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A Material That Throws Heat into Space Could Soon Reinvent Air-Conditioning (technologyreview.com)
52 points by rbanffy 5 months ago | hide | past | web | favorite | 25 comments



Same guys as the IEEE article : https://news.ycombinator.com/item?id=15176721 but a bit more on the practicality of hooking it into existing systems.

One of the interesting side effects of putting a bunch of solar panels on my roof was the shade they provide (they sit about 4" above the roof) and that small gap of air made the attic significantly cooler than before the panels were installed. With these on the 'north side' of the roof (not with water plumbing, just providing a 5 degree C temperature difference) that would be helpful in the Bay Area.


When we built our house we put the actual metal roof several inches above the roof. The actual roof decking NEVER gets hotter than the ambient air temperature (115F max here in Texas). Solar panels or any form of shade structure (including trees) accomplish the same thing.

http://tinyhousemansion.com

Shingles or metal directly on a roof heats the house substantially but is cheaper (costs less in labor). That is why we still do it. (Just like dark roofs look "nice" so we still do that too).


Great link, and with this material you would get 5 degrees C below ambient on their underside, so nearly 9 degrees F lower.

Putting the shingles directly on the roof has benefits like keeping water out, and a floating roof (like my panels) is much more susceptible to wind damage (either to create a sail out of them). The 'old school' fix was of course sod roofs. If you put enough thermal mass between the roof and the interior you avoid significant heating.


What about capturing heat form the sun in the winter? Bay Area is a good example for that with sunny but cold days. Would those pockets of warmth we find in winter from sheltered sunny areas and attic spaces be reflected away too? It seems to me that capturing power from the sun in the summer and storing in maybe possible in the future local batteries is preferable to reflecting away the power? Or both?


Good question, I suppose if you mounted them as strips like a shutter (but with 180 degrees of rotation in the long dimension) you could have strips that were black on one side and this material on the other. When you want heat you turn the black side out to the sun, when you want cooling you put the reflective side out.


From the article:

The point of jackets, mittens, and scarves is to retain as much of that radiant heat as possible, keeping us warm on winter days.

This is AFAIK not how clothing works - excepting special ones like space suits, firemen suits, etc. Clothes are primary to prevent convective cooling, not radiative cooling.

5C cooling in Bay area may be enough for most of the summer outside of top heat days in Jul-Sep. If that is substantially cheaper than installing AC, it could be a good solution for most homes and save lots of energy.


I don't get why there's a magic band that lets the heat escape? Does it happen to be a gap in the spectra of the atmospheric gases?

Also I was wondering what the effect of plain old mirrors on people's roofs would be. The point being to reflect the sunlight of course.


Because physics is full of magic! Sadly I don't have enough physics background to google this phenomenon in a way that provides good results results, so I can't find other sources, but the way I understand it is there is a small band of radiative frequency that the atmosphere (at least on a non cloudy day) is almost completely incapable of absorbing, and so the energy is essentially easy to radiate into space. If I had to guess, it's probably related to the chemical makeup of our atmosphere, and a different gas and particulate makeup would create a different frequency band.

Mirrors don't work in the way you're suggesting because they don't provide enough of a shift in frequency in the energy they rebound (though they would act as an effective shade). For active cooling strategies, what you want is a material that can take the infrared frequencies from a heatpump, and shift them to the specific frequencies that make it hard for the atmostphere to absorb (similar to the use of phosphors in lighting for the visible spectrum).

Edit: I clearly didn't read this particular article closely enough, and the process they're measuring is actually a passive cooling system, which shows the material, or at least it's shade to be cooler than ambient temps. You could still use this material in conjunction with simple radiators to get better performance from a heat pump though.


> Does it happen to be a gap in the spectra of the atmospheric gases?

Yes.


Also I was wondering what the effect of plain old mirrors on people's roofs would be. The point being to reflect the sunlight of course.

The sun can hit the roof all day and it will not heat a normal house...so many layers in between and the heat will not reach below.

IMO it's just a matter of time before we use the sun to offset its effects on homes and offices. A super sunny day gives us a lot of power we could use to cool what the sun is heating. They coincide perfectly for a lot of hours.


> The sun can hit the roof all day and it will not heat a normal house

This is completely false. An attic that's heated up to 130+* will have a significant impact on the temperature of the house it's attached to unless you use an uncommonly ridiculous amount of insulation. This is why things like attic fans are so effective in reducing AC load...


If this is feasible, the great thing about it is it’a entirely passive, basically getting cooling for “free”. (I suppose you still pump a coolant through it but that’s fairly negligible energy cost.)


I could see people in hot climates using these as reverse solar panels for cooling their pools. Nothing refreshing about jumping into a 95* pool on a 100*/90% humidity day...


Will it zap birds?


I don't believe it's radiated in a focused beam, so no chance of zapping. Worst case would be like stepping in front of a mirror I think. I can definitely feel it when I step into the reflection of my house windows.


My thought was blinding pilots...


Infrared, no blinding.


So we're going to shoot these precious entropy differentials away into space?


No, we're gonna shoot some heat to space to gain some of those precious entropy differentials.


Low quality energy. Also, not sure how precious it really is. A car has a better entropy differential at the exhaust pipe.


You prefer the current approach, consuming excess fossil fuels to inefficiently dissipate them into the atmosphere?


http://sites.gsu.edu/geog1112/files/2012/10/Energy_Balance.p...

The earth already emits massive amounts of energy back into space. Nearly all the energy received from the sun is reemitted back into space, if this didn't happen the earth would quickly burn up. Adding reflectors won't make a huge difference as this energy would go back into space anyway.


In equilibrium that's true, but the whole problem is that we're not in equilibrium at the moment. That's what climate change is all about. If we can increase the albedo (reflectance coefficient, basically) of the planet, we can push the Earth down into an equilibrium position at lower temperature.

EDIT: Of course, we would have to significantly alter the albedo, which is probably not possible with rooftop panels alone.


Grandparent poster is suggesting that sending this energy out to space is a waste. It's not, if there were no reflectors, the energy will end up in space anyway. The reflectors emit about 40 watts per square meter. In comparison, the earth emits about 12 trillion watts per hour. So these reflectors won't directly make any noticeable increase on the earth's total solar irradiance.

From the article, it doesn't look like increasing albedo is the researcher's goal for these devices. It looks like they're doing this to reduce air conditioner energy usage, which would reduce CO2 emissions from fossil fuel burning power plants.

Also, the earth's temperature is never really in equilibrium in the first place, as it's always heating or cooling due to the day-night cycle or seasonal axis tilt.


> the earth emits about 12 trillion watts per hour

Delete "per hour". The watt is already a unit of energy per unit time (1 joule per second, specifically). So 12 trillion watts is the same as 12 trillion joules per second.




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