A method that converts ambient temperature fluctuations into electrical power could enable continuous, years-long operation of remote sensing systems, say researchers at the Massachusetts Institute of Technology (MIT), Cambridge, Mass. Their thermal resonator system might avoid the need for other power sources or batteries, they believe.
Instead of requiring two different temperature inputs at once like commercial thermoelectric generators, the thermal resonator uses swings in ambient temperature that occur between day and night to produce power.
Figure 1. This close-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. Source: Melanie Gonick, MIT.
“While thermal energy harvesting has traditionally focused primarily on spatial temperature differences and obvious heat sinks and sources, temperature fluctuations are another great source of thermal energy harvesting to consider with many characteristics that make it very interesting for industrial applications,” notes Anton Cottrill, a graduate student in MIT’s Department of Chemical Engineering who worked on the project.
The thermal resonator uses a tailored combination of thermal effusivity materials. The basic structure is a metal foam, made of copper or nickel, coated with a layer of graphene to provide greater thermal conductivity. The researchers infuse the foam with octadecane, a solid/liquid phase-change material that changes within a particular range of temperatures.
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“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. The combination of the three materials — metal foam, graphene and octadecane — makes it “the highest thermal effusivity material in the literature to date,” explains Michael Strano, a professor in the Department of Chemical Engineering who led the project.
When tested, a tiny sample of material produced 350 millivolts of potential and 1.3 milliwatts of power with a 10°C temperature difference between night and day — enough to power simple, small environmental sensors or communications systems. In addition, the high-thermal-effusivity material is extremely robust; it can withstand a variety of temperatures, weather conditions and physical forces, note the researchers. An article in Nature Communications contains more detail on their findings.
The technology is well suited for tailoring to specific applications, including tuning the material’s properties to enable harvesting heat from the on/off cycling of machinery, believe the researchers.
These temperature variations are “untapped energy,” stresses Cottrill, and could be a 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 hopes to use the system as a way of powering networks of sensors that monitor conditions at oil and gas drilling fields, for example.
The team also has been in contact with makers of wireless sensors and transmitters for monitoring plant processes. Other applications beckon, too. “Monitoring for the purposes of security and safety, for example, is an avenue of industrial interest as well,” says Cottrill.
“The next steps in developing the device are scale-up and interfacing with a rechargeable battery to meet the power requirements of a particular application. This will be a major focus over the next 6 months to 1 year,” says Cottrill.
