A diagram of a contemporary flywheel battery. Image copyright Active Powers, Inc.

Advanced Flywheel Batteries
Tech level: 12

Flywheel batteries are also known as flywheel generators. They share some characteristics with homopolar generators, and some systems integrate features of both.

Flywheels batteries use a disk composed of dense materials rapidly spinning in an enclosed, near-vacuum compartment to store and generate electricity. When electricity from an outside source is applied to the battery, this interior disk is spun faster and faster. After the electricity is cut off, the disk continues to spin, "storing" the energy potential of the electricity with its rotational motion. When one wants to draw energy out of the battery again, the spinning of the disk is used to drive an electrical motor, or alternately it may be used as the motor itself. This places a load on the spinning wheel, slowing it back down.

Flywheel batteries now being developed for vehicle applications can produce peak outputs of 150 kilowatts or more, and one being researched by University of Texas' Center for Electromechanics for use in railroad engines envisions a massive 3 megawatt flywheel battery system. NASA is also developing flywheel batteries for use on the International Space Station and other future space ventures. As the technology progresses, compact and lightweight flywheel batteries for use in personal applications may emerge.

One of the great advantages of using a flywheel battery is that it can store large amounts of potential electrical power for a very long time, exceeding modern batteries both in terms of capacity and longevity. Modern flywheel batteries can store kilowatt-hours worth of electricity, and the more advanced models available today are projected to be able to store their energy for twenty years or more. As the flywheel is employed in a near-frictionless vacuum environment, there is very little to slow it down and can keep spinning for years on end.

Compare this to the best chemical batteries, which invariably store electricity for a year or two at most, and have to be carefully disposed of at the end of their operational lifetimes (typically 3 to 5 years) because they contain a number of caustic chemicals.

The amount of energy a flywheel battery can ultimately hold depends on both the mass of the flywheel and its maximum rate of spin. The more massive the wheel, the more kinetic energy it will contain for any given rotational rate. To ensure the fastest spin possible, a flywheel is suspended using magnetic bearings within a vacuum or near-vacuum chamber.

Of course, there are inevitable complications in developing more advanced versions of this kind of technology. The more massive the flywheel is, the more likely centrifugal force will try and tear it apart the faster it spins. Research into making the flywheel out of advanced composite materials and alloys that can withstand these kind of forces is ongoing, with candidate materials including wheels made out of diamond filament fibers and carbon nanotube fibers. And the more resistant to break up from centrifugal force the wheel is, the faster it can be spun, and the more energy it can store. The current flywheel champ, being developed by NASA, is capable of 60,000+ rpms. Future versions of flywheel batteries envision rpm’s in the hundreds of thousands.

NASA's Flywheel Battery prototype being developed at the Center for Space Power in College Station, Texas. >
Another potential problem is that these dense, rapidly spinning wheels contain a lot of kinetic energy, and should a mishap occur or the battery be badly damaged, the flywheel could be knocked loose and tear up anything in its path. For this reason, flywheel batteries have to be heavily shielded and would often be run at below peak capacity to avoid this kind of potential problem.

Also, a pair of counter-rotating flywheels in the same battery may be necessary to avoid rotational progression problems in applications where the battery may not be well-anchored. These include mobile weapon and space flight applications. Some speculation has been put forth that flywheel batteries placed on satellites and spacecraft could also double as gyroscopes as well as energy storage devices.

Besides applications in transportation and space, advanced flywheel batteries would also be useful in providing power back-ups to installations, building, and private homes; allowing communication and power distribution systems to better handle large surges in their use; provide the high current needs for all-electric or electric-enhanced construction equipment; and provide power for high-energy-consuming weapon systems like railguns, coilguns, lasers, and plasma guns.

Further Information

Article added 2006