Beating the heat: These plant-based iridescent films stay cool in the sun

Cellulose is sustainable, biocompatible, and ideal for radiative cooling applications.

See full article...

 

[SixDegrees]

That's just great! Not only does it work - but it's actually manufacturable and can be retrofitted to existing structures. What's there not to like? The remaining questions are durability and how quickly the performance decays when it gets dirty (also, how hard it is to clean).

It actually isn't clear at all that it can be easily manufactured - or that it has all the other qualities, like robustness and durability, that are necessary in the marketplace, as you note.

 

[henryhbk]

Living in Arizona, I'd love to see it used on my cars. If I could get in the car in the middle of the day and not burn the hell out of my hands and legs I'd be in heaven!

Well shades in parking lots are a huge help. In Kuwait every parking lot has shades over the spaces (in some places like our airport they put PV arrays on top to power parking lot lighting in the evenings. Works remarkably well. Hell the 8.5kw solar array we mounted on our roof acts as a giant parasol to our house (just being 6” above the roof dropped the temperature of the top floor by 2 degrees) plus of course on days of maximum sun is when AC is most needed and it basically covers that load.

 

[henryhbk]

That's just great! Not only does it work - but it's actually manufacturable and can be retrofitted to existing structures. What's there not to like? The remaining questions are durability and how quickly the performance decays when it gets dirty (also, how hard it is to clean).

Not to mention making something durable to be outside standing still vs. durable enough to be on a car are totally different things.

 

[henryhbk]

Perhaps you could exploit the lower angle winter sun shines at – the way solar walls and roof overhangs have already been done for millennia – have the roof angled north so it reflects or radiates away in the summer, with a big south‑facing window or solar wall with a big summer‑shielding overhang passively heating up in winter. Add some 'clever' attic/cellar door openings depending on the season for draft. The roof coating would just augment it more in the summer.

If this was your roofing material it’s not radiating away the heat inside your house, rather the roof getting heated itself. Yes in snowy conditions that’s nice to help clear the roof since as the roof heats under the snow it radiates that heat up and melts the snow.

 

[CrystalCowboy]

... It's funny (read: bleak as hell) that 20 years ago, you wanted south facing windows to catch daylight...

Not in my northern hemisphere four-season climate. The wisdom going back decades (at least) was to have south-facing windows with an overhang, so that in the winter they got direct light, and in the summer they didn't.

Also, artists prefer northern light because it is indirect and diffuse.

 

[The Lurker Beneath]

I've been thinking of expanding my house in my lot. It's funny (read: bleak as hell) that 20 years ago, you wanted south facing windows to catch daylight, but now I can only think of keeping windows to the north and trying to set myself up for solar to the south. I'll trade 50W in LED lighting for kW of cooling, thanks.

Wow, global warming over the last 20 years must have been pretty bad where you are.

 

[Wheels Of Confusion]

Don't traditional passive radiative cooling materials (barium sulfate, calcium carbonate, etc.) work best by reflecting IR in frequencies of the narrow "window" bands that lets it pass through the atmosphere without being absorbed by water vapor, CO2, etc? Do these films work in a similar fashion with optical crystals instead of pigments?

 

[Mechjaz]

Wow, global warming over the last 20 years must have been pretty bad where you are.

It is? I'm not sure if you're trying to poke at me, pretending that I "have global warming especially badly/worse than others" or what, but it used to snow 2-3 times a winter, and now we're lucky to get once. It's been 100° F in September, and had month-long runs where the temperature does not go below 80° even at night. It was not like this even within my memory, so yes, it is really bad where I am. It's really bad everywhere, though I believe it's more noticeable in four-season climates like mine, where summer has become six months and winter has become six weeks.

I'm around 40° N.

 

[jazzylarry]

Lol 20 f'ing years ago and it's still not common :|

There is a reason.

$$s. That windshield is about $1800 of them....

EDIT: I should note that the heads up display windshield is about the same....

 

[brightsafflicted]

Some bacteria like to eat cellulose, so I wonder about the long-term stability of such materials, especially in outdoor applications.

