Radiative Cooling At Night (For Heat Sinks)

Terraformer · 2023-01-28 04:00:26

It's possible to make ice, when the ambient temperature is above freezing, by exposing water on a cloudless night and letting it radiate to the 3K night sky. Does anyone know how large a differential can be generated by this? If we used an evacuated tube radiator, say, so that the dominant source of energy is the night sky, what sort of temperatures could we achieve?

Last edited by Terraformer (2023-01-28 07:46:21)

Terraformer · 2023-01-28 04:45:14

What. Radiative cooling is possible even in the daytime. With results suggesting a 30F (17K) temperature difference being possible to achieve (National Geograph). That's in the day without any vacuum to isolate it from the air.

With a 3K sink to radiate to... how frozen can we freeze things?

Terraformer · 2023-01-28 07:12:37

Oh. Spacecraft shielded from the sun and radiating in a vacuum have got down to just above background temperature, haven't they.

I don't know how to calculate the flux that a radiator at say 250K would have. How fast can we cool things down?

Terraformer · 2023-01-28 07:46:05

Aaand it's just the Stefan-Boltzman law. Assuming a perfect emitter and ignoring heat from the surroundings, at 200K we would be removing 91 W/m^2 worth of heat. 3.6L of ice from room temperature water every hour for each square metre.

But I don't want ice from room temperature water. I want a very large insulated block of supercooled mud, which I can then use as a heat sink to generate power from moderate (3030-33K) temperatures that are achievable by geothermal wells or cheap solar collecters (okay in the solar case we're probably use the radiators, so it would be more like 360K). All using abundant non-toxic materials. Would take up land for sure, and I don't know how much power we'd be able to extract per square metre, but if we're in a desert or something like that people might not mind so much.

Terraformer · 2023-01-28 09:04:57

Okay, so supercooled might not be so easy with just radiative cooling -- the 'clear' night sky is a lot warmer than 3K. It may be difficult to get significantly below freezing, especially if you're not in a desert...

But there are people working on using it to generate power. How we can turn the cold of outer space into a renewable resource

tahanson43206 · 2023-01-28 11:34:16

For Terraformer re (perhaps surprising?) addition to topic ...

Per Google:

About 6,790,000 results (0.43 seconds)
Far-infrared laser - Wikipedia
https://en.wikipedia.org › wiki › Far-infrared_laser
Far-infrared laser or terahertz laser (FIR laser, THz laser) is a laser with output wavelength in between 30-1000 µm (frequency 0.3-10 THz), in the far ...
Infrared - Wikipedia
https://en.wikipedia.org › wiki › Infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light and shorter than ...
Natural infrared · Regions within the infrared · Heat · Applications

I don't know the answer, but am wondering if a laser operating in the infrared range might be more efficient at moving unwanted thermal energy out of a machine operating as a heat engine.

The problem for any engineer attempting to pull energy out of a deep thermal well (or perhaps even a shallow one) is how to efficiently remove energy not used in a heat engine.

Your topic here about radiative cooling reminded me of the need for a way to improve performance of energy collection systems operating at the bottom of thermal wells. Existing techniques (that I know about) move vast numbers of baryons around. Eliminating the movement of baryons entirely, and moving electrons and photons instead, would be a significant improvement over the state-of-the-art.

(th)

Terraformer · 2023-01-28 12:21:34

Maybe we can get quite cold after all. The daytime radiative system I mentioned worked by reflecting wavelengths dominant in the sky (so avoiding absorbing energy) and radiating at a wavelength the atmosphere is transparent to (so the energy can go straight to space). This would also work at night. You'd want to avoid the wavelengths absorbed by water and CO2, I think?

If it can be applied as a cheap enough spectral paint, the whole system could be built using almost entirely (the paint is a bit of a funny one) abundant materials. Maybe not even using evacuated tubes -- it would depend on how much heat was absorbed from the air. Nice and simple.

Terraformer · 2023-01-28 15:06:31

Oh yeah it works alright.

In a 24 hour day-night cycle, the cooler is maintained at a temperature that is at least 33 oC below ambient air temperature, with a maximal temperature reduction of 42 oC, which occurs during peak solar irradiance.

