Electrical Equipment - Wind Energy Systems for Electric Power Generation - page 76

R 2 / 2, and in case of a cylindrical ring of mass m , inner and outer radii R i
and R a , respectively, we get J = m

J = m ·

( R a 2 + R i 2 ) / 2. The equation indicate the prefer-
ence for large rimmed rotors to obtain large values of J , and for high speeds

·

.The
rotor may be a steel flywheel, however carbon-fiber composites are of advantage re-
garding tensile strength. Flywheels have to be specified for a maximum permissible
outer surface speed.
In FES systems, the rotor is accelerated to the permissible speed (charging), and
decelerated for discharging, using a motor-generator set and a power electronic in-
verter. No-load losses in bearings and gas friction have to be minimized in flywheel
storage devices. Vacuum enclosures and magnetic bearings are used to meet these
requirements.
Advanced flywheels are rated for speeds of 20000 min − 1 and above. High energy
densities of 130 Wh/kg and overall energy efficiency of 95% are reached. These
properties and long lifetime make them an attractive however costly choice not only
in stationary systems such as uninterruptible power supplies (UPS), but also in vehi-
cles. Flywheels have meanwhile also found interest as load levelling devices acting
together with wind energy systems.

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