Better energy harvesting with ‘law-breaking’ device

When an object is exposed to sunlight, it absorbs energy from the sun’s rays and converts it into heat. However, there is a limit to how much the object can warm up. This is why a person lying on a beach won’t catch fire.

As objects or people absorb energy from the sun, they also emit energy in the form of heat. You may have felt this when walking past a block wall on a hot summer afternoon.

Kirchhoff’s law of thermal radiation has always explained the relationship between an object’s ability to absorb and emit energy. It states that absorptive and emissive efficiencies are equal at each wavelength and angle of incidence.

However, a new device developed by the lab of Harry Atwater challenges this law. The device breaks the tight relationship between absorbed and emitted efficiencies, which could have implications for sustainable energy harvesting systems and camouflage technology.

The research paper, titled “Direct Observation of Kirchhoff Thermal Radiation Law Violation,” was published in the journal Nature Photonics on July 24.

“For over 150 years, Kirchhoff’s law has been upheld, but this is the first experimental proof that the law can be broken,” says Atwater.

The lead author of the study, Komron Shayegan, explains that decoupling absorptivity and emissivity is important for energy-harvesting systems. By redirecting emitted radiation towards another energy-harvesting object, higher energy conversion efficiencies can be achieved.

The device developed by the researchers combines a material with a strong magnetic-field response and a patterned structure to enhance absorption and emission in infrared wavelengths. The effect of breaking Kirchhoff’s law can be observed by heating the device above room temperature and comparing its emissive and absorptive efficiencies.

Co-authors of the paper include researchers from Stanford University, the University of Houston, and the Liquid Sunlight Alliance.

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