Energy harvested from thin air could power gadgets for years

MIT engineers have created a system that converts ambient temperature changes into energy

Thomas McMullan
9 May 2018

Those living in the UK will know all about sudden temperature changes. Spring is in the air one day, then there’s a snowstorm the next. It’s a nightmare for wardrobe decisions, but all this capricious weather could soon be put to good use thanks to an invention that converts such fluctuations in temperature into electricity.

MIT engineers have developed a way of harvesting energy from thin air; or more precisely, from the ambient thermal energy in the atmosphere. And we’re not talking about a few sparks. This could have the potential to power sensors and communications devices for years without the need of batteries.  

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The system, called a thermal resonator, works by capturing heat on one side of the device before slowly radiating it on the other side. Thanks to the materials used to create the machine, one side always lags behind the other as the system tries to reach equilibrium, and it’s this difference that can be harvested through the process of thermoelectrics.

To get this system to work, the researchers had to make it from a carefully tailored combination of materials. They needed something with an optimal thermal effusity: a combination of how rapidly heat can spread through a material, and how much heat can be stored in the material. The problem is that, in most materials, when one of these properties is high, the other is low. Materials that quickly soak up heat tend to be poor at storing it.

In a paper published in Nature, the researchers describe how they used a copper and nickel foam, coated with graphene to increase thermal conductivity. This was infused with a type of wax called octadecane, which changes between a solid and liquid within the range of temperatures needed for the application.

The octadecane stores the heat, while the graphene means heat can spread very fast through the system. According to graduate student Anton Cottrill, the study’s lead author, this combination creates “the highest thermal effusivity material in the literature to date”.

A test of the system showed that a 10°C drop in temperature between day and night produced 350 millivolts of potential and 1.3 milliwatts of power. It’s not enough to keep your TV running, but that could power a simple communications system or an environmental sensor.

“We’ve built the first thermal resonator,” says Cottrill. “It’s something that can sit on a desk and generate energy out of what seems like nothing. We are surrounded by temperature fluctuations of all different frequencies all of the time. These are an untapped source of energy.”

One great thing about the system is that it doesn’t rely on sunlight to work. Because it responds to ambient temperature changes, it can work in the shade, and this means it can be located pretty much anywhere subject to temperature changes.  

With the concept proven, the engineers want to look at how the system could work with different types of temperature fluctuations, from the on-off cycling of motors in a refrigerator to the rhythms of industrial machinery. There’s even scope for this type of technology to be used off world, with planetary rovers or landers drawing energy from day and night cycles.

Image credits: MIT, Justin Raymond, Melanie Gonick