In this GECP project, we first theoretically show that ultra large temperature reductions up to 60 oC below ambient can be achieved.

Okay so I'm kind of excited about the potential for power production because it's inert and uses abundant materials (okay so the coating may not) *and can probably get past planning*. What efficiency can real heat engines get anyway, as a fraction of their theoretical efficiency?

tahanson43206 · 2023-01-28 15:27:18

For Terraformer re #8 and paper cited...

Conclusions
Our results in passive radiative cooling demonstrate the possibility of reaching the
fundamental limit of radiative cooling by combining photonic and thermal design. It
points to an avenue for further improvement of radiative cooling systems. The selective
emitter we use can be performed at large scales. There is also the possibility of applying
the vacuum system on a larger scale, similar to what has already been done in the solar
thermal industry. Owing to its passive nature, radiative cooling technology does not
consume electrical power and emit greenhouse gas, Therefore, it could have a significant
impact on greenhouse gas emission reduction. In addition, our results in self-adaptive
radiative cooling and thermophotonic circuits could lead to new directions in using
thermal radiation for thermal management and energy harvesting applications.

Four years have passed ... I'm hoping NewMars members may be able to find updates on this work.

(th)

Terraformer · 2023-01-29 13:17:29

Wikipedia article on passive daytime radiative cooling.

Given that visible light isn't a problem at night, a coating optimised for nighttime cooling will be different. Should be able to achieve cooler temperatures, due to the overall amount of heating from the sky being a lot lower. Needs to reflect in the infrared to avoid absorbing photons coming from water vapour in the atmosphere and of course radiate in the 8-14 micrometer window. I think it's doable.

SpaceNut · 2023-01-29 15:41:45

This is sort of the inverse or opposite of evaporation equation.

https://www.engineeringtoolbox.com/evap … -d_690.htm

https://chemistry.stackexchange.com/que … e-of-water

Of course, the sign of input is a negative so that you can get it to freeze.

SpaceNut · 2023-01-29 16:28:06

This topic reminds me of the Pellitier device..Night time solar is not such a crazy idea... by louis and State-change engine/generator by FreudSlip as well as Geothermal and Geostored Energy by Void plus Thermal Energy Storage by tahanson43206 in that we are making use of energy gain for another purpose or in this case loss.

Calliban · 2023-02-02 08:20:08

Terraformer wrote:

It's possible to make ice, when the ambient temperature is above freezing, by exposing water on a cloudless night and letting it radiate to the 3K night sky. Does anyone know how large a differential can be generated by this? If we used an evacuated tube radiator, say, so that the dominant source of energy is the night sky, what sort of temperatures could we achieve?

The absolute lowest temperature that a horizontal radiator can achieve on Earth is given by the 'effective sky temperature'.
https://physics.stackexchange.com/quest … e-of-earth

Earth's atmospheric gases, predominantly water vapour, backscatter infrared light emitted from the surface, even at night. Using the Steffan-Boltzmann equation, the resulting infrared flux can be used to calculate the effective sky temperature for a black body in thermal equilibrium. This is the absolute lowest temperature that a radiator can achieve. But other factors would come into consideration in designing a real device. In order to freeze ice, the radiator needs to achieve a net energy flux to its environment, so must be hotter than the effective sky temperature. You won't be able to make ice this way unless the radiating temperature of the surrounding land is already quite close to freezing. Though you can of course use a heat pump.

That said, the idea of storing energy as heat in a cold source is a good one. Because the latent heat of fusion of water is high, about 90kWh of heat can be stored in each cubic metre of water ice produced. You could use this energy directly for refrigeration, or as a heat sink. Cryogenic energy storage involves liquefaction of air, which is then stored in a tank. Energy is recovered by boiling the liquid air and expanding the high pressure vapour through a turbine. If heat pump is used to produce ice or liquid air, then the heat yielded by the pump may also be valuable. We have discussed the use of warm water for district heating in other threads.

