Flywheel storage is an energy storage method based on mechanical inertia. A heavy rotating disk is accelerated by an electric motor, which acts as a generator on reversal, slowing down the disk and producing electricity. Electricity is stored as the kinetic energy of the disk.
Friction must be kept to a minimum to prolong the storage time. This is often achieved by placing the flywheel in a vacuum and using magnetic bearings, tending to make the method expensive. Larger flywheel speeds allow greater storage capacity but require strong materials such as steel or composite materials to resist the centrifugal forces (or rather, to provide centripetal forces). The use of carbon nanotubes as a flywheel material is being researched. The ranges of power and energy storage technically and economically achievable, however, tend to make flywheels unsuitable for general power system application; they are probably best suited to load-levelling applications on railway power systems.
High-temperature superconducting flywheels (funded by DOE) are currently under development. Such systems would offer inherent stability, minimal power loss, and simplicity of operation as well as increased energy storage capacity.