Passive radiative cooling ceramic with high solar reflectivity(cityu.edu.hk)
cityu.edu.hk
Passive radiative cooling ceramic with high solar reflectivity
https://www.cityu.edu.hk/research/stories/2023/11/10/new-cooling-ceramic-can-enhance-energy-efficiency-construction-sector-and-help-combat-global-warming-cityu-research
99 comments
These surfaces at scale will be a game changer for fighting climate change. They seem like magic. Here’s a key quote from Wikipedia:
> Some estimates propose that if 1–2% of the Earth's surface area were dedicated to PDRC that warming would cease and temperature increases would be rebalanced to survivable levels.
Solar near people and PDRCs away from people (where solar is impractical) along with a reduction in Carbon is a feasible plan today.
> Some estimates propose that if 1–2% of the Earth's surface area were dedicated to PDRC that warming would cease and temperature increases would be rebalanced to survivable levels.
Solar near people and PDRCs away from people (where solar is impractical) along with a reduction in Carbon is a feasible plan today.
Cover 1-2% of the Earth's surface with solar panels and you have so much spare energy that you can do all kinds of crazy stuff, like sucking carbon dioxide out of the air and turning it to diamonds for fun.
Solar panels are limited by cost. If this cooling ceramic is a better version of the barium sulfate thing (i.e. just an extremely cheap-to-produce material that's very white) then this is viable because it doesn't cost hundreds of dollars per square metre to manufacture.
Its main limitation would likely be construction costs, which is a problem solar panels are already starting to run into. Although construction of solar requires setting up cables and not e.g. accidentally snapping hundred-dollar solar panels in half (which would be a non-issue for barium sulfate, since it's dirt-cheap and basically just white paint anyway).
Its main limitation would likely be construction costs, which is a problem solar panels are already starting to run into. Although construction of solar requires setting up cables and not e.g. accidentally snapping hundred-dollar solar panels in half (which would be a non-issue for barium sulfate, since it's dirt-cheap and basically just white paint anyway).
"hundreds of dollars per square meter" not true anymore.
https://allo.solar/panneau-solaire-410w-noir-mono-p-type-tw-...
104.08 EUR tax (20%) included for 410Wp gives 0.254 EUR/Wp
1722 x 1134 x 30 mm = 1.92 square meter gives 53.38 EUR/m2.
Some wooden floor are more expensive than PV per square meter.
People forget that PV, battey, wind costs are still decreasing.
https://allo.solar/panneau-solaire-410w-noir-mono-p-type-tw-...
104.08 EUR tax (20%) included for 410Wp gives 0.254 EUR/Wp
1722 x 1134 x 30 mm = 1.92 square meter gives 53.38 EUR/m2.
Some wooden floor are more expensive than PV per square meter.
People forget that PV, battey, wind costs are still decreasing.
A quick calculation on Wolfram Alpha: to cover 2% of the world's surface at 50EUR/m2 would cost roughly 540 trillion euros, or 5.5 years of current world GDP.
Quoting the GP:
> People forget that PV, battey, wind costs are still decreasing.
Some insist on forgetting it. And the decrease has been exponential, with no reason to believe it will stop at the next order of magnitude.
Anyway, covering 2% of the world with anything is a huge undertaking.
> People forget that PV, battey, wind costs are still decreasing.
Some insist on forgetting it. And the decrease has been exponential, with no reason to believe it will stop at the next order of magnitude.
Anyway, covering 2% of the world with anything is a huge undertaking.
If I'm not mistaken 510 millions square kilometer earth surface, 2% of that 10 millions square kilometer, at 200 Wp/m2 gives 2 PW peak.
Current peak electricity production capacity is about 7.5 TW according to:
https://www.statista.com/statistics/267358/world-installed-p...
Current peak electricity production capacity is about 7.5 TW according to:
https://www.statista.com/statistics/267358/world-installed-p...
There are many open bare places on Earth where solar panels are useless because there are no electricity users nearby: any desert.
If all it takes is to truck the ceramic panels around and dump them to the ground, then it's probably not that expensive.
If all it takes is to truck the ceramic panels around and dump them to the ground, then it's probably not that expensive.
