Ultra-white ceramic cools buildings with high reflectivity | Hacker News Reader

Ultra-white ceramic cools buildings with high reflectivity

newatlas.com

178 pointsthunderbong129 comments

This article is a fairly terrible summary, particularly when it comes to explaining the really neat thing about these coatings: It's not just that they reflect a lot of inbound light, they also radiate heat in a part of the infrared spectrum that can pass transparently through the atmosphere.

https://en.m.wikipedia.org/wiki/Passive_daytime_radiative_co...

Which is one of the major reasons that these specialized coatings are better than just white paint or mirrors or things like that. They can actually cool the structure to below ambient temperature.

This is because the blue sky is very "cold". Point an infrared thermometer up there and see... Very cold indeed.

Which usually objects can't fully exploit, because the heat they radiate away is absorbed by their immediate surroundings, which radiate part of it back towards them. You can "cheat" that by radiating away heat in a spectrum that isn't absorbed by anything, removing it from the area.

Basically. To be clear, it's not that you are worried about getting "the same" heat reflected or otherwise radiated back toward you. (The radiation emitted by your roof doesn't appreciably change the radiation incident on your roof from the atmosphere.) It's that if you are passively emitting at some frequency you are necessarily absorbing at that frequency too (by the 2nd law). And you don't want to be absorbing at the frequency where the atmosphere is opaque and hence warm. In contrast, it's fine to be absorbing at the frequency where the atmosphere is transparent, because then you're just exposed to cold outer space.

I had no idea what you were talking about until I remembered that infrared is just a Radio wave and therefore everything is an antenna.

Antennas that are good at transmit at X frequency tend to also be good at receiving X frequency necessarily. And different 'colors' are nothing more than different antenna resonance points.

Wow. I've never thought of heat in that way. Mind blowing.

You could use chimneys that are coated by the irradiative material on the inside, pointing all the IR up into the sky.

I had to look this up - apparently it's around 0C if you do this! https://mynasadata.larc.nasa.gov/sites/default/files/2018-12...

It would be interesting to put these out at night and have them drop a designated area below the dew point and collect water.

As I understand it, nighttime temps get near the dew point but often don’t hit it because water in the air releases so much heat.

Mold/algae will grow on the wet surfaces in a matter of days, destroying the main promise of the material.

Im not so sure. Mildew has to eat something. Assuming the surface is kept clean and water is collected regularly, it shouldn't be a major problem.

You don't see mildew growing on cars very often, do you? Yet, growing up in Florida, I'd see few covered cars in the morning all the time.

Cars not, they go to wash. Those who don't are indeed visibly dusty/moldy.

Wet roofs and other surfaces -- pretty much all the time, especially in humid climates (like Hong Kong, where the research is made). Same for glass windows (we have window-washer job for that reason).

Surfaces can be cleaned with pressure washer, but unlike purely aesthetical reasons, I'm saying that the main feature of this material may stop working in a matter of days due to even minuscule amount of growth.

At night they would do the opposite what you suggest. A very white surface would stay warmer, a black (in the sense of IR black) surface would radiate heat better and cool below dew.

Wouldn't a black surface be absorbing heat from above it?

The question then is, what heat? From the stars and moon is almost nothing. No idea what it exactly is, but assume that a clear sky at night has about -170 C temp (you are looking at the cold universe, only some CO2 in between to absorb). So the net radiation is always outbound.

And the best way to radiate are black (not the color but as in the definition of heat radiation) objects.

The question I was really asking was answered by this: https://physics.stackexchange.com/questions/159856/why-is-bl...

I.e. surfaces which are good at absorbing radiation are also good at emitting, which I didn't know.

And literally cool the planet. That ir that passes through the atmosphere is heading into space, at least in clear days.

Even the wikipedia page is confusing for someone who knows basic thermodynamics.

