Charge Your Cell Phone Just By Walking

Clara Moskowitz, LiveScience Yahoo News 7 Feb 08;

Charging your cell phone might soon be as simple as taking a walk around the block.

Scientists have created a new machine that harvests energy from the movement of the knees while walking.

Six volunteers wore the devices on their legs while strolling on a treadmill and were able to produce about five watts of electrical power each. That's enough energy to run 10 cell phones simultaneously.

"Since muscles are the powerhouses of the body, my colleagues and I designed our device to generate electricity from the motion of the knee joint," said Max Donelan, director of the Locomotion Laboratory at Simon Fraser University in Canada. "It resembles a knee brace and weighs about 1.5 kilograms [3.3 pounds] including the gearing and generator."

Results from tests of the device were published in the Feb. 8 issue of the journal, Science.

The apparatus captures the energy of a person's movement by coupling an electrical generator to knee motion.

When the wearer extends the knee, a gear on the device turns and spins the generator, which builds up energy.

When the knee is flexed, the device turns off so as not to tax the wearer. The machine senses motion and knows when to activate itself.

The design allows a person to use the tool without expending much additional energy beyond the normal demands of walking. The researchers measured the volunteers' oxygen intake and carbon-dioxide output to make sure it wasn't strenuous to use.

Work on a similar device built into a backpack was announced in 2005.

“People are an excellent source of portable power," Donelan said. "An average-sized person stores as much energy in fat as a 1,000-kilogram battery. People recharge their body batteries with food and, lucky for us, there is about as much useful energy in a 35-gram granola bar as in a 3.5-kilogram lithium-ion battery.”

The device could be used to power computers in remote regions where electricity is scarce, Donelan said.

"The early markets are people whose lives depend on portable power, such as people with artificial limbs," Donelan told LiveScience. "On the military side, soldiers have an incredible dependence on batteries these days, so both these groups could benefit."

Knee dynamo taps 'people power'
Jonathan Fildes, BBC News 7 Feb 08;

"All of the new developments in prosthetics require large power budgets," Dr Douglas Weber of the University of Pittsburgh, and one of the authors of the paper, told BBC News.

"You need power to run your neural interface; you need it to run your powered joint, and so on.

"Getting that power is going to be really important."

Walk and talk

The new device generates power by a process known as "generative braking", analogous to the braking systems found in hybrid-electric cars such as the Toyota Prius.

"Walking is a lot like stop-and-go driving," explained Dr Max Donelan of Simon Fraser University in Burnaby, Canada, lead author of the paper.

"Within each stride muscles are continuously accelerating and decelerating the body.

Hybrid electric cars take advantage of stop-and-go driving using so-called "regenerative braking" where the energy normally dissipated as heat is used to drive a generator.

"We have essentially applied the same principle to walking."

Using a series of gears, the knee brace assists the hamstring in slowing the body just before the foot hits the ground, whilst simultaneously generating electricity.

Sensors on the device switch the generator off for the remainder of each step.

In this way, the device puts less strain on the wearer than if it was constantly producing energy.

Tests of the 1.6kg device produced an average of 5 watts of electricity from a slow walk.

"We also explored ways of generating more electricity and found that we can get as much as 13 watts from walking," said Dr Donelan.

"13 watts is enough to power about 30 minutes of talk time on a typical mobile phone from just one minute of walking."

However, to generate this amount of power the generator had to be constantly switched on, which required more effort from the wearer.

Battery pack

The knee brace is the latest development in a field known as "energy harvesting".

The field seeks to develop devices and mechanisms to recover otherwise-wasted energy and convert it into useful electrical energy.

"We're pretty effective batteries," Dr Donelan told BBC News. "In our fat we store the equivalent of about a 1,000kg battery."

Tapping this power source is not a new idea and has been exploited in everyday devices such as wind-up radios and self-winding watches.

The US defence research agency Darpa has a long-standing project to tap energy from "heel-strike" generators implanted in soldier's boots and powered through the pumping motion of a footstep.

And in 2005, US scientists showed off an energy-harvesting backpack which used a suspended load to convert movement into electrical energy.

However, heel-strike devices generate relatively little energy whilst people using the backpack have to bear the burden of carrying the bag.

"It requires a relatively heavy load - around 38kg - to get a substantial amount of power," said Dr Donelan.

Simulations showed that a soldier carrying the pack and walking at a relatively brisk pace could generate around 7.4 watts of power. "It's about the same amount of power as [the knee braces] produce," said Dr Donelan.
Kit list

The team believes the new device could have many uses.

"I think the early adopters will be people whose lives depend on portable power," he told BBC News.

"On the medical front, portable power is used by those who have amputated limbs to charge their powered prosthetic limbs," he said.

However, Dr Art Kuo at the University of Michigan does not believe it will be simply a case of strapping the device on to an existing prosthetic.

"It would probably involve building a new [prosthetic] knee that uses some existing ideas and then also tries to harvest energy using these principles," he said.

The team also hope the device could be useful for people who have suffered a stroke or spinal chord injury who wear an "exoskeleton" to help them move.

"The current and future emphasis is on powered exoskeletons," said Dr Donelan.

Soldiers may also benefit from wearing the knee brace to power the multitude of devices they now carry ,such as night vision goggles and GPS.

"They treat batteries like they treat food and water - they are so essential to what they do," he said.

Dr Donelan has now set up a spin-out company to exploit the technology and believes it will eventually be possible to develop a small device that can be fitted internally across different joints.

However, in the short term he has his sights set on a light weight, slim-line version of the knee brace.

"That's about 18 months away, so it's not science fiction far in the future stuff," he said.