So in order to save the environment we plan on covering it with white sheets?
Just because we don't want to tell Africa and China to stop burning and polluting and dumping it all in the rivers?
Just because we don't want to tell Africa and China to stop burning and polluting and dumping it all in the rivers?
Just to point out, but it's not Africa, and it's mostly not China that are burning and polluting.
Where I live solar panels are limited by regulations. They pay for themselves after a couple of years so people would plaster them literally everywhere the sun shines if they were allowed to. White paint otoh probably has worse ROI.
The "white paint" parent is referring to can passively cool the surface a few degrees below ambient temperature 24/7 (yes, even at night). I could imagine worse investments.
White paint (if cheap) on a building you are actively trying to cool would seem to have a very good roi.
I think white shingles for the roof would probably be even more effective. IIRC, a lot homes in hotter climates have started using them.
Adding bifacial solar panels is probably a fantastic combination as well.
Adding bifacial solar panels is probably a fantastic combination as well.
A lot less than 1% of the earth‘s surface is buildings.
A negative ROI (return on investment) suggests it won't pay for itself ever, be that in cash or carbon.
If you're actively trying to cool the building then painting it in something that lowers the energy needed to cool it. And as were just talking about paint, carbon and cash cost is likely to be low relative to savings.
So no, painting all houses in this stuff might not reverse global warming, but that wasn't what we were talking about.
If you're actively trying to cool the building then painting it in something that lowers the energy needed to cool it. And as were just talking about paint, carbon and cash cost is likely to be low relative to savings.
So no, painting all houses in this stuff might not reverse global warming, but that wasn't what we were talking about.
Seriously. 1-2% of Earth's surface is a _lot_ of surface.
As a term of comparison, here's a back of the envelope calculation estimating that 0.18% of the surface is covered by roads and parking areas.
https://www.reddit.com/r/theydidthemath/comments/zwni88/requ...
Approximately 1% is covered by any kind of construction: https://www.quora.com/What-percentage-of-Earths-land-is-used...
Almost 15% of land area (not total surface) has been "modified" by humans: https://www.weforum.org/agenda/2021/10/human-impact-earth-pl... This includes agriculture, mining and, somehow, pollution.
https://www.reddit.com/r/theydidthemath/comments/zwni88/requ...
Approximately 1% is covered by any kind of construction: https://www.quora.com/What-percentage-of-Earths-land-is-used...
Almost 15% of land area (not total surface) has been "modified" by humans: https://www.weforum.org/agenda/2021/10/human-impact-earth-pl... This includes agriculture, mining and, somehow, pollution.
Indeed; at 9.2 trillion m^2, the Sahara desert sits right in that range at 1.8% of Earth's 510 trillion m^2.
Yup. We have more than enough harnessable solar power to allow every single person a clean energy budget higher than a typical American, offset all other industrial carbon production, and start to unwind the last century of humanity's carbon impact.
But instead of working towards that end-state, a lot of people (even people on this site) seem to be more interested in shaming others into using less energy, and attacking others for "immoral" uses of energy.
But instead of working towards that end-state, a lot of people (even people on this site) seem to be more interested in shaming others into using less energy, and attacking others for "immoral" uses of energy.
Using less energy makes perfect sense until we achieve this desirable future state of abundant excess energy. That state is not right now, however.
Nope, because all the carbon production until that point can be unwound.
The shitty uncomfortable life that the "use less" camp wants me to live, cannot.
The shitty uncomfortable life that the "use less" camp wants me to live, cannot.
But currently we aren't on a course to actually unwind that carbon production in time.
It's like saying there's an oasis 2 days walk from here, so don't worry about having a shower and possibly using all your drinkable water. You still need to save enough water to actually get to the oasis, and at the moment, we're kinda sorta having a conversation about whose job it is to actually make the trek, so in reality it's going to be at least another day before everyone decides that they need to start marching, and as we only currently have 2.5 days of drinking water. That shower is really irresponsible.
It's like saying there's an oasis 2 days walk from here, so don't worry about having a shower and possibly using all your drinkable water. You still need to save enough water to actually get to the oasis, and at the moment, we're kinda sorta having a conversation about whose job it is to actually make the trek, so in reality it's going to be at least another day before everyone decides that they need to start marching, and as we only currently have 2.5 days of drinking water. That shower is really irresponsible.
