Phase change graphene system harvests thermal energy from day/night cycle

February 16, 2018 //By Nick Flaherty
Phase change graphene system harvests thermal energy from day/night cycle
Researchers at MIT in the US has developed a thermal energy generator that takes advantage of the swings in ambient temperature that occur during the day/night cycle.

The new system, called a thermal resonator, could enable continuous power for remote sensing systems without using batteries.

The thermal resonator does not need direct sunlight and so is unaffected by short-term changes in cloud cover, wind conditions, or other environmental conditions, and can be located in the shadow under a solar panel.
The team needed a material that is optimised for thermal effusivity -- how readily the material can draw heat from its surroundings or release it. This balances thermal conduction and capacity which tend to be contradictory: if one is high, the other tends to be low. Ceramics, for example, have high thermal capacity but low conduction.

The basic structure is a metal foam, made of copper or nickel, which is then coated with a layer of graphene to provide even greater thermal conductivity. Then, the foam is infused with a wax called octadecane, a phase-change material, which changes between solid and liquid within a particular range of temperatures chosen for a given application.

Essentially 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 can then be harvested through conventional thermoelectrics.

"We basically invented this concept out of whole cloth," said Michael Strano, Carbon P. Dubbs Professor of Chemical Engineering at MIT. "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." This combination of the three materials makes it the highest thermal effusivity material in the literature to date, he says.

A sample of the material made to test the concept showed produced 1.3mW of power at 350mV from a 10ºC temperature difference between night and day. This outperforms an identically sized, commercial pyroelectric material --

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