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Comment by voltagex_ | Hacker News Reader

http://science.sciencemag.org/content/early/2017/02/08/scien... is the paper linked in the article

Did not read the article, but the abstract is misleading.

It does not matter if the atmosphere absorbs the radiation from your device or it reaches space, after a few feet it's irrelevant. The important bit is the underside of the tarp which is not going to be below ambient temperature. Making the absolute best it can do the equiviemnt of a fan on the roof which can't replace AC.

Making the absolute best it can do the equiviemnt of a fan on the roof which can't replace AC.

You are right it won't replace the AC, but the technology could lead to a reduction in overall AC use.

Only as much as some tarps over a building and a fan 'cooling' the AC's evaporator coils.

Don't get me wrong it's a neat effect, just not particularly useful.

If you covered things like parking lots and sidewalks, it would reduce the heat island effect and hence air temperature, and so reduce AC some more.

The same reason why cities plant so many trees. https://www.epa.gov/heat-islands/using-trees-and-vegetation-...

This has actual cooling capacity. From the publication:

"A 72-hour continuous measurement of the ambient temperature and the surface temperature of an 8-in-diameter hybrid metamaterial under direct thermal testing. A feedback-controlled electric heater keeps the difference between ambient and metamaterial surface temperatures less than 0.2°C over the consecutive three days. The heating power generated by the electric heater offsets the radiative cooling power from the hybrid metamaterial. When the metamaterial has the same temperature as the ambient air, the electric heating power precisely measures the radiative cooling power of the metamaterial. The continuous measurement of radiative cooling power over three days shows an average cooling power > 110 W/m2 and a noon-time cooling power of 93 W/m2 between 11am – 2pm. The average nighttime cooling power is higher than that of the daytime, and the cooling power peaks after sunrise and before sunset. The measurement error of the radiative cooling power is well within 10 W/m2 (32)."

Fans won't drop temperatures below ambient. This will.

It absolutely does matter, because the beads that are tuned to radiate particular frequencies of infrared will also be tuned to absorb those frequencies as well. If the frequencies aren't transparent to the atmosphere, you'll be reabsorbing just as much as you transmit and end up at equilibrium with the atmosphere.

It is essentially similar to the way in which you can freeze water in a shallow dish on clear nights that are nonetheless slightly above freezing temperature - you are facing a black-body radiator that is well below ambient local temperature.

That's a great explanation. I wish the article had put it in these terms... because reading through it, I kept thinking there was no way to cool the building to below the ambient temperature. But if you're equilibrating with space instead of the atmosphere, that's a whole different kettle of fish.

At night sure, in the day time the sun is adding heat across a wide band of frequencies. This heats up less, but does not drop below ambient temperature in full sunlight.

PS: You don't actually need a thing special at night to get this effect: http://www.asterism.org/tutorials/tut37%20Radiative%20Coolin...

The advantage of targeting the IR window is that it would work even with clouds and water vapor in the atmosphere whereas, plain old thermal radiation needs a clear night to be effective.

Of course it won't pump heat backwards into the sun but they propose a water circulation system, which I imagine could store some daytime heat to release it at night. That and masonry of the walls of the building would store even more.

Reading the article helps. One of these tarps cooled a container of water underneath it by 8°C during an overnight test:

"We further demonstrate the effectiveness of radiative cooling for a relatively large thermal mass using water as a cold storage medium. A plastic water tank was placed underneath the radiative cooling glass-polymer hybrid metamaterial, putting water in close contact with the heat-conducting copper plate. Since the water is stationary in the experiment, its large heat capacity substantially slows down the cooling process. We therefore used a 10-µm-thick HDPE film on top of the Polystyrene foam box in this setup to reduce convective heat loss and improve thermal isolation. The water temperature continuously dropped, reaching more than 8° C below ambient after two hours of exposure."

That's amazing. You could actually cool a tank of water at night for free and pump it though your house during the day in place of AC.

Except it's not limited to nighttime use. Because of the tuned IR window, it works in daytime also, just not as well.

That means less than you would think as a container with the same setup and no meta material at night would also drop below ambient temperature.

A sealed container? I'm not following the physics on this. I can only assume you're talking about evaporative cooling of some sort. This was not an open container.

No, you need to insulate the container from the ground. Then you have conductive and radiative heat gain and loss with the air. Works better on top of a hill, in the middle of a field, under a clear sky. The container has to have minimal thermal mass, and be a food conductor but that's it.

PS: An easy demo is a car roof with the windows open at night in a field.

They did insulate the container from the ground.

Which is why I am saying this. The important thing at night is not delta from ambient temperate, but the delta with and without their covering.

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