Handbook of Energy Storage for Transmission or ... - W2agz.com

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cycles. The rotor is subject to fatigue effects due to the cyclical application and stress
during charge and discharge. The most common failure mode for the rotor is the
propagation of cracks through the rotor over a period of time. Crack propagation can be
difficult to detect in steel rotors, and hazardous failure modes are possible in which large
chunks of steel break off from the rotor during operation. Appropriate design and
operation precautions must be taken in order to ensure safe operation (see Safety, below).
In graphite rotors, cracks tend to propagate longitudinally, or result in de-lamination of
the concentric layers of material. This phenomenon causes the rotor to gradually deviate
from normal operation. Thus, monitoring of suitable operating parameters will ensure
that the device can be removed from service before a hazardous failure mode occurs.
Recharge Time
Like electrochemical capacitors and SMES, flywheels have the advantage of relatively
quick recharge times. Recharge times are comparable to discharge times for both power
and energy flywheels designs. High-power flywheel systems can often deliver their
energy and recharge in seconds, provided that adequate recharging power is available.
Bi-directional power conversion facilitates this two-way action. On the other hand, in
stabilizer applications the controls may be designed to provide a negative feedback where
the rate of charging and discharging depend on voltage or frequency. In this case
charging may occur quickly and discharge slowly or vice versa and most of the time no
charging or discharging is required.
Standby Power Loss
Flywheel systems may have both intrinsic and parasitic power losses which cause the
energy stored to be gradually used up. A certain amount of power must be applied to
maintain a high level of charge if the flywheel is used in a standby mode. The magnitude
of the power loss is dependent on the design of the flywheel, and may have both intrinsic
and parasitic components.
Intrinsic power losses include friction and other forces that cause the rotor to slow down,
and are common to all flywheel designs. Intrinsic power losses can be reduced through
the use of techniques such as vacuum containment and magnetic bearings, but can never
be reduced to zero.
Parasitic power losses include power provided to active magnetic bearings or cooling for
superconducting bearings. Unlike intrinsic losses, parasitic power losses are independent
of the speed of the flywheel. Not all flywheel systems have parasitic power losses.
Electrical Interface
The electrical interface, where the flywheel mechanical or kinetic energy is converted to
electrical energy, and vise versa, can vary greatly with different flywheel types and
applications. There are a number of rotating machine technologies that can be used in
flywheels for generating (during discharge) and restarting (after discharge). These
include permanent magnet alternators as well as dc, synchronous, wound-rotor induction,
and written-pole motors and generators. Additional variety comes when these various
machine technologies are mixed with different forms of power electronic and
electromagnetic frequency conversion technologies. Also in the mix are application-
Flywheels Page 12