[HN Gopher] Ultrawhite BaSO4 Paints and Films for Daytime Radiat...
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Ultrawhite BaSO4 Paints and Films for Daytime Radiative Cooling
Author : jbotz
Score : 57 points
Date : 2021-09-18 11:43 UTC (11 hours ago)
(HTM) web link (pubs.acs.org)
(TXT) w3m dump (pubs.acs.org)
| TYFYC wrote:
| Barium 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.
| zokier wrote:
| That's too dismissive take of the paper. The whole innovation
| here is the formulation of BaSO4 based acrylic paint that
| retains most of the reflective properties while being practical
| as a paint instead of using pure BaSO4 which would be too
| fragile.
|
| Select quotes:
|
| > Overall, the BaSO4 film reached a solar reflectance of 97.6%
| and an emissivity of 0.96 in the sky window (Figure 2b). Its
| solar reflectance is significantly higher than that of the
| commercial white paint (400 mm thickness), especially in the UV
| and NIR ranges. [...] For the BaSO4 paint, a standalone paint
| sample of 400 mm reached similar optical properties (98.1%
| solar reflectance, 0.95 sky window emissivity)
|
| > To demonstrate the reliability of our BaSO4 paint, we
| conducted abrasion test, outdoor weathering, and viscosity
| characterizations. [...] The BaSO4 paint reached a wear index
| of 150, comparable to the commercial exterior paint with a wear
| index of 104. The weathering test was conducted by exposing the
| BaSO4 paint outdoors for 3 weeks (Figure 4b). The solar
| reflectance remained the same within the experimental
| uncertainty.
| yunohn wrote:
| 100% this.
|
| I cringe every time a HN comment starts with "this is nothing
| new; not ground breaking; people should've expected this".
|
| So much for a discussion forum where you're meant to read the
| post and spark insightful commentary.
| mrfusion wrote:
| Restore balance to the force, you must.
| swayvil wrote:
| Would zinc oxide or titanium dioxide be a better option? They
| use that white in everything. Might be durable on a roof.
| ben_w wrote:
| Possibly, possibly not. One thing they really like with this
| is that it radiates heat photons in wavelengths that go
| through the sky rather than being reflected back to the
| ground by the greenhouse effect.
| jbotz wrote:
| > 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 cachaca isn't any more
| expensive.
| tgsovlerkhgsel wrote:
| Definitely not the case in the EU.
|
| You can buy it, but it'll be taxed like drinking alcohol.
|
| However, I'm surprised that other alcohols (e.g. isopropanol)
| that don't suffer from this cannot be used instead of
| ethanol.
| TYFYC wrote:
| It'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 wrote:
| My 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...
| KaiserPro wrote:
| SAme for the UK, it should have some percentage of methanol
| or some sort of difficult to remove bittering agent
| yarcob wrote:
| I can't talk for all countries, but in Austria the cheapest
| alcohol you can buy is "Spiritus", and they add something to
| make it taste bitter. You can use it as fuel, for degreasing,
| cleaning, etc.
|
| You can get pure, food grade ethanol at the pharmacy.
| yunohn wrote:
| > Barium Sulphate is nothing new (...) It is not the easiest
| thing to work with, and is not at all durable
|
| I only read the abstract, but it seems like they've considered
| your stated limitations.
|
| "we experimentally demonstrate remarkable full-daytime
| subambient cooling performance with both BaSO4 nanoparticle
| films and BaSO4 nanocomposite paints"
|
| "providing great reliability, convenient paint form, ease of
| use, and compatibility with the commercial paint fabrication
| process."
|
| Also, you can always get commercial licenses for restricted
| substances like ethanol; that's not an actual problem for house
| paint jobs.
| BugsJustFindMe wrote:
| > _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)_
|
| Only ethanol needs to be denatured. You can easily get 99.953%
| pure isopropyl on Amazon, and it isn't poisoned because
| isopropanol is already poison.
| foobarian wrote:
| I mean ethanol is denatured by adding methanol, which is also
| an alcohol, so technically the mixture is still 100% alcohol
| :-) Except for drawing water due to being hydrophilic etc.
| BugsJustFindMe wrote:
| I think sometimes they use acetone.
| [deleted]
| mrweasel wrote:
| Isn'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 wrote:
| It would so cool to make shirts and hats with this. Has anyone
| looked into that?
| dylan604 wrote:
| Does 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 wrote:
| Could this work on tennis courts or too relectove?
| supernova87a wrote:
| Why don't we cover roofs with those retroreflective beads
| embedded in pavement paint? Wouldn't that reflect even more?
| HPsquared wrote:
| Retroreflectors 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?)
| blincoln wrote:
| I 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-f...
| fhdch wrote:
| TLDR
|
| 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 wrote:
| Specifically, 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 wrote:
| > _I don 't know how it holds up to weather, though._
|
| It does very well. The main terminal of Denver International
| Airport has an enormous PTFE-coated fiberglass fabric roof.
| It's decades old now and has weathered hail, high winds, and
| Colorado sun without any significant wear that I know of.
