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Comment by ar813 | Hacker News Reader

I'm one of the authors / people working on this. Yep, radiators like the ones on the ISS are pretty much the dominant/only ways to get heat out in space.

In response to your questions: yes, it can in fact cool below the ambient air temperature entirely passively (even during the day! -- which adds real value for cooling applications). We've shown in other recent work that you can use this effect to cool as much as 45-50°C below the ambient air temperature, if you insulate the radiator perfectly.

The reason this works here on Earth is that some upward thermal radiation isn't absorbed and re-emitted back to you. So if you start at the air temperature, looking upwards, you will be sending more heat out than the sky sends back to you. This allows an upward-looking surface to cool itself down until the heat going out and coming back to it balance out.

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Can you apply this technology - coupled with AeroGel based insulation of radiators - to shipping containers/container walls?

This would allow for a really low cost refridgerated/temp-controlled containter. Then have a small requirement for a solar panel to pump some fluids. The result would be the ability to have shipping containers that can maintain temps with extremely low cost/energy requirements and you might revolutionize the ability to ship foods around places like Africa.

Also - can this be applied in any way as a "paint"? Such that you paint cars, roofs or whatever with this?

This is brilliant! Specifically the container idea!

> 45-50°C below the ambient air temperature

That's incredible! Isn't that enough to run a heat engine on? If so, does this allow you to extract (nearly) free energy out of the ambient heat?

That differential could run a Sterling engine.

My god, that could be a great asset to communities in water-starved regions of the planet.

You can run a Stirling engine on much lower temperature differentials:

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

https://www.amazon.com/Three-Stirling-Engines-Without-Machin...

That differential was quoted for perfect insulation. Plugging in a heat engine is the opposite of insulation.

It all comes down to units of thermal rejection per area per unit lifetime cost. If either one of those values is less than desirable expectations will fall short.

Have to consider the amount of heat moved, too. It's the same difference as between pressure and volume, or volts and amps. Perhaps it can't move a large volume of heat quickly, but can achieve a decent temperature difference given enough time?

And even if it can, I am reasonably confident (from my arm chair here) that this combination of relying on a narrow band of passive IR emissions to drive a heat engine will be less effective than a photovoltaic cell generating electricity directly from incoming visible radiation across a relatively broad frequency band.

Ah, but only one of those still operates in the dark.

50C is not much for heat engines your limited to under 15% effecency in the best theoretical case. So solar + battery is going to be a net win unless your thinking of flipping panels at night for ~1% more power over pure solar panels which is silly from a cost perspective.

I thought solar was widely favored long term, but it's going to be a long time before it establishes that position.

Does this work when humidity is high? Water absorbs thermal infrared and emits it back.

You can still get cooling, but you get less heat rejection per unit area in high humidity because of the effect you point out.

As someone who is very unknowledgeable about this subject area, what kind relationship between humidity and lost heat rejection would there be?

I ask because as someone who has lived most of his life in southern Florida, this kind of device could reduce electricity bills massively around here, as long as the humidity isn't too much of a problem

You heat the surrounding humid air instead of sending the heat into space.

If there is even a slight breeze blowing, then wouldn't that move enough of the heated humid air out of the way?

No significant difference. The point is that the more opaque the air is to infrared, the more your panels are in thermal equilibrium with the surrounding air (ambient air temperature) rather than space (2.7K). They're in equilibrium with a weighted combination of those. Doesn't matter whether you heat up the surrounding air by 0.01 degrees.

And that surrounding air will radiate thermal infrared back heating the panel. As was pointed out in another thread, this still may work, but not as efficient as long as some infrared is not absorbed.

> if you insulate the radiator perfectly.

I don't quite understand what that means. If your radiator is supposed to radiate away the surrounding heat, shouldn't it on the contrary be connected as much as possible to the environment?

No, because the heat is radiated through infrared -- not conduction and convection. Any heat transfer by conduction/convection is going to be warming the thing you want to cool and will be working counter to the purpose of the device.

Conduction, convection and radiation.

my understanding is what you're talking about only applies for traditional conduction/convection, where you're expecting moving air/water/... to act as a heat-transfer medium, so you want to maximize your surface area of your radiator and fluid flow rates to facilitate that transfer.

what they're doing here is shifting the transmission characteristics of radiation to be within the infrared window (https://en.wikipedia.org/wiki/Infrared_window) so that it's kind of like surrounding air (and water vapor, most importantly) aren't present at all and you're radiating in to the giant cold heatsink that is space.

