Electricity Production From Body Heat

North Carolina State University researchers have developed a new design for harvesting body heat and converting it into electricity for use in wearable electronics. The experimental thermoelectric generator prototypes are lightweight, conform to the shape of the body, and can generate far more electricity than previous lightweight heat harvesting technologies.

The researchers also believe they have identified the optimal site on the body for heat harvesting.

Daryoosh Vashaee, an associate professor of electrical and computer engineering at NC State and corresponding author of a paper on the work said, “Wearable thermoelectric generators (TEGs) generate electricity by making use of the temperature differential between your body and the ambient air. 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 (μW/cm2). Our technology generates up to 20 μW/cm2 and doesn’t use a heat sink, making it lighter and much more comfortable.”

The team’s research paper “Wearable Thermoelectric Generators for Human Body Heat Harvesting”, has been published in the journal Applied Energy.

The new design begins with a layer of thermally conductive material that rests on the skin and spreads out the heat. The conductive material is topped with a polymer layer that prevents the heat from dissipating through to the outside air. This forces the body heat to pass through a centrally-located TEG that is one square centimeter. Heat that is not converted into electricity passes through the TEG into an outer layer of thermally conductive material, which rapidly dissipates the heat. The entire system is thin – only 2 millimeters – and flexible.

Vashaee, 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 added, “In this prototype, the TEG is only one centimeter squared, but we can easily make it larger, depending on a device’s power needs.”

The researchers also found that the upper arm was the optimal location for heat harvesting. While the skin temperature is higher around the wrist, the irregular contour of the wrist limited the surface area of contact between the TEG band and the skin. Meanwhile, wearing the band on the chest limited air flow – limiting heat dissipation – since the chest is normally covered by a shirt.

In addition, the researchers incorporated the TEG into T-shirts. The researchers found that the T-shirt TEGs were still capable of generating 6 μW/cm2 – or as much as 16 μW/cm2 if a person is running.

“T-shirt TEGs are certainly viable for powering wearable technologies, but they’re just not as efficient as the upper arm bands,” Vashaee said.

“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,” he added. “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.”

This is a huge jump in wattage for body heat driven thermoelectric generators. Getting from 1 μW/cm2 on up to 6, 16 and an impressive 20 from early lab experimental prototypes is a real sit up and notice level achievement. One would expect Fitbit and other personal electronic device manufacturers are going to be very curious about this technology. Mass market introduction will drive to the medical uses the team is thinking about. But batteries at these power levels are not huge barriers.

Lets hope that the next time this team gets a press release ready they note more than raw watts. Knowing operating volts would be quite useful and the photo of the Fluke multimeter running 3.1 something isn’t real clear. A microwatt is 10−6 W or a millionth of a Watt. Volts divided into the Watts isn’t going to show a lot of Amps, but realistically, solar cells driving watches and calculators are in this power range, so the team is much closer than one might guess.

Go NC State! Your humble writer could live with an armband for running and charging some devices.