Volume 8, Number 2     March/April 2000

Aeropace Technology Development

Flywheels Make the Grade

A flywheel energy storage system recently achieved full-speed operation at 60,000 revolutions per minute, the highest speed ever demonstrated for a flywheel levitated and spun on magnetic bearings. This makes the flywheel a viable candidate for replacing chemical batteries on the International Space Station (ISS).

Research engineers at NASA’s Glenn Research Center in Cleveland, Ohio, and U.S. Flywheel Systems, Inc., with partners TRW, Texas A&M University, the University of Texas Center for Electro-mechanics and the Boeing Company, combined their expertise for more than five years to improve the flywheel technology that enabled this feat. The team looked at everything from the materials used, to the types of bearings, to more efficient motor/generators, to the algorithms used for system control. Team members developed or identified high-strength carbon fiber/epoxy composites for the rotor, low-loss magnets for the bearings, high-speed electric motor/generators for energy conversion and computer algorithms for motion control. It all came together late last December at the California facilities of U.S. Flywheel Systems, the company that built the system.

"The flywheel energy storage system represents a revolutionary step in energy storage technology. We’re thrilled to have met our sustained operating speed goal," said Raymond Beach, Glenn’s principal investigator and team leader for flywheel development at the center.

The flywheel is a kind of mechanical battery that converts energy to mechanical motion and, when necessary, converts that motion back to energy. On the ISS, electricity from solar arrays will run the motor that spins the wheel. During the shade period of the orbit, the spinning wheel will turn the motor, now acting as generator, to make the electricity that powers science equipment and life support systems.

"The process is very efficient. More than 85 percent of the energy put into the wheel comes out," Beach said. At full operating speed, the flywheel rotor’s linear velocity is two and one half times the speed of sound (or 1,875 miles per hour), and if allowed to spin down without load, it would spin for more than 12 hours before coming to rest.

Flywheels have several advantages over the chemical batteries currently proposed for the ISS. They can be designed to have a lifetime that matches that of the ISS; chemical batteries planned for the ISS will last only about five years and must be replaced during the mission. They operate effectively over a wide temperature range; chemical batteries operate well only within a narrow range of 32 to 50 degrees Fahrenheit. They are more efficient, returning more of the energy put into them than do chemical batteries. They can also provide more power because of their higher energy density.

These advantages mean lower costs of operation for the ISS. In particular, they mean more mass can be devoted to science experiments and facilities and even to astronaut quality-of-life payload. The next step in the development of the energy storage system is to build an endurance test for an engineering model of the size and output power that will fly on the ISS.

Flywheel research at Glenn is part of its continuing effort to provide the power for the future exploration of space and other worlds. The flywheel energy storage system demonstration project is managed at Glenn for Johnson Space Center’s Engineering Research and Technology Research Program Office and ISS Payloads Office.

For more information, contact Dr. Timothy Tyburski at Glenn Research Center. 216/433-8616, Timothy.E.Tyburski@grc.nasa.gov Or contact Laurel Stauber at Glenn Research Center. 216/433-2820, stauber@grc.nasa.gov Please mention you read about it in Innovation.