Power Pack – Generating power with the motion of a back pack

Finally, you can recharge your iPod with Clif bars. When the military needed to recharge batteries on the move, they turned to University of Pennsylvania professor Larry Rome, an expert in muscle power and, it turns out, a capable inventor. His solution the world’s first electricity-generating backpack Rome, who studies fish muscles, says the idea struck him in a Navy meeting. US troops were lugging 50-pound packs, including 20 pounds of batteries for high-tech gadgets. The brass wanted to use muscle power to generate electric power, but the best existing technology was shoe generators, straight out of Get Smart. I said, “That’s a terrible idea,” recalls Rome. “The force of the heel strike is only over a couple millimeters. The right way became obvious: with every step, these guys are lifting 80 pounds 5 to 7 centimeters – that’s potentially 36 watts of mechanical energy.” To turn his brainstorm into hardware, Rome grabbed an old external-frame backpack from college days and called his lab’s “very line machinist” Fred Letterio. In their basement shop full of mills and lathes, the two added springs to suspend the cargo compartment from the pack frame. As the wearer’s stride raises and lowers the pack. the load slides up and down. driving vertical rods to spin a geared DC servomotor up to 5.000 rpm to generate electricity.

With a 40-80 pound load. Rome’s pack generates 7 watts, plenty of juice to simultaneously power a two-way radio, GPS receiver, and night vision goggles (or cellphone, PDA, digital camera, and iPod). The load can be locked for stability on sketchy terrain, and then unlocked to generate power again. Ultimately, the generator pack (patent pending) will weigh just a couple pounds more than a regular backpack. Carrying it burns 3% more energy, but wearers say it’s more comfortable, and the extra work costs only a couple of extra candy bars. (“Food is 100 times more efficient than batteries.”) Green bonus: the technology could keep tons of toxic batteries out of landfills.

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From the column Made on Earth – MAKE 5, page 20 – Keight Hammond.