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Ultrawhite BaSO4 Paints and Films for Daytime Radiative Cooling
- rbanffy 5y agoCould this high reflectivity work for trans reflective display backgrounds?
- imglorp 5y agoWhy are 99% of US roofs still black? Residential, commercial, they're all some variation of black tar. It seems almost cheating to simply change their reflectivity for a huge cooling power savings.
- georgyo 5y agoWhere are you located? In NYC we use black tar to seal the roof, but then it is painted silver or white. You almost never see an actual black or dark colored roof. https://maps.app.goo.gl/HwyKfFLkyFuKdWPt5 https://maps.app.goo.gl/HwyKfFLkyFuKdWPt5
- potiuper 5y agoTar is black. (Zinc-)Titanium dioxide paint is an extra expense; using barium sulfate is crazy as barium is toxic. Problem of dirt. The money would be better spent on alternative sources of power besides fossil fuels.
- Turing_Machine 5y agoSoluble barium compounds are toxic. Barium sulfate is nearly insoluble and has very low toxicity. People routinely drink the stuff as a radiocontrast agent before getting an x-ray.
- potiuper 5y agoAnd many of those roofing tiles will be burned at disposal, or in a house fire; Barium sulfide is poisonous - see carbothermal reduction.
- Turing_Machine 5y agoVast amounts of barium sulfate are used to whiten paper, which is often burned, and no one worries about that.
- pfdietz 5y agoIsn't that titanium dioxide?
- adrian_b 5y agoTitanium dioxide a.k.a. titanium white is a different white pigment. Most white paints used nowadays contain titanium white. Nevertheless, many white plastics contain other white pigments instead of titanium white. Titanium white itself is inert, but when it is exposed to the sun light it can catalyze the destruction of organic substances like plastics or paint binders that have not been selected to be resistant to this. So when using titanium white outdoors, the white paint must be specified as UV resistant. I have not heard about paper that uses titanium white. That might be due to the higher cost of titanium white, which might be too much compared to the cost of paper.
- h2odragon 5y agoSheet metal roofing is gaining ground in my area (USA Southeast). There's a little catalog of colors available from every supplier, they all cost the same so just pick one! ... most people seem to like dark red, dark brown, and black; still. I cannot comprehend it. 15 years ago I put a white, metal roof on my house, my summer cooling costs are half or less my neighbors' with the shingled roofs, and they've replaced their shingles in that time frame. The crop duster aircraft also use my house as a navigation marker and we get to see them frequently.
- zbrozek 5y agoMy town put a limit on the reflectivity of structures, efficiency be damned.
- ben_w 5y agoReflectivity (metallic) or albedo (diffuse)?
- zbrozek 5y agoBoth, but only with usefully-detailed specification on albedo.
- h2odragon 5y agothey may have reason: https://www.atlasobscura.com/articles/when-buildings-attack-melted-cars-ruined-art-and-other-troubles-with-solar-convergence https://www.atlasobscura.com/articles/when-buildings-attack-...
- deleted 5y ago[deleted]
- denimnerd42 5y agoI don't think most people are going to retrofit metal roofs onto asphalt shingle houses because the house are 30+ years old and no one is sure how long they are going to live there and what the house is going to be worth. A metal roof on a million dollar house is no big deal but on 200-300k structure with unknown lifespan to the owner? hardly makes sense. Same cost benefit as solar panels really. The benefit is over 15-20+ years usually where many people can't even justify spending a point on their mortgage because they'll be out in less than 6 on average. I'm struggling with the same decision right now. I have a small (2k sqft is small in this area) 300k structure and I don't know whether it will be worth lot value in 10-15 years or still worth 300k+inflation+value additions. The surrounding neighborhood with 1-2million dollar structures says "lot value" even if it's pristinely maintained.
- mikepurvis 5y agoFarther north in Ontario, the heating season is way longer than the cooling one, so it does kind of make sense to optimize for heat retention. The other side is that having a roof which warms itself in winter is better for reducing snow accumulation. Though that’s also less of a consideration for a slippery metal roof.