Two questions:

1) Does it emit more IR at higher temperatures? In other words, would pumping hot water through/under it produce larger temperature drops?

2) Is it weather proof, or do you still need to find a coating that protects it while not blocking the IR?

Yes, it emits more radiation/ heat when hotter. A nice feature of radiation is that scales as T^4.

It's pretty durable on its own. That being said, as we've moved to the product stage we've packaged it in a way to protect it further.

Black body radiation scales with T^4, but you're relying on one particular band. I suspected it scaled, but T^4 seems rather unlikely ;-)

I would assume it scales at K*T^4 where K is a constant < 1 and gets smaller as the band narrows.

Most materials scale with T^4. You would need a very weird radiance spectrum to do anything different.

That said, we are talking about something that has a weird radiance spectrum as a selling point, so, expect some deviation from the standard T^4 curve.

I'm not sure why, but I've always thought it interesting that radiation scales as a fourth power. It's unusual to see forth power relationships in fundamental scientific processes.

Could you point the beams at something and heat it up? It seems like a waste just sending it into space.

I can't think of any scenario where you would be better off recapturing that IR beam instead of installing photovoltaics and capturing the stronger sunlight.

Also, removing heat from the Earth is probably a positive at this point in time.

The problem is that the heat is in the wrong place, right? We're not going to stop needing electricity. If you've got an IR beam and you can point it at something you can heat whatever it is and run a generator.

You need to cool the building anyway, so the added energy from displacing the beam to a (presumably turbine) generator would essentially be 'free.'

This is more efficient than the photovoltaic because you don't have to produce install and maintain a photovoltaic (all of which costs some energy), and you were going to cool this building with this method anyway (so there's little downside to harvesting the energy aside from complexity, which is a concern).

I assume you'd need line of sight from the panel to the generator, which it does seem would be tricky, but doesn't seem insurmountable. (Put the collector on a tower for example, in most areas of the country I think it wouldn't have to be higher than a few hundred feet). It just depends on how much heat you're displacing and whether you can collect enough to run the generator.

If it were "I'm just trying to produce energy", I'd agree that the photovoltaic is better, but if this system happened to be dual use and doing two things we already want to do, it seems like it would be a pretty substantial win.

My understanding of this is that the amount of heat transmission depends on the temperature of your target. You point it at outer space (cold), and you get a win. You point it at some sort of IR collector (necessarily at ambient temperature, or higher), and you've lost your cooling ability.

Right. One must remember that the target sink is going to be throwing radiation right back at the source emitter. Theoretically, they will exchange energy until they reach equilibrium between emission and absorption. Luckily for us, it will take a long time to heat space up enough to make a difference...

This also ties into why a magnifying glass cannot make something hotter than its source of light.

https://what-if.xkcd.com/145/

"Lenses and mirrors work for free; they don't take any energy to operate. If you could use lenses and mirrors to make heat flow from the Sun to a spot on the ground that's hotter than the Sun, you'd be making heat flow from a colder place to a hotter place without expending energy. The second law of thermodynamics says you can't do that. If you could, you could make a perpetual motion machine."

Thanks for that, it really helped me understand the problem with my thinking.

So if you point it at a satellite, or the ISS, and run a generator? Would that be a good backup power source for solar currently used?

That's a great question. The thought of an orbiting IR collector crossed my mind while typing that comment, but I didn't want to think about it hard enough to address that. :)

The "beam" this generates is really more of a floodlight than a spotlight, so only a tiny part of the energy would hit your satellite. If you could focus it down to a narrow beam which hit only your satellite, my gut feeling is that you would simultaneously be focusing the surface heat of the satellite onto your panel. So once again, your panel would only "see" a hot surface, and would lose the benefit of cold space.

Also, as others have calculated in other comments, the amount of energy coming from the sun dwarfs what these panels radiate, so you are better off just pointing your satellite collectors at the sun.

> Also, as others have calculated in other comments, the amount of energy coming from the sun dwarfs what these panels radiate, so you are better off just pointing your satellite collectors at the sun.

Especially in space! We lose a good amount of irradiation energy from the sun to the atmosphere.