> PDRCs can be broadband in their thermal emittance capacity, meaning they possess high emittance in both the solar spectrum and atmospheric LWIR window (8 to 14 μm), or selective emitters, meaning they narrowband emit longwave infrared radiation only in the infrared window

It's not really that you want to avoid emitting thermal radiation per se at the frequencies where the atmosphere is opaque. It's that by the second law your thermal (i.e., passive) emissivity is equal to your absorptivity, and you want your absorptivity to be low in frequencies where the atmosphere is opaque and hence is warm. In other words, by basic thermo your passive behavior at each frequency is some value ranging between insulative (reflective) to absorbtive (transparent). If insulative, you will neither thermally emit nor absorb at that frequency. If absorbtive, you will both thermally emit and absorb at the frequency. So you want to be insulative at the frequencies where you would be thermally coupled to the warm atmosphere and you want to be conductive at the frequencies where you would be thermally coupled to cold outer space.

Most materials, aside from metals, of sufficient thickness have high high infrared emissivity. All white paints, for example, already did radiate heat upwards through the atmospheric window. These more reflective films are no more emissive than previous paints (they're all quite emissive to begin with!)

The only difference, optically speaking, really is that they are better solar reflectors.

The difference is that the wavelengths in which they reflect and emit are different. Where you emit, you absorb. White paint has a lower solar reflectance because it emits and absorbs across a broader band. The better PRDC coatings absorb/emit more tightly in the atmospheric IR window.

The youtuber NightHawkInLight made a video about a similar coating (he is using CaCO3 whereas this here is using Al2O3) https://www.youtube.com/watch?v=KDRnEm-B3AI

It was also discussed here https://news.ycombinator.com/item?id=36579995

Would be interesting to know how the performance compares.

Tech Ingredients youtube channel made something similar: https://www.youtube.com/watch?v=dNs_kNilSjk

The top comment there is actually by NightHawkInLight, so they seem to be having a friendly competition/mutual inspiration in pushing this DIY technology forward :)

EDIT: curious to see if Tech Ingredients' barium sulfate or NightHawkInLight's calcium carbonate will work out better in the long run. The latter looks a lot easier to manufacture though.

There’s been a ton of development in super reflective materials since the first demonstration of daytime radiative cooling at Stanford in 2014. Ultimately though, reflectivity does drop with dirt and pollution exposure over time, even for these kinds of materials. So most super reflective materials plateau at a lower number long term outdoors.

Also, just for reference, Spectralon, a sintered PTFE reflectance standard has had this level of solar reflectance for decades. So, in a sense, not that much new here. a sintered ceramic is gonna be expensive and will have a hard time competing with the simplicity of a paint based approach. Super white paints using various pigments have been well studied the last 5 years.

IMO simply being reflective is a dead end. Future materials will have to absorb wideband energy and actively emit it in an atmospheric bandgap to be effective enough to matter. I would think this would be an excellent application for quantum dots and have been sort of low-key waiting for an announcement or paper to drop for the last year or two.

We also need to reduce aircraft contrails. Contrails increase high-altitude clouds (cirrus and related), which blocks the heat that would have been lost by radiative cooling. It isn't hard to reduce contrails. We have the atmospheric data telling us the height of and where clouds are likely to form. Aircraft need to change altitude by 2000 ft from what was planned to avoid the cloud-forming regions.

research paper https://acp.copernicus.org/articles/19/8163/2019/

AI is helping reduce the problem https://blog.google/technology/ai/ai-airlines-contrails-clim...

Reducing contrails would cause more warming unless we have a ton of these coatings out there

That's not how clouds in the atmosphere work. Lower clouds reflect infrared back up and away from Earth. When large oceangoing vessels reduce their soot emissions, the formation of low altitude clouds declined letting more infrared in to the surface and increase the amount of heating in the lower atmosphere and surface of the earth. On the other hand, contrails hold heat in and prevent it from radiating out into space.

The "ideal" situation would be lots of low altitude clouds and no high-altitude clouds like Cirrus and contrails. This conjecture comes from the known property of low altitude clouds reflect the heat up and eliminated heat retention properties of high-altitude clouds.

For more detail:https://earthobservatory.nasa.gov/features/Clouds#:~:text=Lo....

We already pay to clean lots of buildings, and rain should help with that too, no? Seems like this would be a very solvable problem to me, but maybe I'm misunderstanding the problem fundamentally.

