How could you build a simple inverse greenhouse box?

[askingask]

So basically, I stumbled across this concept of radiative cooling. There are a couple of YouTubers who posted videos on this topic.

What I understood, was that in passive daylight radiative cooling, one applies some kind of coating or film with high sunlight reflectance and high longwave infrared emissivity.

Now, there was also one video which talked about an „inverse greenhouse“. Instead of having glass which is transparent to sunlight but blocks infrared, you have a material that blocks sunlight, but is transparent to infrared.

This however sounds different then the first method.

I don‘t care about any large scale application for buildings yet. And I‘ve seen a company already doing that anyway. So I‘m asking for, is more a proof of concept. Ignoring conduction, how would you build a box that would have a net cooling effect. It would behave the opposite to a greenhouse.

 

[Tazerfish]

https://www.physicsforums.com/threads/what-materials-if-any-are-transparent-to-long-wave-ir.876231

There's been a similar post 8 years ago.
Though, intuitively, I can't really see it working.

High density polyethylene is fairly transparent in IR, but it's also transparent in the visible. Any metal will usually be reflective/opaque in all wavelengths...
If you have a material that reflects and scatters visible light, I'd expect it to do the same in IR or even absorb it.
If the material *absorbs* shortwave while being transparent to IR, then it will keep heating up in the sun because it can't radiate it heat away. Only conduction and convection can effectively cool it then. Odds are, it emits enough thermal radiation at *some* wavelengths to cool, perhaps ruining the reverse greenhouse effect.

If this was easy, someone would have already done it.
But maybe, just maybe zinc seleniide could work. They make these orange infrared lenses out of it.
But they obviously still transmit red light, otherwise you wouldn't be able to see through it.

If it has to be really reflective in the visible, maybe try germanium. But that stuff sets you back 30 bucks for 5 grams and if you melt it, you may mess up it's optical properties with contaminants.
Maybe there's some other material that can do it.
This is my best shot.
Hope it helped ^^

 

[askingask]

https://www.physicsforums.com/threads/what-materials-if-any-are-transparent-to-long-wave-ir.876231

There's been a similar post 8 years ago.
Though, intuitively, I can't really see it working.

High density polyethylene is fairly transparent in IR, but it's also transparent in the visible. Any metal will usually be reflective/opaque in all wavelengths...
If you have a material that reflects and scatters visible light, I'd expect it to do the same in IR or even absorb it.
If the material *absorbs* shortwave while being transparent to IR, then it will keep heating up in the sun because it can't radiate it heat away. Only conduction and convection can effectively cool it then. Odds are, it emits enough thermal radiation at *some* wavelengths to cool, perhaps ruining the reverse greenhouse effect.

If this was easy, someone would have already done it.
But maybe, just maybe zinc seleniide could work. They make these orange infrared lenses out of it.
But they obviously still transmit red light, otherwise you wouldn't be able to see through it.

If it has to be really reflective in the visible, maybe try germanium. But that stuff sets you back 30 bucks for 5 grams and if you melt it, you may mess up it's optical properties with contaminants.
Maybe there's some other material that can do it.
This is my best shot.
Hope it helped ^^

I‘ve read this post before, and from what I understand, this only works at night, hence why he only needs long wave IR transmissivity.

Maybe through a clever contraption, could one get the desired effect? Build a box thats simply highly reflective on all sides, except for the bottom side, which is IR transparent. Obviously not perfect, but maybe the visible light, which enters, is low enough in intensity, that the IR light, which leaves, carries more energy out. You get net cooling. My gut tells me it won‘t work though.

 

[Vanadium 50]

clever contraption

This sounds a lot like a perpetual motion machine.

You cannot build a device that spontaneously moves heat from a cold place (making it colder) to a warm place. Thermodynamics won't let you. The closest you can come without doing work is to take a region that is already cold and slow the transfer of heat in.

 

[Bystander]

This sounds a lot like a perpetual motion machine.

"The frosted windshield on a cold night" has already been reported; "clever use" of emissivities was the response to my report.

 

[askingask]

This sounds a lot like a perpetual motion machine.

You cannot build a device that spontaneously moves heat from a cold place (making it colder) to a warm place. Thermodynamics won't let you. The closest you can come without doing work is to take a region that is already cold and slow the transfer of heat in.