 

[Emon]

This reminds me of a recent Tech Ingredients video about making paint that both allows radiative cooling but prevents radiative heating:


View: https://youtu.be/dNs_kNilSjkb


It's beating commercial paints and is made of very common ingredients, with the only downside seeming to be the many layers (and drying time) required for application.

I highly recommend this channel to anyone around here. Any questions you have will probably be answered by the end of the video, this guy is thorough

 

[DovePig]

Do these films work in a similar fashion with optical crystals instead of pigments?

I was wondering that previously and it turns out, yes indeed (going by the first link in the article). With the added bonus that these are both coloured (pigments would normally offset any "gains" from radiative heat transfer), have high emissivity in that narrow 8‑13 μm atmospheric window and high reflectivity in all the other wavelengths (while still exhibiting solid structural colour by not just absorbing some wavelengths like pigments do). Something that's hard or even impossible to achieve with traditional materials, leaving radiative cooling just for nighttime (and previous daytime radiative cooling photonic structure coatings weren't colourful or were more expensive).

 

[Frodo Douchebaggins]

There is a reason.

$$s. That windshield is about $1800 of them....

EDIT: I should note that the heads up display windshield is about the same....

I'd pay it tbh.

Also I do wonder what's different about a HUD windshield (I have a HUD and know it's a much more expensive piece of glass but haven't yet googled to find out why)

edit: Finally googled it. It's polarized from the factory I guess.

 

[graylshaped]

I would say this is an amazing technology, but Nature beat us to it.

 

[Robin-3]

As someone who lives in a country with cold winters and hot summers, these kinds of passive systems that can only do one thing, namely cool surfaces down or heat them up, aren't really suitable for anything here. You don't want to radiate any excess heat at all during winters, you want to keep it all, and during summer you want to radiate as much of it as possible, so you'd need something that can be turned on/off or otherwise prevented from operating.

It's not uncommon for temps to hit -30°C (-22°F for Muricans), but we also hit +35°C (+95°F) every summer, so that's a pretty big range in temperatures. I wanna see someone come up with some good solution that works for both ends.

I was thinking along these lines (Ottawan here - not quite as extreme as what you've got but close).

If this was available as roofing material, during the winter I assume the snow would keep it from working (due to the clear-sky aspect). Walls would be trickier, I imagine - I wonder if there's a way to engineer so that cold-weather contraction would alter the structure just enough to negate the cooling effect?

("Sure, they should be able to do that!" say I, who am not at all a materials scientist.)

 

[niftykev]

Ceramic or crystalline tint on all the glass (including the windshield… you can get very-nearly-clear films that dramatically cut solar energy) will make an enormous difference for that. On my last car I skipped the windshield film because I was a little skeptical of the claims, but when I'd be driving with the sun to my left I wouldn't feel it hitting me at all through the driver-side glass (50% visual light transmission film… so not particularly dark though visibility tinted from the outside), then I'd make a left hand turn and suddenly feel the heat beating down on me… not making that mistake again. When I get this car tinted next week, the windshield is happening too.




That + an EV which can precondition the cabin is a big win, and the AC doesn't have to work as hard when driving, either.

It absolutely fucking baffles me why pretreated glass with better heat rejection isn't a factory option from manufacturers, at least on the higher end.

Can I get that on my house window? My office windows are basically floor to ceiling windows that take up easily 90% of that entire wall of the room. And it faces east, with minimal shade. It gets super warm even when not gaming.

 

[Green RT]

For those leaving cars parked in the hot sun...
A car cover makes a dramatic difference. It takes just a few seconds to put on and take off, but it keeps the interior much more comfortable. It is far more effective than the window shades people use and it take not much more time to deploy and store.

Edit: Added 'not much'. It is a little more trouble than window shades but worth the extra effort.

 

[Wheels Of Confusion]

For those leaving cars parked in the hot sun...
A car cover makes a dramatic difference. It takes just a few seconds to put on and take off, but it keeps the interior much more comfortable. It is far more effective than the window shades people use and it take not much more time to deploy and store.

