Flywheel

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', has 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. 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 in an evacuated ten kilometer radius tube, providing reasonable power storage. This would also prove some technology needed for a lunar colony.