The less rain you get (i.e. during a drought) the more you tend to get dust in the air. We went through this last summer; my dog would kick up clouds of dirt dust running through our yard.

Worse than this, though, is the durability of the material. Lots of these super reflective paints don't hold up very well to rainwater (which itself is not especially clean or PH neutral) or seasonal extremes.

Typically, you wouldn't want to paint the sides of buildings, as that'll just reflect the light mostly down. You want it on rooftops, which most people don't pay to clean frequently or at all.

This makes sense; I guess it just becomes a new added expense with no major benefit aside from maybe saving on some cooling costs? Seems like a thing you'd be able to convince a company to buy into completely, but only once it's cheap and durable.

Maybe it's just dirty coal plants and the nearby streets, but rain seems to be the main reason why our windows get dirty.

You could add a lotus-effect coating, but given that window cleaners are still in business I assume that is quite expensive at scale.

>Ultimately though, reflectivity does drop with dirt and pollution exposure over time, even for these kinds of materials.

I guess this will be the new white washing?, like people used to do on buildings 100 years ago.

For regular buildings, the differences between 99.5%, 99.6%, etc. mostly matter to the supplier's Marketing Dept. and Sales Dept. (Vs. whoever's in charge of HVAC for the building it's installed on.)

Though some of the other properties - especially withstanding ~1,000 °C - make it sound pretty useful for military equipment which might be facing weapon-grade lasers or nuclear fireballs.

> For regular buildings, the differences between 99.5%, 99.6%, etc.

It's going to be <80% after a week in real life due to dust &co anyways

Reaction I: Point...but no way it'll be <80% after a week!

Reaction II: OTOH, the article says zilch about the stuff being dirt-repelling, or trivial to clean. And considering just how bad the air often gets, in some of the world's largest cities...

> the article says zilch about the stuff being dirt-repelling

They talk about "nanostructures", I read it as "dust traps"

Just looking at my window stills give me a good idea of how they'll look if they're anywhere remotely close to a road, and that's in a "clean" western european city

I was reading that in some humid areas they pipe out their grey water or fresh water and disperse it onto their roofs to combat radiant heat from the sun to keep homes cooler.

Can you give some references to this? I might have a place I could use this.

https://www.energy.gov/energysaver/radiant-cooling%0A%0A

https://handbook.ashrae.org/Handbooks/A15/IP/a15_ch54/a15_ch...

https://aircondlounge.com/is-radiant-cooling-efficient/

https://diy.stackexchange.com/questions/847/would-putting-a-...

There’s some YouTube videos also. Radiant cooling.

Installation method is going to make more difference than that too I’d have thought.

What the article needed was a comparison with 'normal' white paint, otherwise reporting 99.6% is not meaningful. The linked story https://newatlas.com/materials/super-white-paint-teflon-98-s... mentions that traditional paint reflects 85% of solar radiation.

85% -> 99.6% is an improvement, but not like a game-changer.

85% -> 99.6% is a ~35X reduction is absorbed solar energy. Definitely a game-changer. (Assuming it holds up in real-world use, etc.)

No, it isn't necessarily. 99.6% to 99.9999996% would be a "one million X" reduction in absorbed solar energy, but it would be of no consequence whatsoever because it would be well below the noise floor of any useful effect (see other people's comments about dust, etc.). The multiplicative factor of the reduction itself is not the relevant thing here. The real effect on a building would have to be measured.

yes, but when buildings painted white already get warm from sunlight, we can safely say that a ~35x reduction will be meaningful, as opposed to the example you provided where there will be no observable effect as you correctly pointed out.

Roofing your house in a fire prone area. Combined with hardyboard siding and you could survive quite a bit.

Roofing tile was the first thing I thought of as well. In Phoenix having a ceramic, heat tolerant, moldable, reflective material seems optimal for tiled roofs, or even as an addition to flat roofs.

I wonder if it can be used in spray applications like other ceramic coatings.

While I agree this is marketing stunts in general, sometime the difference between 99.5 and 99.6 reemission is 0.5 vs 0.4 percent absorption. so one way of seeing it the second one is 20% better than the first one at not heating the building.

