Machine Design

Wheel may fly high

A flywheel energy-storage system recently reached 60,000 rpm

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, and may lead to flywheels replacing chemical batteries on the International Space Station. Researchers at NASA's Glenn Research Center, Cleveland, and U.S. Flywheel Systems Inc., Calif., along with several other partners labored for five years to improve basic flywheel technology. The latest feat marks the highest speed ever demonstrated for a flywheel -- a mechanical battery that converts energy to mechanical motion and can convert that motion back to energy -- while levitated and spun on magnetic bearings.

In practice, a flywheel energy system, also known as an electromechanical battery, uses an electric motor to spin a wheel or rotor to store energy. The energy stored in the wheel then drives an electric generator which, in turn, powers an electrical load. The resistance of the load, reflected back to the wheel, gradually reduces the wheel speed. The high wheel speed is important, as the greater a flywheel's rpm, the more energy it stores.

Team members, also including researchers from TRW, Texas A&M University, the University of Texas Center for Electromechanics, and Boeing, developed high-strength carbon-fiber/epoxy composites for the flywheel's rotor, low-loss magnets for the bearings, high-speed electric motors and generators for energy conversion, and computer algorithms for motion control.

This particular flywheel, enclosed in an aluminum vacuum vessel, consists of a high-speed brushless dc motor, upper and lower sets of radial magnetic bearings that provide high magnetic flow, a high-power axial magnetic bearing, a lightweight rotor made of carbon-fiber composites, high-speed mechanical touchdown bearings, and a motor/generator drive.

On the space station, a motor powered by electric solar arrays will spin the wheel. When the station is shaded, the spinning wheel will turn the motor -- now acting as a generator -- to produce electricity for science equipment and life-support systems. According to researchers, more than 85% of the energy supplied to the wheel can be output, a much higher rate of return than chemical batteries. This type of flywheel has a service life equal to that of the space station, as opposed to the five-year life of proposed chemical batteries, and can operate over a much wider temperature range. In addition, the new device reportedly stores more energy/pound than chemical batteries.

The system uses no hazardous chemicals and can be applied to a variety of commercial applications including uninterruptible power supplies, pulsed power devices, and electric or hybrid vehicles.

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