A flywheel is a simple rotating wheel used to store energy or stabilize something. The energy it stores is equal to its moment of inertia -- a physics term that basically means the mass of the object times the square of its distance from the axis of rotation -- times the square of its angular velocity divided by 2. Flywheels help stabilize drive shafts subject to alternating pressures, such as piston engines or piston pumps. The stabilizing effect comes from the flywheel resisting changes in its rotational speed.
Flywheels are used as power storage devices for high-power science experiments that would produce an unacceptable power spike if removing power from the electric grid. Such flywheel batteries might operate in a vacuum, to prevent energy loss due to air friction, and will be periodically sped up again to compensate for rotation speed lost due to energy dissipation from heat and vibration. Good flywheel designs will dissipate as little heat and vibration as possible, retaining energy for the target application.
A flywheel stores different quantities of energy depending on its mass and rotation speed. For instance, a bicycle wheel has a mass of about 1 kg (2.2 lb), diameter of about 70 cm (28 in), and a rotation speed of about 150 rpm (rotations per minute). This adds up to a stored energy of 15 J (joules). Next, consider a wheel on a train moving at 60 kph (37 mph), with a mass of 942 kg (2,076 lb), diameter of 1 m (3.3 ft), and a rotational speed of 318 rpm. This flywheel would have a rotational energy of about 64 kJ (kilojoules), roughly 43,000 times greater than the bike wheel.
Flywheel batteries dedicated to energy storage have energies much larger than both these previous examples, mainly due to extreme rotation speeds. One example made by a flywheel company in Ottawa boasts a 100 kg (220 lb) mass, diameter of 60 cm (27 in), and a rotation speed of 20,000 rpm. This flywheel battery can store about 10 MJ (megajoules), enough to light 100 100-watt light bulbs for 1,000 seconds. This flywheel battery design is not much larger than a refrigerator. An even larger flywheel, of the type used as an electric power backup, may hold 100 MJ of power. These types of flywheels may be used by casinos, hospitals, data centers, or in industry to maintain power in case of a failure or fluctuations in input.