Taking People Power to a New Level
Some people exercise by power walking. But what if walking could actually provide electrical power? Researchers have developed an electrical generator mounted on the knee that turns walks into watts.
The device, which in its current form looks a little like a simple knee brace with cyborg bling, harnesses power from part of the stride.
J. Maxwell Donelan, the lead researcher and a professor of kinesiology at Simon Fraser University in British Columbia, compared the device to the regenerative braking used to produce electricity for hybrid cars.
The generator does not capture the motion throughout the entire stride, since that would subject the user to a dragging feeling with each step. Instead, the gearwork disengages at the beginning of the step and re-engages as the leg swings back from a stride.
This means that the only drag occurs at the tail end of the stride, when muscles are actually working to slow the leg down. It does not detract from the energy required for moving forward, and in fact, by slowing down the leg at that stage of the stride, ends up relieving the muscles of some of the effort.
One device on each leg can produce about five watts of electricity, Dr. Donelan said. That is enough to run 10 cellphones, or potentially, medical devices like insulin pumps or prosthetic limbs. The power generated could be stored in a battery.
Dr. Donelan suggested that the device could also be a boon to soldiers, who may carry some 30 pounds of batteries to run their increasingly high-tech gear for a 24-hour mission. A system that could be used to replace batteries or extend their life might ease their burden and increase their abilities in the field, he said.
Harvesting energy from human movement has long been a dream of scientists in the field of biomechanics. The energy stored in body fat is the equivalent of a battery that weighs more than a ton, Dr. Donelan said.
But harnessing that power has proved an enormous challenge. People can now buy hand-cranked flashlights, for example. But “no one really wants to crank a hand crank for eight hours a day,” Dr. Donelan said.
Instead, he said, it should be possible to harvest power from an activity that people might be doing anyway, like walking.
Other efforts to tap the power of movement have included shoe-mounted devices and systems that channel the energy from the bouncing motion of a backpack. The shoe-motion systems, however, have so far been able to produce less than a watt of energy, and the backpack systems require that the special energy-generating backpack be worn.
For people who need to carry heavy loads, the backpack provides a way to generate power from effort they would be expending anyway — and even seems to make carrying the load somewhat easier, said Larry Rome, the University of Pennsylvania professor who developed it. The backpack system, in its most recent configuration, adds only about four pounds to the burden the user would already be carrying, Dr. Rome said, and it can generate some 20 watts of power.
Dr. Rome expressed admiration for Dr. Donelan’s device. “What was extremely clever about it was the design came from their very deep understanding of how people walk,” he said.
Dr. Donelan said that wearing his 3.3-pound knee device in its current form could take some getting used to. “You definitely notice it,” he said. “It is heavier than a typical knee brace.”
The weight on the side can also be awkward at first. Once a user begins walking with it, however, the generator adds no effort to movement. Because the device assists the leg’s slowing-down motion at the end of a stride, Dr. Donelan said, “you miss it when it’s gone.”
The device is described in the current issue of the journal Science.