"can be" is very far from "is going to be" and even further from "is going to be paid for".
Also, we have a lot of solar energy .. during the day. The storage/transmission problem is solvable but not solved.
Also, we have a lot of solar energy .. during the day. The storage/transmission problem is solvable but not solved.
Who’s gonna pay for the unwinding though?
Whatever you we use that energy on will probably create heat though.
To put that in perspective, in 2014 the estimate was artificial structures covered 0.6% of the Earth's surface. It might take a while to double the amount of buildings, roads etc. and cover it all with PDRCs
Are artificial drivable and walkable surfaces counted?
Quoting https://science-atlas.com/faq/how-much-of-the-earth-is-inhab...: "According to the FAO Global Land Cover SHARE database, produced in 2022, 0.6% of Earth’s land surface is defined as ‘Artificial surfaces’. Artificial surfaces include any areas that have an artificial cover as a result of human activities such as construction (cities, towns, transportation), extraction (open mines and quarries) or waste disposal. This figure gives us an estimate of roughly 900,000 km2 of human-covered land worldwide."
You can't run tires on a reflective surface and expect it to remain reflective.
Thas wasn't quite what I was implying, but actually, figuring out economical high albedo roads would be fantastic.
Iron fertilization of equatorial oceans would make for very high albedo algae blooms. The ocean absorbs the vast majority of absorbed solar radiation. It has been pointed out that at low angles it reflects almost all light, but that is when the energy has gone through the most atmosphere and the water or land would absorb much less anyway. Napkin math scaling up previous fertilization experiments indicates about 1.5 billion USD per however long the blooms last, to trigger a snowball earth. We'd stop when it got cold enough.
Easily verified from ice core data. The UN Convention on Biological Diversity banned large-scale iron fertilization experiments in 2008. Cynically, I don’t think anyone’s figured out how to make money on it either.
If the 1.5 billion USD per event estimate is correct, it has to be banned because it's cheap enough that people don't need to make money on it — it's about what Oxfam raises every 18 months.
> UN Convention on Biological Diversity
I really wish people would stop inferring financial motives when other, far more obvious motivations are right there.
I really wish people would stop inferring financial motives when other, far more obvious motivations are right there.
I think in the grand scheme of things we are more likely to start making asphalt and concrete out of basalt, for the CO2 absorption.
Until you drive on them on a sunny day...
Or a winter day at high latitudes. I'm convinced the Illuminati (it's in the name, no?) arranged the roads so you're always driving into the sun wherever and whenever you're going.
> These surfaces at scale will be a game changer for fighting climate change.
Aren’t the materials just reflecting heat into the atmosphere, which due to human activity has greater heat carrying capacity?
The materials would be good for reducing energy consumption (cost-benefit ratio tbd).
Aren’t the materials just reflecting heat into the atmosphere, which due to human activity has greater heat carrying capacity?
The materials would be good for reducing energy consumption (cost-benefit ratio tbd).
The thing that the surface is radiating to is the cold blackness of outer space. This is why it feels colder on a cloudless day or night.
As always, the technology is promising but I’ll only really believe it when I can buy it at Home Depot.
As always, the technology is promising but I’ll only really believe it when I can buy it at Home Depot.
You can DIY it: https://www.youtube.com/watch?v=N3bJnKmeNJY
To save people a bunch of time: no, you can't. The best he comes up with is something incredibly fragile that is damaged if you so much as look at it wrong, and any sort of protective covering or coating dramatically alters its effectiveness.
Nobody has come up with a DIY "paint" or coating that is durable.
Nobody has come up with a DIY "paint" or coating that is durable.
It may not be a durable paint ready for most if any practical applications, but you can in fact try it yourself.
> This is why it feels colder on a cloudless day or night.
This is not entirely true. It's not because your thermal energy is radiated to something colder, but rather because clouds contain and reflect the thermal energy that would otherwise be lost to space.
This is not entirely true. It's not because your thermal energy is radiated to something colder, but rather because clouds contain and reflect the thermal energy that would otherwise be lost to space.
