Flywheel energy storage is a method for storing energy using a rapidly spinning flywheel. The flywheel, which generally spins in a vacuum, stores energy as rotational energy. Energy can be removed from the system or added to the system by means of an electric motor/generator. Flywheels spin at a very high number of revolutions per minute (RPM) and can store a significant amount of energy. The durable nature of flywheel energy storage systems and their ability to rapidly absorb or discharge large amounts of energy make them excellent candidates to replace or supplement conventional batteries for use in electric vehicles.
Older flywheel energy storage systems relied on purely mechanical bearings to support a rapidly rotating flywheel. Modern versions instead use magnetic bearings. As a result, they are more durable, because the moving parts inside the system are not subject to constant friction. Some modern flywheel systems even employ superconducting magnets, although the cost and temperature requirements for these systems limit their usefulness.
The reduced friction of magnetic bearings also increases the utility of flywheels in storing energy. This is because less kinetic energy is lost to friction while the flywheel is spinning. The increased storage efficiency is particularly important when a flywheel is used to store energy for longer periods of time, because mechanical flywheel systems will dissipate much of their mechanical energy over the first hour after charging.
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Flywheels are used in many applications where rapid charging and discharging are important. This type of energy storage has been used successfully to power motor vehicles and works especially well as a form of regenerative braking. The fact that flywheels act as large gyroscopes when they are spinning requires special engineering so that the handling of vehicles is not adversely impacted, and it can serve to stabilize vehicles.
Flywheel batteries have been suggested as a means to power next-generation weapon systems. They have the ability to store more energy than conventional batteries and to discharge that energy faster. This led the United States Army to consider using flywheels to power weapons using electromagnets driven by flywheels.
The largest problem with flywheel energy storage is the potential for damage and injury if a charged flywheel is broken. These systems store energy as kinetic, rotational energy, so serious damage to the flywheel housing can cause the flywheel to shatter. Charged flywheels typically spin at 40,000-60,000 RPM, so this can release dangerous shrapnel. Modern flywheel systems employ advanced flywheel housings to limit the danger from this sort of event.