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Forget those hand-cranked radios. Forget pocketfuls of heavy D-cells. And especially forget those nifty Faraday-effect survival flashlights⁠⁠—you know, the ones you have to shake back and forth until you have carpal tunnel syndrome and it starts to look vaguely obscene. The ultimate human-powered camping gadget is here⁠⁠—the backpack generator.

For years, researchers have tried to extract electricity out of motions that people make anyway. That’s a tall order, as most of the energy of walking is expended within the body itself. Barring surgery, the only ways to get at it have been through shoes or exoskeletons. Using an exoskeleton to capture energy from gross limb motion is cumbersome, to say the least⁠⁠—especially as extra effort is required to generate the electricity. So most researchers have turned to shoes.

Piezoelectric shoes and their more evolved kin haven’t performed too well. When the foot strikes the ground, it exerts force, but the foot stays planted. Since work = force X distance, essentially no work is performed between heel and ground. The shoes get around that by having a lot of give, but that just makes walking unpleasant and annoying.

Enter Lawrence C. Rome, Taeseung Yoo, and others at the University of Pennsylvania. They reasoned that a backpack actually joggles up and down a lot, so why not try to suck energy out of that? Inspired by self-winding watches, they put together a backpack with the weight on springs. When a person walks, the weight bobs up and down, driving a generator. They stuck the rather heavy (20-38 kg, 44-84 lbs) backpacks on some volunteers, and folks started walking.

The result was magical. The backpack devices⁠⁠—keep in mind now, these are merely prototypes⁠⁠—generated a peak wattage three hundred times that of the best trick shoes (about 7.4 watts compared to 10 or 20 milliwatts). Seven watts isn’t bad⁠⁠—it’s enough to run three cell phones, two GPS devices, and your iPod. Moreover, when loaded with the springy backpacks, walkers were more efficient than those carrying equally heavy traditional backpacks. The researchers predicted the backpacks would cost an extra 48 watts of metabolic power, but the walkers only expended 19, resulting in power savings around 60%. Talk about your power walkers.

The cause for the improved walking hasn’t been thoroughly investigated yet, but it’s clear that the volunteers changed their usual gait significantly. The hypothesis is that the walkers time the up-bounce to happen right when switching legs. At this point, both legs have to use energy to jerk the body’s center of gravity into the next arc. If the backpack load is headed up right about then, the weight on the legs is less, and there’s less wasted energy.

A reviewer (Arthur Kuo) points out “ there is no obvious reason why the backpack cannot be improved to reduce muscle work requirements even further.” (ref 1, below) The authors also remark that the backpack could run an electric fan or coolant pump for hikers’ comfort. Future campers may fight over who gets to wear the heavy electric pack.

This is no perpetual motion scam. The body still pays for the energy put out, so one might have to eat more often. Since the most average of foods has an energy density 100 times that of the best batteries, the researchers suggest that folks bring food rather than batteries. So instead of a “free lunch”, the motto is “pack a lunch”.

Still, one thing Rome, Yoo and the other folks don’t explicitly address is the issue of storing the energy. They tout the advantages of food over batteries, but as currently conceived, one would have to pace back and forth to keep talking on her cellphone. Still, there’s no denying this would eliminate the need for replacement batteries, as it could keep charging the ones hikers left with in the first place.