I’m just waiting for a version of a vacuum thermos that keeps my drink hotter, longer. Put this on the inner side of the outside surface and paint the inner side of the inside surface with the blackest black available. Almost like a thermal diode. Reverse the sides and you have a cold-cup instead…
I’d buy two: one for hot and one for cold!
I’d buy two: one for hot and one for cold!
This would not work in your thermos! This material is reflective in the visible but emissive in the infrared. The only thing that matters in the thermos is being reflective in the infrared.
Thermodynamics does not permit a thermal diode to exist.
Thermodynamics does not permit a thermal diode to exist.
I think it's even simpler than that. Any black body that is colder than its surroundings heats up. Any black body warmer than its surroundings will cool down.
This thermos would only keep your coffee hot if you dropped it into a furnace.
This thermos would only keep your coffee hot if you dropped it into a furnace.
You could make a colored flask which absorbs visible light (from the sun) but reflects thermal wavelengths (from the drink). But due to infrared leakage and thermal conduction, I don't know if it would keep drinks above 100 degrees F for anywhere near as long as a standard vacuum flask.
What about real materials, e.g. not a black body?
The neat thing about wearing white in the desert is that it keeps you cool during the day and warm at night. Wearing black does the opposite.
A perfect black surface on a spacecraft can do more than charcoal black paint, but only by a matter of degrees. If memory serves its less than a 2x multiplier between off the shelf and exotic materials. But when ounces cost thousands of dollars, you’re going to go exotic.
A perfect black surface on a spacecraft can do more than charcoal black paint, but only by a matter of degrees. If memory serves its less than a 2x multiplier between off the shelf and exotic materials. But when ounces cost thousands of dollars, you’re going to go exotic.
"The Second Law of Thermodynamics: Heat cannot, of itself, pass from one body to a hotter body."
https://www.youtube.com/watch?v=bYZyipx01tc
https://www.youtube.com/watch?v=bYZyipx01tc
My "solution" is a $5 mug heating pad that pulls 16W. It only heats when there's a weight placed on. Probably accounts to 10kWh for the whole year to keep a warm cup of coffee ready for the whole day.
Every random thermos out there already has a reflective surface for insulation.
The material is apparently hydrophilic, so it can absorb a lot of water. That means things like algae and gunk will fill the material, rendering it less effective.
From reading through the supplementary materials for the paper [0], it seems that the authors are aware of this flaw and they found a way to make it hydrophobic. The approach they tested involved soaking the ceramic in a bath with a commercially available fluorosilane [1] that is used to make things superhydrophobic. Fig. S20 in the supplementary materials has a chart that shows the treated ceramic being very good ("solar reflectivity...remains at ~99.0%") but not quite as stellar as the untreated ceramic.
[0] https://www.science.org/doi/full/10.1126/science.adi4725
[1] https://www.sigmaaldrich.com/US/en/product/aldrich/667420
[0] https://www.science.org/doi/full/10.1126/science.adi4725
[1] https://www.sigmaaldrich.com/US/en/product/aldrich/667420
Most of these clever optical solutions for all of our problems have an "it gets covered in dust eventually" problem. You can also get the opposite effect -- heat absorption with little radiative heat loss -- using a very thin layer of silver, but again, dust.
We've been working on self-cleaning windows, but I have no idea if any of those materials are transparent in the desired band.
How much maintenance does this take to keep its reflectivity? Does it get colonised by algae? If it needs to be scrubbed with bleach once a year, I can see that being an issue.
Has anyone understood why they keep bringing the leidenfrost effect in the discussion? I really don't know what this has to do with radiative cooling.
Great video about making home-made high reflectivity paint: https://youtu.be/dNs_kNilSjk
Aluminium plate painted with this paint is cooler when exposed to the sun not only than unpainted plate but also ambient air which feels really magical for me.
Aluminium plate painted with this paint is cooler when exposed to the sun not only than unpainted plate but also ambient air which feels really magical for me.
I'm having some trouble with the idea that all of that heat is going to go back into space.
I can see my neighbor's roof. If I can see it, that means that it's radiating in all directions, not just into space.
