People Power: Capturing The Body's Energy For Work On and Off Earth

People Power: Capturing The Body's Energy For Work On and Off Earth

By Erik Baard
Special to SPACE.com
posted: 07:00 am ET
28 November 2001

Covert military operations and space shuttle missions are both burdened by the fact that they rely on an inefficient, energy-wasting machine: the human body. Considering one of the biggest logistical problems planners face is getting power to equipment in remote places like Afghanistan or the moon, researchers are devoting their efforts to cut some of those losses through "energy harvesting" from the human body.

If that gives you creepy images of people wired up as batteries a la "The Matrix," stop fretting. What NASA and the Pentagon want to do is scoop up electrons from what bodies in normal activity produce: heat, motion, flexing and stretching, compression, urine, and body heat. This is quite different from other human-powered schemes that take extra exertion, like spring or dynamo flashlights and radios that are wound-up by a special handle, flashlights that are squeezed by the user to generate charge, or flywheels that store energy from a cord that is pulled.

A simple version of a piezoelectric, energy-harvesting device, these sneakers broadcast a location signal while the wearer walks.

According to the Center for Space Power and Advanced Electronics, a NASA commercial center in Alabama, the human body is on average 15% fat, capable of producing 11,000 watt hours. When the average Joe eats his daily bread, he takes in 3,300 watt hours. The charge rate is about 7kW if the waiter starts pushing you out the door after a half hour lunch, according to the Center. "Clearly the amount of energy consumed by an individual is sufficient to provide power for electronic devices if a suitable method can be found to convert a small fraction of that energy to electricity," the Center concludes in a report on the subject.

Broken into usable terms, waiting to be harvested are 81 watts from a sleeping person, 128 from a soldier standing at ease, 163 from a walking person, 407 from a briskly walking person, 1,048 from a long-distance runner, and 1,630 from a sprinter, according to the center. But of course there’s not 100% capture. Body heat, for example, can only be converted with 3% efficiency with current thermoelectric materials.

Advances in nanotechnology and materials science are causing energy needs to drop at the same time production and transfer of it is increasing. The military applications are humble still, though, aimed at small gear like personal battery chargers, medical sensors, displays, gun sights, and range finders. Boots that turn the compression of a compound into voltage have already powered a radio, according to the Defense Advanced Research Projects Agency. NASA is hoping to feed a range of body monitors, electronics and mission-specific devices. In the commercial sphere, companies are racing to power simple things like watches for now, with an eye to scaling up to handheld PDAs in the future.

Some of the most promising mechanisms for passively converting human body functions into electricity are:

Piezoelectric devices: Piezoelectric substances, like some ceramics, also generate electrical energy from mechanical strain but without the need for voltage to be applied. This well-understood material is the core of "heel strike" devices that generate electricity from walking. "Generating 1-2 watts per shoe is not out of the question. A major issue that remains is the durability of these devices," Dr. Robert J. Nowak, program manager for energy harvesting at Darpa, wrote to SPACE.com. Great for soldiers, bad for astronauts: "giant steps are what you take, walking on the moon."

Urine-based fuel cell: Yes, you can turn pee into power and not just by turning a turbine after a few beers. First subject urea to enzymatic hydrolysis to make carbon dioxide and ammonia, and then oxidize the ammonia to nitrogen and water. But the center notes that "one problem with the system is the need for alkaline conditions that may require transport of sodium hydroxide, a hazardous compound. Also, to achieve power generation in the range of 0.5 - 1W, a system to concentrate the breakdown products of urea, such as reverse osmosis, will be necessary." But for astronauts and soldiers on the run, "one attractive feature of this fuel cell concept is the production of water as a by-product of the system."

Inertial energy scavenging: You can own a piece of this technology already – some Seiko watches are powered by a weight that swings as you move, driving a tiny generator. No one expects to generate much electricity from these systems, but deployed in each element needing electricity they could do the trick in concert. Also, while gravity is absent in space, inertia is not.

People Power: Capturing The Body's Energy For Work On and Off Earth (cont.)

Electromagnetic generator: Large muscular groups (especially legs) can generate electricity by simple motions against gravity and small direct current permanent magnet motors. But the center cautions, "there is little or no efforts within the scientific community to design efficient small generators of the type needed for harvesting of human energy."

Thermoelectric materials: These materials convert body heat into electricity by using combinations of materials (metals or today, new ceramics) that are poor thermal conductors and good electrical conductors. When two of them at different temperatures come into contact, electrons migrate, charging a battery or creating usable current through something called the Seebeck Effect. The trouble is that you need great temperature differences to get significant energy, and "on Earth most places are pretty close to body temperature," notes Dr. Henry Brandhurst, director of the center. And what about in the cold depths of space? For the inner solar system at least, photovoltaic panels seem like a better bet, he says.


A simple version of a piezoelectric, energy-harvesting device, these sneakers broadcast a location signal while the wearer walks. Image courtesy the IEEE. Click to enlarge.

But that skepticism hasn’t slowed down efforts at NASA to improve the technology, and one company is already pushing it as an off-the-shelf product. Applied Digital Solutions is unveiling "Thermo Life." The company is "working closely with a watch manufacturer," says Keith Bolton, the chief technology officer. Already the technology has proven itself capable of keeping analog watches ticking, he reports.

And Dr. Rama Venkatasubramanian of the Research Triangle Institute reported in Nature this month a breakthrough in new materials that could double or triple the output of thermoelectric generators.

Electrostrictive polymers: These materials create charge when stretched after voltage is induced through them. No prototype has been made, and there are concerns about how quickly this material might wear out. But it does dovetail nicely with NASA’s conception of the spacesuit of the future, which will be skin tight to maintain mechanical pressure on blood systems in place of the ambient Earth air pressure replicated in "puffy" suits today.

Electrostatic force arrays: (Also called Integrated Force Arrays) A cousin of electrostrictive polymers, this is a new technology. It’s expensive and untested in power generating applications, or for simple durability.


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