Technical Description

Flywheels store energy mechanically as kinetic energy by bringing a mass into rotation around an axis.  According to classical, mechanical physics the kinetic energy of a rotating mass m in distance r from the point of rotation can be expressed as:

E_kin=1⁄2∙I∙ω^2

where I is the moment of inertia – equal to m∙r^2– and ω is the angular velocity. It is seen from this expression that the kinetic energy of a rotating flywheel increases proportionally to the mass and to the distance from the rotation point squared. The energy also increases proportionally to the angular velocity squared. To maximize the stored energy for a given mass and rotation speed, the mass should be separated from the rotation point as much as possible. On the other hand, the centrifugal force acting on the mass is defined as:

F_c=m∙r∙ω^2

and thus, the requirements to the materials binding the mass to the rotation center - increases proportionally to the separation distance. This fact sets limits to the maximal available distance because of the properties (tensile strengths) of known, available construction materials.

Whereas flywheels were formerly mainly constructed of metallic materials, modern flywheels are usually constructed – at least partially - by polymer/fiber composite materials. Flywheels are appropriate for fast dynamic energy storage for applications like peak shaving or long energy storage times. Large flywheels should preferably be designed from composite materials due to the high rotational speeds and the bigger strength to weight offered by these materials. Metallic rotors are mainly used for simple seconds to minutes energy storage systems like UPS (uninterruptable power supplies).

Drawing showing a cross section of the flywheel system

Schematic diagram of flywheel energy storage system

Prices for flywheels and comparison of prices