|
| EDIT:
|
| The same product has since been used in a bunch of other
| stadiums and other large structures, including the Georgia Dome
| where it survived a near-miss by a tornado with only minor
| tears:
|
| https://spinoff.nasa.gov/Spinoff2009/ip_2.html
|
| > _" The material is pound-for-pound stronger than steel while
| weighing less than 5 ounces per square foot. It offers up to 24
| percent solar translucency while providing as much as 75
| percent solar reflectance"_
| [deleted]
| exabrial wrote:
| Sorry dumb question... but I though _black_ was the best radiator
| (black body radiation and all that)... what am I missing?
| scythe wrote:
| Absorption 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.
| gbrown wrote:
| Came 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 wrote:
| The 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.
| [deleted]
| rbanffy wrote:
| Could this high reflectivity work for trans reflective display
| backgrounds?
| imglorp wrote:
| Why 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.
| swayvil wrote:
| Speaking 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.
| georgyo wrote:
| Where 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
| beef234 wrote:
| I live in Florida and see black and dark brown tar shingles
| installed at least 95% of the time. I even live in a
| hurricane-prone area and it's surprisingly rare to see a
| metal roof installed.
| dylan604 wrote:
| Where in Florida is NOT hurricane-prone?
| jsilence wrote:
| "I live in Florida..."
| mikepurvis wrote:
| 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. Though that's
| also less of a consideration for a slippery metal roof.
| zbrozek wrote:
| Low 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 wrote:
| > 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 50deg 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 wrote:
| Yes, 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.
| potiuper wrote:
| Tar 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 wrote:
| Soluble 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 wrote:
| And many of those roofing tiles will be burned at disposal,
| or in a house fire; Barium sulfide is poisonous - see
| carbothermal reduction.
| adrian_b wrote:
| Even if some of the sulfate will be reduced to sulfide,
| making the barium soluble, the sulfide will be oxidized
| again by the air after some time, immobilizing any
| remaining barium.
|
| The barium that might have leached will be precipitated
| by the carbon dioxide from the air into barium carbonate.
|
| Any form of soluble barium left in the environment will
| end soon as either barium sulfate or barium carbonate,
| which are insoluble, thus non-poisonous.
|
| Barium is one of the least dangerous metals with high
| atomic number. You have to do something really stupid and
| deliberate with it in order to be able to poison anyone.
| Turing_Machine wrote:
| Vast amounts of barium sulfate are used to whiten paper,
| which is often burned, and no one worries about that.
| pfdietz wrote:
| Isn't that titanium dioxide?
| adrian_b wrote:
| Titanium 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.
| PragmaticPulp wrote:
| Commercial 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.
| denimnerd42 wrote:
| look up SRI shingles. They are asphalt composite shingles with
| reflectivity granules. Still not as good as a metal roof but
| much cheaper.
| h2odragon wrote:
| Sheet 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.
| dylan604 wrote:
| How loud is it in your house when it rains/hails? I have a
| metal awning covering my back porch, and it is painfully loud
| during hail (granted not that frequently). Does the attic
| insulation help deaden/dampen the sound?
| zbrozek wrote:
| I have a metal roof and it's no louder than the shingle
| roof in my childhood home. I think the big difference vs
| your awning is that my roof is fully supported by the
| home's sheathing and is screwed down in many places. This
| limits the opportunity for the metal to resonate.
| h2odragon wrote:
| It's noticeable; not much louder than a shingle etc roof
| tho because we have a "R9" fiberglass insulation layer
| between the roof studs and the metal. A retrofit roof would
| have roof decking to help with that, and you could probably
| arrange similar insulation too.
|
| Its nowhere near as loud as an aluminum skinned mobile
| home.
| zbrozek wrote:
| My town put a limit on the reflectivity of structures,
| efficiency be damned.
| ben_w wrote:
| Reflectivity (metallic) or albedo (diffuse)?
| zbrozek wrote:
| Both, but only with usefully-detailed specification on
| albedo.
| h2odragon wrote:
| they may have reason:
| https://www.atlasobscura.com/articles/when-buildings-
| attack-...
| [deleted]
| baybal2 wrote:
| I'm surprised that steel roof is not the cheapest option in
| most of US, and in the rest of the world.
|
| Shingles are almost always more expensive because of
| laborious installation.
| denimnerd42 wrote:
| I 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.
| zdragnar wrote:
| Older rural houses are getting new metal roofs out here all
| the time. It's actually a really common trend out this way.
|
| It is one of those upgrades you can add to a house that
| actually retains its value when you go to resell... Most
| metal roofs have lifespans that are minimum double that of
| shingles and, per your example, solar panels.
|
| Of course, most people buying in the countryside aren't
| intending on moving out in fewer than 6 years; 7 minimum is
| generally considered the break even point around here.
| h2odragon wrote:
| I'm talking mobile homes and an area with an average home
| value of under $100k. One of the big selling points of
| metal roofing here is durability, i can point to another
| neighbor (3mi down the road) whose house was built sometime
| in the 30s but is now a more valuable redneck shotgun shack
| because of the (burgundy) metal roof. Edit: come to think
| of it, it may have been tin roofing originally; it had been
| shingled before the metal tho.
|
| It sounds like your area is one where the location and the
| market determine value far more than anything _on_ the
| land, so treat it as "build to suit" commercial property
| and assume anything built on it is disposable.
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