A film that can cool buildings without the use of refrigerants
economist.com
economist.com
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.
You are right it won't replace the AC, but the technology could lead to a reduction in overall AC use.
Don't get me wrong it's a neat effect, just not particularly useful.
"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 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.
PS: You don't actually need a thing special at night to get this effect: http://www.asterism.org/tutorials/tut37%20Radiative%20Coolin...
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.
"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."
PS: An easy demo is a car roof with the windows open at night in a field.
Presumably Dr. Yin was responsible for energy absorption and Dr Yang for energy emission.
Almost every new house here includes a big air conditioner, and during hot months the euphemistically named 'load shedding' is often performed: http://www.afr.com/news/politics/sa-power-crisis-may-spread-...
Thus any improvements to (more) passive cooling technologies are very welcome
Where I'm from(Poland) summers can get stupidly hot, 40C on some days, it's difficult to sleep at night, yet I don't know anyone who has air conditioning at their house. It's a huge luxury, due to the cost of the unit + cost of electricity to run it.
I never realized how much I hated this until I started spending a lot of time in a warm country that uses AC sparingly.
However in the hottest parts of Texas the summers are brutally hot and humid even at night, and the insects are a force of nature. AC is kind of a requirement. I would imagine that if AC were magically outlawed tomorrow, 90% of the population or more of most of the South would pack up and leave by the end of August.
This works if you have good fans and a lot of icewater. Using aircons too much ends up making you sick, going in and out of the heat. In Thailand we had two golden retrievers who had their own fans in the coolest parts of the house, they would sleep through the heat and then be very active in the early morning and evening.
It should also be mentioned that we are far enough south that the length of day and night doesn't change that much over the year -- the night is always long enough for radiational cooling to do it's job. That is not the case in places like New York or Boston where the nights are so short in August that it never cools down. It's always comfortable in the morning here before the mid-morning heat hits.
Contrast this with visiting hot parts of the United States where you go from your airconditioned house to your airconditioned car to your airconditioned big box store to your air conditioned office. That's a terrible way to live.
With caveats: If we imagine a city radiating all its energy as infrared, it could cause weather abnormalities.
Note: this is similar to, but not the same, as geothermal heating.
That would be pretty cool, no pun intended.
I tend to think this is like a gas mantle in a gasoline (propane) lantern. The mantle is doped with rare earth that emit more strongly in the visible section than a black body would. On this case the temperature is much lower of course and the glass beads are sized to emit strongly in an the infrared band that isn't absorbed by the atmosphere.
It appears that the researchers have tuned the emission to a frequency that is not reabsorbed by the atmosphere but emitted unimpeded into space. Pretty clever engineering.
https://en.wikipedia.org/wiki/Polariton
So, couple this, in terms of the dispersion of energy: "Our hybrid metamaterial is extremely emissive across the entire atmospheric transmission window (8-13 μm) due to phonon-enhanced Fröhlich resonances of the microspheres."
And this in terms of the absorbtion: "In the case of large microspheres, modal interference between higher order modes makes the hybrid metamaterial strongly infrared-absorbing."
And finally this in terms of reflectance: "The hybrid metamaterial strongly reflects solar irradiation when backed with a 200-nm-thick silver thin film prepared by electron beam evaporation... The measured spectral absorptivity (emissivity) of the sample (Fig. 3D) indicates that the 50-μm-thick film reflects ~ 96% solar irradiation..."
The new film works by a process called radiative cooling. This takes
advantage of that fact that Earth’s atmosphere allows certain wavelengths
of heat-carrying infrared radiation to escape into space unimpeded. Convert
unwanted heat into infrared of the correct wavelength, then, and you can
dump it into the cosmos with no come back.
the work is done by the huge temperature difference, about 290°C, between
the surface of the Earth and that of outer space.But what about places with erratic weather, or even just seasonal changes? It seems like this will need to be some way to flip this stuff.
I wonder if you could integrate it with a solar cell, so you would get both cooling and electricity.
It would be nice if they could figure out how to turn the effect off at the film, instead of having to pipe water.
Does anyone have an idea of how long this might take to get to market as a rooftop heat exchanger for domestic use? I guess it largely depends on durability of the film; presumably the heat exchanger part of the system is a known quantity.
As an achievement in science this is cool, but... we should be reducing energy consumption while also not polluting the surface of the planet. Where's this stuff going to end up when houses covered with it are torn down in 30-100 years?
I would guess that there are examples of much larger consumers of plastic.
But then, after thinking a bit about it (in my semi frozen office because someone turned out the heating during the night), won't this increase the heating costs a lot during the winter?
Also, the demonstrated energy transfer density of this stuff is high enough that the area of your roof is enough cooling to make a series dent in your cooling cost.
In contrast ordinary objects (the computer case, a tree, a wall), do transmit those frequencies.
This would have no effect - it's absorbing the frequencies emitted by the wall, and transmitting to the wall, the net effect is zero.
i.e. it only works under open sky. I suspect even clouds would block this from functioning. (Not sure about this - maybe water has a "window" at this frequency?)