Power harnessed one step at a time

Engineers call it 'crowd farming.' If it works, you could help power city lights just by taking a stroll.

In the push to harvest alternative energy, scientists have tapped a number of novel sources: the sun, corn, old cooking oil. But how about the simple act of walking?

For two architecture students at the Massachusetts Institute of Technology in Cambridge, Mass., the sound of footsteps is an echo of energy gone to waste. They figure that the stomp of every footfall gives off enough power to light two 60-watt bulbs for one second.

"Now imagine how many people walk through a train station each morning, or walk down the street in Hong Kong," says James Graham, who, with fellow MIT graduate student Thaddeus Jusczyk, is helping to develop the growing field of "crowd farming."

They devised a special floor of sliding blocks that can turn motion energy (such as from a footstep) into electrical energy. As commuters march across the floor, it would collect tiny flickers of power from each stride and channel that energy.

According to their design – which this summer won a prestigious award from the Holcim Foundation for Sustainable Construction in Zurich, Switzerland – 28,527 footsteps could power a train for one second – 84,162,203 paces could launch a space shuttle.

The problem with their plan: Right now, it only exists on paper. But others have developed real-world examples of plugging into people power. Over the past few years there's been a boom in technology that harnesses piezoelectricity – the science of drawing power from mechanical stress, including motion. While the crowd-farming push has its critics, the discipline is growing, and businesses are signing on. [Editor's note: The original version's definition of piezoelectricity needed clarification.]

"This is a really exciting time because there's been a lot of growth all of a sudden," says Steve Anton, whose review article on recent piezoelectric advances ran in the June issue of the journal Smart Materials and Structures. "For a long time the research was confined to the lab, but a number of real applications have started coming out."

Take POWERLeap. This project, built by sustainable designer Elizabeth Redmond of Chicago, is a scaled-down, but glammed-up, version of Mr. Graham's scheme. When pedestrians trot across one of the flooring system's four decorated glass tiles, LED lights flicker to life underneath their feet.

"I installed it on a sidewalk in Ann Arbor [Mich.] and people were really surprised and excited by the lights glowing from the street," says Ms. Redmond. "When people find out that they were powering the lights, and there were no batteries involved, you get a whole other wave of wows."

Redmond is now crafting a new design for POWERLeap, thanks to a $10,000 grant from flooring giant Mohawk Industries of Calhoun, Ga. Graham says the two of them have been e-mailing about combining their interests.

Power-generating backpack

Human energy can also be harnessed to power a cellphone or charge a battery. Henry Sodano, an engineering professor at Arizona State University in Tempe, Ariz., has developed a backpack that serves as a portable, wearable way to keep gadgets juiced.

His team created piezoelectric straps that draw power from the bag's natural bounce. At a normal stride, the stress on the bands can pull in 45.6 milliwatts (mW) – just shy of what's needed to perpetually power an iPod nano MP3 player, and more than enough to keep a Motorola Razr mobile phone charged.

"We could power a Razr in standby using 9mW of power and store the remaining 36.6mW of power, allowing us to talk for one minute for every 10 minutes walked," he says. "Or you could charge an LED headlamp while you walk in the day and use it at night while you camp."

The catch: for the straps to collect the full 45.6mW, they need to support a 100-pound knapsack. "That's a lot," Mr. Sodano admits with a laugh. But he designed the straps for the US military to use. Since soldiers are used to encumbering loads, the special straps in their packs will capture practically free energy, he says.

Finding a balance of how much to leech off a person's movement is the most difficult problem for human-powered technology. All energy has to come from somewhere. So if you're the one charging the device, then, to some extent, you're the one feeling the drain.

Self-winding watches work well because they are so light to begin with that the added weight of the self-winding mechanism is almost unnoticeable. But watches also require barely any power.

"To get anything substantial out of these devices, they would have to weigh a ton, and that's something few consumers will agree to," says Peter Glas­kowsky, a technology analyst for Envisioneering, a market-research firm, in Seaford, N.Y. "The energy you're saving by not using batteries is actually coming from you, and therefore its coming from food. If you add up the energy used to grow, package, ship, and eat, food is an extremely inefficient energy source."

Many plans for battery-charging shoes have been abandoned because walking in them was too taxing, says Mr. Glaskowsky. Most models so far have relied on an extra thick sole that depresses and generates electricity with each step – like a mini version of POWERLeap built into the boot. "A lot of people complain that it feels tiring after a while, like walking in sand," says Glaskowsky.

(Mr. Anton, a graduate student at Virginia Tech, plans to unveil a new shoe design in October that places a piezoelectric charger in a box above the heel. This would add weight, he says, but eliminate the deflating feel of the sole-based style.)

Similarly, the One Laptop per Child foundation ditched their plan to attach an auxiliary hand crank to the side of its "$100 laptops," deciding the strain was not worth what little power it provided. The group then signed on for a "yo-yo" pull-cord charger, but are now leaning toward using simple solar panels.

Simple movements may soon extend battery life

Some piezoelectric engineers have found successful, subtle ways to feed off human motion. A consortium of mostly British companies is on track to release by the end of the year an in-body microgenerator that will convert energy from joint movements, heartbeats, and breathing, says manager Martin McHugh of Zarlink Semiconductors. The tiny device will help both decrease the size and extend the life of batteries attached to pacemakers and other medical instruments, saving patients from costly, surgical replacements.