New Spin For Flywheel Technology

Publications ranging from Wired Magazine to Discover described them as the next major source of clean power. Enthusiastic inventors claimed flywheels could be the primary power source for a variety of needs, even automobiles. The Chrysler unit of Daimler-Chrysler once planned to develop a 500-hp entry for the LeMans race powered by a flywheel turbine powerplant.

Aside from a few modest successes, flywheels have not lived up to this sort of hype. The basic science hasn't been in place to conquer fundamental problems that crop up when wheels spin at the 30,000 rpm-and-higher speeds necessary to deliver performance promised by the most wild-eyed flywheel proponents.

The conundrum is explained by NASA flywheel project Chief Engineer Ray Beach. NASA's Glenn Research Center in Cleveland has had flywheels under development for several years with the idea of applying them in scenarios ranging from orbiting space stations to satellites. But engineers there found a lot of holes in the technology when they first started serious work on the subject.

"People have been talking for 20 years as though this technology was right around the corner. It hasn't been around the corner," says Beach. "When NASA first got into flywheels about seven years ago, we assessed the different technologies available and felt the physics was there to build a unit that would handle the requirements for a space application. Initially we worked with several flywheel firms. We had some success but we found we were significantly off from what you needed to put something into space."

THE BASICS OF HIGH-TECH FLYWHEEL DESIGN
The allure is easy to understand. Flywheels potentially have a high-energy density and are pollution-free. That's why they have garnered interest through the years as a source of clean power for trains, trucks, and automobiles.

Many of those drawn to flywheel research have focused on getting high energy density by means of a light wheel that spins fast, usually 30,000 to 100,000 rpm. Managing the high inertia that results from such a design has proven to be problematic on a variety of fronts. Flywheel energy storage potential is proportional to mass moment of inertia and the square of rotational speed. Speed is limited by the strength-to-density ratio of the rotor material, so strong-but-light composite materials have gotten most of the attention.

Problem is that composites can delaminate or be subject to failures in the fiber/matrix material under the right conditions.

Flywheel researchers say they now have a handle on how to design wheels that won't burst during their projected lifespan. Even so, containment schemes able to hold debris flying off a high-energy wheel have gotten attention as well.

Flywheels operate in at least a partial vacuum to avoid the aerodynamic drag losses that would otherwise result. This means they must incorporate a vacuum pump (if they are not sitting in the vacuum of outer space), which itself dissipates power. The vacuum can complicate the dissipation of heat generated by ohmic losses in the bearing electromagnets and rotor. In addition, active magnetic bearings require sophisticated computer control to maintain levitation.

These systems usually have a rotating-field generator with the magnetic field supplied by rare-earth permanent magnets. The specific strength of these magnets is typically much less than that of the composite flywheel. Thus they must spin at much lower tip speeds and sit near the inner portion of the flywheel. This may compromise the power density of the generator.

The experience eventually led NASA to seek help from academia. "In the end, we found all the flywheel companies we worked with had similar capabilities. Most of the ones we've had experience with are small and have really sharp people. And most of them are underfunded so they are generally selling more than what they really have had in hand," says Beach.

One of the institutions tapped by NASA was Texas A & M's Center for Space Power, which has been researching low-loss and lowmass magnetic bearings. The term " bearing" is actually something of a misnomer here. Magnetic bearings on flywheels actually refer to a levitation system that raises the rotor shaft with magnetic fields so there is little or no physical contact while it turns. Electronic controls monitor the shaft position in real time and adjust fields to keep the rotor shaft centered.

Similarly, NASA went to the Center for Electromechanics (CEM) at the University of Texas in Austin for help with rotor technology. The problem NASA encountered initially was that rotor technologies wouldn't provide the kind of tip speeds (linear velocity at the outside radius of the flywheel) needed to give energy densities space applications require. Researchers at U of T came up with an all-composite rotor-plus-arbor design. This approach replaces composite material close to the shaft with a light and flexible structure dubbed an arbor. The arbor expands outward with higher rpm's at the same rate as composite material at the outer-most wheel diameter. This keeps the arbor from loading the outer-most ring elements and boosts flywheel life.

CEM has worked on several advanced flywheelprojects over the years, including efforts sponsored by Darpa (Defense Advanced Research Projects Agency) as well as feasibility studies for transit buses and a 3-MW, 15,000-rpm unit for trains. One conclusion coming out of this work: "Space is probably the best application for advanced composite flywheels right now," opines CEM program manager Joe Beno. "Economically it makes the most sense because there is such a premium put on weight and efficiency. But once you get flywheels made in sufficiently large quantities and their price comes down, they can be reasonable for a lot of areas."

One of these areas may be mass transit. In studies funded partly by the Houston Metropolitan Transit Authority, CEM used a 150-kW flywheel hitting 40,000 rpm as a replacement for chemical batteries on a hybrid-electric city bus. The wheel could be made to last the life of the bus, unlike chemical batteries which needed periodic replacement. CEM figures the flywheel was responsible for boosting fuel efficiency on a demonstration bus by 30% and acceleration by almost a factor of two.

CEM's transit bus project sheds light on some of the practicalities of fielding flywheels in vehicles. For example, researchers studied how bumps and potholes might affect a spinning wheel. They came up with a magnetic bearing for the device that kept the wheel suspended to within a 20-mil tolerance for impact loads of 3 g or less. More severe jarring (which topped out at about 8 g in tests there) would put the wheel momentarily down on a backup mechanical bearing.

Also of concern was the containment of wheel debris in the event of a failure. The need to reliably contain high-energy-density flywheels was highlighted nine years ago by the death of an engineer in Germany during a spin test of a composite flywheel. The same year, Darpa sponsored the Flywheel Safety Project to find means of safely keeping a disintegrating wheel from causing damage.