- zbrozek 5y agoLow sun angle in the winter vs summer means a bit more thought needs to go in to making the decision of whether net retention or net reflection is preferable. My strategy is to cover the entire roof in solar panels and to try to find useful things to do with the electricity. Sadly that does not send much power back into space, and my panels are only 22% efficient. Plus, in practice, you'll want a ton of insulation between the interior space and the roof anyway. So if you've done a good job there it's kind of a moot point wrt/interior comfort. But sending that power back into space rather than absorbing it can reduce thermal cycle stress on the roof and keep the outdoor space nearby cooler.
- sedachv 5y ago> Farther north in Ontario, the heating season is way longer than the cooling one, so it does kind of make sense to optimize for heat retention. The other side is that having a roof which warms itself in winter is better for reducing snow accumulation. So much wrong here. Asphalt shingles neither "retain heat" nor "warm themselves." You really do not want either property in a roof covering in a climate where it snows because that will cause the formation of ice dams. If you have snow on your roof, it obviously covers the black shingles - now you have a white roof anyway. If it is winter and you do not have snow on your roof, it is still not worth it - solar irradiance at 50° latitude (approximately Timmins, Ontario) is going to be about a factor of 7 less in January than in June. You are much better off with a white roof in northern Canada to reduce peak heat stress in the summer, which will continue to get worse and worse due to greenhouse gasses.
- eloff 5y agoYes, plus while every Canadian home has heating, not all homes have air conditioning. Optimizing the roof for the summer makes a lot of sense. We had a steep roof in the Okanagan which rose about 20ft to the peak. That extra room was problematic in winter for heating but a life saver in the hot, sunny summer. The loft was unbearable during the summer. A neighbor with a low roof would run a sprinkler on his roof all summer long, that's how bad the heat would get.
- swayvil 5y agoSpeaking as a person presently engaged in roof repair. Not just black. Dark, fractulate, nigh-furry extremely light-absorbing black. It's like they designed it to soak up sun and get hot. So hot that it starts to melt. So hot that you need to use special nails so they don't pull through the half-melted black goo. Anything would be an improvement. I like white painted steel.
- denimnerd42 5y agolook up SRI shingles. They are asphalt composite shingles with reflectivity granules. Still not as good as a metal roof but much cheaper.
- PragmaticPulp 5y agoCommercial buildings have white membranes on the roof. They’re flat so you can’t see it from the ground. Residential buildings usually have sloped roofs visible from the ground so the roof color is an architectural choice. Depending on the locale, current building codes can require enough roof insulation that very little heat flows through the roof in either direction. The insulation has an added benefit of preventing heat loss in the winter.
- TYFYC 5y agoBarium Sulphate is nothing new. It's used widely on integrating spheres, and I have painted some myself. The video that they show painting it on a piece of glass and comparing that to commercial paint is very misleading. It is not the easiest thing to work with, and is not at all durable. Once on a surface, it dries to a white chalky texture and just touching it will get it dirty and diminish its reflective properties. You would have to have so many additives to make it weatherproof and durable enough for outside that it would end up being no better than standard white paint. When I used it to coat an integrating sphere, it had to be thinned with 100% alcohol (impossible to obtain unless your a pharmacist because all non-drinking alcohol is poisoned so you can't get drunk without paying a liquor tax) so that all the thinning agent would dry out without leaving any residue behind. Anything left in the Barium Sulphate will diminish its reflectivity.
- jbotz 5y ago> all non-drinking alcohol is poisoned ...in the USA. In many (most?) other countries pure ethanol can be bought easily over the counter, and where I live even at any gas station. Yes, you could drink the ethanol from the gas station here (it has no additives, but better dilute it first), but nobody does because the cheapest cachaça isn't any more expensive.
- TYFYC 5y agoIt's a peeve of mine. One of those inconvenient truths of history that people don't like to talk about or don't know about. We (the US government) started poisoning alcohol WITHOUT TELLING PEOPLE. The fact that the practice continues is ridiculous.
- fmajid 5y agoMy father worked for Saudi petrochemical company SABIC in the 80s. Some bright spark decided to go to the ethanol factory and get some high purity hooch from a tap and make an end run around Saudi alcohol restrictions. Unfortunately he made a wrong turn and went to the methanol factory instead…
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- mrweasel 5y agoIsn't that going to be absolutely blinding, especially when wet? In Denmark some cities already have rules about glazed tiles, disallowing its use in many areas because it blinds neighbors and potentially traffic.