Edit: Added 'not much'. It is a little more trouble than window shades but worth the extra effort.

It's true. A large part of the heating of a car comes from the metal car body getting saturated and radiating heat inwards. Parking in the shade when possible is a cheap way to avoid this. Unfortunately, in the US too many parking lots have zero shade, or only a few stunted crepe myrtles that everyone else has already sniped by the time you get there.

Fucking Hell, what I wouldn't give for a national wave of "Let's put solar panels over the parking spots, it's win-win" initiatives.

 

[Mechjaz]

Well shades in parking lots are a huge help. In Kuwait every parking lot has shades over the spaces (in some places like our airport they put PV arrays on top to power parking lot lighting in the evenings. Works remarkably well. Hell the 8.5kw solar array we mounted on our roof acts as a giant parasol to our house (just being 6” above the roof dropped the temperature of the top floor by 2 degrees) plus of course on days of maximum sun is when AC is most needed and it basically covers that load.

This is the arrangement I covet most of all. I have a single story, and not a very tall one at that, but the attic gets deadly hot. It's a terrible stress on the structure of the roof and the heat pump to try to keep things reasonable in the house.

Last night I installed a new gable fan though, I think & hope that's going to be a huge return on my investment. The last one was from the 60's; I don't know how long ago it died, but after a brief heat pump outage last summer, I know that a single active method to cool the house isn't enough.

If nothing else, it was fun to take all my experience building PCs and seal off all the turbulent crevices and backdrafts.

Strangely related, I have a 200mm case fan from who knows where and when working as a gable fan in my shed. It doesn't do much to change out 1000 cu ft of hot air, but it's not hurting.

 

[Kyuu]

This is the arrangement I covet most of all. I have a single story, and not a very tall one at that, but the attic gets deadly hot. It's a terrible stress on the structure of the roof and the heat pump to try to keep things reasonable in the house.

Last night I installed a new gable fan though, I think & hope that's going to be a huge return on my investment. The last one was from the 60's; I don't know how long ago it died, but after a brief heat pump outage last summer, I know that a single active method to cool the house isn't enough.

If nothing else, it was fun to take all my experience building PCs and seal off all the turbulent crevices and backdrafts.

Strangely related, I have a 200mm case fan from who knows where and when working as a gable fan in my shed. It doesn't do much to change out 1000 cu ft of hot air, but it's not hurting.

If by gable fan you mean a fan moving air out of the attic, I'm sorry to tell you but that's a waste of energy and can be actively harmful.

If you'd like to know more, I recommend this article on Energy Vanguard, probably the best source for building science on the web.

 

[Frodo Douchebaggins]

Can I get that on my house window? My office windows are basically floor to ceiling windows that take up easily 90% of that entire wall of the room. And it faces east, with minimal shade. It gets super warm even when not gaming.

Don't see why not? I'd suspect the film doesn't care what sort of glass it's applied to.

 

[Frodo Douchebaggins]

For those leaving cars parked in the hot sun...
A car cover makes a dramatic difference. It takes just a few seconds to put on and take off, but it keeps the interior much more comfortable. It is far more effective than the window shades people use and it take not much more time to deploy and store.

Edit: Added 'not much'. It is a little more trouble than window shades but worth the extra effort.

Can be rough on the paint though, especially with regular use and anything less than perfect care with cleaning the car and the cover.

 

[Mechjaz]

If by gable fan you mean a fan moving air out of the attic, I'm sorry to tell you but that's a waste of energy and can be actively harmful.

If you'd like to know more, I recommend this article on Energy Vanguard, probably the best source for building science on the web.

Damnit. It's hard to imagine that pulling cooler air (90° vs 140°) from the soffit vents through the attic is bad, but damnit. I also have a giant whole house fan that I use occasionally to forcibly draft air through windows up into the attic. I'll follow up when it heats up for the year - it's been strangely cool so far.

I do have AC, as mentioned, and additional insulation to roll out to prevent my ceiling from getting hot. I hope to replace the roof with a white or natural metal roof in the next 1-2 years.

