FLYWHEELS
A flywheel is a simple mechanical energy storage device comprising a large wheel on an axle fitted with frictionless bearings. The flywheel stores kinetic energy as a result of its rotation. The faster it rotates, the more energy it stores. Provided there is a means to extract this energy again, the system can be used for a variety of energy storage applications.
Traditional flywheel-based systems have been in use as mechanical energy storage devices for thousands of years. Millstones, potters wheels, and hand looms have all used them to both store energy and smooth out the peaks in energy delivery by hand or foot. Simple flywheel energy storage devices are also 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.