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System harnesses temperature fluctuations to generate power

lose-up view shows the thermal resonator (black box) with its radiative cooling fins across the top. The device is filled with a phase-change material that allows it to capture energy from changing temperatures. Credit: Melanie Gonick

A system has been developed with the potential to enable continuous, years-long operation of remote sensing systems, for example, without requiring other power sources or batteries, according to MIT.

Rather than previous methods used in thermoelectric devices, which have requiring two different temperature inputs at the same time, MIT has said its thermal resonator is able to produce electricity via the swings in ambient temperature.

Professor Michael Strano, MIT, said: “We basically invented this concept out of whole cloth. We’ve built the first thermal resonator. 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.”

The researchers explained that the power levels generated have so far been modest, but an advantage of the system is it does not need direct sunlight.

As the thermal resonator is able to generate energy from ambient temperature changes –even in the shade – the team said it will be unaffected by short term changes in environmental condition and can be located anywhere that’s convenient. This includes underneath a solar panel, in perpetual shadow, where it could even allow the solar panel to be more efficient by drawing away waste heat.

Anton Cottrill of MIT and the study’s lead author, explained that the system has outperformed an identically sized, commercial pyroelectric material by a factor of more than three in terms of power per area.

The team tailored the material for the system so it had thermal effusivity characteristics. This featured a metal foam structure, made of copper or nickel, which they coated with a layer of grapheme and then infused with octadecane.

In MIT’s experiments, the material was said to, in response to a 10-degree-Celsius temperature difference between night and day, have produced 350 millivolts of potential and 1.3 milliwatts of power.

According to Cottrill, ‘the phase-change material stores the heat and the graphene gives you very fast conduction’ when it comes time to use that heat to produce an electric current.

Essentially, as Prof Strano explained, one side of the device captures heat, which then slowly radiates through to the other side. One side always lags behind the other as the system tries to reach equilibrium. This perpetual difference between the two sides is can then apparently be harvested through conventional thermoelectrics. The combination of the three materials, Prof Strano added, makes it ‘the highest thermal effusivity material in the literature to date’.

The team believe that by tuning the properties of the material, it may be possible to harvest other kinds of temperature cycles, such as the heat from the on-and-off cycling of motors in a refrigerator, or of machinery in industrial plants.

“We’re surrounded by temperature variations and fluctuations, but they haven’t been well-characterised in the environment,” Strano continued.

The team have claimed that this is the first system that can be tuned to respond to specific periods of temperature variations, such as the diurnal cycle.

Cottrill described the variations as ‘untapped energy’, which MIT see as a potential complementary energy source in a hybrid system that, by combining multiple pathways for producing power, could keep working even if individual components failed.

Saudi Arabia’s King Abdullah University of Science and Technology, who has partly funded this research, said it hopes to use it as an orthogonal source to power networks of sensors that monitor conditions at oil and gas drilling fields.

Volodymyr Koman, an MIT postdoc and co-author of the new study, added that the system could also be used to offer low-powered, long-lasting power sources for landers or rovers exploring the moon.

Bethan Grylls

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