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.