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Scientists Develop "Paint" To Help Cool the Planet
AaronW writes Engineers at Stanford University have developed an ultrathin, multilayered, nanophotonic material that not only reflects heat away from buildings but also directs internal heat away using a system called "photonic radiative cooling." The coating is capable of reflecting away 97% of incoming sunlight and when combined with the photonic radiative cooling system it becomes cooler than the surrounding air by around 9F (5C). The material is designed to radiate heat into space at a precise frequency that allows it to pass through the atmosphere without warming it.
Cars being greenhouse ovens is a terrible issue. Not just because it's uncomfortable getting into the car on a hot day, but because people accidentally kill pets and children by leaving them in a hot car every year (and others not so accidentally). Surely there's better tech than what we use today to prevent our automobiles from becoming lethal ovens.
Certainly there is. You can just cook your kids and pets at home, no need to waste the gas going out at all. Home ovens have been large enough to do this for decades now. People are so wasteful!
--Hannibal
Scientists Develop "Paint" To Help Cool the Planet
They're calling it "White".
Seriously, though, it's a mirrored silver paint with some nanoparticles mixed in to make it even cooler (pun intended). But if people aren't painting their roofs white and silver today, do they really think their paint will change that?
On the other hand, a radiator that reflects sunlight sounds promising for other applications, like heatsinks for space probes.
How can I believe you when you tell me what I don't want to hear?
According to Wikipedia's entry on hafnium, "Hafnium reserves are projected to last under 10 years if the world population increases and demand grows."
Vegetarian Stew
Serves 20
Peel, core, and slice one vegetarian.
Place in trunk of black car for two days.
Season to taste.
Passively cooling an object below ambient temperature seems... counterintuitive. I think I understand what's going on here, but I'd like to see some more thorough discussion. Particularly, I'd like to know how you can find any passband in which an object at ambient temperature radiates more heat than it takes in from direct solar exposure, except the bands blocked by the atmosphere.
Sorry, not possible, as per the first law of thermodynamics.
Nope, quite possible per the first law of thermodynamics, as well as the second and zeroth laws. If the atmosphere is transparent and the object is exposed to the sky, heat can radiate from the object to space (which, even accounting for solar exposure, has a mean effective temperature well below that of air temperature in many places ~ 230K). If the air is still and the object can reflect most of the incident radiation, there is no reason why the object can't cool below air temperature. It is a completely separate mechanism of heat transfer to a different heat sink.
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For those fortunate enough to have institutional access, the research paper is here.
Quickly picking some highlights:
The atmospheric transmission window is between 8 and 13 microns. They achieved 4.9C below ambient in direct sunlight at 850 watts per square metre. Cooling power was 40.1 watts per square metre. Emissivity (equivalently absorptivity) averages about 70% in the 8-13 micron window (estimated from a plot.)
Here's a quick back-of-the-envelope calculation
90% reflective white paint: absorbs 85W/m^2
97% reflective foil: absorbs 25.5W/m^2, an improvement over white paint of ~60W/m^2
This film: emits 40W/m^2, an improvement over simple foil of ~60W/m^2.
So in this scenario, the special film gives twice the benefit compared to just going for something simple and reflective. (The 90% for white paint is guess-work. The 97% for 'foil' is just matching the special film. Perhaps someone can update the calculations with better founded values.)
The summary title is highly misleading.
It is not paint, it is a manufactured film. It cools buildings, not planets. Yes, with enough you could cool the planet, but if you wanted to take that route, it would be much more cost effective to just use aluminium foil and use a marginally larger area of it (or, indeed, white paint.) Back in the real world, the way this invention cools the planet is by reducing electricity demand for air conditioning. (I saw another article about this in which one of the authors makes exactly this point.)
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Yes, but the equilibrium temperature of the object will be between the air temperature and the sky temperature. The object will be cooler than the air, and so heat transfer due to convection will go from the air to the object. However, the object will still be warmer than the sky, so heat transfer due to radiation will go from the object to space.
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Direct solar exposure is reflected, then the reflected light and internal heat radiation is modulated to a frequency that passes easily through the atmosphere. I'd have to question the efficiency of modulation, which seems to be where their breakthrough occurred.
"Ambient" is important to define here. The temperature of the air is not actually playing much of a role in the black body equation. If the sky was made of more buildings at ambient temperature, then the story would be different, but other than the sun it's mostly an open pit into which anything radiated never returns. Also keep in mind that that figure may be referencing the temperature of the air near the whole building including the lower floors; it is cooler up high on tall buildings.
The idea is that the heat provided from within the building and the heat from the 3% of sunlight that gets through the mirror all pools and the mirror material then converts it to a specific passband. So you have more heat pooling than what comes in on that passband.
How effective this system remains when contaminated with a coat of dust is a question. Also comparative advantage to absorbing the heat/light and using it to power AC.
Someone had to do it.
