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.
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.
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.
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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