A flywheel is a simple mechanical energy storage device comprising a large wheel on an axle fitted with frictionless bearings. A flywheel stores kinetic energy as a result of its rotation. The faster it rotates, the more energy it stores. In order for a flywheel to be effective as an energy storage device there must be a way of feeding energy into the flywheel and a means of extracting it again.

Simple flywheel energy storage devices are fitted to all piston engines to maintain smooth engine motion. The engine flywheel is attached physically to the engine camshaft and as the pistons cause the camshaft to rotate they feed energy into the flywheel. For electricity storage applications, energy will normally be fed into the flywheel using a reversible motor generator.

The faster a flywheel rotates, the more energy it will store. Conventional flywheels are fabricated from heavy metal discs made of iron or steel. However these discs are only capable of rotating at low speeds. For power applications, new lighter composite materials are being developed, capable of rotating at 10,000-100,000 rpm without fracturing under the immense centrifugal force they experience. Such devices must be housed in exceedingly strong containers which will prevent the pieces of the flywheel scattering like shrapnel in the event of a catastrophic failure.

Energy storage systems must operate with low energy loss. This is accomplished in flywheel systems by using magnetic bearings to eliminate bearing friction and by operating the flywheel in either a vacuum or in a container filled with a low-friction gas such as helium.

One of the problems with flywheel energy systems is that the flywheel will rotate at varying speeds depending on how much energy it contains. If a conventional motor generator is used to extract electrical energy from the flywheel, this will translate into a variable frequency output. Grid electricity, however, must be generated at a constant AC frequency. Various electromechanical and electronic means of overcoming this difficulty have been found.

Flywheels have the attraction of virtually zero maintenance and infinite recyclability. They have proved to be one of the best and cheapest ways of maintaining power quality during power failure or network voltage or frequency dip. Response time is fast and in the case of power failure a flywheel system can bridge the period between the power outage and a long-term back-up system such as a generator set coming on line.

The largest flywheel system so far built is a 1-MW unit comprising 10 100-kW flywheels used to maintain system voltage on the New York transit system. Storage capacity is 250 kWh, sufficient to provide 1 MW for 15 min.

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