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 — an established method for converting temperature fluctuations to electricity — by factor of more than three in terms of power per area, says graduate student Anton Cottrill.
“The phase-change material stores the heat,” said Cottril, “and the graphene gives you very fast conduction” when it comes time to use that heat to produce an electric current.
While the initial testing was done using the 24-hour daily cycle of ambient air temperature, tuning the properties of the material could make it 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-characterized in the environment,” said Strano. The research was partly funded by a grant from Saudi Arabia’s King Abdullah University of Science and Technology (KAUST), which hopes to use the system as a way of powering networks of sensors that monitor conditions at oil and gas drilling fields, for example.
“They want orthogonal energy sources,” said Cottrill, ones that are entirely independent of each other, such as fossil fuel generators, solar panels, and this new thermal-cycle power device. Thus, “if one part fails,” for example if solar panels are left in darkness by a sandstorm, “you’ll have this additional mechanism to give power, even if it’s just enough to send out an emergency message.”