A flywheel is a mechanical device specifically designed to efficiently store rotational energy (kinetic energy). Flywheels resist changes in rotational speed by their moment of inertia. The amount of energy stored in a flywheel is proportional to the square of its rotational speed and its mass. The way to change a flywheel’s stored energy without changing its mass is by increasing or decreasing its rotational speed. Since flywheels act as mechanical energy storage devices, they are the kinetic-energy-storage analogue to electrical inductors, for example, which are a type of accumulator. Like other types of accumulators, flywheels smooth the ripple in power output, providing surges of high power output as required, absorbing surges of high power input (system-generated power) as required, and in this way act as low-pass filters on the mechanical velocity (angular, or otherwise) of the system.

Common uses of a flywheel include:

  • Smoothing the power output of an energy source.
  • Energy storage systems
  • Delivering energy at rates beyond the ability of an energy source.
  • Controlling the orientation of a mechanical system, gyroscope and reaction wheel Flywheels.


Carbon-composite flywheel batteries have recently been manufactured and are proving to be viable in real-world tests on mainstream cars. Additionally, their disposal is more eco-friendly than traditional lithium-ion batteries.