by josh perry, editor
[email protected]

researchers at the massachusetts institute of technology (mit) in cambridge, mass. have developed a new thermal resonator that uses the swings in ambient temperature that occur in the change from day to night to produce electricity.

the team’s test device, which has been deployed on the roof of an mit building for several months, was used to prove the principle behind their new energy-harvesting concept. (justin raymond/mit)

according to a report from the mit website, this technology has the potential to create continuous remote sensing systems that would not need batteries or outside power sources, among other possible applications.

the resonator is small enough to sit on a desk and researchers insist that it can take advantage of an “untapped source of energy” from temperature fluctuations in the ambient, although the power produced is “modest” for the time being.

“the advantage of the thermal resonator is that it does not need direct sunlight; it generates energy from ambient temperature changes, even in the shade,” the article explained. “that means it is unaffected by short-term changes in cloud cover, wind conditions, or other environmental conditions, and can be located anywhere that’s convenient — even underneath a solar panel, in perpetual shadow, where it could even allow the solar panel to be more efficient by drawing away waste heat.”

to produce electricity from these fluctuations in temperature, the researchers focused on the property of thermal effusivity, which is how well a material draws and releases heat from its surroundings. to avoid properties with high thermal conductivity but low capacity, they engineered a metal foam made of copper or nickel that is then coated with graphene and infused with a phase-change material, octadecane.

“a sample of the material made to test the concept showed that, simply in response to a 10-degree-celsius temperature difference between night and day, the tiny sample of material produced 350 millivolts of potential and 1.3 milliwatts of power — enough to power simple, small environmental sensors or communications systems,” the article continued.

as the heat transfers from one side of the material to the other, one side is always lagging behind, which creates a temperature gradient necessary for thermoelectricity. researchers will study whether or not this material can also draw heat from sources such as the cycling of motors or machinery.

the research was recently published in nature communications. the abstract read:

“materials science has made progress in maximizing or minimizing the thermal conductivity of materials; however, the thermal effusivity—related to the product of conductivity and capacity—has received limited attention, despite its importance in the coupling of thermal energy to the environment.

“herein, we design materials that maximize the thermal effusivity by impregnating copper and nickel foams with conformal, chemical-vapor-deposited graphene and octadecane as a phase change material. these materials are ideal for ambient energy harvesting in the form of what we call thermal resonators to generate persistent electrical power from thermal fluctuations over large ranges of frequencies.

“theory and experiment demonstrate that the harvestable power for these devices is proportional to the thermal effusivity of the dominant thermal mass.

“to illustrate, we measure persistent energy harvesting from diurnal frequencies, extracting as high as 350 mv and 1.3 mw from approximately 10 °c diurnal temperature differences.”