The ISS has no shortage of warmth. Their challenge is keeping cool. Adding IR heat will only make things worse and require the use of more (solar) energy to remove that heat again through its cooling panels, which themselves work like this invention by radiating IR heat into space!

Maybe we could use it as a distributed way of beaming power to satellites or space probes. Have them all be capable of tilting and network them all together.

Low temperature thermal energy is really worthless unless it's in somebody's home in winter or another case where you need to keep things slightly warm in a cold environment. You can't store it or transport it very far at all. It's the waste product of just about every kind of machine. Look at the steam pouring out of this cooling tower -

https://en.wikipedia.org/wiki/Cooling_tower#/media/File:Cool...

Even a power station doesn't bother trying to use that for anything.

A common use is space heating in buildings in winter, but you won't be running an air conditioner in that case, nor will your neighbors want it.

You're not wrong, but it's also important to remember that they're running a heat engine to extract work. The amount of work that can be extracted is dependent on the temperature differential between the heat source and the heat sink. The water being cooled there is to keep the heat sink cold. And since the heat sink will always be colder than the heat source, they can't recycle any of that heat back to the heat source without expending energy to move it. Which would kind of ruin the point.

It's not that the heat isn't useful, or even that it's not a large amount of heat. It's that the heat is in the wrong place, and they need to efficiently get rid of it.

To me, it seems like a nice way to help alleviate climate change - though my guess would be that it's not enough energy to make a big difference.

If, at minimum, every air conditioner in the world becomes 21% more efficient, then that's a bit of a dent in climate change.

I guess I was mostly thinking about the effect of reusing the IR beam. It gets a little confusing to me in that AC units move more energy than they expend. So for example a 10 EER means an AC unit moves 10x more energy than it takes to run. So if a 10 kW AC unit becomes 8 kW to run (~20% more efficient), a the same time the panel would beam out ~5% of 100 kW of moved energy? Or 5kw. If you reuse 5kW somewhere, then you lose ~70% of the potential climate change heat savings? 5/(5+2)?

I'd have to recheck al the assumptions here because I put high odds that I constructed this back of the envelope set of numbers with some mistake in it...

It also means that air conditioning may become accessible to some people for whom it wasn't before, e.g. because they rely on solar power, or because electricity is expensive.

You can heat things up to the cooler temperature.

If you point it at something, and it heats up, it will radiate back heat up the thing you are trying to cool. The whole point of pointing it to space is that space is cold (in the radiation sense).

Wow, cool. Well, thanks for doing this sort of research! Better cooling could save us a whole lot of energy :)

I assume this is negatively affected by cloud cover, but I'm curious to what degree.

I was curious too, because the article was very specific about the wavelengths emitted. I found some sources saying that the 8-13 um wavelengths are in a band that isn't absorbed much by water vapor, e.g. https://wattsupwiththat.com/2008/06/18/a-window-on-water-vap....

I find hard to believe that without your special cover, the heat would travel 10-100km up to atmosphere and then reflect right back onto the radiator.

What I am missing?

You're missing the bits of atmosphere right under (and inside) your nose.

It's also not like other wavelengths are reflected right back, but when the atmosphere can absorb them, it will also emit them. (Radiation tends to be symmetric like that.) Conversely, the emitting panel will absorb this radiation, until equilibrium is reached.

That means that you need to use wavelengths where the air isn't literally glowing, if you want radiation cooling to work at all.

That's cool. But presumably any surface insulated in this way will reach equilibrium with the atmosphere/space in a similar way, correct?

Wondering if the radiation tech could work in sending the heat generated from other sources like clothes dryers etc as well?

Is this related to how ice can form in the desert at night?

this is such amazing work! how expensive are these radiators before and after scaling production? ignoring the pumps, how long should they last without repair?

The great thing is that we can use existing manufacturing processes (which we are already using) to do this extremely cheaply at large volumes. So the cost at scale will be competitive and make economic sense for customers. Our focus is on packaging this into a compelling product that delivers large electricity savings. Lifetimes of 20+ years are expected.

So are those normal thin-film deposition processes? I don't know much about those, but am interested in them - are any of the materials particularly tricky to work with?

Or can you just sputter the oxides one-by-one onto a silver substrate?

Thanks for jumping in! How many installed square feet of real estate do I need to be equivalent to my 3 ton air conditioner (which needs 4 square feet)?

any chance yall are hiring? :)

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