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.
Or absorbs at visible light frequencies and reflects at lower thermal infrared frequencies.
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.
This is of course a neat tech demo, but I don't really see the application. Thermal electrical generation seems to be crap compared to photovoltaics and for just heating stuff up, black paint must be orders of magnitude cheaper, even if it might not be a "perfect" absorber of sunlight.
Materials that are perfectly black in visible and near-infrared light, and mirrors in thermal infrared would be naturally warm. Materials that are mirrors in visible light and perfectly black in far infrared would be naturally cool.
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.)
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 :
> 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.
That's what I was thinking of. They laminated a reflective material under a thin sheet that had a particular absorption spectrum, with a thermally conductive bond, and put fused silica on top of it to protect against weather.
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'd hazard that this would be an obvious "first application" for this technique. If they can improve the absorption efficiency by 130%, that translates to reducing the number of mirrors by something like 25%, so a fairly significant cost saving. Alternately, if you can capture and store that energy without increasing costs too much you end up with a more efficient power plant per unit-area of land. Sounds good to me!
Various potential applications in biomedical, environmental, and energy fields.[1]
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."
Yes, any application requiring direct heat avoids electric heater losses. Alternatively, using this to generate electricity runs up against thermal electric losses.
I was ready to debunk your comment, but then remembered that existing solar thermal panels are at ~90% efficient already. Is it somehow easier / more efficient to build a thermoelectric generator from a metal, vs existing flat-plate + liquid collectors?
Solar thermal panels might be wonderful for heating up water or buildings, where the target temperature is fairly low. However, as the target temperature goes up, the target starts radiating that heat energy back out, which is where this innovation could be useful.
So, it's useful for applications where you want a higher temperature than most solar heating applications now.
In mediterranean countries hot water is regularly heated with rooftop tanks / heaters. This could mean higher temperatures achieved with that relatively easily.
To be clear - selective solar absorbers that absorb the solar spectrum well and have low infrared emissivity (so they don't radiate heat well), are a mature concept and can be achieved by multilayer coatings pretty well. This is a nice manufacturing process and could also be a competitive approach compared to conventional thin film deposition. However, I don't think the specific performance is dramatically different from things that are used in commercial solar thermal systems.
Various potential applications in biomedical, environmental, and energy fields.[1]
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."
Maybe not! The mold in the first picture is made by Kaupert, a company specializing in chocolate molds. Of course they might mean that Kaupert made the base mold and Morphotonix added the holograph ...
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.
The Article also talks about hydrophilic and hydrophobic patterns using the same setup. I would be interested in seeing the properties of a channel or via through the tungsten etched with these patterns. It might change the thermal absorption rate of water passing through the hole or some other neat behavior like a change to the capillary effect. This could improve concentrated solar applications even further.
> "...etching a full-color photograph of a family into the refrigerator door; or proposing with a gold engagement ring that matches the color of your fiancee’s blue eyes."
> 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.