Short note: Flywheel energy storage

A flywheel energy storage system (FESS) stores energy as rotational kinetic energy in a spinning rotor (mass). When extra electricity is available, a motor accelerates the flywheel; when energy is needed, the same machine works as a generator and slows the flywheel down.

Energy stored: E = ½ I ω², where I = moment of inertia and ω = angular speed. Since E rises with the square of speed, high speeds (20,000–50,000 rpm) are used with strong, light rotors (carbon fibre).

Components

Rotor (flywheel), motor/generator, magnetic or low-friction bearings, vacuum chamber (to reduce air friction) and power electronics.

 Grid <-> [Power electronics] <-> [Motor/Gen]
                                      |
                              [Flywheel in vacuum]

Advantages

• High power density, very fast response (milliseconds), efficiency of 85–95%.
• Very long life (> 100,000 cycles), little maintenance, no chemical hazard, and wide temperature range.

Disadvantages

• Low energy capacity: it can supply power only for seconds to minutes; self-discharge due to friction (standby losses).
• High initial cost and safety concerns (rotor failure), and requires containment.

Applications

Frequency regulation and power quality in the grid, UPS in data centres, regenerative braking in trains and buses, smoothing wind and solar power, and spacecraft.