Twirlix

Meet Twirlix, a heavy flywheel who stores energy by spinning.

A spinning object has kinetic energy even when it is not going anywhere, because every bit of it moves in a circle. For rotation, mass is replaced by the moment of inertia I, which measures how much mass there is and how far it sits from the axis. Speed is replaced by the angular speed ω in radians per second, so the energy is K = ½Iω².

Take a solid disc flywheel of mass 20 kg and radius 0.3 m. Its moment of inertia is I = ½mR² = 0.5 × 20 × 0.09 = 0.9 kg·m². Spinning at 100 rad/s, about 955 revolutions per minute, it stores K = 0.5 × 0.9 × 100² = 4500 J. Double the spin to 200 rad/s and it stores 18,000 J, four times as much.

Engineers use flywheels to smooth out the jerky output of engines and to store energy for short bursts. Some energy storage systems spin their flywheels inside a vacuum to cut air drag.

Spin cannot rise forever. The stress in the rim grows with ω², so every flywheel has a safe top speed, and friction in its bearings slowly drains the stored energy.