Since 2010 the Institute of Electrical Measurement and Measurement Signal Processing has been working in the field of Flywheel Energy Storage Systems (FESS). An FESS is an energy storage device, similar to a battery, but instead of storing the energy chemically, it is stored in kinetic form in a rotating disc. Typically, the rotor is accelerated by an electric motor (charging process) and decelerated when the motor is switched to generative operation (discharging). The stored energy is a function of the angular velocity squared, which is why there is a trend towards faster and more compact flywheels. The following image shows the topology of various FESS concepts.