What goes around comes around" is not just a popular expression and the title of a Bob Marley song, it is also a good description of what is happening these days with flywheel energy storage. The technology is coming around again after undergoing a round of improvements in materials, magnetic bearing control, and power electronics.
Of course, scientific and technical advances by themselves are not enough to renew interest in a technology, however good it may be. The advanced wizardry must also serve a genuine need. Today's flywheel batteries, which depend on a rotating mass to store energy, score well in both areas: they embody several exciting technological advances, and they are serious contenders for a variety of important energy-storage applications. They are, for example, competitive with chemical batteries in applications like transportation or improving power quality, which involve many charge-discharge cycles and little in the way of long-term storage.
Progress in power electronics, particularly in high-power insulated-gate bipolar transistors (IGBTs) and field-effect transistors (FETs), underlies higher-power flywheel operation. While the stored energy is determined by the speed, mass, and geometry of the wheel, the limits on input and output power are in general set by the power electronics. With these higher-power devices, fewer individual components are needed, so the power electronics package can be comparable in size to the flywheel plus motor-generator combination.
The growing density of energy storage is to be attributed mainly to advances in fibers, resins, composite manufacturing techniques, and manufacturing quality control. Together, these have made it possible to construct flywheels strong enough to operate reliably at high speed. Exploiting such developments, US Flywheel Systems (Pasadena, Calif.) has operated a composite flywheel at 60 000 r/min with a corresponding rim speed of about 1 km/s. On the lifetime reliability front, the University of Texas at Austin has subjected a composite flywheel spinning at about 48 000 r/min to more than 90 000 charge-discharge cycles with no loss of functionality.
In an increasingly electrical world, the need to store electric energy is growing, both to help improve power quality and to accommodate distributed generation. Most schemes for realizing those goals involve the storage of energy near the load, which, as shall be shown, makes flywheel batteries prime candidates for the job.
Nor are grid-connected applications the only likely beneficiaries. In transportation, hybrid vehicles need to store power. While an internal combustion engine supplies them with constant power, an electric motor powered by a temporary store (today most often a nickel metal-hydride battery) supplies extra energy for acceleration, deceleration, and (in the future) electrically actuated active suspensions. Flywheels are also finding a place in hybrid gas-turbine/electric trains, where battery banks would be too large and heavy.
Space vehicles, especially those in orbit around the earth, should also make a good home for flywheel batteries. In earth orbit, after all, the sun is the prime energy source, so that energy must be stored for the parts of the orbit when the satellite is in darkness.
In military affairs, the recently released modernization plans for both the U.S. Navy and the U.S. Army indicate their intention to depend more heavily on electricity for the propulsion of both ships and manned and unmanned ground vehicles, as well as for the weapons, navigation, communications, and intelligence systems they carry. This multipurpose use of electric energy tends to call for more energy storage because the various systems often use energy at different rates—that is, at different power levels. With appropriate energy storage, the primary power source may be absolved from handling the peak power load.
Alternative forms of storage
Flywheel energy storage systems are attractive for the types of applications for which a designer might also consider conventional electrochemical batteries or superconducting magnetic energy storage (SMES). With the latter, energy is stored in the magnetic field surrounding a coil of superconducting wire. When cooled to cryogenic temperatures, superconducting wires exhibit zero resistivity, which means that a current circulating in such a coil can persist for a very long time without loss. Of course, keeping the coil at cryogenic temperature itself consumes energy, which is one reason why SMES systems are not considered suitable for long-term energy storage.
Given the state of development of both flywheel batteries and SMES systems, it is to be expected that costs for both can be lowered with further technical development. On the other hand, electrochemical batteries, above all the lead-acid variety, already have a tremendous economy of scale that has driven costs down about as far as they are likely to go. The comparison table suggests that lead-acid batteries and flywheel systems are competitive on the basis of life-cycle cost for some applications today.
In all probability, all three technologies will remain viable. Each has attributes different enough from those of the others to maintain a market niche unless a disruptive technological breakthrough occurs in one area.
Modern flywheel batteries have been designed for a variety of applications. Their initial niche will be for individual flywheels capable of storing approximately 1-500 megajoules. As the table shows, the peak power ranges from kilowatts to gigawatts, with the higher powers aimed at pulsed-power applications. All these systems are in use, in testing, or under construction, except that the one for the hybrid combat vehicle is still in the design phase.