Flywheel Energy Storage

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Introduction

Flywheels are inertial energy storage systems. They were first used in steam engines, in the 1970s this technology was used for centrifugal enrichment of uranium and then rediscovered as a potential new energy storage system. Flywheels store kinetic energy in a rotor; its diameter ranges from a few inches up to several meters, and speeds up to 200,000 rpm. Usually a generator/motor in- and outputs electrical energy to the flywheel.

Capacity

Flywheel energy storage systems have capacities from 0.5 kWh to 500 kWh. The energy stored is ½Jw², where J / (kgm²) is the moment of inertia of the flywheel system and w / (rad/s) is the angular velocity of the flywheel.
The flywheel itself loses less than 0.1% of the stored energy per hour, if magnetic bearings are used. Thus, flywheels can’t store energy for a long period of time, but the energy stored in a flywheel systen can be released in a relatively short period of time. This makes it very useful for UPS (Uninterruptable Power Supply) systems and also for vehicles. It acts as a mechanical battery.

Composition

The basic components are a rotor, a shaft, a vacuum housing, bearings, and a generator/moter.
Electrical energy is necessary to drive the system. It’ is fed into the motor which converts it into rotational energy. The energy is stored in form of kinetic energy in the rotor. When energy is needed, rotational power is taken out of the rotor and converted back into electrical energy.


The rotor is made of material that has a high tensile strength and a low density, e.g kevlar. Such a material is necessary because the kinetic energy rises with the angular speed of the rotor, but only linear it’s mass.
The bearings are usually magnetic bearings since no mechanical bearings are available for such high angular speeds. In addition, the loss is lower. Therefore also a vacuum housing is used. The shaft has no special requirements.