Breaking the century-old law! New device will harvest energy better
If you put an object in the sunlight, it will start to heat up. This is because it absorbs energy from sunlight and converts that energy into heat. If you leave the object outside, it will continue to get hotter, but only up to a certain point. After all, people lying on the beach in the sun don't catch fire.
As an object (or person) absorbs energy (sunlight), it also releases energy (infrared radiation or heat). For example, this is what you experience on a summer afternoon when you walk past a wall and feel the heat it emits.
The ability of an object to absorb and emit energy in the form of electromagnetic radiation - the link between its absorption and emission efficiencies is known as Kirchhoff's law of thermal radiation. (Kirchhoff's law of thermal radiation)". This law is a concept developed by Gustav Kirchhoff in 1860, which states that at every wavelength and angle of incidence, the absorption and emission efficiencies are equal.
Now, a new device developed in the lab of Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science, breaks the tight relationship that usually exists between an object's absorption and emission efficiencies. The invention could also have important implications for the development of sustainable energy harvesting systems and certain camouflages.
The paper describing this work, "Direct Observation of Kirchhoff Thermal Radiation Law Violation," was published July 24 in the journal Nature Photonics. Nature Photonics.
"Kirchhoff's law has held true for more than 150 years, and although theoretical violations of the law have been suggested before, this is the first time it has been experimentally demonstrated that the law can be broken."
Komron Shayegan, the first author of this new study and a graduate student in electrical engineering, further explains, "Equivalence as specified by Kirchhoff's law has always been a guiding principle for designing devices that absorb and emit energy in the form of radiation because by designing and measuring the absorption properties of a material, we have free access to the emission properties. Recently, however, there has been a new twist in the design of emitter/absorbers-namely, we are trying to go beyond the simple one-to-one equivalence between an object's emissivity and absorptivity."
"One motivation for decoupling the two is in energy harvesting systems. For example, if an energy-harvesting object, such as a photovoltaic (solar panel), re-emits some of the energy it absorbs back into the energy source (the sun) as heat, then humans lose that energy. Theoretically, if PV or other energy harvesting objects re-radiate the absorbed radiation from the energy source to another energy harvesting object, higher energy conversion efficiencies can be achieved."
"Our research shows that it is possible to break the equality of Kirchhoff's law of thermal radiation by placing a device in a moderate magnetic field. The device itself combines a material with a strong magnetic field response and a patterned structure that enhances absorption and emission at infrared wavelengths. What is particularly exciting is that we were able to observe this effect simply by heating the device above room temperature and directly comparing the emission and absorption efficiencies."
Schematic of magnetically tunable emission and absorption of a GMR structure coupled to n-InAs, and comparison of zero-field absorptivity and emissivity.
Violation of Kirchhoff's law in absorptivity and emissivity measurements.
Co-authors of the study include Souvik Biswas, formerly of Caltech and now of Stanford University; Bo Zhao of the University of Houston; Shanhui Fan of Stanford University; and Harry Atwater, Otis Booth Leadership Chair in Engineering and Applied Sciences.
Reference link:
[1]https://phys.org/news/2023-07-energy-harvesting-law-breaking-device.html
[2]https://www.nature.com/articles/s41566-023-01261-6
[3] https://divisions.caltech.edu/newspage-index/better-energy-harvesting-with-law-breaking-device