So you need to somehow pump it into space.

 

[askingask]

"The frosted windshield on a cold night" has already been reported; "clever use" of emissivities was the response to my report.

Now what interests me here, is the application of emissivity instead of transmissivity. Water has high emissivity. They used to use that property in the desert to freeze water in the night.

 

[Tazerfish]

From my understanding, emissivity—which has to be the same as absorptivity at that wavelength—is not very important. It just changes how much cooling you get, not what temperature you can achieve.
Plus, there is a gazillion materials that have emissivities near 1, and you can't get any better than that. Water really isn't special.

Making ice in a desert is admittedly cool, but that trick is sadly pretty finnicky and not easily replicated. The infrared temperature of the sky might not even be cold enough sometimes.
A quick forum search turned up that it's typically -10 to -20C at midlatitudes. In the subtropics, with hot descending air, you're just barely below freezing, most days.
Next you have to contend with all the heat flows from the surroundings into the water, ruining your radiative cooling.

A box made of metal might work.
Put a lid of germanium at the top.
Germanium even is pretty reflective in the visible!

Hell, maybe you could produce some actual cooling even in sunlight.
However, it sure as hell won't be *useful* or cost effective.

Maybe you can tell us what you're really after here.

 

[askingask]

From my understanding, emissivity—which has to be the same as absorptivity at that wavelength—is not very important. It just changes how much cooling you get, not what temperature you can achieve.
Plus, there is a gazillion materials that have emissivities near 1, and you can't get any better than that. Water really isn't special.

Making ice in a desert is admittedly cool, but that trick is sadly pretty finnicky and not easily replicated. The infrared temperature of the sky might not even be cold enough sometimes.
A quick forum search turned up that it's typically -10 to -20C at midlatitudes. In the subtropics, with hot descending air, you're just barely below freezing, most days.
Next you have to contend with all the heat flows from the surroundings into the water, ruining your radiative cooling.

A box made of metal might work.
Put a lid of germanium at the top.
Germanium even is pretty reflective in the visible!

Hell, maybe you could produce some actual cooling even in sunlight.
However, it sure as hell won't be *useful* or cost effective.

Maybe you can tell us what you're really after here.

People on youtube have already made videos on „passive daylight radiative cooling“. There are a few materials which are white and have high emissivity. I guess Limestone kinda works out here. Another thing is, that you could use any transparent and highly emissive material, and just put a strongly reflective mirror under it. Most incoming light goes through the material and gets reflected by the mirror. While the transparent and emissive material is cooling down.

 

[russ_watters]

What I understood, was that in passive daylight radiative cooling, one applies some kind of coating or film with high sunlight reflectance and high longwave infrared emissivity.

Now, there was also one video which talked about an „inverse greenhouse“. Instead of having glass which is transparent to sunlight but blocks infrared, you have a material that blocks sunlight, but is transparent to infrared.

Not special glass, just use a sun shade and point the device away from the sun.

I'm not sure how well it would work during the day, though, with scattered light from the atmosphere. Even at night I think the difference is only a few degrees.

 

[askingask]

Not special glass, just use a sun shade and point the device away from the sun.

I'm not sure how well it would work during the day, though, with scattered light from the atmosphere. Even at night I think the difference is only a few degrees.

Honestly sounds too good to be true. But it makes kind of sense. Very simple.

 

[russ_watters]

Honestly sounds too good to be true. But it makes kind of sense. Very simple.

Works for the Webb Space Telescope.

 

[Nik_2213]

Can you arrange solar-powered shutters ? Electric or thermal ??

Passive louvres are a simple way to regulate solar heating, but technique varies by latitude..

 

[sophiecentaur]

"The frosted windshield on a cold night"

The 'cold sink', on a clear night is almost equivalent deep space for which the effective / net temperature is very low. An ancient technique for producing ice in the desert at nights involved large shallow pools of water which would freeze and the ice was removed and stored underground (by slaves, of course) for use in the rich guys' drinks.

 

[askingask]

For those who are still interested into this, I‘ve found an article which claims to achieve sub zero temperatures in daytime.
https://scholar.google.com/scholar_...n#d=gs_qabs&t=1724616438354&u=#p=d7qMGmM5zxAJ