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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.
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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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.
The (paywalled) research paper states: "The use of HfO2 is, however, not essential, and can be replaced with titanium dioxide (TiO2), which is less expensive."
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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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