Harnessing "People Power"


A new, knee-mounted device harvests energy from the end of a walker’s step, in the same way that hybrid-electric cars recycle power from braking. With a device on each leg, volunteers generated about 5 Watts of electricity, while using little additional energy. That’s enough power to run 10 cell phones at the same time, and twice the power needed for computers in developing regions. The devices start with a knee brace that’s rigged with a simple motor and a single-gear clutch system. Six volunteers wore the devices while walking slowly on treadmills. Max Donelan and colleagues measured each person’s oxygen use and carbon-dioxide production to learn how much energy it takes to work the devices. They also measured power output with the gear disengaged, locked in place for the reverse swing only, or engaged for the full stride. The reverse-swing mode allowed walkers to produce about 5 Watts of electrical power, without breaking a sweat. By running, one volunteer even generated 54 Watts of power, though the cost of this extra effort remains unknown. Noting that a half-billion children live without electricity, the authors suggest that perhaps this “people power” strategy might prove useful in remote regions for powering computers. More immediately, the device might help drive robotic arms, for people with missing limbs or hands. It might also extend the battery-life of implanted insulin pumps and lighten the load that soldiers carry into the field. Researchers have tried tapping human energy by using shoe-mounted devices, and by bouncing a load up and down in a special backpack. But shoe generators crank out a mere 0.8 Watts of energy, and the backpack is heavy. This research appears in the 07 February 2008 issue of the journal Science.

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VIDEO is available.

Dr. Donelan behind a close-up of the knee brace.

[Image © Science]


The biomechanical energy harvester comprises an aluminium chassis (green) and generator (blue) mounted on a customized orthopedic knee brace (red), totaling 1.6 kg mass, with one worn on each leg.

[Image © Science]


The chassis contains a gear train converting low velocity and high torque at the knee into the high velocity and low torque for the generator, with a one-way roller clutch allowing for selective engagement of the gear train during knee extension only and no engagement during knee flexion.

[Image © Science]


The schematic diagram shows how a computer-controlled feedback system determines when to generate power using knee angle feedback, measured with a potentiometer mounted on the input shaft. Generated power is dissipated in resistors.

[Image © Science]



Download video 1. In this energy harvesting mode, we programmed the device to engage only during the end of the swing phase, producing electrical power while simultaneously assisting the knee flexor muscles in decelerating the knee. The mask and mouthpiece measure the metabolic cost or "effort." We use the cabling and computer to engage and disengage power generation and for quantifying the amount of generated power.

Download video 2. In this energy harvesting mode, the
device harvests energy whenever the knee is extending irrespective of whether the knee is accelerating or decelerating.

[Video © Science]


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