In my defense, the second bullet has nothing to do with me, and the hope is to lower the temp of the attic such that heat doesn't roll down into the house. (Yes, I know how that sounds, but I've opened my attic access in the dead of summer and felt a column of hot air force itself down into house where it's located.)

To overcome the problems I listed at the beginning of this article, you’d need a situation where the following are true:
The house doesn’t have air conditioning, or the AC is turned off when the attic ventilator runs.
The house has no natural draft water heaters, furnaces, or boilers.

 

[Pink Daisy]

I've been thinking of expanding my house in my lot. It's funny (read: bleak as hell) that 20 years ago, you wanted south facing windows to catch daylight, but now I can only think of keeping windows to the north and trying to set myself up for solar to the south. I'll trade 50W in LED lighting for kW of cooling, thanks.

Not so bleak as you might think. The big difference here isn't global warming but the switch from incandescent light to LED light. 20 years ago if you blocked off the daylight you put in even more energy to light the place with a glowing hot wire. LEDs will do the job with less energy than the sun shines in.

And if you want to let daylight in you can use IR blocking glass that keeps out much more of the heat than older glass, in addition to the old technique of large overhangs that shade the windows in the hottest parts of the day.

 

[Pink Daisy]

As someone who lives in a country with cold winters and hot summers, these kinds of passive systems that can only do one thing, namely cool surfaces down or heat them up, aren't really suitable for anything here. You don't want to radiate any excess heat at all during winters, you want to keep it all, and during summer you want to radiate as much of it as possible, so you'd need something that can be turned on/off or otherwise prevented from operating.

It's not uncommon for temps to hit -30°C (-22°F for Muricans), but we also hit +35°C (+95°F) every summer, so that's a pretty big range in temperatures. I wanna see someone come up with some good solution that works for both ends.

At -30 the best bet is a cold roof with snow on top. And once it's covered in snow this stuff won't make much difference because there's no particular radiation to reflect. It may emit some additional energy into the snow so keeping the roof cold to avoid ice dams might be even more important. If you commonly get a bare roof at -30 due to anything other than way too little insulation, well, you have my sympathy.

 

[jimsteph]

Well shades in parking lots are a huge help. In Kuwait every parking lot has shades over the spaces (in some places like our airport they put PV arrays on top to power parking lot lighting in the evenings. Works remarkably well. Hell the 8.5kw solar array we mounted on our roof acts as a giant parasol to our house (just being 6” above the roof dropped the temperature of the top floor by 2 degrees) plus of course on days of maximum sun is when AC is most needed and it basically covers that load.

I agree, but only about 1/3 of the parking spots in the company-provided lot are covered, and you have to get in a whole lot earlier than I'm willing to do in order to claim one. At my previous job I lobbied for years to get covered parking, and they still don't have it:

Unfortunately, in the US too many parking lots have zero shade, or only a few stunted crepe myrtles that everyone else has already sniped by the time you get there.

My friends call this the Arizona Shade Imperative: the compulsion to skip the mid-range parking spots (a close parking spot trumps all) to search for ones that have just a smidgeon of shade from a wayward palo verde branch.

Fucking Hell, what I wouldn't give for a national wave of "Let's put solar panels over the parking spots, it's win-win" initiatives.

You, sir, have my vote!

 

[CrystalCowboy]

This is the second Ars article in a week on structural color.

The common theme seems to be "structural color is neat. Here are wildly impractical ideas about how this could translate into commercial products." I conclude that it must be grant renewal time.

 

[Bernardo Verda]

Perhaps you could exploit the lower angle winter sun shines at – the way solar walls and roof overhangs have already been done for millennia – have the roof angled north so it reflects or radiates away in the summer, with a big south‑facing window or solar wall with a big summer‑shielding overhang passively heating up in winter. Add some 'clever' attic/cellar door openings depending on the season for draft. The roof coating would just augment it more in the summer.

My current apartment is actually worse (too hot) in late spring and early fall, than in mid-summer, because there's not enough overhang. Mid summer is much more comfortable, because the (south-facing) sliding door and windows on the balcony (and the balcony too, for that matter) get more shade for a greater part of the day.

