" Wind, water, solar . . . Who needs the tired, old elements? Each of those energy sources has had its proverbial day in the sun. It's time to move aside, ye olde renewables, and make way for a truly abundant and unceasing power source: our limbs, our clothes, our cars.
The world is a frenetic, kinetic kaleidoscope that hums with the constant motion of 6 billion people and their accouterments. Take a good look at the crushing multitudes on K Street at rush hour or at the vehicles rumbling around the Beltway. There's energy in them thar commuters. All that motion might well hold a key to much-sought energy independence. We just need to bottle some of that juice.
Certain materials produce an electrical current whenever they're flexed, bent or otherwise deformed. This occurs because these so-called piezoelectric materials -- which are as diverse as quartz crystals, leaded ceramics and bone -- contain pockets of positive and negative charge. When the material is flexed, those charges shift around, creating the potential for electrical current. Thanks to this piezoelectric effect, such materials can be used to convert the motion that distorts them into electricity.
A few piezoelectric-based devices have been around for years, including pedal-powered bicycle lights and hand-held and flintless gas-grill lighters. The science behind them dates back to 1880, when 21-year-old Pierre Curie, who would later share a Nobel Prize in physics with his wife, Marie, co-discovered the piezoelectric effect with the help of his older brother.
But engineers are only now beginning to see piezoelectricity as a source of abundant energy. They're using novel materials and techniques to harvest the "free" (i.e., untapped) energy that people generate when they move. Think of it as a giant energy recycling program. And it literally uses flex fuel.
A journey of a million kilowatt-hours begins with simple footsteps. Engineering teams worldwide are embedding piezoelectric materials in flooring and paving materials so the ground can absorb the energy from our moving feet. One company claims that five hours of busy pedestrian traffic over a portion of sidewalk could power a dim streetlight all night.
Some engineers have designed backpacks, shoes and other wearables that draw energy from the wearer's motions. Two years ago, in the journal Science, Canadian researchers described a modified knee brace that produced five watts when worn walking. That's enough to power several cellphones. Before long, though, phones may effectively charge themselves.
This year, Nokia filed a patent for a piezoelectric device that could let a phone battery draw a small charge each time it gets jostled in your pocket. If your phone still somehow conks out? In theory, just shake it and dial.
Engineers are also weaving piezoelectric fibers into clothing and other textiles. Some are experimenting with microscopic piezoelectric wires of zinc oxide. When flexed, these wires generate currents that could charge a battery or power portable electronics connected to the garment. Other researchers have embedded piezoelectric materials in silicone rubber that can be stretched, say, across someone's chest to draw power from the rise-and-fall motion of his lungs.
You don't have to get all out of breath about it, either. Even couch potatotes like me produce free energy. It's called body heat. Thermoelectric materials, which are akin to piezoelectric ones, can produce electricity in response to temperature changes within the material rather than from shape changes. That means the radiant warmth of my skin is a potential source of power.
Perpetua Power Source Technologies, for instance, is working to create garments that use such heat to support a "wireless body area network" capable of, for example, monitoring a sick person's vital signs. By the time I develop hypertension from sitting around too much, my clothing may be capable of alerting me to my high blood pressure.
Of course, my body heat is nothing compared with the furnace that is a car's engine. That's one place where thermoelectric power could really come into its own. Some experts estimate that only a quarter of the fuel a typical car consumes is utilized to power the vehicle. (Hybrids do a bit better in this department, thanks to their electrodynamic regenerative braking, which produces energy when vehicles slow down, a form of energy harvesting.) Researchers are testing systems that could salvage some of that heat emitted by the engine and convert it back into usable energy.
A particularly interesting breakthrough was reported last month when Zhong Lin Wang and his colleagues at the Georgia Institute of Technology published a study of two tiny piezoelectric generators that they had created. Each is about the size of a paper clip. When squeezed, one produces roughly the same voltage as an AA battery. The other does the same when it's bent. Together, these devices are the first that engineers have been able to create that can pack so much voltage in such a small package. Such generators could be used to operate small sensors powered by sources as simple, Wang said, as sound waves, vibrations or even the flutter of a flag.
In other words, there's free energy in the brave wave of the Star-Spangled Banner. When it comes to energy independence, you can't get more patriotic than that."
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