Engineers in the USA have developed an energy-harvesting patch that produces electricity from body heat to power wearable devices.
The wearable thermoelectric generator (TEG), developed by a team from the US North Carolina State University, adjusts to the shape of the body and transforms heat to electricity with much better efficiency than other similar devices, claim the researchers.
"Wearable thermoelectric generators 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 describing the device. "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 centimetre squared.”
The new device, generating up to 20μW/cm2, thus offers a twenty-fold improvement.
Instead of a heat sink, the technology relies on a layer of thermally conductive material that is attached to the skin, spreading out heat. The conductive material is coated with a polymer that prevents the heat from dissipating through to the outside air. As a result, the body heat has to pass through a centrally-located TEG one centimetre squared in size.
Heat that is not converted into electricity passes through the TEG into an outer layer of the thermally conductive material and is rapidly dissipated. The entire system is only 2mm thin and flexible.
"In this prototype, the TEG is only one centimetre squared, but we can easily make it larger, depending on a device's power needs," said Vashaee.
To maximise the amount of electricity generated, the device is best to be placed on the upper arm. Although skin temperature is higher around the wrist, the irregular contour of the wrist limits the surface area of contact between the TEG band and the skin. Wearing the band on the chest reduces air flow and thus limits heat dissipation - since the chest is normally covered by a shirt.
The researchers also tried to incorporate the device into a T-shirt but found that such a solution would decrease the energy yield to 6μW/cm2. When the wearer was running, the energy harnessing T-shirt generated 16μW/cm2 of electrical power.
The researchers hope their device could be used in future to power wearable health monitors such as heart monitors or devices predicting asthma attacks.