Engineers are working on technology that could turn a t-shirt into a power plant.

Researchers at North Carolina State University have developed a new design for harvesting body heat and converting it into electricity for use in wearable electronics.

The experimental prototypes are lightweight, conform to the shape of the body, and can generate far more electricity than previous lightweight heat-harvesting technologies.

Wearable thermoelectric generators (TEGs) make use of the temperature differential between the human body and the ambient air to generate electricity.

Previous approaches made use of heat sinks - which are heavy, stiff and bulky – and could only generate one microwatt or less of power per centimetre squared (μW/cm2).

The new design generates up to 20 μW/cm2 and does not use a heat sink, making it lighter and much more comfortable.

It 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, forcing the body heat to pass through a centrally-located, 1cm2 TEG.

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 flexible and just 2 millimetres thick.

In tests so far, researchers have found that the upper arm is 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 – thus limiting heat dissipation - since the chest is normally covered by a shirt.

When the researchers incorporated the TEG into t-shirts, they found that they were capable of generating between 6 μW/cm2 and 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,” says Associate Professor Daryoosh Vashaee, who worked on the project as part of the Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) scheme.

“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 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.”

Their latest paper is accessible here.