Shanks’s pony charges battery

Some technologies are talked of and ‘talked-up’ long before there is any real prospect of their being delivered, and many of the low-power energy harvesting theories seem to fall into this category.

energy harvesting shoe-2I can’t remember how long ago I first heard about the possibility of harvesting energy from the thrust and release expended when walking, but I do remember friends telling me that the resultant power harvest would be so vanishingly small as to make it ‘not worth the effort’.

I’m an optimist, however, and happy to see that the University of Wisconsin has thought it worth investigating.

They have released to the media a report of their research suggesting that, in the spirit of that old proverb ‘every little helps’, harvesting walkers’ or runners’ energy could be worthwhile. A bit of sole power.

The researchers claim to have “an innovative energy harvesting and storage technology” that could “reduce our reliance on the batteries in our mobile devices”.

Small is beautiful

The researchers, Tom Krupenkin, a professor of mechanical engineering at University of Winsonsin’s Madison College of Engineering, and J. Ashley Taylor, a senior scientist in Madison’s mechanical engineering department, have enabled an energy harvester to be embedded in a shoe to capture energy produced and store it for later use.

They foresee both military opportunities in the idea, and the potential to provide a source of power for people in remote areas and developing countries that lack adequate electrical power grids.

“Human walking carries a lot of energy,” Krupenkin says. “Theoretical estimates show that it can produce up to 10 watts per shoe, and that energy is just wasted as heat. A total of 20 watts from walking is not a small thing, especially compared to the power requirements of the majority of modern mobile devices.”

Krupenkin says even a very small amount of that energy could be enough to power mobile devices such as a smartphone or a torch.

energy harvesting shoeBecause of the relatively small displacements and large forces of footfalls, the researchers have been thinking about how to directly convert mechanical motion into electrical energy.

Reverse electrowetting

They have used “reverse electrowetting,” which they pioneered in 2011, where as a conductive liquid interacts with a nanofilm-coated surface, the mechanical energy is directly converted into electrical energy.

The method can generate usable power, but it requires an energy source with a reasonably high frequency – such as a mechanical source that’s vibrating or rotating quickly.

“Our environment is full of low-frequency mechanical energy sources such as human and machine motion, and our goal is to be able to draw energy from these types of low-frequency energy sources,” Krupenkin says. “So reverse electrowetting by itself didn’t solve one of the problems we had.”

The Bubbler

To overcome this, the researchers developed what they call the “bubbler” method, which combines reverse electrowetting with bubble growth and collapse.

Their bubbler device – which contains no moving mechanical parts – consists of two flat plates separated by a small gap filled with a conductive liquid. The bottom plate is covered with tiny holes through which pressurized gas forms bubbles. The bubbles grow until they’re large enough to touch the top plate, which causes the bubble to collapse.

The fast, repetitive growth and collapse of bubbles pushes the conductive fluid back and forth, generating electrical charge.

“The high frequency that you need for efficient energy conversion isn’t coming from your mechanical energy source but instead, it’s an internal property of this bubbler approach,” Krupenkin says.

The researchers say their bubbler method can potentially generate high power densities – lots of watts relative to surface area in the generator – which enables smaller and lighter energy-harvesting devices that can be coupled to a broad range of energy sources.

The proof-of-concept bubbler device generated around 10 watts per square meter in preliminary experiments, and theoretical estimates show that up to 10 kilowatts per square meter might be possible, according to Krupenkin.

“The bubbler really shines at producing high power densities,” he says. “For this type of mechanical energy harvesting, the bubbler has a promise to achieve by far the highest power density ever demonstrated.”

Unsurprisingly, Krupenkin and Taylor are seeking to partner with industry and commercialize a footwear-embedded energy harvester through their startup company, InStep NanoPower. I wish them well!

J Ashley Taylor (left) and Tom Krupenkin showing prototypes in their lab

J Ashley Taylor (left) and Tom Krupenkin showing prototypes in their lab

Images: UW – Madison College of Engineering



One comment

  1. What’s wrong with piezo-electric methods? Or a magnet and coil?

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