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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 v

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.)

BlueTemplar 7y ago
Yeah they should team up with the guys that made the "anti-solar panel" : https://www.sciencenews.org/article/device-harnesses-cold-night-sky-generate-electricity-dark https://www.sciencenews.org/article/device-harnesses-cold-ni... (They just painted aluminium black...) ---- 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" ?? ---- EDIT2 : Ok, same guy(s), slightly different device : https://www.asme.org/topics-resources/content/new-solar-energy-device-beams-heat-into-space 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.
- logfromblammo 7y ago
  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 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.

BlueTemplar 7y ago

Yeah they should team up with the guys that made the "anti-solar panel" : https://www.sciencenews.org/article/device-harnesses-cold-night-sky-generate-electricity-dark https://www.sciencenews.org/article/device-harnesses-cold-ni... (They just painted aluminium black...) ---- 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" ?? ---- EDIT2 : Ok, same guy(s), slightly different device : https://www.asme.org/topics-resources/content/new-solar-energy-device-beams-heat-into-space 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.

logfromblammo 7y ago
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 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.