Power walk: Footsteps could charge mobile electronics
low, in the not-so-distant future you could charge it simply
by plugging it into your shoe.
An innovative energy harvesting and storage technology
developed by University of Wisconsin-Madison mechanical
engineers could reduce our reliance on the batteries in our
mobile devices, ensuring we have power for our devices no
matter where we are.
In a paper published Nov. 16, 2015, in the journal
Scientific Reports, Tom Krupenkin, a professor of
mechanical engineering at UW-Madison, and J. Ashley
Taylor, a senior scientist in UW-Madison's Mechanical
Engineering Department, described an energy-harvesting
technology that's particularly well suited for capturing the
energy of human motion to power mobile electronic devices.
The technology could enable a footwear-embedded energy
harvester that captures energy produced by humans during
walking and stores it for later use.
Power-generating shoes could be especially useful for the
military, as soldiers currently carry heavy batteries to power
their radios, GPS units and night-vision goggles in the field.
The advance could provide a source of power to 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 tapping into just a small amount of that
energy is enough to power a wide range of mobile devices,
including smartphones, tablets, laptop computers and
flashlights. For example, a typical smartphone requires less
than two watts.
However, traditional approaches to energy harvesting and
conversion don't work well for the relatively small
displacements and large forces of footfalls, according to
the researchers.
"So we've been developing new methods of directly
converting mechanical motion into electrical energy that are
appropriate for this type of application," Krupenkin says.
The researchers' new energy-harvesting technology takes
advantage of "reverse electrowetting," a phenomenon that
Krupenkin and Taylor pioneered in 2011. With this approach,
as a conductive liquid interacts with a nanofilm-coated
surface, the mechanical energy is directly converted into
electrical energy.
The reverse electrowetting 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.
"Yet 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."
To overcome this, the researchers developed what they
call the "bubbler" method, which they described in their
Scientific Reports study. The bubbler method combines
reverse electrowetting with bubble growth and collapse.
The researchers' 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 speedy, 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."
Krupenkin and Taylor are seeking to partner with industry
and commercialize a footwear-embedded energy harvester
through their startup company, InStep NanoPower.
Their harvester could directly power various mobile devices
through a charging cable, or it could be integrated with a
broad range of electronic devices embedded in a shoe, such
as a Wi-Fi hot spot that acts as a "middleman" between
mobile devices and a wireless network. The latter requires
no cables, dramatically cuts the power requirements of
wireless mobile devices, and can make a cellphone battery
last 10 times longer between charges.
"For a smartphone, just the energy cost of radio-frequency
transmission back and forth between the phone and the
tower is a tremendous contributor to the total drain of the
battery," Krupenkin says.
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