Flywheel batteries work by accelerating a pair of rotors (flywheels) to a very high speed and maintaining the energy in the system as rotational energy. The energy is converted back by slowing down the flywheels.
The space environment has a number of advantages for flywheel energy storage:
- The natural vacuum eliminates energy losses due to atmospheric drag.
- Cryogenic temperatures of space enable superconductor magnetic bearings that minimize friction in the system, without further refrigeration.
- Energy losses due to friction, hysteresis etc. can be utilized to heat the spacecraft.
- Due to the absence of living beings, minimal safety precautions, in case the spinning flywheel 'explodes', have to be made.
- Because of their angular momentum, flywheels can act as reaction wheels for attitude control as well, even while storing energy.
Furthermore, in vehicles, such as a lunar rover, flywheels can stabilize motion due to the gyroscopic effect.
Current best theoretical energy densities of flywheel batteries are around 200 Wh/kg.
Magnetically Supported Flywheel Hoops
The best theoretical energy density calculation for a flywheel assumes that the flywheel is prevented from flying apart by its own structural strength. So larger flywheels with lower centrifugal force for the same rim speed get no advantage because the larger wheel has more mass to support. However a flywheel need not be limited to its own structural strength to keep from flying apart. If a flywheel consists only of the rim and the radius is ten kilometers, then 1005 meters per second squared acceleration at the rim would be generated by a rotation rim velocity of 3170 meters per second. This 102.6 g centrifugal force could be countered completely by magnetic force holding the iron rim in place with no hoop stress and no requirement for structural strength to resist the hoop stress. The structural strength of Luna itself would be used to hold the flywheel together with the force being transferred to the flywheel magnetically just as force to hold a magnetically supported rail road train above the tracks is transferred magnetically. This would result in a flywheel with 5.02 * 106 watt seconds per kilogram or 1400 watt hours per kilogram. This would not be the kind of flywheel that one would use to power a vehicle rolling around Luna. It would be used for stationary facility power storage. For higher power densities people would design larger diameter flywheel rims. There is the added complication of designing an electric motor generator to ride on the magnetically supported iron rim, but this should only reduce the power density by a small fraction. This technology could be used on earth with an evacuated donut shaped tube to hold the flywheel. A one meter diameter evacuated tube would be curved to form a ten kilometer radius donut. The hoop rotating at 3170 meters per second inside, riding on magnetic levitation, would provide reasonable power storage on earth. The proven technology would later be used on Luna with no need for the vacuum chamber.