In a world where everything from our automobiles to our underwear may soon run on electricity, more efficient portable power is a major concern. After a century of stagnation, chemical and ultracapacitor batteries have recently made some strides forward, and more are on the horizon. But the most promising way of storing energy for the future might come from a more unlikely source, and one that far predates any battery: the flywheel.

In principle, a flywheel is nothing more than a wheel on an axle which stores and regulates energy by spinning continuously. The device is one of humanity’s oldest and most familiar technologies: it was in the potter’s wheel six thousand years ago, as a stone tablet with enough mass to rotate smoothly between kicks of a foot pedal; it was an essential component in the great machines that brought on the industrial revolution; and today it’s under the hood of every automobile on the road, performing the same function it has for millennia—now regulating the strokes of pistons rather than the strokes of a potter's foot.

Ongoing research, however, suggests that humanity has yet to seize the true potential of the flywheel. When spun up to very high speeds, a flywheel becomes a reservoir for a massive amount of kinetic energy, which can be stored or drawn back out at will. It becomes, in effect, an electromechanical battery.

The capabilities of such a device are as extraordinary as its unique design. A traditional lead-acid cell-- the battery most often used in heavy-duty power applications-- stores energy at a density of 30-40 watt-hours per kilogram: enough to power a 100-watt bulb for about 20 minutes. A flywheel-based battery, on the other hand, can reach energy densities 3-4 times higher, at around 100-130 watt-hours per kilogram. Unlike the battery, the flywheel can also store and discharge all that energy rapidly without being damaged, meaning it can charge up to full capacity within minutes instead of hours and deliver up to one hundred times more power than a conventional battery.

What’s more, it’s unaffected by extreme temperatures, boasts an efficiency of 85-95%, and has a lifespan measured in decades rather than years.

While the average person has probably never heard of a flywheel battery, the concept is starting to be taken seriously by commercial and governmental interests. Large corporations see flywheel energy systems as ideal for power backup applications because of their long lifespan and low maintenance. Power companies often use them for load-leveling purposes: maintaining a steady flow of electricity between power generation peaks, or storing surplus energy during low-demand periods to prevent brownouts later on. Applications such as laboratory experiments that require huge amounts of electricity are sometimes powered by a flywheel, which can be gradually charged up over time rather than placing a massive drain on the power grid all at once. And NASA is funneling considerable resources into developing flywheel systems, which they believe could completely replace batteries in space applications. Apart from a marked superiority in energy density and lifespan, flywheels have the unique advantage of providing energy storage and attitude control for a spacecraft or satellite in one easy package. When two flywheels aboard a satellite spin in opposite directions at equal speeds, the satellite will maintain its attitude; when energy is transferred between the wheels to speed one and slow the other, the satellite will rotate.

But it’s closer to the ground that we find perhaps the most exciting potential application for a flywheel power system. With the modern world's increasing awareness of the economic and environmental drawbacks of oil-powered automobiles, the electric car has taken on an almost mythical status. Despite decades of development, a practical electric automobile seems as far away as ever, and the limitations of current batteries are largely to blame—they’re sorely lacking in power, storage capacity, charge speed, durability, and lifespan. Flywheel energy storage could well be the solution, and we don’t even have to delve into the theoretical to imagine how such a system would work. In an almost forgotten piece of transportation history, the flywheel-driven vehicle was briefly a reality.

The Gyrobus
The Gyrobus
The Gyrobus was an obscure public transportation vehicle that saw service in Switzerland, Zaire, and Belgium during the 1950s. Electric buses were already common at the time, but they were restricted to traveling along a grid of overhead electric lines. The idea behind the Gyrobus was to free a bus from this prison of wires. Instead of a conventional engine, the bus carried a three-ton rotating steel wheel attached to an unusual electric motor. When the bus was parked at a charging station, the motor would accelerate the flywheel up to around 3000 RPM; then, when it was time to take off, it became a generator, converting the flywheel’s kinetic energy back into electricity which drove the bus’s wheels. The charging process took between 30 seconds and 3 minutes, and once charged a Gyrobus could travel 3-6 miles at speeds of 30-40 mph.

A host of problems with the design ensured a short life for the Gyrobus experiment. The bus’s flywheel sat on a standard bearing which frequently broke under the strain, and which rapidly drained the wheel’s energy through friction. The resulting need to recharge the bus every few stops proved to be a significant hassle. Furthermore, the massive wheel made a Gyrobus far heavier than a regular bus, and far less efficient. The Gyrobus was simply more money and trouble than it was worth.

The flaws in the Gyrobus’s design were serious obstacles facing any flywheel-powered vehicle, but almost all of them have since been overcome. The justification for the bus’s massive steel wheel, and all the problems that came with it, was basic physics: the heavier a rotating object is, the more energy it holds. Increasing the object’s rotational speed, which raises its energy exponentially quadratically rather than linearly, is a far more efficient way to add energy. But spinning a steel wheel too much faster would tear it apart. The Gyrobus’ designers were therefore stuck with favoring size over speed, but this is not the case for modern engineers. The solution came in the 1970s, when materials both stronger and lighter than steel began to appear. Today, carbon fiber flywheels exist that can be spun fast enough to hold 20 times more energy than steel wheels of equal mass—and these materials continue to improve. The delicate and energy-draining bearings that hindered the Gyrobus have also been made obsolete. It’s now taken for granted that any flywheel energy system will use magnetic bearings, which levitate the wheel within a vacuum enclosure so that it spins in a nearly friction-free environment.

