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Who needs batteries when you can power up with a walk?

By Chris Davis (China Daily USA) Updated: 2016-01-06 11:09

The one thing that all of the 3,600 exhibitors' products now at the Consumer Electronics Show in Las Vegas have in common is that they all need electricity. Energy - be it wind, sunlight, tides, friction, gravity - is all around us, and figuring out new ways to harness it and turn it into electricity has led to some of the most inventive breakthroughs in science.

One such got unveiled at MIT today. A team led by Professor Li Ju has developed a completely new method of using electrochemistry to tap into the energy of small, human-scale motions, like walking, and could provide a virtually unlimited power supply.

Previous attempts to harness electricity from small motions have been based on what scientists call the triboelectric effect, which is basically friction (like the sparks from rubbing a balloon against a wool sweater). Another source has been crystals that produce minute voltage when they're bent or compressed. Those have worked well for tapping into motion with high frequency, such as the vibrations of machinery.

But for typical human-scale motion such as walking or exercising, those systems have their limits.

Who needs batteries when you can power up with a walk?

Li's new system works like a battery, with a twist, literally. Two thin sheets of lithium alloy act as electrodes, separated by a layer of porous polymer soaked with a liquid electrolyte that is especially efficient at conveying lithium ions between the plates.

When the layered composite is bent or pinched, even slightly, the pressure difference squeezes ions through the polymer (like the reverse osmosis process used in water desalination), producing a counteracting voltage and an electrical current to an external circuit, which can then be used to power other devices.

The concept came to Li at a conference in Boston in 2013 where the phenomenon underlying the new device was being discussed as an obstacle, or "parasite", in the battery community, because voltage put into a battery caused bending that led to electrode cracking and "hemorrhaging" of ions.

Li, who is the Battelle Energy Alliance Professor in Nuclear Science and Engineering at MIT and studied at the University of Science and Technology of China, wondered why not turn the whole process around? If nature has this property, why not reverse it and put it to use. Bend it and wring electricity out.

Li e-mailed one of his graduate students, Sangtae Kim, who happened to be on vacation in Paris, and asked him if he could build a device based on the concept.

He did. It took about a year to come up with a prototype and another year to fully understand how it worked. It's about the size of a postage stamp, paper thin and patent pending. But bending and unbending it, or squeezing it, does produce electricity, however small the amount, for now.

One of the first applications they are thinking of trying are sensor networks, small devices that can be powered with minimal current.

"Imagine a toddler walking," said Kim. "If we install this device into the shoe of the baby to power-up a GPS with wireless capability, we can basically track where the toddler is because the device will power the GPS without the need of ever changing a battery."

Similar things can be done with adults. Human walking, step by step, generates 80 joules of energy per second through the sole of the foot and all of it is wasted entirely. "If we can recover some of it into electricity, we can do some useful things," Kim said.

Another obvious use is to put the devices in road pavement so they can be used as an auxiliary power unit for the local electricity grid, Kim said. Localizing power sources cuts down on the amount of electricity lost in transmission over distances.

After building the device the researchers determined how much energy it was theoretically supposed to generate per bending. And the prototype was generating only about 15 percent that. But, Kim said with confidence, "this can easily be solved with an engineering breakthrough we are working on."

Bottom line: The device converts mechanical to electrical energy, Li explained in an MIT press release, therefore, "it is not limited by the second law of thermodynamics; so in principle, it could be 100 percent."

Li, whom Kim considers a "genius", also said the system could easily be manufactured in any desired size and is amenable to industrial manufacturing processes.

It certainly adds a whole new meaning to power walks.

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