- New 2mm thick patches convert body heat into electricity
- Layer of thermally conductive material sits on skin and spreads out heat
- Polymer layer on top forces heat into device that makes electricity
- Found the optimal place to harvest body heat is on the upper arm
- Researchers have also incorporated the patch into T-shirts
Soon working up a sweat won’t just increase your heart rate on your Fitbit – it will also power the device.
Researchers are currently developing a new design that harvests body heat and converts it into electricity that can power wearables and smartphones.
The system uses a body-conforming patch with a conductive layer that forces body heat through a centrally-located wearable thermoelectric generator, where it is converted into electricity.
This innovation derives from North Carolina State University, who states their prototype can generate far more electricity than previous lightweight heat harvesting technologies – 20 μW per centimeter squared, compared to 1 microwatt or less.
‘Wearable thermoelectric generators (TEGs) generate electricity by making use of the temperature differential between your body and the ambient air,’ says 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 (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 began with a layer of thermally conductive material that rests on the skin and spreads out the heat.
A polymer layer was then placed on top to prevent heat from escaping from the body.
This components also forces the body heat to pass through a centrally-located TEG that is one cm2.
Heat that is not converted into electricity passes through the TEG into an outer layer of thermally conductive material, which rapidly dissipates outside the body.
The team began with a layer of thermally conductive material that rests on the skin and spreads out the heat. A polymer layer was then placed on top to prevent heat from escaping the body, this forces the body heat to pass through a centrally-located TEG that is one cm2
This new system is just 2 millimeters thick and the team says it is also very 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,’ says 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.
While creating this new power generator, the team discovered that the upper arm is the optimal location for harvesting heat.
TURN YOURSELF INTO A WALKING CHARGER
Researchers at the Massachusetts Institute of Technology have developed a button sized self-charging battery that can scavenge energy from low temperature sources of heat.
The device can charge itself at temperatures between 20°C(68°F) and 60°C (140°F), far lower than other heat-harvesting technologies.
The button sized battery designed by Dr Gang Chen and his team at the Massachusetts Institute of Technology is a prototype for a new way of charging mobile devices from surrounding heat
Dr Gang Chen, head of the mechanical engineering department at MIT, who led the work, said the technology could lead to new mobile phone batteries that can be charged without needing to be plugged in.
The two centimetre wide battery works by exploiting the relationship between temperature and voltage known as the thermally regenerative electrochemical cycle.
This cycle means that a battery charged at high temperatures can deliver more electricity at lower temperatures than has been used to charge it in the first place because of the energy absorbed as heat.
Dr Chen and his colleagues found they use this to create a ‘heat engine’ to generate electricity purely from the heat surrounding the battery.
By tuning the battery’s electrodes, which were made from lead and ionic iron, Dr Chen and his colleagues were able to produce a device that could achieve this at low temperatures.
This they believe, would allow a phone to be charged by harvesting energy from body heat and then cooling down when it is removed from a pocket.
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.
They also found that wearing the band on the chest limits air flow, as 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.
This new system is also just 2 millimeters and the team says it is also very flexible. In addition, the researchers incorporated the TEG into T-shirts .
‘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 says.
‘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.’