical regenerative variable-frequency drive typically uses 6 IGBTs to convert DC electric power to mechanical energy and back; the IGBT arrangement used in maglev flywheels is no different.

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There has to be a system to speed the rotor up; one imagines that the same system would be used to get the energy back out.

I suppose that an electromagnetic system would be the way to do it. There are enough designs out there that it should be possible to google one. Let's see, here's a good link, the California Energy Commission report on the subject of flywheels.

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You'd think if the system were electromagnetic they'd store the energy in a capacitor. – Mark Eichenlaub Mar 16 '11 at 5:20
@MarkEichenlaub: Why? Do you think that capacitors can store more energy per cubic meter of space they take up or per dollar to produce them? – endolith Nov 22 '11 at 4:21

I think a homopolar generator setup is used in some situations. Static magnetic field is introduced through the conductive wheel, and a current is induced between the center and outer rim of the disc. You can generate huge pulses of current in this setup, on the order of millions of amperes.

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but wouldn't that give quite a jolt, slowing down the rotor considerably?

If you are trying to extract energy isn't that the point?
If you apply a force to the flywheel you extract kinetic energy, whether you do this by grabbing it with a clutch, letting in air or electromagnetically doesn't really matter

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