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> cannot cool a surface below the ambient temperature. Only active cooling can cool anything below the ambient temperature. 100% reflectivity can prevent it ab

> cannot cool a surface below the ambient temperature. Only active cooling can cool anything below the ambient temperature. 100% reflectivity can prevent it absorbing more heat, if you can achieve it, but it doesn't cool anything down. > “Barium sulfate has reflective properties that allow us to hold below ambient temperature,” said Felicelli That's a bit more like it: 3/4 of the way down the page, someone finally points out that paint can't cool anything, it can only stop it warming up.

jcims 4y ago
>Only active cooling can cool anything below the ambient temperature. Not exactly: https://en.wikipedia.org/wiki/Radiative_cooling https://en.wikipedia.org/wiki/Radiative_cooling Couple of videos explaining the effect https://www.youtube.com/watch?v=N3bJnKmeNJY https://www.youtube.com/watch?v=N3bJnKmeNJY https://www.youtube.com/watch?v=5zW9_ztTiw8 https://www.youtube.com/watch?v=5zW9_ztTiw8
orev 4y ago
An object in a room at ambient temperature would slowly warm up to equal the same temperature from the conduction of the warmer air around it. If the temperature of an object can “hold below ambient”, it means it must be able to allow the object to shed heat energy, at least at the same rate or faster than it absorbs the ambient heat. I.E. cooling.
dahart 4y ago
What’s the difference in practice, in summer weather? If the ambient temp is 30C in the morning and heats up to 40C in the afternoon, for example, then a paint that keeps things at 30 could replace active cooling on a red house with a brown roof whose surfaces exposed to the sun might even heat up to 50. It seems fair to call paint that effectively reflects the sun’s heat a “cooling” method, because it’s cooling relative to what you’d have otherwise, right?
planede 4y ago
> Only active cooling can cool anything below the ambient temperature. It's more complex than that. Through thermal radiation objects on the surface of Earth are also thermodynamically connected to the surface of the Sun (~6000K) and Space (~4K). The way to cool below ambient temperature with passive cooling is to isolate from the Sun, radiation from nearby objects and and conducting heat from surrounding air as much as possible, while making the thermal connection to Space as strong as possible. 1. How to isolate from the Sun: surface reflective in the spectrum where the thermal radiation from the Sun is the strongest, which is not too surprisingly the visible light spectrum (hence this article) 2. How to isolate from surrounding objects: insulation against conduction + surface reflective in the spectrum where the surrounding objects' thermal radiation is strongest (somewhere in infrared). 3. Thermal connection with space: you want to emit in the infrared spectrum as strong as possible. 2 and 3 are seemingly in contradiction, as a surface can't be a good reflector and good emitter at the same wavelength at the same time. But just have a geometry where the surface satisfying point 2 is pointed towards surrounding objects, while the surface satisfying point 3 radiates towards space.

jcims 4y ago

>Only active cooling can cool anything below the ambient temperature. Not exactly: https://en.wikipedia.org/wiki/Radiative_cooling https://en.wikipedia.org/wiki/Radiative_cooling Couple of videos explaining the effect https://www.youtube.com/watch?v=N3bJnKmeNJY https://www.youtube.com/watch?v=N3bJnKmeNJY https://www.youtube.com/watch?v=5zW9_ztTiw8 https://www.youtube.com/watch?v=5zW9_ztTiw8

orev 4y ago

An object in a room at ambient temperature would slowly warm up to equal the same temperature from the conduction of the warmer air around it. If the temperature of an object can “hold below ambient”, it means it must be able to allow the object to shed heat energy, at least at the same rate or faster than it absorbs the ambient heat. I.E. cooling.

dahart 4y ago

What’s the difference in practice, in summer weather? If the ambient temp is 30C in the morning and heats up to 40C in the afternoon, for example, then a paint that keeps things at 30 could replace active cooling on a red house with a brown roof whose surfaces exposed to the sun might even heat up to 50. It seems fair to call paint that effectively reflects the sun’s heat a “cooling” method, because it’s cooling relative to what you’d have otherwise, right?

planede 4y ago

> Only active cooling can cool anything below the ambient temperature. It's more complex than that. Through thermal radiation objects on the surface of Earth are also thermodynamically connected to the surface of the Sun (~6000K) and Space (~4K). The way to cool below ambient temperature with passive cooling is to isolate from the Sun, radiation from nearby objects and and conducting heat from surrounding air as much as possible, while making the thermal connection to Space as strong as possible. 1. How to isolate from the Sun: surface reflective in the spectrum where the thermal radiation from the Sun is the strongest, which is not too surprisingly the visible light spectrum (hence this article) 2. How to isolate from surrounding objects: insulation against conduction + surface reflective in the spectrum where the surrounding objects' thermal radiation is strongest (somewhere in infrared). 3. Thermal connection with space: you want to emit in the infrared spectrum as strong as possible. 2 and 3 are seemingly in contradiction, as a surface can't be a good reflector and good emitter at the same wavelength at the same time. But just have a geometry where the surface satisfying point 2 is pointed towards surrounding objects, while the surface satisfying point 3 radiates towards space.