Lightweight Device Uses Body Heat to Make Electricity

Lightweight Device Uses Body Heat to Make Electricity
October 7, 2016

Technology and electronics are making their way into more and more aspects of our daily lives. There are apps that can adjust the thermostat, track what kind of sleep you're getting on your mattress and even check in on your dog.

Wearable technology allows us to measure how many calories we’ve burned with our shoes and check email from our watches, and thanks to new technology being developed at NC State University, we may someday be able to charge devices with a T-shirt.

Researchers in NC State’s electrical and computer engineering department have designed a new lightweight thermoelectric generator (TEG) capable of harnessing human body heat to generate electricity. The paper describing this research was published in the journal Applied Energy.

Study author Haywood Hunter with TEG shirtHeat has been used to generate electricity for decades. Heat can create steam, which spins a turbine, which makes electricity. Thermoelectric generators skip a few steps and go directly from heat to electricity by creating a temperature gradient. When one side of an electrically conductive material is hot on one side and cold on the other, electrons will naturally migrate from the hot side to the cold side, and moving electrons are what we know as electrical current.

Over the past few years, engineers have been attempting to create wearable TEGs that could use human body heat to power wearable devices. NC State electrical and computer engineering professor Daryoosh Vashaee and his colleagues developed a lightweight, flexible TEG with a similar power output to many much heavier models.

A central issue in making a high-power TEG is maintaining a temperature gradient—heat on one side and cold on another—because the greater the temperature difference between the two sides of the TEG, the faster electrons will flow. 

The heat element comes from the body, so the challenge is keeping the other side of the wearable TEG cool. Earlier models, Vashaee says, often made use of heat sinks, which are large bulky components whose sole purpose is to drain heat. The issue with heat sinks, however, is that as good as they are at draining heat from the TEG, they are too rigid and heavy to be useable in clothing.

The TEGs developed at NC State work differently. The TEG itself is a centimeter-wide square surrounded by a conductive material that takes the electrical current to whatever device is being charged. Every part of the system except for the TEG itself is covered with heat-insulating polymer that funnels heat through the TEG piece. Outside of the TEG is a thermal conductor that pulls heat away from the TEG and dissipates it to the outside air. The whole device is about two millimeters thick and can bend to get maximum contact with the skin.

The device works best in places where there is plenty of body heat. Vashaee and his colleagues found that the upper arm has the best combination of heat generation, ventilation to the outside air and a flat surface for good contact. They have, however, developed shirts with TEGs built into the chest. The t-shirts cannot generate as much power as something on the upper arm, but they certainly look a lot cooler.

The engineers designed this new TEG as part of the National Science Foundation’s research center for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST). As convenient as it would be to charge a phone with a T-shirt, the primary goal of this work is to power wearable health-monitoring devices so patients could keep an eye on their health and provide around-the-clock data to doctors with no need for bulky batteries that need changing and charging.

Still, we can dream of a day when we can play Pokemon GO all day, with our phones staying powered simply by walking around.

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina.

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