I can see my neighbor's roof. If I can see it, that means that it's radiating in all directions, not just into space.
> If I can see it, that means that it's radiating in all directions, not just into space
That depends on what you mean by "see". If you paint a square on a white paper with a magical paint that absorb 100% of the light, you can still see that there's a black square, because it's sitting on a contrasting background.
Even if the material is perfectly radiating everything to space, i.e., the roof sends no light to your eyes, it's still just a patch of blackness among normal things, and you can see the shape of it, just like a puddle on the road at night, as you walk with your back to the moon.
That depends on what you mean by "see". If you paint a square on a white paper with a magical paint that absorb 100% of the light, you can still see that there's a black square, because it's sitting on a contrasting background.
Even if the material is perfectly radiating everything to space, i.e., the roof sends no light to your eyes, it's still just a patch of blackness among normal things, and you can see the shape of it, just like a puddle on the road at night, as you walk with your back to the moon.
It's about the angle of reflection. When light reflects off a surface, it leaves at an angle equal to the angle it comes in at.
Most of the light hitting the roof will be reflected back up into space. Some small percentage of it will scatter and disperse into all other directions.
If you were in a plane at exactly the right angle between the roof and the sun, you'd see a glare much stronger than you'd see at street level.
Most of the light hitting the roof will be reflected back up into space. Some small percentage of it will scatter and disperse into all other directions.
If you were in a plane at exactly the right angle between the roof and the sun, you'd see a glare much stronger than you'd see at street level.
Google "diffuse reflection" and "specular reflection" if you're interested to understand why.
Things do not radiate uniformly, neither behave like mirrors (specular). The truth for most objects is somewhere in between.
You can see it with a plastic object as well, when it is positioned so that the angle to the normal vector (a vector perpendicular to the surface) of the light source and to your eye are the same (as in, you would be able to see the light source if it were a mirror), you see much more light than when you are simply looking at it "from the side".
Things do not radiate uniformly, neither behave like mirrors (specular). The truth for most objects is somewhere in between.
You can see it with a plastic object as well, when it is positioned so that the angle to the normal vector (a vector perpendicular to the surface) of the light source and to your eye are the same (as in, you would be able to see the light source if it were a mirror), you see much more light than when you are simply looking at it "from the side".
I'd imagine it's (total amount of sunlight hitting the visible portion of that roof) * (… the solid angle the window occupies, relative to the roof?)
The first portion of that equation is hot: roofs have a lot of sq. ft.'age, and a fair bit of sunlight strikes them. But the second part is well the other direction: the window probably doesn't occupy a whole lot of the field of view of the roof: i.e., mostly, any ray emitted randomly from any point on the roof is likely going to be skyward, so assuming it emits the heat/light out equally in all directions, your window only gets a small fraction.
The portion coming in your window doesn't go back out to space but I don't think that's a big deal?
The first portion of that equation is hot: roofs have a lot of sq. ft.'age, and a fair bit of sunlight strikes them. But the second part is well the other direction: the window probably doesn't occupy a whole lot of the field of view of the roof: i.e., mostly, any ray emitted randomly from any point on the roof is likely going to be skyward, so assuming it emits the heat/light out equally in all directions, your window only gets a small fraction.
The portion coming in your window doesn't go back out to space but I don't think that's a big deal?
That problem can be solved by adding a mirrored ring around the cooling surface, so it can only "see" the sky:
https://www.youtube.com/watch?v=5zW9_ztTiw8&t=404s
https://www.youtube.com/watch?v=5zW9_ztTiw8&t=404s
It doesn't need to go back into space. If you don't want the scattered sunlight to heat your house, you can always add the same ceramic to your house...
I'm pretty sure that doesn't work. The surface works because it's emitting a band of radiation that the atmosphere is mostly transparent to. If it sees radiation in that same band it's going to absorb it, not emit. Point enough of these at each other and you'll approach zero.
The material works by having very low emissivity / absorptivity at sunlight wavelength so that sunlight is scattered back out, and by having high emissivity at typical earth temperature blackbody peak wavelength, so it radiates out heat effectively.
Sunlight that is scattered off the roof can scatter back off other roofs before heading into space. Likewise radiation that is emitted by the roof can be absorbed and then re-emitted by other roofs before heading off into space. Neither of these preclude the roof from doing its job.
