4 ms·

DIY radiative cooling paint from YouTuber NightHawkInLight - https://www.youtube.com/watch?v=N3bJnKmeNJY&list=PL1a2HkcVbmAWExiWT__qQypLEwkvijnIM https://www.you

by jcims 10mo ago

DIY radiative cooling paint from YouTuber NightHawkInLight - https://www.youtube.com/watch?v=N3bJnKmeNJY&list=PL1a2HkcVbmAWExiWT__qQypLEwkvijnIM https://www.youtube.com/watch?v=N3bJnKmeNJY&list=PL1a2HkcVbm... It has pretty impressive performance. Tech Ingredients did one or two vids as well - https://www.youtube.com/watch?v=dNs_kNilSjk https://www.youtube.com/watch?v=dNs_kNilSjk Was thinking of whipping up a batch for my rv.

kumarvvr 10mo ago
This concept is used all over India to cool down homes that are on the top floors. https://www.amazon.in/EXCEL-CoolCoat%C2%AE-Reflective-Coating-Summer/dp/B09BFZ67XH?th=1 https://www.amazon.in/EXCEL-CoolCoat%C2%AE-Reflective-Coatin... Basically, have a highly reflective white coat on your roof, to reduce temperatures by about 3 Degrees Celsius. Almost all homes in Urban India are made from concrete and bricks, which can hold a lot of heat. I myself have been in houses that use this to cover only some rooms of the house (mainly the bedroom), and the temperature difference is definitely noticeable. It also makes the room livable in the extreme hot summers in India.
- kragen 10mo ago
  This is the opposite. It says, "Refelects [sic] 90% of solar infrared rays," because of its "High IR reflective Pigments [sic]," so its emissivity in the infrared is 0.1, but the IR-selective paints we're talking about here are optimized for high infrared emissivity, which means they absorb a lot of infrared. Maybe there's some wiggle room here because solar infrared is mostly near IR and MWIR, and the place where we want high emissivity (absorptivity) is longwave IR, but to the extent that the advertisement makes any claims about infrared emissivity, it claims very low infrared emissivity, not high. A paint with low emissivity across the spectrum will slow down the temperature rise when the sun is up, but also slow down the temperature drop when the sun is down. This can still make rooms livable, but it isn't the same as what you get with regular whitewash, where the temperature of the roof is actually lower than the temperature of the air around it.
  - jcims 10mo ago
    It kind of blew my mind when I first learned about this whole phenomenon (mostly from the YouTube series I posted). Not all white paints are equal and it’s kind of interesting to think that something that looks mostly identical to our eyes has very different (passive) properties in the infrared. I think one of the things in the paints that Ben adds is a set of microspheres that reject incident incoming infrared beyond a certain angle but allow it to pass through when radiated. Something like that.
    - kragen 10mo ago
      He usually garbles the scientific theory in his videos, but I trust that he's honestly reporting his experiments, and that his theoretical errors are honest mistakes rather than intentional attempts to mislead. You should be aware that there are rigorous constraints on how much absorptance can differ from emittance, known as Kirchhoff's Law of Thermal Radiation: https://en.wikipedia.org/wiki/Emissivity#Absorptance https://en.wikipedia.org/wiki/Emissivity#Absorptance because without them you could get mechanical power generation from a uniform bath of thermal radiation, which would give you a perpetual-motion machine.
  - schiffern 10mo ago
    To be fair it does say "reflects solar infrared rays," which I would interpret as "the IR from the Sun" (aka NIR). The product datasheet[0] claims a thermal emittance (aka LWIR) of 0.82. Having such a high value is typical for non-metallic surfaces.[1] [0] https://5.imimg.com/data5/CA/RO/MY-653008/excel-cool-coat.pdf https://5.imimg.com/data5/CA/RO/MY-653008/excel-cool-coat.pd... [1] https://ntrs.nasa.gov/api/citations/19840015630/downloads/19840015630.pdf https://ntrs.nasa.gov/api/citations/19840015630/downloads/19...
    - kragen 10mo ago
      Thank you for finding this! I was wrong, and I'm pleased that the product is in fact what the great-grandparent claimed. And the NASA publication is very helpful for putting this in context.
kragen 10mo ago
IIRC, the papers they're working from mention that lime works very nearly as well as the baryta they're using. Guess what people have been painting their houses white with for several thousand years?
- marcosdumay 10mo ago
  Not with the optimal mixture for maximum-packing of limestone nanospheres. That effect is almost not perceptible in normal milled limestone.
  - kragen 10mo ago
    It would be very surprising if that were true, given the LWIR emittances in the NASA paper cited in https://news.ycombinator.com/item?id=46212591 https://news.ycombinator.com/item?id=46212591. I totally believe that you can get significant percentage improvements with the right mix of particle sizes, but the LWIR emittance of just about all nonmetals was upwards of 75% the emittance of an ideal blackbody. That puts a ceiling of about ⅓ (4:3) on the possible improvement on that end. And plain old whitewash has a reflectivity upwards of 90% in the visible, while the best known materials, such as polished silver, are something like 96%, so you might get 3:2 on that end—but probably won't even get 2:1. https://nvlpubs.nist.gov/nistpubs/bulletin/07/nbsbulletinv7n2p197_A2b.pdf https://nvlpubs.nist.gov/nistpubs/bulletin/07/nbsbulletinv7n... is one older reference on this, but https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=903 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=90... gives plots of silver mirrors' reflectance by wavelength, polarization, and angle of incidence. As a side note, what people have been painting their houses with for thousands of years isn't normal milled limestone; it's slaked lime, which forms limestone by absorbing CO₂ from the air over about a month. Modern whitewash has milled limestone mixed into it, but the morphology of the final surface isn't very similar to milled limestone in a transparent binder.

