New Backpack Generates Its Own Electricity

John Roach
for National Geographic News

September 8, 2005

A backpack that generates its own electricity may soon allow rescue workers, explorers, and soldiers to power their equipment while on the go, a team of scientists reported today.

The backpack will derive its power from the motion of the person carrying it.

The invention, known as the suspended-load backpack, frees handheld computers, cell phones, and dozens of other devices from the constraints of limited battery life and the reach of the power grid.

"To have electricity is crucial in remote places," said Lawrence Rome, a biologist at the University of Pennsylvania in Philadelphia who led the device's development. Rome's research focuses on how muscles function while walking and running.

The idea for the backpack grew out of a request from the U.S. Office of Naval Research to develop a way to generate electricity from body movements.

At the time, U.S. soldiers in Afghanistan were bogged down with 80-pound (36-kilogram) packs full of GPS instruments, night-vision goggles, and other gear that required an additional 20 pounds (9 kilograms) of batteries.

"Once I started to design the pack, I realized that everyone has the same problem," Rome said. Explorers surveying the rain forest in Africa and field scientists cataloging microbes in Chile, for example, need electricity to power their equipment.

The backpack is based on the external-frame design once popular with backcountry trekkers. With loads weighing between 44 and 84 pounds (20 to 38 kilograms), the pack can generate more than seven watts of electricity from the up-and-down movement of the cargo compartment.

Seven watts is sufficient to power a handful of electronic gadgets at the same time, including a cell phone, an MP3 player, a handheld computer, night-vision goggles, and a water purifier.

Rome and his colleagues describe the backpack in tomorrow's issue of the journal Science.

How It Works

When walking, a person's hip moves up and down about two to three inches (five to seven centimeters) with each step. The movement occurs as the body vaults over each foot.

When a person carries a loaded backpack, the pack too moves up and down the same distance at the same time. By detaching the frame of the pack from the load it carries, the scientists were able to capture the energy of this up-and-down movement.

The backpack frame sits on the carrier's back while the load is suspended from the frame by vertical springs. The springs allow the load to slide up and down with the same motion of the hip, but lagging by a fraction of a second.

"You get a differential movement between the frame of the backpack and the load, and that's where we generate the electricity," Rome said.

The pogo-stick like movement of the load turns a gear connected to a generator at the top of the pack. This gear rotates coils of wire within a magnetic field inside the generator, creating electricity.

The amount of power generated depends on how much weight is in the pack and how fast the user walks. Researchers measured a maximum output of 7.4 watts. Gadgets like cell phones require less than one watt.

Efficiency

While the separate movement of the cargo compartment and the frame sounds awkward and uncomfortable, Rome said "it's actually more comfortable than a normal backpack."

In a commentary accompanying the Science report, Arthur Kuo, a mechanical engineer at the University of Michigan in Ann Arbor, writes that the backpack is easy to wear and works surprisingly well.

"Why it works so well is unclear," he writes. "Perhaps the device reduces the mechanical work required of muscles to walk while carrying a load."

Rome speculated that the added comfort may stem from the springs, which reduce the need to jerk the load up and down.

"Imagine that you step up on a rock with a rigid backpack. That weight has to go up instantly—it can't hang on the ground. … [I]n a suspended backpack, when you step up, the load lags behind you," he said. The effect is like that of a shock absorber, he explained.

In addition, Rome and colleagues found the backpack altered the gait of test wearers, causing their up-and-down hip movement to become smaller.

The researchers also learned that the self-powering backpack weighs only slightly more than a conventional pack—equivalent to carrying around an extra candy bar.

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