- mrfusion 5y agoIt would so cool to make shirts and hats with this. Has anyone looked into that?
- dylan604 5y agoDoes it get even whiter by using a fabric detergent with a whitening agent? ;-) Then again, will these white fabrics still glow blue in blacklight since they don't need these whitening agents? Pure white in blacklight would be cool.
- mrfusion 5y agoCould this work on tennis courts or too relectove?
- blincoln 5y agoI wonder how Teflon (PTFE) compares to this stuff. In the DIY multispectral photography community, a lot of people use it as a reference material because not only is it pure white to our eyes, it also reflects (diffusely) UV-A, UV-B, and SWIR very well. I found a chart and paper[1] showing that a proprietary variation of it called Tetratex continued that theme well into the SWIR range, but unfortunately the authors didn't measure it into the MWIR or LWIR ranges. It's easiest to find at the hardware store as very thin tape for wrapping pipe threads, but I bought a few bars of it (about 5mm x 30mm x 300mm) on eBay years ago, so I know it comes in a form that would be more convenient for making into roofing or siding material. I don't know how it holds up to weather, though. According to that paper, it needs to be at least 0.5mm thick to reflect longer wavelengths, so anyone who's curious about testing it should find the thicker stuff like I did. [1] https://www.researchgate.net/figure/Reflection-coefficient-for-PTFE-tapes-as-a-function-of-wavelength-The-insert-shows-a_fig7_254060766 https://www.researchgate.net/figure/Reflection-coefficient-f...
- fhdch 5y agoTLDR But for teflon to passively cool, it would have to be strongly absorbent in the IR. The way it works is this: the sun’s rays are at 5500 K. So you pick the set of materials that reflect light in the visible gap (the atmosphere takes care of the rest). This keeps the material from getting heat. Then, from this set, you pick a material that is strongly absorbent in IR. If it absorbs in IR, itll emit there too, there y cooling itself
- URSpider94 5y agoSpecifically, it needs to absorb/emit in a frequency range called the “sky window”, where the atmosphere is transparent. In this range, the object is basically seeing the cold of space at a few Kelvin, so that almost no energy is absorbed and a lot is emitted. The trick is to design a system where the total power absorbed across the solar irradiance spectrum is less than the power that is emitted in the sky window, so that the system is a net power sink (in this case to the tune of 100W per sq. meter or so).
- nate_meurer 5y ago
- exabrial 5y agoSorry dumb question... but I though _black_ was the best radiator (black body radiation and all that)... what am I missing?
- deleted 5y ago[deleted]
- gbrown 5y agoCame here with the same question, but there’s a good answer elsewhere in the thread that it’s about wavelengths: reflect where the sun is bright, and among those materials find one that is the most absorbent/emissive in the IR.
- saba2008 5y agoThe sun. Black is both best radiator and best absorber. So it works perfectly in space (where you minimize incoming energy by turning it perpendicular to sun), and not so good on surface. This material works by being reflective in spectrum, where most of sunlight energy is (which makes it white, as visible spectrum ~= sunlight on earth surface), and being 'black' (good radiator and absorber) in other spectra.
- scythe 5y agoAbsorption spectra and emission spectra are identical for thermodynamic reasons. But emissions happen in the infrared and absorption from sunlight happens at visible frequencies. So if you reflect visible light and absorb infrared light, you can stay cool in the Sun. Conversely, applying ultra-thin metal coatings to carbon-black objects can make them reflect infrared and absorb visible light, which is ideal for solar-thermal collectors.
- supernova87a 5y agoWhy don't we cover roofs with those retroreflective beads embedded in pavement paint? Wouldn't that reflect even more?
- HPsquared 5y agoRetroreflectors focus their reflection specifically in the direction of the light sources using reflection and/or refraction, which involves some losses. I'd expect retroreflectors to sacrifice total brightness/emissivity in favour of directionality. Probably better to just have a reflective surface (mirror finish perhaps?)