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Fly-Wheeling in the Night.

Flywheel Energy Storage Modules (FESM) will soon replace batteries on Earth-orbit satellites, while saving millions of dollars in launch-weight premiums and creating millions more in revenues.

An alternate energy management and power conversion technology solves problems in electrical backup power supplies, aviation and industrial high-speed power conversion systems, space satellite energy storage and attitude control systems. This flywheel-based technology is expected to replace the chemical batteries presently used for energy storage and attitude control on Earth-orbit satellites. Between fifty and sixty Earth-orbit satellites are launched each year, with industry projections for the growth of commercial satellites predicting an increase to more than 100 per year by the year 2003.

Present Earth-orbit satellite technology uses solar panels to power all systems while the satellite is in the sunlight. For low earth-orbit (LEO) satellites, there is typically about a 60-minute period where the satellite is in a sunlit orbit and about 30-minutes of an eclipsed cycle, during which trickle-charged Nickel-Hydrogen batteries are used.

The Flywheel Energy Storage Module (FESM) developed and manufactured by Optimal Energy Systems, Inc., Torrance, CA, combines a motor/generator unit for power conversion with the flywheel for energy storage to replace the Ni-H batteries. While in sunlit orbit, the motor will spin the flywheel to a fully charged speed, while the generator mode will take over to discharge the flywheel and power the satellite during the eclipse phase. The company's present flywheel technology is about four times better than present battery technology on a power stored vs. weight comparison.

Weighing less than 130 lbs, the FESM is 18.4-in, in diameter by 15.9-in, in length and is projected to deliver 2 kW-hr of useful energy for a typical 37-minute d LEO eclipse cycle. Initial cost savings would be in launch-weight premiums, based on a rate of between $10,000 and $15,000 per pound. There may be as much as 1000 pounds in the battery systems on future low-Earth-orbit (LEO) satellites planned to be launched within the next five years. Larger geosynchronous-Earth-orbit (GSO) satellites may require even larger battery systems--the current average for commercial GSO storage is 2,400 lbs of batteries, which is decreased to 720 lbs with an equivalent FESM. With transponder payloads at 66 lbs per unit, this could make room for 25 additional transponders on each satellite, resulting in a corresponding payload revenue increase.

Flywheel Technology

The Optimal Phase II flywheel rotor will be the primary component in the FESM. The subscale prototype version of this element weighs 24 lbs and has safely demonstrated 900 Watt-hours of energy storage (about 83 W-hr/kg). The FESM is expected to provide more than 40 W-hr/kg maximum energy storage for 200,000 cycles.

This flywheel design, lighter in weight and smaller in size for a given energy than other flywheels on the market, repeatedly demonstrates very low stresses at extremely high speeds of up to 60,000 rpm. With this technology in hand, the company expects to offer satellite energy systems that store as much as four times the energy per pound, and delivering more than three times the power of that in competing electrochemical batteries. In achieving these energy levels, the company has derated its maximum operating speeds for the flywheels to such an extent that the expected life of its flywheel energy storage systems will be as much as twice as long on a LEO satellite than existing electrochemical battery systems.

The limitation in the development of high-energy flywheels has always been in devising a method for attaching the composite rotor to the shaft without destroying the multi-ring composite rim as it spins to very high speeds. All flywheels expand and grow in diameter as they increase in rotational speed. A composite rim expands at a much higher rate than the solid metal hubs typically used on flywheels, causing tension between the rim and the hub. Optimal's flywheel hub is geometrically designed to expand at the same rate as the composite rim, creating a stress-free interface between rim and hub, thus preventing the fiber matrix from pulling apart when the flywheel spins at extreme speeds. This growth-matching technology creates a zero-stress interface that remains for any speed at which the flywheel is operated.

The Motor/Generator

The Optimal electrical machine has very low stresses at supersonic speeds and is more than 95% efficient. The very low loss is due to a patent-pending rotating back-iron and iron-less stator, which eliminates eddy current losses. The high magnet velocity results in high efficiency and high peak power The design uses high-strength graphite composite materials in combination with high strength titanium and steel alloys that can withstand very high stress levels without failing. These materials must be machined carefully; J&S Fabrication, Inc., Port Townsend, WA, performs the machining, as well as some assembly work for Optimal (see article on page 30).

The breakthrough that surpasses other high-speed electrical machines is the manner in which rare earth magnets are spun at such elevated speeds. The critical limitation in the development of a supersonic electrical generator has been in devising a method for holding the highly magnetic rare earth magnets without breaking them as they reach excessive speeds. Although rare earth magnets can withstand extreme compressive stresses (even greater than most alloy steels), they are very weak in tension and fail at very low stresses when placed in tension. Past designs found the machines self-destructing before they could reach sufficiently high enough speeds to achieve any appreciable improvement in efficiency and power output.

The new rotor design creates a uniform compression stress "cradle" for the magnets that changes to fit the magnet shape as the electrical machine increases in speed. The more the rotor increases in speed, the more uniform are the stresses in the magnets, owing to the unique shape of the magnets. Laboratory tests have further indicated that, in fact, the magnets tend to eliminate their tensile stress as the rotor increases in speed. The rotor has achieved magnet array speeds of more than 1.25 times the speed of sound.

Optimal also produces a Flywheel Power & Momentum Module (FPMM) that provides momentum storage at one third the weight of existing momentum wheel products while additionally storing 108 W-hr of energy for satellite secondary power supply. Other applications for this technology may be found on the ground--in the increasing demand for environmentally-friendly power management alternatives, specifically in uninterruptible and load-leveling power supplies.
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Copyright 2001 Gale, Cengage Learning. All rights reserved.

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Author:Gooch, Stephanie
Date:Jul 1, 2001
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