At the end of the day, the roof will effectively reflect back sunlight from the high temperature sun, while radiating infrared radiation into the low temperature sky, and have a neutral energy balance with the surrounding earth / houses / trees / objects which are at a similar temperature to it.
Sunlight that is scattered off the roof can scatter back off other roofs before heading into space. Likewise radiation that is emitted by the roof can be absorbed and then re-emitted by other roofs before heading off into space. Neither of these preclude the roof from doing its job.
At the end of the day, the roof will effectively reflect back sunlight from the high temperature sun, while radiating infrared radiation into the low temperature sky, and have a neutral energy balance with the surrounding earth / houses / trees / objects which are at a similar temperature to it.
> Sunlight that is scattered off the roof can scatter back off other roofs before heading into space.
… but that's not what they're asking about?
Imagine you're standing in a second story window, looking out. Out the window, you're seeing your neighbor's roof, coated with this stuff. A lot of sun is hitting that roof — How much heat is coming in through that window, off that roof?
There have been stories in the past of reflective surfaces cooking nearby apartments, so I get the concern. I think in this case it boils down to it not being reflective, mostly, so it's fine.
… but that's not what they're asking about?
Imagine you're standing in a second story window, looking out. Out the window, you're seeing your neighbor's roof, coated with this stuff. A lot of sun is hitting that roof — How much heat is coming in through that window, off that roof?
There have been stories in the past of reflective surfaces cooking nearby apartments, so I get the concern. I think in this case it boils down to it not being reflective, mostly, so it's fine.
You’re missing the point here. Almost willfully so.
Two pieces of this material in an overlapping field of reflection are putting out exactly the wavelength the other will absorb. I can’t defend my building from being baked by yours by coating it with the same stuff. I am simply going to return the favor in the process.
Not to mention the big problem of creating a new case of the haves versus the have-nots.
Two pieces of this material in an overlapping field of reflection are putting out exactly the wavelength the other will absorb. I can’t defend my building from being baked by yours by coating it with the same stuff. I am simply going to return the favor in the process.
Not to mention the big problem of creating a new case of the haves versus the have-nots.
Sure — but how visible is the roof from above versus the sides?
It’ll expel that fraction to space regardless of its neighbors.
It’ll expel that fraction to space regardless of its neighbors.
Less than 70% on non-commercial structures. And on commercial structures the roof can be a negligible percentage of the total building envelope.
This is cool stuff, but it’s just stuff. It’s not a silver bullet. We here are all old enough, many of us by half, to no longer get sucked in by magic thinking.
This is cool stuff, but it’s just stuff. It’s not a silver bullet. We here are all old enough, many of us by half, to no longer get sucked in by magic thinking.
The bulk of the roof can be assumed to be oriented to the sky, or it wouldn't get much sun and be of no concern. So it will radiate towards the sky mostly.
The infrared radiation will indeed go out in all directions. The way I understood it there will be less of it, because the roof is cooler. So if this material replaces a traditional surface, you will get more visible light scattered your way, and less infrared light.
The infrared radiation will indeed go out in all directions. The way I understood it there will be less of it, because the roof is cooler. So if this material replaces a traditional surface, you will get more visible light scattered your way, and less infrared light.
What does it mean for a layman? It says it reflects around 99% sunlight and 96% infrared. Does it mean if placed outside in hot summer days the underside will stay at ambient temperature?
It'll likely stay a few degrees below ambient. Ordinarily, that doesn't happen because the surroundings are emitting and absorbing infrared roughly in balance with things at the same temperature, but if you can make a material that emits most of its infrared at a particular frequency that the atmosphere is nearly transparent to, its heat will leak out into space.
This could be very interesting material for surfaces in greenhouses where you want to have as much light as possible but during the summer don't want to have the heating effect.
[deleted]
momirlan(3)
The region around Almeria is covered by greenhouses that use white plastic sheets to protect their crops. The region has effectively cooled by 0.3 degrees whereas other areas have increased 0.5.
https://www.agroberichtenbuitenland.nl/actueel/nieuws/2019/0...