A friend of mine recently installed solar panels on his roof to generate electricity. They were quite expensive, but my friend considers it an investment that will pay for itself in lower electricity bills over a period of decades. Of course, the panels generate electricity only when the sun is shining. The house is still connected to the electrical grid just like every other house in the neighborhood, but in such a way that it uses electricity from the grid only when the demand is greater than the output from the solar panels. When the panels are producing more electricity than is being used, the electric meter spins backward, and the electric company effectively buys the excess power. So if there were a power outage, the house would have electricity only during the day.
Homes that are off the grid and use solar panels or windmills to produce electricity must store the excess for times when insufficient power is being produced. The usual way to do this is to install a large bank of lead-acid batteries, similar to the ones used in cars. When electricity is being generated, itâ„¢s stored in the batteries, and when itâ„¢s needed, itâ„¢s drained from the batteries. The very same principle is used in hybrid gasoline-electric cars, on the International Space Station and in a number of satellites and spacecraft. Itâ„¢s also fundamental to an uninterruptible power supply (UPS), which you can purchase to keep your computer going for a while or provide emergency lighting in the event of a power outage.
But thereâ„¢s a problem with using batteries for storing electricity: they wear out. Even the most sophisticated modern batteries used in cell phones and laptops can only be discharged and recharged a finite number of times; sooner or later, they refuse to hold a charge. Depending on the type of battery and how itâ„¢s used, the lifespan can be as little as three to five years. Now, buying a new laptop battery every few years for $50 is one thing, but buying enough batteries to power a whole house is going to be enormously expensive. Meanwhile, those old batteries will need to be disposed of very carefully, because they contain toxic elements. And letâ„¢s not forget that such high-capacity batteries are both heavy and bulky. If youâ„¢re using them to power a space station, youâ„¢re going to face considerable inconvenience in replacing them.
Although chemical batteries are likely to be around for a very long time, those with a need for high-capacity, long-term electricity storage are eagerly looking for alternatives. One such alternative is based on a very old and simple device: the flywheel. A flywheel is simply a heavy spinning wheel that stores kinetic energy and then releases it as needed. Flywheels are common in mechanical devices from pottersâ„¢ wheels to automobiles to clocks as a means of regulating or smoothing motion that comes in spurts. Because a flywheel can build up a good bit of inertia, it can keep a mechanism moving during lulls in energy input.
Now people are turning flywheels into batteries. Conceptually, a flywheel battery is very simple. Hook up a motor to a flywheel to spin it when electricity is supplied (storing the energy as kinetic energy). When you want to retrieve energy from the flywheel, hook it up to a generator. (In fact, the motor and the generator can be one and the same.) So you put electricity in and get electricity out, and in the meantime itâ„¢s stored as the motion of a spinning wheel.
Putting a New Spin on It
As you might expect, however, itâ„¢s not quite that simple. Because of the forces of gravity and friction, any flywheel will eventually dissipate all its energy and spin down. So for long-term storage, you want a design with as little friction as possibleâ€which can be accomplished using magnetic bearings to make the wheel float and enclosing it in a vacuum to eliminate air resistance. Capacity is another issue. The greater the mass of the wheel and the faster it spins, the more energy it holdsâ€though you improve efficiency more by increasing the speed than you do by increasing the mass. However, the faster you spin a flywheel, the more centrifugal force will build up, potentially causing it to shatter. So materials must be chosen (or created) very carefully, and the entire assembly must also be well shielded, in case the wheel shatters due to a defect or other problem. In the last decade or so, technological solutions to these problems have begun to present themselves, and modern flywheel batteries, which put out about ten times the power for their weight as lead-acid batteries, are beginning to appear with life expectancy ratings of 20 years or more.
Two problems that have not yet been solved are cost and scalability. Although itâ„¢s possible to purchase a flywheel battery to act as a backup power supply for your home or business, it will set you back many thousands of dollarsâ€enough to pay for quite a few yearsâ„¢ worth of batteries. And you wonâ„¢t see a flywheel battery small enough to power handheld devices or large enough to power a city block. Still, in an era that values devices with no moving parts as a design triumph, itâ„¢s fascinating to watch a good old-fashioned spinning wheel emerge as the battery of the future
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 humanityâ„¢s oldest and most familiar technologies: it was in the potterâ„¢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 itâ„¢s under the hood of every automobile on the road, performing the same function it has for millenniaâ€now regulating the strokes of pistons rather than the strokes of a potterâ„¢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 cellâ€œ the battery most often used in heavy-duty power applicationsâ€œ 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. Whatâ„¢s more, itâ„¢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 itâ„¢s closer to the ground that we find perhaps the most exciting potential application for a flywheel power system. With the modern worldâ„¢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 blameâ€theyâ„¢re sorely lacking in power, storage capacity, charge speed, durability, and lifespan. Flywheel energy storage could well be the solution, and we donâ„¢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 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 flywheelâ„¢s kinetic energy back into electricity which drove the busâ„¢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 busâ„¢s flywheel sat on a standard bearing which frequently broke under the strain, and which rapidly drained the wheelâ„¢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 Gyrobusâ„¢s design were serious obstacles facing any flywheel-powered vehicle, but almost all of them have since been overcome. The justification for the busâ„¢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 objectâ„¢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 Gyrobusâ„¢ 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 massâ€and these materials continue to improve. The delicate and energy-draining bearings that hindered the Gyrobus have also been made obsolete. Itâ„¢s now taken for granted that any flywheel energy system will use magnetic bearings, which levitate the wheel within a vacuum A magnetic bearing
enclosure so that it spins in a nearly friction-free environment. A magnetic bearingFlywheels in a system like this can
glide along for months once theyâ„¢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 wasnâ„¢t until the 1990s that the technology began to approach the necessary sophistication. Like the mechanism in the Gyrobus, Bitterlyâ„¢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. Bitterlyâ„¢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 chargeâ€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 carâ€contending with swerves, stops, and bumpsâ€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, Chrysler's Patriot prototype
and drain excess energy through eddy currents, random electrical flows in the system. Chrysler's Patriot prototypeAnother
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 Bitterlyâ„¢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 flywheelâ„¢s containerâ€but designers must balance strength with mass in order to keep a vehicleâ„¢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
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, itâ„¢s striking how many of civilizationâ„¢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 theyâ„¢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 potterâ„¢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.
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