August and July are tied for being the hottest months since record-keeping began 136 years ago, according to scientists at NASA. But what if, instead of cranking up the AC, all that resulting body heat could have been put to good use?
That’s what researchers at North Carolina State University set out to discover recently, and on Monday announced they had developed prototypes for harvesting body heat and converting it into electricity for use in wearable electronics.
“Wearable thermoelectric generators (TEGs) generate electricity by making use of the temperature differential between your body and the ambient air,” explained Daryoosh Vashaee, an associate professor of electrical and computer engineering at NC State and corresponding author of a paper on the work. “Previous approaches either made use of heat sinks—which are heavy, stiff and bulky—or were able to generate only one microwatt or less of power per centimeter squared. Our technology generates up to 20 microwatts of power per centimeter and doesn’t use a heat sink, making it lighter and much more comfortable.”
Here’s how it works: a layer of thermally conductive material, topped with a polymer, rests on the skin and prevents body heat from dissipating by forcing it to pass through a centrally-located TEG. Heat that is not converted into electricity passes through the TEG into an outer layer of thermally conductive material that then disperses it. The entire system is only 2 millimeters thick and flexible.
“In this prototype, the TEG is only one centimeter squared, but we can easily make it larger, depending on a device’s power needs,” Vashaee said, who worked on the project as part of the National Science Foundation’s Nanosystems Engineering Research Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) at NC State.
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Notably, the researchers discovered that the optimal location for heat harvesting is not the wrist, where the skin temperature is higher, but the upper arm. That’s because the irregular contour of the wrist limited the surface area of contact between the TEG band and the skin.
Somewhat interestingly, the researchers found that while T-shirts integrated with TEGs were capable of generating as much as 16 microwatts of power per centimeter squared if the person wearing it was running, upper arm bands were still more efficient.
“The goal of ASSIST is to make wearable technologies that can be used for long-term health monitoring, such as devices that track heart health or monitor physical and environmental variables to predict and prevent asthma attacks,” Vashaee said. “To do that, we want to make devices that don’t rely on batteries. And we think this design and prototype moves us much closer to making that a reality.”