Checking my understanding here: in a very simplified sense, is this saying that we’re taking energy, be it from ambient temperature during the evening or that plus solar radiation during the day, and essentially bottlenecking/‘forcing’ it to radiate in the spectrum that’s atmospherically transparent and can thus escape to space? Seems the use of insulating it in a vacuum would be to minimise heat loss until it’s cooled to the respective temperature that radiates in that specific spectrum.

Incredibly cool (hah) paper to read, thank you.

You might like NightHawkInLight's videos on the topic[0] and Tech Ingredients[1] - they've been working on making paint using this technology with an at-home DIY process and experimenting with it.

[0] https://www.youtube.com/watch?v=KDRnEm-B3AI&t=285s

[1] https://www.youtube.com/watch?v=dNs_kNilSjk

Tech Ingredients is amazing

You should take an infrared thermometer and point it to the clear sky. If the sky is very clear, you will get temperatures much lower than ambient. You just let heat radiate away, and this is enough to cool.

this is a great paper, thanks

a thing to keep in mind with research papers is that in many cases they're concerned with finding how to get the largest effect, or demonstrate the effect in a way that's most clearly due to the reasons they claim and not some experimental error, or measure the effect most precisely, and only in rare cases are they concerned with how to get the effect most cheaply, a consideration which conflicts with the others

there are a lot of cheap materials that are transparent in the thermal infrared, like ldpe, potassium chloride, sodium chloride, silicon, and rock crystal. they're mostly a pain in one way or another (not that i have any experience with this)

Why isn't this replacing aircon over the whole world? Unless that IR-transparent window is made of gold dust, it has to work out cheaper than the energy cost of running an air conditioner.

If my napkin math is right, solar panels give around 2x higher electricity production per square meter than these devices give cooling power. Then that goes into a COP of ~3 for your aircon, and the solar panels have have a factor of 6x better cooling performance per area covered.

Then factor in that electricity can be used for lots of other useful things than cooling, and that solar power variation during the day is perfectly matched with cooling demand, it's a no brainer.

my air conditioner is rejecting 3000 watts to the outdoors right now, even though my bedroom is the same temperature as the nighttime outdoors. doing that with radiative cooling would require 8 square meters of emitter area at 400 watts per square meter and a clear, low-humidity sky (not necessarily night, but in the daytime, you additionally have to keep the sun off the panels). my air conditioner's condenser is significantly less than 1 square meter as seen from the sky and also works when it's cloudy or humid outside

basically forced-air convection heat transfer is a motherfucking miracle, and vapor-compression refrigeration even more so

I am disappointed you could not come up with a more concrete term for an improvement over a 'motherfucking miracle'. :)

But, yes, heat pumps are magic shite.

magic shite!

Adding to everyone else, but local climate makes a big difference. This isn't very useful unless you have dry still air, and it works best at night, but could maybe work during the day if you have a good view of the sky but have blocked the sun.

Most places with dry still air don't need specific cooling at night, because "everything" already cools due to this effect, and it doesn't stay hot for long after the sun goes down. Areas that need a lot of cooling overnight tend to be humid, which disrupts radiative cooling. Also, it doesn't take much airflow for convection to transfer more heat than radiation, and most places have variable wind... so you can't really count on it.

> Most places with dry still air don't need specific cooling at night, because "everything" already cools due to this effect, and it doesn't stay hot for long after the sun goes down.

Exactly this. This is reflected as well in the article, even if not promintently: they have 30C Celsius at midday (which is hot) and 4-5C at night (which is cold). 25C between day and night is close to desert behavior as far as diurnal air temperature variation [1] is concerned.

[1] https://en.wikipedia.org/wiki/Diurnal_air_temperature_variat...

Here in Brazil we use a system with copper pipes in an insulated "box" to heat water during the day using this energy from the sun. A couple of years ago, in a cold winter day with a minimum of 5 degrees (celsius), just after the sunrise the water froze, broke the pipes and the glass above it. I couldn't understand what happened because the ambient temperature was above freezing point, maybe it was something like this effect?

Fascinating! That does seem like the most likely explanation.

This reminds me of the ancient ice ponds that made ice thousands of years ago in Persia. I read somewhere that they were able to make ice through a combination of radiative and evaporative cooling at night temperatures around the same as you experienced, about 5C.

Yes, I like this explanation, when that phenomenon happened in our house I thought one of our neighbours was dumb because he just put a blanket over the collector on the roof, as he said, "to keep it warm". But now I think he was right. Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve the problem of water frozing from radiation in our solar heating.

>Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve the problem of water frozing from radiation in our solar heating.

This doesn't exist and can't even in principle because it would be a fundamental violation of thermodynamics. Basically, it would be a Maxwell's demon for radiation that would allow you to arbitrarily reduce entropy.

But, it might be possible to have a material that has different characteristics at different temperatures, as long as it's symmetric. If it's nearly opaque to IR in its cold state, hopefully sunlight at dawn would warm it rapidly enough that it would automatically "shut off" on cold nights, and still "turn on" shortly after dawn even on cold days.

Light meter controlled / tracking louvers would be the quick fix.

Yes I think that'd be ok. You'd basically just be taking advantage of the thermal gradient between the radiating body and the object being heated. It'd be analogous to adding more insulation as it heats up and removing insulation as it cools down.

You'd be slowing the flow of heat into your reservoir just as much as you'd be slowing down the loss of that heat later though.

You might still get some benefit in preventing freezing at the expense of needed a larger area to get the same amount of heat flow in to your water system though.

I don’t think maxwells demon is impossible, it just is a device for converting información to energy. In theory, many such devices may be possible for the conversion of information to both energy and matter. This seems like it might be support the simulation hypothesis.

We call this a one-way mirror.

https://en.m.wikipedia.org/wiki/One-way_mirror

They are not perfect (intensity & wavelength limitations), but they exist.

See also mirages and total internal reflection.

Relevant link: https://en.wikipedia.org/wiki/Yakhch%C4%81l

The handwavey explanation in my head is that “ambient” for this emitter is not just the immediate physical environment, but also deep space, which is very very cold. This wouldn’t work if the temperature of space and the air were the same.

An emitter is also an absorber so if space were as hot as the emitter then it would not shed heat.

Yeah, on a clear night the pipes radiate heat to space but nothing much radiates the other way as space is cold and dark. It's also why the tops of cars get frosty on clear nights.

Simple plastic film stretched over the opening would be a good start, perhaps two layers with some air between. You lose a little in radiation but gain heaps from stopping convection.

Could this be used for power generation at large scale, while cooling the Earth at the same time?

Consider this: quite powerful solar thermal generators [1] and furnaces [2] have been built, which use mirrors to concentrate sunlight. Do they create intense cold at night? No, because you can't focus coldness. The device in the article, and the one described above, depend on shielding a volume from terrestrial heat sources, and are effective only because the heat being transferred is minimized.

[1] https://en.wikipedia.org/wiki/Gemasolar_Thermosolar_Plant

[2] https://en.wikipedia.org/wiki/Odeillo_solar_furnace

Will it work equally well on nights with cloud cover?

No. It doesn't work the same as clouds are at ~0C+ whereas space is like -270C (~3K).

Theres videos on youtube by a dude that does Science experiments at home making radiant paint from common stuff, it's really good and easy to use and shows solid reductions below ambient.