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Comment by jack_riminton | Hacker News Reader

I’m missing the basic premise here, how does pointing an insulated box at the sky at night cool whatever’s in the box?

A black body will emit radiation (and receive radiation) until it's in equilibrium with its surroundings. In theory, the black bit of box radiates heat into space, and receives some of the CMB until both are at ~3K. It's how we cool satellites and spacecraft.

But the effect is quite small, and I'm suspicious of it (as the nearest surroundings are the hot planet, which transfer heat more effectively!)

Snow is a near perfect black body radiator

Therefore imagine the impact of reduced snow and ice due to global warming. It’s free cooling and we’re throwing it away.

The albedo is the part that's bad to lose. I don't think snow emits especially strongly in the frequencies relevant for this. A highly imperfect blackbody emitter could cool more effectively.

This was a key plot point in the 1990s video game: Midwinter

https://en.wikipedia.org/wiki/Midwinter_(video_game)

If you grow crops, your crops can freeze when it's a clear sky at night with stagnant air even though the air temperature only goes close to freezing.

In some agri areas, you'll see "wind machines" to blow the warmer air around the crops to keep them from radiating their heat and frosting.

https://www.ontario.ca/page/wind-machines-minimizing-cold-in...

Wow unradiant heat, or more accurately radiant cooling I suppose. Fascinating

It's possible to make ice in the desert with this effect. Not too hard to believe.

It’s also commonly freezing in the desert at night

it's easy to calculate how big the effect is with units(1)

    You have: stefanboltzmann tempC(20)**4
    You want: W/m**2
     * 418.76592
     / 0.0023879689
    You have: stefanboltzmann tempC(0)**4
    You want: W/m**2
     * 315.65782
     / 0.0031679874

so in the temperature range of interest you're radiating 300–400 watts/m², derated for your surface emissivity at the relevant infrared wavelengths. as i understand it, the temperature will drop until the heat emitted through radiation is counterbalanced through heat that seeps in through the insulation and through the stagnant, stratified air above your pan of water, and, probably more importantly, through greenhouse-effect radiation from the warm atmosphere and any opaque objects you have unfortunately included in your field of view

tens or hundreds of watts of cooling is quite significant indeed when we're talking about making some ice or cooling some food

There are some IR bands that are transparent between the ground and space so if you optimize your emissions in those bands (because you are not an idealized black body) and insulate yourself from conduction and convection heating, you can beat your local surroundings and cool yourself lower than ambient to a useful degree.

https://en.wikipedia.org/wiki/Infrared_window

There are materials and even paints that have emission spectrums designed for this.

For example: https://www.parc.com/technologies/self-cooling-paint/

Normally you would just be in the same thermal equilibrium with everything else having similar emission spectrums you'd give and receive similar amounts of radiative heating and cooling. Tweak your spectrum and you tip the balance in your favor.

> as the nearest surroundings are the hot planet, which transfer heat more effectively!

The box is insulated. In one case it's an unused refrigerator that opens on the top with more insulation apparently surrounding it. The planet may be hot but on a cold night the ground does nothing to warm me. Soil is not a good conductor of that heat at the surface nor is the air between it and the container. The planet stays "hot" because it is "insulated" as well.

The ground does a lot to warm you, radiating terrestrial temperature BBR to your entire lower hemisphere, basically it's the same ~100W/sqm.

Sleeping under some tree cover on a clear night vs sleeping under the clear sky makes heaps of difference.

The black body temperature of the night sky on earth is much warmer than the temperature of the CMB in the universe. You can test this by aiming an IR temperature gun on a clear night.

Relevant stack exchange: https://physics.stackexchange.com/questions/153839/what-is-t...

It's an issue for amateur astronomers too as you have a mirror in a long tube pointed directly at the sky: https://britastro.org/2021/dealing-with-dew

That's why he insulates his box from the hot planet and enjoys the fact that the air is largely IR-transparent.

Almost all surfaces radiates heat. You can heat and cool using radiation. It’s the same way satellites is cooled, when you can’t cool with convection (interact with other atoms).

Black absorb more radiation, but it also radiates more

Great question!

“PDRC surfaces are designed to be high in solar reflectance (to minimize heat gain) and strong in longwave infrared (LWIR) thermal radiation heat transfer through the atmosphere's infrared window (8–13 µm) to cool temperatures even during the daytime.”

https://en.m.wikipedia.org/wiki/Passive_daytime_radiative_co...

I also posted this as a separate post for visibility. It’s an amazing technique that more should be aware of.

I guess that not having other warm objects pointing at the water, will not warm the water (example: a wall radiates infrared so it heats the water if the wall has "sight" over the water).

Insulating the walls of the container with the water servers the same purpose - not have the water get heat from the surroundings.

I'm guessing he just froze water when the temperature dropped below zero at night, and everything else is nonsense.

I wonder how you prove/disprove this. I imagine if you had a control box with water exposed to the air that wasn't "radiating heat," then that should show if the solar freezer was indeed working or not.

You could build a few test configurations.

One as stated, insulated fridge, open on top.

One, insulared fridge, open on top, under an open air shelter, no walls, just a roof, where the roof is a significant distance away from the top of the fridge, so airflow is not impacted, but there's no view or the sky. Possibly variations on this with the roof being ir transparent or not.

Another test configuration could have the fridge closed, but with forced air ventilation.

He did try a basic control with his first version: “i got ice consistently over several nights, even though there was no frost on the ground, and pots of water sitting next to the box cooker did not freeze.”

Ah you're right! I totally missed that line. Cheers.

Humans have been using this technique (radiating heat into the cold of space) to freeze water in hot places, for a very long time.

https://www.realclearscience.com/blog/2018/07/09/how_people_...

That's an interesting article. I'm aware of this sort of radiative cooling, but usually convection and conduction, along with evaporative cooling will be much, much more important than radiative cooling. I'd like to see some kind of report or article showing the cooling potential in W/m^2. I remember someone made a metamaterial optimized for emitting heat in the wavelength of the atmospheric window, but the value was quite low.

You might be thinking of the work led by Professor Shanhui Fan @ Stanford some years back;

https://news.stanford.edu/2017/09/04/sending-excess-heat-sky...

This is being commercialized, but I don't know how successful it has been to date.

https://www.skycoolsystems.com/

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