this weekWheel of Fortune

October 9, 2004

Flywheel Batteries

A new spin on energy storage

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, it’s stored in the batteries, and when it’s needed, it’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. It’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.

Battery Up
But there’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 it’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 let’s not forget that such high-capacity batteries are both heavy and bulky. If you’re using them to power a space station, you’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 potters’ 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 it’s “stored” as the motion of a spinning wheel.

Putting a New Spin on It
As you might expect, however, it’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 possible—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 holds—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 it’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 dollars—enough to pay for quite a few years’ worth of batteries. And you won’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, it’s fascinating to watch a good old-fashioned spinning wheel emerge as the battery of the future. —Joe Kissell

More Information about Flywheel Batteries…

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