In order to cool below the ambient air temperature using passive radiative cooling during daytime why is it important to radiate heat into space? [closed]

Question

Why not radiate it instead of the surrounding air, like Air conditioners? I know an air conditioner does not transfer/dump heat to the outside by radiation, but by phase transformation-specifically condensation..but AC's do dump heat into their immediate environment. not up to space/upper atmosphere. This question is in regards to Sky cool systems (https://www.youtube.com/watch?v=CMaZfuInd9c&lc=Ugzi8-NoZHgor7JAa1V4AaABAg.9f29-BNHdQR9f2mwQeWzRI) which makes meta material film to be used on rooftops of buildings to cool the building directly-or to cool water pipes. The film is made out of meta materials,that in addition to reflecting away all sunlight incident on it, also radiates away infrared in the 8-13 micrometer range. These mid infra red wave lengths are not "trapped" by greenhouse gases in the lower atmosphere so radiate straight into upper atmosphere or space.

Is it because if the material was radiating/emitting infrared merely into the surrounding air (instead of radiating out to space)- part of that radiation would just be returned back to the surface/material radiating it after being absorbed and re emitted by the gases in the surrounding air? Thus the requisite cooling would not happen in daytime direct sunlight? So is this sort of like an inverse greenhouse gas effect? Is it necessary to radiate away in the spectrum where the infrared escapes to space, to achieve the requisite cooling?

Or is it because passive radiative cooling cannot transfer heat from the inside (colder) to the outside (sunny and warm) air-only to the further colder environment of space or the upper atmosphere... For transferring heat from cold to warm one needs to work against entropy. Air conditioners/heat pumps overcome entropy by using electric power to compress vapors. By using meta materials Sky cool is essentially a creating a heat channel to space...

Also how is this approach of using metamaterials (which radiate away in the specific wavelength bands of mid infrared, that are not absorbed by GHGs),different from using regular infrared reflective paint (which I presume radiates away at all wavelengths of the infrared)? Are my questions clear?

Answer 1

The key word in your question is “passive”. Passive heat transfer only goes from hot to cold. Heat transfer from cold to hot requires work, like in an air conditioner.

So, in a typical summer day you might have the sun at several thousand K, the ambient environment at a little over 300 K, the building that you want to cool to a little under 300 K, and deep space at about 3 K.

Of those possible heat reservoirs only space is less than the building, both the sun and the ambient environment are warmer, so to have heat passively flow out of the building it must go directly to space.

Answer 2

I am not sure if I am allowed to post what I think might be an answer to my own question. <Edit: You are.> I am very new to any Stack Exchange, or asking questions on the Internet. I just joined yesterday. <Edit: Reading the FAQs can help.> I found this answer on another Physics Stack exchange thread to a question very similar to mine...

<Edit: In general, if you ask a very similar question and someone notices, your question will be deleted.>

similar question

The answerer replied: " Kirchoff's law states that body emits radiation at a given temperature and frequency exactly as well as it absorbs the same radiation... Say instead of aiming for the transparency window they designed the cooler to emit at frequency X that the atmosphere does absorb. By Kirchhoff's law of thermal radiation, since the atmosphere absorbs at X it also emits at X. And likewise since the cooler emits at X it also absorbs at X. That would result in the cooler absorbing extra radiation from the atmosphere at frequency X, which would heat it up. " Could you please clarify this answer in quotation marks? Wouldn't the same thing happen to a cooler like the Sky Cool material designed to emit at frequency Y (8-13 micrometer or the atmospheric transmission window) wouldn't this cooler also absorb at Y because it emits at Y, thus also heating up? Or am I misunderstanding Kirchoffs law of thermal radiation? I have little background in Physics/engineering. Thank you for the clarificaitions.

Answer 3

If you radiate infrared and some of it will be absorbed by the atmosphere, then yes, some of it will radiate back to you. If the atmosphere is cooler than you are, you'll radiate away more than you get back.

If you radiate infrared that won't be absorbed by the atmosphere, the atmosphere will radiate some also and will radiate some of that to you. But both you and the atmosphere will radiate some in the other direction, out into space. The atmosphere won't absorb what you radiate so it will be cooler. That works better than when the atmosphere absorbs it.

I remember reading in an old book about camping recipes, they made a reflector oven by taking three sides of a large cardboard box and covering the insides with aluminum foil. Since it was a corner, it tended to reflect stuff back in the same direction it came from. They used it as a solar heater. They used it to reflect heat from a fire, so their food cooked faster. They slept on the opposite side of the fire so the reflector would provide somewhat more heat. And they had a trick -- at night they aimed it at the sky and suspended a bowl of water in front of it. They claimed they got some ice that way, when the outdoor nighttime temperature was around 50 degrees F. The water radiated heat, and the reflector sent it into space and didn't reflect nearly as much back from the night sky.

I haven't done that myself.