Lasers etch a 'perfect' solar energy absorber
rochester.edu
rochester.edu
> reduces heat dissipation at other wavelengths
Most of the time we use the "blackbody" approximation; and we're familiar with the idea that black objects radiate and absorb heat well, while white or shiny ones do not. This lets us make something that absorbs like a black object and emits like a white one. In effect its own surface is like a tiny greenhouse effect.
Until the object starts to glow red-hot, it will have low radiative losses. Once it gets that hot--maybe 3000K--it will radiate very well, until cooling enough to radiate mostly at the longer reflective wavelengths.
"Control over the absorption spectral range of surfaces is of major importance for a wide range of applications, such as selective solar absorbers, thermal emitters, structural colouring water condensation and daytime and night-time radiative cooling. In particular, for a solar-thermal energy absorber operating at high temperature, the absorber should be an SSA since the main cooling mechanism is thermal radiation... an ideal solar light absorber has nearly 100% absorbance within the solar spectrum and negligible thermal emittance within the blackbody radiation spectral range at mid-to-high temperatures (100–500 °C), i.e., an SSA. SSAs can thus maximise the temperature of solar absorbers and increase the efficiency of a heat engine driven by solar radiation."
[1] https://www.nature.com/articles/s41377-020-0242-y (full text paper)
So, it's useful for applications where you want a higher temperature than most solar heating applications now.
If you paired those materials, you could collect solar energy on the hot end, and radiate thermal energy into empty space on the cool end, and put a Stirling engine between them. Then you have a heat engine that does not dump its waste heat into the atmosphere. At the theoretical limit, that means your passively-radiating cool end can approach 3 K, instead of 300 K.
So you take a huge polished stainless steel dome, and etch the outside to be black at visible and near-infrared wavelengths, from 200 nm to 8000 nm, and you take some smaller domes, etch them to be black between 8000 nm and 14000 nm, and put them at the focal points of some parabolic reflectors, all in the shadow of the first dome, aimed at empty space. (8000 nm to 14000 nm is the "infrared window", where the atmosphere is mostly transparent to those wavelengths.)
https://www.sciencenews.org/article/device-harnesses-cold-ni...
(They just painted aluminium black...)
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EDIT: No, wait, that's a different team that the one I had in mind ? I seem to remember them using a complex material that specifically took advantage of this "infrared window" ??
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EDIT2 : Ok, same guy(s), slightly different device :
https://www.asme.org/topics-resources/content/new-solar-ener...
> A thin wafer of germanium had the right properties: It is fairly opaque at visible wavelengths, absorbing most incoming sunlight, while being generally transparent at the mid-infrared.
> Because most of the energy in the solar spectrum is in the visible and near-IR range, Fan said, germanium could capture solar energy for use in thermal or photovoltaic applications, while allowing mid-IR energy to escape for radiative cooling.
> The Stanford team tested the concept with an experimental device that placed a germanium wafer in front of a mid-infrared emitter.
> As reported in a recent paper in the journal Joule, the wafer absorbed enough sunlight to warm up by 24 degrees Celsius, while the emitter sent enough radiation through the infrared “window” to cool itself by 29 degrees Celsius below ambient temperature.
Laser-etching of normally reflective metals makes the material less complex. For one, you don't have to match thermal expansion coefficients any more.
The night-sky radiative cooling concept is thousands of years old-- https://en.wikipedia.org/wiki/Yakhchal --but we have better materials now. India and Persia made ice by filling shallow trays with water, insulating them underneath with straw, and exposing the water to a calm, clear, night sky.
I wish they would include comparison to tungsten painted black.
Could improve the efficiency of solar thermal power stations:
"Control over the absorption spectral range of surfaces is of major importance for a wide range of applications, such as selective solar absorbers, thermal emitters, structural colouring water condensation and daytime and night-time radiative cooling. In particular, for a solar-thermal energy absorber operating at high temperature, the absorber should be an SSA since the main cooling mechanism is thermal radiation... an ideal solar light absorber has nearly 100% absorbance within the solar spectrum and negligible thermal emittance within the blackbody radiation spectral range at mid-to-high temperatures (100–500 °C), i.e., an SSA. SSAs can thus maximise the temperature of solar absorbers and increase the efficiency of a heat engine driven by solar radiation."
[1] https://www.nature.com/articles/s41377-020-0242-y (full text paper)
Edit: Oh sad day, it seems the chocolate may have just been a demo of the etching and molding technology.
Either way I see a lot of value in this technology, even if it's just for novelty and a piece of that chocolate would cost 100€, I think there's still a significant market in the luxury segment.
Yeah... they should have stopped at car colors.