 

[MickeB]

As someone who lives in a country with cold winters and hot summers, these kinds of passive systems that can only do one thing, namely cool surfaces down or heat them up, aren't really suitable for anything here. You don't want to radiate any excess heat at all during winters, you want to keep it all, and during summer you want to radiate as much of it as possible, so you'd need something that can be turned on/off or otherwise prevented from operating.

It's not uncommon for temps to hit -30°C (-22°F for Muricans), but we also hit +35°C (+95°F) every summer, so that's a pretty big range in temperatures. I wanna see someone come up with some good solution that works for both ends.

Some 6-8 years ago there was a different startup (Stanford related?) with a similar idea. Not sure what happened to the company, they haven't updated their website in several years.

Either way, they seemed to have given some thought to this very issue, and explicitly stated that their target customers would be places like data centers, or facilities with industrial sized freezers. In short, places where there already are hard working heat pumps dumping heat out of the building, that are basically running 24/7/365.
The thermal film would them be placed on aluminum frames with channels for coolant, mount them with plenty of space between the frames and the roof, and hook them up to dump the heat from the heat pump into outer space, and in the process boosting the efficiency of the existing heat pumps (AC or freezer).

And since the panels aren't making good thermal contact with the roof, if you don't want the extra cooling, you simply turn off the circulation pump and the panels will primarily cool down exterior air, not the building.

 

[ianmcf]

I’m confused… is it cooling the air or ‘reducing the heating of the air’? I assume the latter.

 

[graylshaped]

My current apartment is actually worse (too hot) in late spring and early fall, than in mid-summer, because there's not enough overhang. Mid summer is much more comfortable, because the (south-facing) sliding door and windows on the balcony (and the balcony too, for that matter) get more shade for a greater part of the day.

Hot part of the year for me is Sept-Oct. This year has been weird. Most mornings, when walking the dog, I realize it rained overnight, which is I guess nice for Orange County, CA. Typical daytime temp to date this year has been in the low 60F-ish. I would love it to crank up to the 80's so my kid and his friends can enjoy the community pool.

 

[szielins]

Fucking Hell, what I wouldn't give for a national wave of "Let's put solar panels over the parking spots, it's win-win" initiatives.

I don't disagree per se, but it turns out there are real reasons they're not more common. Adding solar-panel shades to an otherwise open-to-the-sky parking lot means adding electrically-wired structures that can hold the panels high off the ground and survive the occasional ramming by cars, where "car" includes "ludicrously heavy light truck". The structural brute force needed is considerable.

Ignoring economies of scale for the moment, https://www.ecowatch.com/solar/solar-carport guesstimates a single spot runs $18-25K. That's high enough to be daunting. Yeah, you get some electricity out of it--but it's not a lot, it comes when every other solar installation is already swamping the grid with joules, the panels themselves need to be maintained, and they lose capacity with time.

 

[Boskone]

Fucking Hell, what I wouldn't give for a national wave of "Let's put solar panels over the parking spots, it's win-win" initiatives.

I suggested just that at work. Car cover plus energy production? Sweet.

The building admin guy responded to the suggestion stating the building roof couldn't take the weight of solar arrays. The roof never came into the picture, but whatever.

He's just about worthless anyway.

 

[DovePig]

I’m confused… is it cooling the air or ‘reducing the heating of the air’? I assume the latter.

Both, actually. here the PRDC 'works', as I understand it, by (apologies for any inaccuracies or shortcuts I might have made):

1. having high reflectivity in most of the light spectrum, with the added twist here that it can change the reflected light's colour without absorbing the opposite colour, like normal pigments and dyes do heating up in the process.
2. having high emissivity in the very narrow part of the infrared spectrum the Earth's atmosphere is transparent to, preferentially radiating any heat to outer space instead of just radiating it to heat the surrounding air if at all, think the direct opposite of greenhouse effect.
3. The coating cools the coated surface even in daylight by preferentially radiating in the narrow infrared atmospheric window, plus it reflects the visible and other infrared light, not heating up from absorbing these.

