flywheels

Flywheels are energy storage devices that store kinetic energy in a rotating mass. The energy stored is E = 1/2 I ω^2, where I is the rotor's moment of inertia and ω its angular speed. To maximize energy for a given mass, flywheels use a rim-heavy design, increasing I.

A typical flywheel consists of a rotor within a strong housing, made from steel, aluminum, or advanced composites such as carbon fiber. To minimize energy loss from air drag, rotors run in vacuum, and bearings are magnetic or hybrid magnetic/mechanical.

Operation: energy is added by accelerating the rotor and removed by braking. Flywheels provide high power and fast response over short intervals, with long cycle life and no chemical degradation. Losses from bearing friction, windage, and temperature limit efficiency.

Applications include regenerative braking in vehicles and rail systems, uninterruptible power supplies, grid stabilization, and pulsed-power systems. They also support spacecraft attitude control.

Advantages: high power density, long life, rapid response. Disadvantages: safety risks if the rotor fails, need for vacuum and precise balance, and cost for large-scale storage.

History: the concept dates to early machinery; modern high-speed flywheels were developed in the 20th century for energy storage and stabilization.