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Comment by ColinWright | Hacker News Reader

This reminds me of using a solar oven in reverse.

Take a heavily insulated box with a window on one side and put a parabolic reflector in it. Point the reflector at the clear night sky, and you can create ice at the focus.

All objects are in a balance between incoming and outgoing heat, so when all the heat going out it sent into a sink - such as the clear night sky - the object will drop in temperature.

The effect isn't huge, but it's real. The system described in the submitted article here has nothing like the energy generating potential required to become a sensible source of renewable energy, but it's a nice effect, and a nice system.

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Use this as the low-T side of a heat engine, and you've got a nice little generating technology. Maybe filtering the outgoing radiation to enhance an available low-absorbance band (or bands) and you might be able to help yourself to the T of the background sky. I've actually thought about this a fair bit for things like low-T geothermal resources (increasing the Carnot Delta T by working on the low-T side). Any optical physicists want to prove to me that I'm full of crap?

I’m trying to wrap my head around this. While shielding the focus from receiving any thermal radiation via surrounding objects ( ground, etc) you also provide a 360 degree thermal radiation heat sink, making it as easy as possible for whatever is at the focus to shed heat?

Yup.

http://solarcooking.org/radiant-fridge.htm

Your description does not appear to be correct. Ice is not being formed at the "focal point," ice is being formed under the reflector.

You can't focus coldness. "Cold" radiation is simply less radiation. When you focus it, you get more radiation, not less of it. More radiation means more energy.

Instead you're simply seeing the effect of radiative cooling. See "Nocturnal ice making in Early India and Iran" [1]. By insulating the sides of a pan of water, and leaving the only window for radiation pointed to the night sky, you're ensuring that more radiation is leaving the water than entering it. Thus the water cools down.

So it would work equally with an insulated box with high walls. It's not about focusing the coldness.

[1] https://en.wikipedia.org/wiki/Radiative_cooling#Nocturnal_ic...

I have a feeling we're talking past each other, and I'm not going to bother addressing everything you've said point-by-point. Experiments show that have a parabolic reflector configured correctly results in a stronger effect. You're not focusing coldness, but the radiation from the body being cooled emerges in every direction, so putting it at a parabolic focus ensures that all the radiated heat gets directed outwards to the night sky.

Yes, simply having an insulated container and having an opening pointing at the sky does work, but it's not the best you can do.

The focusing effect will help, because you're focusing the infrared radiation from the object away.

If you have an insulated box with tall sides, radiation from the object will reflect from the sides of the box back to it, keeping it warm (or less cold).

Off topic, but what's with the spammy links at the bottom of this page? Did the site get hacked?

Screenshot: https://imgur.com/a/Oz4XEhy

Not seeing it. You got hacked? Plugins maybe?

They are there, and deliberately coded to render as invisible.

This is the URL: http://solarcooking.org/radiant-fridge.htm

Pull the source and look at the last few lines. Alternately, hover your mouse over the area at the bottom of the page and look at the URLs it would go to if you click.

I've tried contacting the author but the email no longer works. I've not yet contacted the site owner.

I did view the source, but I was expecting exactly what GP screencapped rather than a single line. So, yep, it's there.

What is sort of practical temperature differential can you achieve? Eg, would you expect to get ice if ambient was 50-deg F? How about 80-deg F?

Can someone who knows how this works explain what's going on in terms of the 2nd law of Thermodynamics?

Not much is going on in terms of the 2nd law, because this device is not a closed system (heat is coming in and then later going out) so the 2nd law doesn't apply.

Consider a small body at the focal point of a parabolic reflector, and that reflector pointed at empty sky at night.

You effectively have the body radiatively thermally connected to space, and space is cold. So heat radiated from the object goes out to space, and nothing comes back, and thus the object gets colder.

If doesn't work if there are clouds, because clouds are quite warm.

> ...because clouds are quite warm.

And are, therefore, radiating heat back into the small body via parabolic reflector.

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