Harvesting Energy From Humans

The human body contains enormous quantities of energy. In fact, the average adult has as much energy stored in fat as a one-ton battery. That energy fuels our everyday activities, but what if those actions could in turn run the electronic devices we rely on? Today, innovators around the world are banking on our potential to do just that.

Movement produces kinetic energy, which can be converted into power. In the past, devices that turned human kinetic energy into electricity, such as hand-cranked radios, computers and flashlights, involved a person’s full participation. But a growing field is tapping into our energy without our even noticing it.

Consider, for example, a health club. With every step you take on a treadmill and with every bicep curl, you turn surplus calories into motion that could drive a generator and produce electricity. The energy from one person’s workout may not be much, but 100 people could contribute significantly to a facility’s power needs.

That’s the idea behind the Green Microgym in Portland, Oregon, where machines like stationary bikes harvest energy during workouts. Pedaling turns a generator, producing electricity that helps to power the building. For now, body energy supplies only a small fraction of the gym’s needs, but the amount should increase as more machines are adapted. “By being extremely energy-efficient and combining human power, solar and someday wind, I believe we’ll be able to be net-zero for electricity sometime this year,” says the gym’s owner, Adam Boesel. His bikes, by the way, aren’t the first to put pedal power to work. In some parts of the world, cyclists have been powering safety lights for years with devices called bicycle dynamos, which use a generator to create alternating current with every turn of the wheels.

Dance clubs are also getting in on the action. In the Netherlands, Rotterdam’s new Club WATT has a floor that harnesses the energy created by the dancers’ steps. Designed by a Dutch company called the Sustainable Dance Club, the floor is based on the piezoelectric effect, in which certain materials produce an electric current when compressed or bent. (The most common example is a cigarette lighter, in which a hammer causes a spark to be emitted when it strikes a piezoelectric crystal.) As clubgoers dance, the floor is compressed by less than half an inch. It makes contact with the piezoelectric material under it and generates anywhere from two to 20 watts of electricity, depending on the impact of the patrons’ feet. For now, it’s just enough to power LED lights in the floor, but in the future, more output is expected from newer technology. In London, Surya, another new eco-nightclub, uses the same principle for its dance floor, which the owners hope will one day generate 60 percent of the club’s electricity.

Gadget Power

Beyond body-powered gyms and dance clubs, ideas are also in the works to provide electricity for more ordinary, useful things. Researchers are creating ways to power small mobile devices like cellphones, MP3 players and laptops when there is no access to conventional energy sources.

Max Donelan of the Locomotion Laboratory at Simon Fraser University in British Columbia, in collaboration with American and Canadian researchers, is developing an electromagnetic generator fitted to a standard knee brace. The prototype, which Donelan unveiled last February, turns a one-minute walk into enough current for a half-hour cellphone conversation.

The knee generator uses sophisticated electronics to ensure that it grabs only excess energy. A computer measures the angle of the knee during every step to determine when to engage and disengage the generator. In the course of an ordinary stride, we use muscle energy both to accelerate the leg forward in an arc and then to brake its downward motion. The generator kicks in only during the swing phase of a footstep when the muscles are already braking, so it doesn’t take power away from your step and slow you down. The electricity then flows through a wire to charge or power a battery or device.

At more than three pounds, the generator, called the Bionic Energy Harvester, is cumbersome. But thanks to lighter gears and a framework made of lightweight materials such as carbon fiber, the latest model, which is expected in the next year or so, should weigh closer to one pound. A microcomputer will replace a standalone computer that is wired to the unit in the current prototype.

Such a device has many possible uses. The Canadian military is partially funding Donelan’s research because soldiers carry as many as 30 pounds of batteries for communications and navigation equipment—a load that could be significantly lightened by an alternative energy source. Public-safety workers such as firefighters and police could also use the technology to power handheld equipment during emergencies. In the future, artificial limbs that require batteries may instead be designed with Donelan’s technology. And next-generation devices could run gadgets like cellphones, global positioning systems, iPods and digital cameras. This could be particularly useful for hikers and mountain climbers, who spend much of their time away from power sources.

Other generators in development use the same electromagnetic principle as the Bionic Energy Harvester. For instance, Larry Rome of the University of Pennsylvania has created the Lightning Pack, a backpack that captures energy from the natural up-and-down movement of your hips. As you walk, a bag bounces on a spring, which connects through gears to an electrical generator. Wires carry the electricity to your batteries or gadgets. The output is impressive: 20 watts, enough for nearly all portable devices, Rome says. But the bag is impractical for most people because it needs to weigh 80 pounds to generate 20 watts. (The heavier the load, the more mass that oscillates up and down, and the greater the kinetic energy potential.) The U.S. Marine Corps, however, is interested and has commissioned a pack for soldiers.

Multitasking Clothing

A far cry from an 80-pound backpack, energy harvesters the size of a thread are being developed by Zhong Lin Wang and two colleagues at the Georgia Institute of Technology. These mini-generators can be woven into T-shirts or other clothing and will collect energy from the body’s smallest movements, piping electricity to mobile devices.

Wang’s generators use piezoelectricity on a small scale. For the prototype, he grew zinc-oxide crystals on yarnlike Kevlar fibers. The crystals jut out on nanowires like thousands of small bristles and, when rubbed against each other, they bend and create electricity. In the prototype, two centimeter-long fibers produced 16 picowatts, or 16 trillionths of a watt. It’s a minuscule amount of electricity, but the output grows as more fibers are added. The researchers predict that clothing with these fibers could generate up to 80 milliwatts of electricity per 11 square feet of fabric, which is almost enough to power a cellphone or other mobile electronic device.

Before we see garments that generate electricity—which could happen in about five years—Wang and his colleagues must overcome several challenges. The biggest problem is that these nanofibers can’t get wet. A lining that zips out when laundering the garment could be the solution, and Wang is also exploring the possibility of waterproof nanofibers.

His next goal is to make the fibers more efficient. To this end, he is experimenting with different kinds of polymers and seeking better methods of combining the materials and collecting the electric charge. But even if the nanofibers don’t become much more efficient, they might still be able to power gadgets entirely by body movement. Electronic devices continue to get smaller, requiring less power, and higher-capacity batteries will store the energy that is accumulated over a longer period of time—bringing us that much closer to an era when our movements are no longer wasted.

Soon, we might not even have to consciously move to create power. Wang is working on a polymer film that would surround his power-generating fibers and allow them to be implanted into our bodies. There they would harvest kinetic energy from the steady dilation and contraction of blood vessels, providing a source of electricity for pacemakers, insulin pumps and other medical devices—making for a truly powerful breakthrough.