That's the alleged beauty of photonic metamaterials and why they are so 'trendy' nowadays, they can manipulate light on a different level than normal dyes, just like butterfly wings. Obviously, the devil's always in the details, so any practical application needs to be cheap (within the order of magnitude of normal paint 'cheap'), easy to clean and durable. We'll see if they manage to do that, as it can be a really hard path from basic science to everyday implementations...

 

[Bernardo Verda]

I suggested just that at work. Car cover plus energy production? Sweet.

The building admin guy responded to the suggestion stating the building roof couldn't take the weight of solar arrays. The roof never came into the picture, but whatever.

He's just about worthless anyway.

These days, a lot of resistance to common sense solutions is just ideological opposition to acknowledging reality.

(That's in addition to the usual phenomenon of people in certain jobs just being resistant to anything new -- or being expected to actually do their jobs. It might be merely my personal experience, but "building managers" as a group, are surprisingly likely to meet all three criteria.)

 

[CrystalCowboy]

Roofs with cool roof coatings can be as much as 100° F cooler than roofs covered with traditional, dark-colored roofing materials, demonstrating energy savings of as much as 10-70%, according to EPA.

And that's just based on reflectivity, not fancy pantsy photonic materials.

 

[szielins]

Roofs with cool roof coatings can be as much as 100° F cooler than roofs covered with traditional, dark-colored roofing materials, demonstrating energy savings of as much as 10-70%, according to EPA.

And that's just based on reflectivity, not fancy pantsy photonic materials.

That number appears on the American Coatings Association site, yes, but I don't see it at the EPA site it points to. ACA points to https://www.epa.gov/heatislands , which has a subsection https://www.epa.gov/heatislands/using-cool-roofs-reduce-heat-islands , which includes the considerably more reasonable "Together, these properties help roofs to absorb less heat and stay up to 50–60°F (28–33°C) cooler than conventional materials during peak summer weather."

I'm sure you could get a hundred degree F difference between VantaBlack and a mirror under the right circumstances, but regular paints versus regular shingles/tar won't be THAT dramatic.

 

[SixDegrees]

This is the second Ars article in a week on structural color.

The common theme seems to be "structural color is neat. Here are wildly impractical ideas about how this could translate into commercial products." I conclude that it must be grant renewal time.

Several years ago, someone figured out that chocolate could hold imprinted grooves well enough to emboss it with diffraction gratings, opening up the possibility of wildly-colored chocolate without the use of dyes. A few dazzling samples were made.

Then it vanished, because it just couldn't stand up to the manufacturing and shipping process. Too fragile, and it wound up looking really shitty when it was unwrapped.

Neat idea. But execution is often a harsh mistress.

 

[JMTronicHobbyist]

"The surface emits its own heat into space without being absorbed by the air or atmosphere," - How can the surface be absorbed into the atmosphere?

Another question from what I've read in the comments: how can these materials tell whether the sky is clear? If there's a cloud above the material won't the infrared just bump into some water at the end of its journey toward space and heat that water a little, but far from the material itself? Seems like the cooling should still work if it's not too humid even if there's a cloud overhead. Even in high humidity I'm sure some IR would still escape, even if a photon hits an h2o that h2o would have to either hit the surface or another molecule, sending it careening into the surface to cause a net zero transfer, but some photons should travel further or hit molecules which then fly away from the surface, taking the heat with them. I would expect performance to decrease as humidity increases but not a binary "clear sky and dry air it works, cloudy and humid it doesn't work". Am I just understanding it wrong?

 

[Boskone]

These days, a lot of resistance to common sense solutions is just ideological opposition to acknowledging reality.

(That's in addition to the usual phenomenon of people in certain jobs just being resistant to anything new -- or being expected to actually do their jobs. It might be merely my personal experience, but "building managers" as a group, are surprisingly likely to meet all three criteria.)

I think it's just generic laziness. That's kind of his M.O.

Skim the message, don't bother with details, write it off as to much trouble and come up with an excuse.