Flywheels are not a new concept, but the technological development in the last 10 years or so made it feasible to become a commercial product. How a flywheel storage system works? The concept is actually pretty simple: it absorbs energy from the grid and stores it in a high-density rotating flywheel. The flywheel thus acts as a kinetic energy battery. Since it spins at very high speeds (>20000 rpm) it is capable of providing very high bursts of energy in a very short period of time. It consists of an electric motor enclosed in a vacuum container and suspended on magnetic bearings. This way almost no losses are encountered due to inertia in the bearings and the surrounding air. After the power loss the motor acts as a generator, converting stored kinetic energy into electric energy.
There are many possible uses for flywheels: trains, cars, uninterruptible power supplies, pulse power, but one of the most interesting is the frequency regulation of the grid. By observing frequency changes it is possible to determine if the power demand is higher (frequency drops) or lower (frequency rises) than the supply. Given the property of the flywheels to deliver high amounts of energy in short periods of time, it sounds like a very promising solution to a very complex problem of power management.