by jcims 10mo ago

kumarvvr 10mo ago

This concept is used all over India to cool down homes that are on the top floors. https://www.amazon.in/EXCEL-CoolCoat%C2%AE-Reflective-Coating-Summer/dp/B09BFZ67XH?th=1 https://www.amazon.in/EXCEL-CoolCoat%C2%AE-Reflective-Coatin... Basically, have a highly reflective white coat on your roof, to reduce temperatures by about 3 Degrees Celsius. Almost all homes in Urban India are made from concrete and bricks, which can hold a lot of heat. I myself have been in houses that use this to cover only some rooms of the house (mainly the bedroom), and the temperature difference is definitely noticeable. It also makes the room livable in the extreme hot summers in India.

kragen 10mo ago
This is the opposite. It says, "Refelects [sic] 90% of solar infrared rays," because of its "High IR reflective Pigments [sic]," so its emissivity in the infrared is 0.1, but the IR-selective paints we're talking about here are optimized for high infrared emissivity, which means they absorb a lot of infrared. Maybe there's some wiggle room here because solar infrared is mostly near IR and MWIR, and the place where we want high emissivity (absorptivity) is longwave IR, but to the extent that the advertisement makes any claims about infrared emissivity, it claims very low infrared emissivity, not high. A paint with low emissivity across the spectrum will slow down the temperature rise when the sun is up, but also slow down the temperature drop when the sun is down. This can still make rooms livable, but it isn't the same as what you get with regular whitewash, where the temperature of the roof is actually lower than the temperature of the air around it.
- jcims 10mo ago
  It kind of blew my mind when I first learned about this whole phenomenon (mostly from the YouTube series I posted). Not all white paints are equal and it’s kind of interesting to think that something that looks mostly identical to our eyes has very different (passive) properties in the infrared. I think one of the things in the paints that Ben adds is a set of microspheres that reject incident incoming infrared beyond a certain angle but allow it to pass through when radiated. Something like that.
  - kragen 10mo ago
    He usually garbles the scientific theory in his videos, but I trust that he's honestly reporting his experiments, and that his theoretical errors are honest mistakes rather than intentional attempts to mislead. You should be aware that there are rigorous constraints on how much absorptance can differ from emittance, known as Kirchhoff's Law of Thermal Radiation: https://en.wikipedia.org/wiki/Emissivity#Absorptance https://en.wikipedia.org/wiki/Emissivity#Absorptance because without them you could get mechanical power generation from a uniform bath of thermal radiation, which would give you a perpetual-motion machine.
- schiffern 10mo ago
  To be fair it does say "reflects solar infrared rays," which I would interpret as "the IR from the Sun" (aka NIR). The product datasheet[0] claims a thermal emittance (aka LWIR) of 0.82. Having such a high value is typical for non-metallic surfaces.[1] [0] https://5.imimg.com/data5/CA/RO/MY-653008/excel-cool-coat.pdf https://5.imimg.com/data5/CA/RO/MY-653008/excel-cool-coat.pd... [1] https://ntrs.nasa.gov/api/citations/19840015630/downloads/19840015630.pdf https://ntrs.nasa.gov/api/citations/19840015630/downloads/19...
  - kragen 10mo ago
    Thank you for finding this! I was wrong, and I'm pleased that the product is in fact what the great-grandparent claimed. And the NASA publication is very helpful for putting this in context.

kragen 10mo ago

IIRC, the papers they're working from mention that lime works very nearly as well as the baryta they're using. Guess what people have been painting their houses white with for several thousand years?

marcosdumay 10mo ago
Not with the optimal mixture for maximum-packing of limestone nanospheres. That effect is almost not perceptible in normal milled limestone.
- kragen 10mo ago
  It would be very surprising if that were true, given the LWIR emittances in the NASA paper cited in https://news.ycombinator.com/item?id=46212591 https://news.ycombinator.com/item?id=46212591. I totally believe that you can get significant percentage improvements with the right mix of particle sizes, but the LWIR emittance of just about all nonmetals was upwards of 75% the emittance of an ideal blackbody. That puts a ceiling of about ⅓ (4:3) on the possible improvement on that end. And plain old whitewash has a reflectivity upwards of 90% in the visible, while the best known materials, such as polished silver, are something like 96%, so you might get 3:2 on that end—but probably won't even get 2:1. https://nvlpubs.nist.gov/nistpubs/bulletin/07/nbsbulletinv7n2p197_A2b.pdf https://nvlpubs.nist.gov/nistpubs/bulletin/07/nbsbulletinv7n... is one older reference on this, but https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=903 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=90... gives plots of silver mirrors' reflectance by wavelength, polarization, and angle of incidence. As a side note, what people have been painting their houses with for thousands of years isn't normal milled limestone; it's slaked lime, which forms limestone by absorbing CO₂ from the air over about a month. Modern whitewash has milled limestone mixed into it, but the morphology of the final surface isn't very similar to milled limestone in a transparent binder.