Backpack generates its own electricity

The cargo compartment of the new backpack, which was filled to weigh between 20 and 38 kilograms (44 to 83 pounds) for a variety of trials, is suspended by springs and moves up and down with respect to the frame. It is this movement of the cargo compartment with respect to the frame that turns a gear connected to an electric generator that is mounted to the frame. Inside this generator, coils of wire within a magnetic field turn when you walk, leading to the production of electricity.

In one backpack “test drive,” the backpack consistently harvested about 7 watts. For comparison, cell phones, global positioning system receivers, and LED headlamps each use less than one watt. So, it appears that walking with the backpack could allow the wearer to run multiple devices at the same time. The generated electricity could also be used to charge batteries.

An 80 pound backpack worn by a disaster relief worker or field scientist often includes 20 pounds of replacement batteries, so the new backpack could allow people to carry fewer replacement batteries.

Power walking for real
There are many creative, though not always efficient, ways to generate electricity on a small scale. For example, a spinning hamster wheel generates enough energy to power a cell phone.

Science This illustration shows the "suspended-load" backpack. The pack frame is fixed to the body, but the cargo compartment is suspended with springs from the frame. During walking, the cargo compartment rides up and down (lower inset). To generate electricity, researchers attached a green toothed rack to the "load plate" of the backpack (upper inset). When this toothed rack moved up and down during walking, it meshed with a gear mounted on a generator which generated electricity.

Efficiency is a critical issue for any energy harvesting technique. Clearly, carrying a hamster and all its supplies into the wilderness is much harder than carrying a cell phone replacement battery. Similarly, if carrying the new backpack burned huge numbers of calories, compared to a regular backpack of the same weight, then you would need to carry so much extra food that simply carrying extra batteries might be more efficient.

A series of treadmill tests measuring how much human energy it takes to walk with the backpack revealed that the backpack can serve as an efficient source of electricity. It’s true that you burn a few more calories when you walk with the backpack than a traditional backpack of the same weight. But, you are generating electricity at the same time, and the weight of the extra food that you must carry is far less than the weight of the batteries that would be required if you didn’t have an electricity-harvesting backpack, making this source of energy a “good deal.”

Leg vaulting
The backpack inventors are still in the process of figuring out why the new backpack lets you walk so efficiently. It appears that your leg muscles save energy during the transition between steps: when both feet are on the ground, your legs are bent and your leg muscles are working hard.

Also, each leg appears to supports a greater share of the weight of the pack than usual at the “top” of each step, when only one leg is on the ground. This leg is relatively straight. During this phase of each step, when the body is vaulting over the extended leg, adding extra weight doesn’t make much of a difference.

It is these subtle differences that could make this backpack more comfortable to wear and better for your back, said Arthur Kuo, an engineer from the University of Michigan in Ann Arbor, MI. Kuo wrote a background article that accompanies the new Science research.

“The backpack’s suspension system might make it more comfortable to wear and the gain in comfort might be well worth it, even without the energy-harvesting capacity,” he said.

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