Scientists make unique knee-brace power generator

Reuters February 7, 2008

Will Dunham

Talk about a knee-jerk reaction. Scientists in the United States and Canada said on Thursday they have developed a unique device that can be strapped on the knee that exploits the mechanics of human walking to generate a usable supply of electricity.

It generates enough power to charge up 10 cell phones at once, the researchers report in the journal Science.

Researchers have been working on ways to harness the motion of the human body to create power.

A shoe-mounted device was nice and light, but did not generate much electricity. A backpack device that generated power as it bounced up and down while a person walks generated a lot of electricity, but was heavy to lug.

The new energy-capturing knee brace, its inventors said, seems to find a happy medium -- generating decent amounts of power while still being relatively light.

The scientists envisioned numerous applications for such a device. It could be of value to hikers or soldiers who may not have access to electricity, they said. It also could be built into prosthetic knees or other implantable devices whose users occasionally must undergo surgery for a battery replacement.

Arthur Kuo, a University of Michigan mechanical engineer who worked on the device, said it works similarly to the way that regenerative braking charges a battery in hybrid cars.

These regenerative brakes collect kinetic energy that normally dissipate as heat when the car slows down. The knee device collects energy lost when a person brakes the knee after swinging the leg forward to take a step, the researchers said.

"It generates a fairly substantial amount of power compared to previous devices and it does so in a way that doesn't affect the user very much," Kuo said in a telephone interview.

"You could easily power 10 cell phones at once. There are some low power computers that you could power. You could imagine devices like GPS locaters, satellite phones," he said.

With a device placed on each leg, volunteers walking on treadmills generated about 5 watts of electricity walking at a leisurely 2.2 mph (3.5 kph). Each of the devices weighs about 3.5 pounds (1.6 kg), which Kuo said was still too unwieldy.

"Even though we've demonstrated this new way to generate power, we don't mean to say this is a usable product at this time. The principle limitations are that our prototype is pretty heavy and bulky," Kuo said, adding that he thinks it can be made smaller and more practical.