This is often the case for percentages "close" to 100%, e.g: with LED efficiency, for powerful LEDs the problem is not the quantity of light emitted but dissipating the heat that may need active cooling, so what may look like a few percent improvement is luminosity may actually be a much stronger decrease of the size of active cooling,

Or semi-transparent mirrors in physics, where you really make a difference between 99.95 and 99.96% reflectance, because what you really look at the transmitance.

Yeah, I mean how does this material actually perform on a roof? Is there a measurable difference in the marginal gain of 95% to 99.6% when analyzing inside temperature of a house or energy reduction as a result?

There seems to be many factors when looking at the actual cause and effect especially when cost is introduced.

Ideally this could switch to ultra-black for the Winter months ;)

I mean you joke but there's no reason you couldn't.

There's an old solar heating method that does something similar. Build a large glass box with a wooden frame, and in the middle of that box place a bunch of bricks that have been painted black.

Place an air vent at the bottom and a tube out of the top, and route that tube into your house, and cover the inlet with a heavy sheet of plastic.

Place the entire contraption in the sunniest spot on your property. When the sunlight hits the bricks they will heat up, and the convection will cause an updraft.

Once the updraft pressure is high enough to lift the plastic, it will spray hot air into your home, and when it is too cold the plastic will stop a backdraft from drawing air out of your home.

Additionally, the bricks serve as thermal mass so the heat will continue for several hours after sundown.

You can improve the performance by replacing the open air inlet with a tube that goes to a colder place in your home so that the convection circulates the air in your house.

This is an amazing demonstration of low-technology. Is this documented anywhere with diagrams?

IDK, I read about it in an issue of Mother Earth News I think?

Found one of the ones I've read:

https://www.motherearthnews.com/sustainable-living/renewable...

I recently bought a property that has a cooling system similar to this. The difference in my case is that there are tunnels underneath the foundation containing rocks. Once the temp inside the house reaches a certain point, there are fans that blow the accumulated hot air in the “greenhouse” through those tunnels. It works, but I’m tearing it all out. It’s an eyesore and I rather have proper temp control.

This is the state of the art type of system for greenhouse heat retention, one that I dream of owning. I can't imagine how an underground system would be an eyesore, the ducts going to the roof?

The glass panels lean against the side of the house. It was built in the 80s and wasn't all that well implemented.

I wonder if e-ink technology can scale to large sized roof tiles.

I live in the north and I think it's remarkable that we don't use some form of white/black transitions or convertible awnings etc for regulating temperature.

Black is a better emitter of heat, so if your home is warmer than the surroundings, it will actually make you colder. That said, infrared radiation is a small contributor to cooling at those temperatures, good insulation is the biggest factor by far.

I kind of just wish we would skip it and put solar panels on everything.

https://www.frontiersin.org/articles/10.3389/fenvs.2014.0001...

"Solar panels reduce both global warming and urban heat island."

DIY version.

https://hackaday.com/2023/07/22/that-ultra-white-paint-that-...

I predict the rise of the "Roof Cleaner," as a vocation.

That stuff will need to be kept pristine.

That already exists though, I just paid a guy in a truck a bundle of money to clean my roof. Maybe this is mostly a thing in the PNW though?

Building exteriors are regularly cleaned all over the world. It's over a billion dollar industry in the US alone.

It would probably be even larger if more US roofs were pristine white instead of dark asphalt.

Yeah, but these will be a different breed. I know a number of folks that power-wash buildings and roofs. It's a cottage industry, hereabouts.

Definitely a PNW thing. The rain here causes moss and other problems that aren't prevalent back east.

I can't wait for more molten car incidents when it becomes even easier for curved designer buildings to focus massive amounts of light at random points on the ground.

This is diffuse reflection (rough), not specular like a mirror.

Diffuse reflections still scare roughly with cos of incidence angle, so diffuse-only panels arranged into parabolic or spherical shell would still focus light, even if much less efficiently.

I assume you're referring to the walkie-talkie building in London: https://globalnews.ca/news/1334377/car-melting-london-skyscr...

