Scientists in Canada and the US have developed a leg-driven knee-mounted generator that can power up to 10 mobile phones just by harvesting the energy normally wasted when we walk.
Simon Fraser University researcher Max Donelan and his colleagues have based their invention on the "generative-braking" principle used in some environmentally-friendly cars. In this system instead of using the brakes to slow the car, the vehicle's momentum is instead turned into electricity that can then be used to start the vehicle moving again later.
The leg-generator works the same way. It consists of a modified knee brace which drives a small generator through a one-way clutch. It can be set to kick-in only towards the end of a leg swing when muscles would need to be applied to stop the leg's movement. In this way the energy that it saves the wearer by slowing the leg is instead turned into electricity.
In trials, with a device strapped to each leg, volunteers were easily able to produce electricity at the rate of 5W, which is six-fold greater than other foot-wear based systems in which the foot-falls are used to generate power. The system also seems to compete very favourably with the next best alternative, which is a backpack in which a suspended load moves up and down to drive a generator as a person walks.
The team point out that most manual electricity generating devices occupy the full attention of the user but this system leaves the user free to attend to othe tasks. The power it produces, say the researchers, could be used to run communication devices and personal computers, especially in countries with no reliable power supply.