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Flywheels are a way of storing energy in the rotational momentum (kinetic energy) of large wheels sealed in a vacuum. They are used as a way of storing and releasing large amounts of electrical energy, and although their method of storage is non-chemical, arrays of flywheels are sometimes called batteries. Because the method of storage is mechanical rather than chemical, the amount of energy stored is only limited by the size of the flywheel, its ability to rotate quickly without tearing apart, and the motor/generator design. Flywheels have found use in specialized applications for which large, quick bursts of power are needed, such as in military rail guns or to provide the electricity to ignite fusion reactions.
Flywheels are usually supported by magnetic bearings that don't require any external power input. Hence, they are relatively inexpensive, reliable, and low-maintenance. Flywheels have been around for centuries, but it's only recently that we've developed the high-quality vacuums, magnetic bearings, and durable materials necessary to store substantial amounts of energy in flywheels. Besides being able to store substantial amounts of energy, other advantages of flywheels include their practically unlimited lifespan, unlimited recharges, and ecologically clean nature.
Many flywheels are made out of steel, iron, or various composites. The maximum rotations per minute for these materials is about 1,000-5,000, depending on the size of the wheel. Experiments have used carbon fiber-based flywheels for superior performance. Possibly the best material for flywheels is carbon nanotubes, or buckminsterfullerene, a substance with tensile strength several times that of diamond. However, the material's cost makes it currently prohibitive for use in flywheel systems.
Flywheels have been touted as the ideal energy storage systems for uninterrupted power supplies (UPS) and, in the future, electric automobiles. Because flywheels are meant to spin very fast, they need to have uniform density and be free of any microscopic flaws. A tiny crack can be magnified and cause the wheel to rip apart and fly in multiple directions, in a catastrophe sometimes called flywheel explosion. Flywheels have a conversion efficiency of about 80%, which is considered substantial.