Already lots of groups doing solar panel cleanings. I imagine the venn diagram between people who invest in solar and in these roofs may have some overlap.

The iRobot Roofba!

Funny that in a lot of Sci-Fi of the non-dystopian type white and shiny materials often seem to be widely used. I suppose they made it like that due to white looking clean, sleek and welcoming, so it would be cool if it actually turned out that way.

It codes as "clean" and "controlled".

It also implied clean energy tech at a time when that was also mostly science fiction.

I've wanted blankets like this for my roof in the summer and the black alternative for winter. (I suppose my roof is black, but the equivalent for the exterior walls for my home)

I'm almost surprised this isnt already a thing.

Sounds like you want trees. In the summer they have leaves which provides shade to your roof. In the winter they lose their leaves and let the sun through. Best of all: its automated.

The gutter cleaning and leaf raking/mulching that results most certainly is not automated, lol.

You can install nearly-perfect gutter guards.

I've also personally just stopped raking leaves entirely. Where I want some grass in my front yard I'll mulch with my mower which is good for the grass, but otherwise I just leave the leaves alone and it results in a soft, typically non-muddy ground like a forest floor.

I'm not proselytizing this course of action, do as you please. But I think it's nice and something most don't consider.

I received an HOA fine for reading this comment.

You will receive a 75 dollar per day fine until the issue is resolved.

Yep. Pine needles are the worst. Gutter guards are a timesaver.

Compared to putting a blanket on the house, cleaning gutters sounds trivial.

Trees large enough to shade your home will tear up your foundation and house rodents (squirrels) that will nest in the walls and A/C units. Trees and pests will deposit leaves, branches and nuts on the roof which will clog gutters and rainspouts. In storms, trees will tear the building down.

If you want a building then remove the trees. If you want trees then create a park and put trees there. Separate the treed areas from the buildings with distance and/or (maintained) root barriers. Otherwise the trees et al will tear the buildings down.

I saw a 12-story high-rise last week whose foundation has been penetrated by "nice hardwood trees" they planted at the base 20-odd years ago.

Similarly buildings don't belong on beaches or on cliffs. The water/wind will always win. Camp on the beach, party on the cliff but never build on the beach or cliff.

This is a very dystopian approach to living.

Plenty of us live with a bit of nature nearby without destroying each other.

I live in a building that was constructed 92 years ago. There are plenty of trees around it and the building is doing just fine.

Yep. It's usually the wrong tree species, wrong placement, or poor maintenance that leads to problems. I had 100'+ pine trees in my hard. Beetles were somewhat of a problem but the woodpeckers were all over them and systemics helped.

Grew up in an house with 60' Australian willows that became too fragile and messy and were too close to a structure. Also, never get lemon trees because thorns from hell.

Big trees very close to a house? Not the best idea, you're always a storm away from serious damage to the house. Tree roots will also eventually damage any kind of surface around the house or house foundation. And trees with shallow roots are really probe to get knocked over in heavy wind.

There are different types of large trees with variations in structural strength of root systems that can be planted far enough away to not be a fire or mechanical hazard but close enough for shade. Minimizing surface roots by choosing the right species. Where I grew up, there was a storm with straight line winds 110 mph that took out every fence but only half of the trees and very few of the old trees.

why not solar panels?

> the black alternative for winter.

Vantablack maybe?

Though, you'd probably need good connections in the US DoD to arrange it. ;)

Anyone who isn't Anish Kapoor can use Black 3.0 instead since he refuses to share Vantablack.

https://www.culturehustleusa.com/products/black-3-0-the-worl...

Vantablack is only limited to Anish Kapoor for artwork. This isn't artwork. However, it's also finicky to apply, so Black 4.0 is much easier.

Black 2/3/4 all have problems with reflectance. Apparently Vantablack doesn't.

Haven't seen Vantablack in person, so not super sure. Black 2.0 and 3.0 I've used though, and wasn't super impressed.

I 3d print busts to paint and give to friends and family. If I put a few coats of black 3.0 on it gets very hot in the sun to the point it can melt the glue that holds the parts together (the plastic isnt thermoplastic so that's fine). I think it would work great for this purpose.