Liquid air energy storage is one of several thermal energy storage systems that look promissing. The maximum achievable recovered energy density of liquid air is only about 700KJ per kg (0.2kWh). This is about 5% of the energy density of diesel. So this has limited potential as an alternative transportation fuel. But as a static grid energy storage mechanism, it compares very favourably to things like batteries. Unlike pumped storage, it can be constructed on any terrain and has a greatly superior energy storage density. It would work best if it could be integrated into a gas turbine powercycle. About half to two-thrirds of the work energy produced by a gas turbine is consumed by the compressor. Starting with liquid air, removes that parrasitic energy loss completely. So you get 2-3 times as much energy output for each unit of natural gas, compared to a conventional GT.

Last edited by Calliban (2023-02-02 08:32:34)

Terraformer · 2023-02-02 08:31:50

Calliban,

The trick is to not radiate as a blackbody If you read the links I've posted, it's possible to create a radiator that reflects the wavelengths coming from the sky and radiates in the atmospheric transparency window of 8-14 micrometres, enabling cooling *even in direct sunlight*.

Calliban · 2023-02-02 09:04:11

Terraformer wrote:

Calliban,
The trick is to not radiate as a blackbody If you read the links I've posted, it's possible to create a radiator that reflects the wavelengths coming from the sky and radiates in the atmospheric transparency window of 8-14 micrometres, enabling cooling *even in direct sunlight*.

I'm pretty sure that you cannot sustain thermal disequilibrium using a reflective coating. If you could, then you have a perpetual motion machine. Just link your flat plate coated emitter to a non-coated reciever and run some kind of generator perpetually across the temperature gradient.

One way of getting around the sky temperature problem is to put your radiator above the effective sky. You can do that by placing a radiator at the top of a hill or mountain. You are then taking advantage of the lapse rate of the atmosphere, which is 10°C per km in the troposphere.
https://en.wikipedia.org/wiki/Lapse_rate

Alternatively, build a tall stack (say, 1km high) and draw colder air down the stack. You would need enough fan power to compress the air to sea level pressure. But because the higher altitude air is dryer, it will still be colder than ambient air at sea level even after compression. This would effectively be a heat pump, using the Earth atmosphere as a radiator. I am guessing that the reason no one has done this is the high capital cost of building such a tower. In the real world, it is cheaper to produce ice using a ground level heat pump with air or water cooling.

Terraformer · 2023-02-02 09:17:35

Calliban,

If that would violate the law of thermodynamics, then we need to rethink physics, because this has been experimentally verified. Just follow the links I've posted to the Stanford paper.

It doesn't, of course. You're still moving heat from a hot place to a cold place (the 3K background), you're just finding a way to get around the warm clouds that are in the way. It wouldn't work without the ability to radiate to a 3K background. No perpetual motion here, unfortunately.

tahanson43206 · 2023-02-02 11:03:41

For Calliban re Terraformer's initiative ....

Please see if you can validate the premise Terraformer is advancing...

I am following this development with keen interest.

A technology based upon creating a low temperature heat sink would appear to have significant application on Earth.

It ** should ** work even better on Mars, where the atmosphere is (presumably) less of a block.

(th)

Terraformer · 2023-02-02 11:44:06

The relevant post.

Terraformer wrote:

Oh yeah it works alright.
In a 24 hour day-night cycle, the cooler is maintained at a temperature that is at least 33 oC below ambient air temperature, with a maximal temperature reduction of 42 oC, which occurs during peak solar irradiance.
In this GECP project, we first theoretically show that ultra large temperature reductions up to 60 oC below ambient can be achieved.
Okay so I'm kind of excited about the potential for power production because it's inert and uses abundant materials (okay so the coating may not) *and can probably get past planning*. What efficiency can real heat engines get anyway, as a fraction of their theoretical efficiency?

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#1 2023-01-28 04:00:26

Radiative Cooling At Night (For Heat Sinks)

#2 2023-01-28 04:45:14

Re: Radiative Cooling At Night (For Heat Sinks)

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#9 2023-01-28 15:27:18

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#10 2023-01-29 13:17:29

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#11 2023-01-29 15:41:45

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#12 2023-01-29 16:28:06

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#13 2023-02-02 08:20:08

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