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magnetic bearings. These bearings would enable much smaller standby losses than even
HTS bearings, and would greatly increase the reliability and simplify the control systems
of high-speed flywheels.
Composite Materials
Another area of intense interest to flywheel developers and manufacturers is the use of
new composite materials in flywheel design. The term “composite material” is used to
describe materials which have complex microstructures, often consisting of several
materials in combination, and which defy classification by composition, crystal structure,
or physical properties. The key point is that composites have a higher tensile strength
and are lighter than steel. The expected benefit in flywheel applications is an order of
magnitude increase in the practical wheel speed.
In the case of flywheels, the most common composite materials under consideration are
graphite fiber composites and glass fiber composites. Both these materials are typically
composed of fine fibers in a parallel arrangement, held together by a binder. This
arrangement produces low-weight materials with very high tensile strength - often greater
than that of steel - in the direction of the fibers. In flywheel applications, the lighter
weight reduces the hoop and radial stresses within a spinning rotor at a given radial
velocity, and the higher tensile strength allows a much higher maximum stress before
yield. These effects combine to allow composite rotors to achieve much higher rotational
speeds (and therefore, higher energy storage potential) than steel rotors.
At present, the specific research with respect to flywheels is generally dedicated to
characterization, adaptation and qualification of new materials to a rotary application.
Composite materials do not act linearly. As a result they tend to be much more difficult
to characterize and understand than materials such as steel. The adaptation of such
materials to rotary applications can also be difficult. For example, fiber composite rotors
cannot be cut out of a block of existing material; they must be constructed in such a way
that fibers are wound around the circumference of the rotor, to absorb maximum stress.
For these reasons, research into new rotor materials can be costly and time-consuming.
Research Activities
There are three major directions in flywheel rotor material research. The first is towards
stronger, lighter materials, which allow higher rim speeds and energy density. The
second is towards cost reduction of composite rotors. The third is towards more effective
and more repeatable manufacturing techniques, producing safer and more reliable rotors.
In general, all the material developments involve these three directions to some degree.
The Composites Technology Center at Pennsylvania State University in University Park,
PA has concentrated its efforts in flywheel technology on the development of lighter,
stronger, and cheaper composite rotors. Penn State investigators are working on
perfecting a carbon filament winding system, which will improve the strength of rotors
while allowing the use of cheaper carbon materials. They have also been involved in
investigations into the use of exotic materials such as carbon filaments, carbon nanotubes,
and higher temperature materials, all of which can improve flywheel performance in the
future.
Flywheels Page 22