Cool. Might be workable after all. Well, if a special price can be figured out for bulk purchases. ;)

My dad lived in a trailer for a little while, and he painted the roof white one summer. It lowered the inside temperature by a few degrees - I don't recall the exact number, but enough that it was noticeable.

We have so much to do before "ultra-white" really matters;

Most buildings aren't white, or even close to white;

Streets -- and parking lots in many areas take up much more surface area, and they're mostly painted black, and they create heat islands. Phoenix changed the color slightly and saw great results https://www.fox10phoenix.com/news/is-phoenixs-cool-pavement-...

Extra passive cooling on a building has more immediate local effects, namely if my house is cooler I don't have to run the A/C as much. A/C is a _huge_ power suck, so avoiding even a little is huge.

These ceramic tiles were also discussed 2 days ago at https://news.ycombinator.com/item?id=38227486

Meanwhile, in places like Australia - councils regulate roof colours and there are laws against light and reflective roofs - dark roofs are actually forced upon.

how can they justify teh cost of cooling in the summer?

They don’t care.

Will it cause something like snow blindness if I look at it?

One bummer is that modern building codes ask for better insulating roofs. Think 15 cm mineral wool. Because of this it does not so much matter anymore how hot the roof becomes at the exterior.

I doubt if you earn back the investment.

I wonder if you could just use mirror tiles? What would be the reflectivity then?

I initially thought the same, but my current model is that the mirror would also have a very low emissivity so you can't passively cool (only not heat so much).

Standard mirrors are only ~98% reflective (https://en.wikipedia.org/wiki/Mirror#Reflectivity). For example, "infinity mirrors" will fade as the number of reflections increase (https://physics.stackexchange.com/questions/305329/are-infin...).

>But these coatings have their issues, including durability.

Was thinking if that would be a good solution for frying pankings made from dark bricks that's 50'C during summer, but probably not?

It might be a good coating for the inside of fires, kilns etc. though only for ones that are under the temperature rating.

How do these tiles compare to a green roof where there is some cooling via water evaporation through plants?

Wouldn't mold/algae grow on its surface, almost immediately disrupting its magic feature?

Interesting. Barium Sulfate (Spectraflect) was likely the previous record holder at 98%.

Depending on the wavelength, Spectralon (sintered PTFE powder) is a bit better. At least until it gets dirty.

But can I put it on my teeth?

Would ultra-black siding help warm my house appreciably in Minnesota winter?

Is this better than mirrors or space blankets?

Mylar will shred and is loud. Might as well do an aluminum roof if you don't mind the noise.

Mirrors will crack and can reflect sunlight in unwanted ways (directly into your neighbor's line of sight, into planes, etc.)

Right, I meant in terms of reflectivity.

in places like Taiwan - buildings tend to be covered with tile outside.

now it occurs to me - must be way for passive cooling.

Can you imagine how awful a city with buildungs made out of this material would be? It would be completely blinding.

I want this as a paint pigment please...

While this material is very exciting, wherever the reflection lands upon can cause real damage and is a problem.

It's not uncommon for developers of office buildings that use a lot of glass, get sued (and win) by neighbors because the light reflected from the glass building causes damage to their property.

In this case it's diffuse reflection, not a mirror, so it doesn't cause any burns, hot spots or similar stuff on near locations. It would just look like a very white surface.

I might be missing it, but where in the article do you see it diffuses the reflection?

The picture, where it looks white and not like a mirror.

It says it 'efficiently scatters almost the entire spectrum of sunlight'. Due to the scattering, the reflection is diffuse.

It's also inspired in a beetle that has a beautiful matte but very bright white shell: https://newatlas.com/beetle-scales-white/53789/

I'm curious to know how effective this becomes after your roof gets dirt & dust on it.

Having a "white" roof isn't a new thing. But most don't do it because they quickly become dirty and look bad.

An extreme case of this sort of thing:

https://www.nbcnews.com/sciencemain/london-skyscraper-can-me...

Reply on news.ycombinator.com