Electricity produced from temperature variations

Sources of energy lurk everywhere around us, in the surrounding air, in the form of temperature fluctuations. Researchers at MIT propose to exploit them thanks to their brand new invention, which lives to the rhythm of the day-night alternation to convert such temperature variations into electricity.

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Drawing energy from temperature differences or fluctuations is nothing new, but existing concepts have drawbacks. Thus, thermoelectric devices depend on a static temperature gradient: they convert into electricity the temperature differences existing simultaneously between two points of the device. This principle makes it possible, for example, to recharge connected watches using the heat of the skin.

More efficient than thermoelectric systems, pyroelectric sensors generate electricity from frequent variations in temperature, such as those due to the passage of an individual. They are used in particular as infrared detectors for alarm systems or automatic ignition.

In this context, a team from MIT presents an innovative device consisting in exploiting the cycles of temperature fluctuations of the ambient air, for example between day and night. Anton Cottrill, Michael Strano and their team at MIT's Department of Chemical Engineering called their invention thermal resonator. They describe this new device in a publication in Nature Communications.

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Tools are also being developed to produce energy from ambient noise.

“We invented this concept from scratch. We have made the first thermal resonator,” says Professor Michael Strano in an MIT statement. The device is portable and operates independently. It can be “simply put on a table and generate energy, as if from nothing. With a variation of 10°C between day and night, the device produces up to 350 mV and 1.3 mW of power, enough to operate miniature electronic and communication devices, as well as sensors.

It doesn't seem like much, but at this stage the device already produces three times more energy than pyroelectric devices of equivalent size currently available on the market. In addition, it works for the long term, without batteries, regardless of the environment and weather conditions, unlike renewable energies such as solar and wind, whose energy production is interrupted by bad weather or lack of wind.

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The device, which the researchers dubbed a “thermal resonator,” turns fluctuations in ambient air temperature into energy. It was developed to exploit the day-night cycle, which governed the choice of materials. © Melanie Gonick, MIT

The device uses the day-night cycle

In the bowels of the mysterious device hides an arrangement of different materials. The basic structure is a metallic foam (therefore a porous structure) made of copper or nickel, covered with a layer of graphene. The researchers filled the foam with a wax of octadecane, a material that oscillates between a solid state and a liquid state depending on the temperature. "This phase-change material stores heat and the graphene provides very rapid thermal conductivity", specifies Anton Cottrill, first author of the publication, which makes it possible to produce electricity.

The choice and arrangement of materials is not trivial. The invention is in fact based on thermal effusivity. This property of materials to absorb or impart heat to their surroundings depends on thermal conductivity (the diffusion of heat through the material) and heat capacity (the ability of the material to store heat). It is with the aim of optimizing this property that the researchers designed the device.

The prototype was installed on the roof of MIT, where it was tested for several months over periods of approximately 15 days. It can be seen, in black on the right of the image, behind a weather station in white, and next to a measuring device, in black on the left. © Justin Raymond, MIT

The thermal resonator, to supplement other sources of energy

On the other hand, the thermal resonator can theoretically adapt to different cycles of temperature changes, more or less important, such as those induced by the day-night alternation, or by the regular switching on and off of motors. refrigerators or machine tools. However, the researchers decided to test the concept on temperature variations due to day-night alternation. All this governed the choice and arrangement of materials during the construction of the prototype, but with a few adjustments, the thermal resonator will be able to exploit other temperature cycles in the future.

For the researchers, the device is above all intended to be used in parallel with other energy-producing technologies. The hybrid system thus formed would continue to operate even in the event of failure of one of the components. For example, the device installed under a solar panel would convert heat losses into energy and increase the efficiency of the panel.

The invention could also find its use elsewhere in the solar system, for the exploration of planets and moons, because it could sustainably supply energy to rovers and landers. It would even be possible to manufacture the system on the spot, from the elements present on the planet, instead of doing it on Earth, according to Volodymyr Koman, one of the co-authors of the publication.

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