A magnetic bearing
A magnetic bearing
Flywheels in a system like this can glide along for months once they’re fully spun up, and under experimental conditions some have spun for up to two years without outside influence. If some friction is present, the wheel can be kept at full charge indefinitely by trickling in just enough energy to overcome it.

With these advancements, it seems that it may at last be time to see the return of the flywheel-powered vehicle. These new machines may bear little resemblance to the Gyrobuses of yesteryear, however. The design that received the most attention in the last decade was the brainchild of Dr. Jack Bitterly, chief engineer for the company US Flywheel Systems. Bitterly had dreamed since the 1970s of building an entirely flywheel-driven car, but it wasn't until the 1990s that the technology began to approach the necessary sophistication. Like the mechanism in the Gyrobus, Bitterly’s system featured a combination electric motor/generator to add and draw power from the flywheel; but this flywheel was made of computer-molded carbon fiber and spun silently on magnetic bearings at 100,000 RPM. Enclosed in a reinforced vacuum container, the whole contraption weighed less than one hundred pounds and could deliver a steady 20 horsepower, or 50 hp in shorter bursts. Bitterly’s idea was to put 16 of these units into a regular-sized car, which would generate 800 hp and travel 300 miles on a single charge—about the same range as a tank of gasoline, but at a cost of around 5-10 dollars. Despite some interest from major car companies, Bitterly and US Flywheel Systems were unable to secure enough support to get their design off the ground.

A number of obstacles held back development of a practical flywheel car, and they remain to this day. First, magnetic bearings are not yet up to the task demanded by a moving vehicle. Keeping a flywheel spinning in a laboratory or in the weightless vacuum of space is one thing; spinning it within the inertial jungle of a speeding car—contending with swerves, stops, and bumps—is an entirely different matter. The bearings must adjust on-the-fly to the sizable g-forces produced by ordinary driving in order to prevent energy loss and damage from flywheel “touchdown.” Even in perfect conditions, current magnetic bearings are not without flaws: they are expensive, unreliable, and drain excess energy through eddy currents, random electrical flows in the system.

Chrysler's Patriot prototype
Chrysler's Patriot prototype
Another problem unique to flywheel designs is the gyroscopic effect, which causes spinning objects to resist changes to their orientation. Obviously this is not a desirable trait when a vehicle is attempting to turn corners.

Finally, safety is a constant concern. A compact flywheel system such as Bitterly’s carries roughly the kinetic energy of a military tank traveling at highway speed, all of which must be released very quickly if the flywheel breaks apart or falls off its axle. Numerous deaths have resulted from just such failures throughout the history of modern flywheel design. This issue ultimately caused the scrapping of the Chrysler "Patriot," a hybrid racing vehicle built in the early 1990s. The car featured a 58,000 RPM flywheel as part of its drive system, but the power of the wheel could never be safely and practically contained. The difference between a potentially deadly failure and a harmless disintegration is the strength of a flywheel's container—but designers must balance strength with mass in order to keep a vehicle’s weight down. The perfect materials and design for such a container have not yet been found.

None of these problems are insurmountable. Magnetic bearings have plenty of potential for improvement and cost reduction: the biggest advance might come from passive magnets made out of superconducting materials, which would eliminate the problems with energy drain and most of the control hardware. The gyroscopic effect, meanwhile, can be largely canceled by mounting the flywheel enclosure on a gimbal or by pairing each flywheel with a counter-rotating partner. And the risk of flywheel failure can be managed; after all, engineers long ago managed to tame gasoline, a far more dangerous energy storage medium that has surrounded us for the last century.

NASA's 41,000 RPM G2 flywheel
NASA's 41,000 RPM G2 flywheel
As with most technologies, the time needed to develop these solutions is a matter of interest, ingenuity, and money. Frustrated by the lack of available funding for a full-fledged automobile project, most flywheel companies, including US Flywheel Systems, have shifted their focus to large-scale business and space projects. This change could be seen as a setback, but in the end it may simply be a more roundabout route to the same goal: once flywheels are proven in such demanding functions as powering the International Space Station, they will be taken seriously for more everyday tasks as well.

When examined closely, it’s striking how many of civilization’s energy and environmental problems can be traced back to inadequate energy storage. Humans happily rely on storage methods with efficiencies as low as 20%, wasting far more energy than we actually use. Automobiles continue to be a top contributor of pollution because they’re driven by a crude and dirty energy medium, and alternative “clean” energy sources such as wind and solar are restricted by the lack of an effective "potter's wheel" to keep the power flowing during down periods. When civilization first harnessed the power of the wheel, the achievement brought about a new era for humanity. Today the wheel seems poised to bring about another such change, and though the impact this time might not alter civilization as we know it, it may yet prove to be revolutionary.

Written by Christopher S. Putnam, posted on 12 October 2007. Chris is a writer and bomb-disposal expert for the Damn Interesting A-Team. He posts from an undisclosed location in Saskatchewan, Canada.
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