WASHINGTON (AP) _ As soldiers, hikers and students can testify, it takes energy to haul around a heavy backpack. Now, researchers have developed a backpack that turns that energy into electricity.
It doesn't crank out a lot of juice _ just a bit more than 7 watts _ but that's enough to run things like an MP3 player, a personal data assistant, night vision goggles, a handheld global positioning system or a GSM cell phone.
The development could eventually allow field scientists, hikers, explorers, soldiers and disaster workers to produce their own electricity.
The researchers used a backpack fastened to the carrying frame by springs. The up-and-down motion caused by walking powers a small generator, producing electricity that can be used directly or stored in a capacitor or battery.
The device, developed by Lawrence C. Rome of the University of Pennsylvania, and colleagues, is reported in Friday's issue of the journal Science.
The electricity-generating frame weighs about 10 pounds, Rome said in a telephone interview. He's working to lighten it, so it will weigh only a couple of pounds more than a standard backpack.
Power generated increases as the load in the backpack gets heavier, he said. Tests ranged from loads of about 40 pounds to about 80 pounds.
Rome developed the new backpack at the request of the Office of Naval Research, which was looking for ways to reduce the need for service members to carry lots of batteries to power equipment while on duty in Afghanistan.
The researchers studied the movement of people walking, and concluded that the hips move up and down between 1.6 inches and 2.7 inches with each step.
They then set about trying to exploit that movement.
The result is the ``suspended load backpack.'' It uses a rigid frame similar to regular backpacks, but instead of being attached directly to the frame, the load is suspended by springs, allowing it to move up and down as the person walks. That movement turns a small electrical generator producing current. In tests on a treadmill, walking on level ground and uphill both produced current, Rome said.
Arthur D. Kuo of the University of Michigan's Department of Mechanical Engineering and Biomedical Engineering said the backpack is novel ``because it generates useful amounts of electrical power, while costing less metabolic power than would be expected.''
Indeed, carrying the backpack uses only a little more energy than carrying a standard backpack of the same weight, said Rome, a biologist who also does research at the Marine Biological Laboratory in Woods Hole, Mass. He said volunteers testing the device altered their gait slightly to move more efficiently.
``Metabolically speaking, we've found this to be much cheaper than we anticipated. The energy you exert could be offset by carrying an extra snack, which is nothing compared to weight of extra batteries,'' Rome said. ``Pound for pound, food contains about 100-fold more energy than batteries.''
The concept resembles that of a self-winding watch where power is generated by the movement of the wearer, commented Kuo, who was not part of Rome's research team.
The research was funded by the National Institutes of Health, Office of Naval Research and the University of Pennsylvania Research Foundation.
A company called Lightning Packs LLC has been formed to improve the suspended-load backpack and to develop an ergonomic backpack based on the prototype. Lightning Packs has applied for patents on both inventions.
Rome said he hopes to have the new version ready for testing in six months to a year.