A Backpack That Generates 7 Watts of Electricity

19:00 08 September 2005
NewScientist.com news service
Paul Marks

A backpack that generates electricity as its wearer strolls along has been developed by experts in human locomotion in the US.

By harnessing the loping up-and-down motion of our hips as we walk, the backpack’s freely-moving load bounces up and down, generating up to 7 watts. That is more than enough to power cellphones with power-draining functions like colour widescreens or Wi-Fi and GPS connections.

The developers hope their suspended-load backpack will be a particular boon for troops, field scientists, explorers and disaster relief workers in remote locations.

The generator has been developed by Larry Rome and his colleagues at the University of Pennsylvania, US, with funding from the US Office of Naval Research. Their aim was to relieve soldiers already carrying 36-kilogram backpacks of the need to carry many spare batteries to power their GPS, communications and night-vision devices.

“The extra weight [of the batteries] compromises the amount of food, medicine and armament they can carry,” Rome explains.

Upside-down pendulum
An earlier answer to this was the “heel strike” generator – a piezoelectric crystal-based device in a boot heel that generates a battery-charging current when crushed by the wearer’s weight on every footstep. But these gadgets only produce power in the region of 10 to 20 milliwatts. A basic cellphone uses between 1 and 2 watts.

So Rome’s team has worked out how to electrically capture some of the energy a backpack wearer expends when carrying a load.

Their trick is to make use of the fact that a walking person moves like an upside-down pendulum. “One foot is put down and then the body vaults over it, causing the hip to move up and down by 4 to 7 centimetres,” he says. And as the hip goes up and down by that vertical distance, so does any load, with the backpack-wearer expending the energy to make it do so.

To retrieve some of that energy as electricity, the team separate the load-carrying sack from the backpack’s frame by mounting it on a spring-loaded plate that is free to move up and down on rail-like rods.

Gaggle of gadgets

The result is that as the wearer walks, their hip motion makes the load oscillate up and down (see a video – mpeg format, 15 MB). To harvest energy from the load’s motion, a toothed rod fixed to the mobile load-plate meshes with a gear wheel on a dynamo fixed to the top of the frame. The load-motion generates a current which can either run a gaggle of gadgets or charge a battery.

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Journal reference: Science (vol 309, p 1726)