Your understanding of the process could use some polishing too.
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What the material is doing (or is claimed to do, anyway) is to re-radiate incident radiation at a wavelength that can pass through through the atmosphere back out to space without being absorbed (i.e. it won't heat up the atmosphere). Since the surface can absorb heat due to convection from the air, it can re-radiate that heat as well into space. This material is not merely reflective, its radiation properties are such that essentially acts as a refrigerator; it can pull heat from the air and radiate it to space.
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It's not the ambient temperature of air that's key here, it's the ambient temperature of space, which is about 2,7K.
All objects are constantly radiating energy and receiving energy back from other things that are radiating. When two objects in radiative exchange are roughly the same temperature, this balances out. But when one is hotter than the other, the hotter one loses more energy than it takes in, and vice versa. And it's not just a little difference - radiative heat loss is proportional to the absolute temperature to the fourth power, that's a pretty big exponent. So when you're exchanging energy with space, which is so cold that it takes very sensitive instruments to be able to measure *anything*, well, that heat is simply lost.
You can see this effect for yourself by noting how cloudy nights are usually warmer than clear nights. Clouds are cold, but they're not as cold as space!
The effect of the combination of radiation, absorption, and reflection, with different band peaks for each phenomenon, manifests itself in atmospheres as a greenhouse effect (positive or negative) versus the radiative equilibrium temperature.
You look beautiful! Incidentally, my favorite artist is Picasso.
Would you care to be more specific? My explanation is pop-science simplified, but I don't see an error in it.
More detailed explanation:
In the 8-13 micron (wavelength) window, atmospheric transmittance averages about 80% (estimated from a plot in the paper.) So the energy received is about 20% of what you'd get from a black body at atmospheric temperature (plus 80% of what you'd get from space, which is negligible in comparison.) So the brightness temperature at 8-13 microns is lower than ground level atmospheric temperature. How much lower depends on the average temperature of the atmosphere along the line of sight, and where 8-13 microns falls on the black body curve at that temperature (even this is oversimplifying) and I can't be bothered figuring that out. However, if we can reflect/insulate all energy except 8-13 micron radiation, then our thermal equilibrium temperature will be the brightness temperature at 8-13 microns to which we are exposed. This is, as noted, less than atmospheric temperature at ground level.
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What the material is doing (or is claimed to do, anyway) is to re-radiate incident radiation at a wavelength that can pass through through the atmosphere back out to space without being absorbed (i.e. it won't heat up the atmosphere).
More importantly: If the wavelength were one that was absorbed by the atmosphere, it is also one where the atmosphere radiates heat back toward the paint.
If your frequency slot is one with "absorption", you "see" the temperature of the atmosphere - a bit cooler than the surface of the (greenhouse-effect boosted) planet, but not by enough to be exciting.
If your slot is one that is essentially fully transparent, you "see" the cosmic background (except for the tiny part of the sky that shows the sun's or moon's disk). That's about 2.7 degrees K, call it -457 Fahrenheit. Liquid helium is substantially warmer at -452.2.
The slow radiation of heat at the sky is almost completely overwhelmed by conductive and other transfers of heat into the paint, of course. Of the 530ish degrees F difference from room temperature, only nine are left.
But that's nothing to sneeze at. The inside of my well-insulated desert house gets up to about 85 in the day without air conditioning. If I could drop that by nine degrees it would be a relatively comfortable 76. (It would likely actually drop more, because the lower temperature of the surface would slow the heating and tend to even the daily cycle of temperature out further.) 85 or more is debilitating. 76, with drastically low humidity (dew point typically about 35), is actually comfy.
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Yes, scientists do, because of the sheer amount of evidence it is happening. You can choose to call them all idiots, or refuse to believe in the scientific method, but all that does is demonstrate that you care more about not changing your attitudes than you do about learning, which reflects poorly on you, your family, your upbringing, and your culture. Why you seem to think it makes you look good is beyond me, but then seeing as you are railing against the scientific method, it's not entirely surprising.
This is non-trivial amount of land. I'm not sure how much space tree needs to grow properly - let's assume 10m^2. In my country, we have around 40mil people. 10 trees per year per person is 40.000.000 * 100m^2, which is 4.000.000.000m^2, which gives 4000km^2. Each year.
Entire land area of country is around 300.000km^2. 30% of it is already forest. 60% is agricultural land.
What you are suggesting is planting 1.3% more area of forests each year. In 50 years, there would be no agriculture anymore - just 90% forest and 10% rest. In another 5-6 years, there would be just forest.
I think that you can achieve same effect without planting forests. Just starve all people out by destroying all agriculture lands in one big go, planet will heal itself. Not that there will be anybody to care.
That'll be the Walkie Talkie building
http://www.bbc.co.uk/news/uk-e...
The problem isn't reflectiveness alone, the shape of the building played a part too.
Why UNIX?