How and Why

A bounteous source of power may lie right beneath our feet

Ben Harder
Tuesday, April 27, 2010

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.


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