Technology Review – University of Wisconsin-Madison researchers have come up with a microfluidics technique that scavenges considerably more energy from human footfalls and converts it into electric power. Previous attempts to make energy-harvesting shoes have yielded less than a watt of power, but the new approach could lead to a shoe-mounted generator that produces up to 10 watts, says Tom Krupenkin, a mechanical engineering professor who led the work.
“A lot of energy is simply wasted as heat while we walk,” says Krupenkin. “If one can convert this into electrical energy, numbers come out to be up to 10 watts per foot.” Cell phones and smart phones need about 1 to 2 watts, while small laptops need 10 to 12 watts. Power-generating shoes could be an important breakthrough for soldiers, who currently carry heavy batteries to power their radios, GPS units, and night-vision goggles.
Walking exerts a lot of force on the heel and toe, and cushioned soles can compress by about a centimeter with every step. Energy harvesters convert this force and displacement into electrical energy.
The new concept, presented in a Nature Communications paper, involves microscopic droplets of a conductive fluid flowing between electrodes coated with dielectric films. The droplets—the researchers used mercury or a gallium-based alloy called galistan—can be sandwiched between flat plates coated with the film or can be enclosed in a coated microchannels.
It might be years before you can buy a power-generating shoe, though. So far, the researchers have only made an array of 150 droplets that gives a few milliwatts of power. However, they calculate that a device with 1,000 droplets in a four-meter-long, one-millimeter-wide channel, which would cover an area of 40 square centimeters and fit in a shoe sole, could generate a few watts.
Krupenkin and his colleagues have established a startup, InStep NanoPower, to develop and, potentially, commercialize the technology. The company has a first-generation benchtop-sized prototype device. They expect to the third generation harvester could be embedded in footwear. “This type of product will have to be a collaborative project between Instep and a shoe manufacturer,” Krupenkin says. “We can’t expect anything on the market earlier than two years.”
Experimental studies show that up to 20 W of power per foot is simply lost as heat during normal walking. This might sound like a lot, but in fact this is still a small fraction of about 300 W of total metabolic power that the person typically generates during walking
The power generated by the footwear-embedded harvester can be used in one of two ways. It can be used directly to power a broad range of devices, from smartphones and laptops to radios, GPS units, night-vision goggles and flashlights. In this case the power can be delivered to the device using a number of methods, ranging from simple wiring to conductive textiles to wireless inductive coupling.
Alternatively, a Wi-Fi hot spot can be integrated into the harvester to act as a “middleman” between mobile devices and a wireless network. Such an arrangement dramatically reduces power consumption of wireless mobile devices and allows them to operate for much longer time without battery recharge. No direct physical connection between the mobile devices and the harvester unit is required in this case.