Very Cooool White

[marting]

I recently pointed an IR temperature reading remote "thermometer" at blue sky on a cloudless day near noon. The reading was -20C.

Looking at the energy spectrum of sunlight vs blue sky, it appears that one could come up with a surface that was highly reflective or even diffusive at wavelengths of 200nm to say 3000 nm but highly emmisive at longer wavelengths. If such a surface was possible, it should be possible to have more energy radiated into blue sky than absorbed from the shorter wavelengths of sunlight. Such a surface would be cooler than the air temperature.

My question is, does such a surface exist? If not, how close can one come to a net radiator in warm climates? Does anyone know of any research optimising this?

marty

 

[Mosquito]

I recently pointed an IR temperature reading remote "thermometer" at blue sky on a cloudless day near noon. The reading was -20C.

Looking at the energy spectrum of sunlight vs blue sky, it appears that one could come up with a surface that was highly reflective or even diffusive at wavelengths of 200nm to say 3000 nm but highly emmisive at longer wavelengths. If such a surface was possible, it should be possible to have more energy radiated into blue sky than absorbed from the shorter wavelengths of sunlight. Such a surface would be cooler than the air temperature.

My question is, does such a surface exist? If not, how close can one come to a net radiator in warm climates? Does anyone know of any research optimising this?

marty

Not to kill your dreams of a perpetuum mobile, but your surface would be AT air temperature, not below it, as it would be heated by the physical contact with the air. And this assumes that its total capacity for radiating heat is, at air temperature, higher than its capacity for capturing solar radiation.

I've had the same idea myself, but it would mean free lunch, and that ain't happening anywhere near here, I tell ya'.

Mosquito

 

[Ririon]

Puh!

I was THIS close to selling my oil stocks...

It would have excellent roofing material in tropical regions, though.

Ririon

 

[marting]

Not to kill your dreams of a perpetuum mobile, but your surface would be AT air temperature, not below it, as it would be heated by the physical contact with the air. And this assumes that its total capacity for radiating heat is, at air temperature, higher than its capacity for capturing solar radiation.
Mosquito

No. For one thing the surface temp of objects often drops many degrees below ambient at night. The surface temp would be in equilibrium with conductive/convective loss to air vs net radiated and absorbed light energy. There is no law of physics being broken here. One can easily show that a perfect "white surface" at wavelengths of say 3000nm or less and a perfect "black" at longer wavelengths would radiate more light energy than it absorbs at 20C in a noon sun, open field, with no clouds and thus cause it to become cooler than ambient in direct sunlight.

And by a lot!

 

[Roboramma]

Wait, I don't get it. Would this material be cooler than ambient temperature if it was stored in a dark room? How?

If not, how would shining light(from the sun or wherever) on it make it lose energy?

 

[69dodge]

Wait, I don't get it. Would this material be cooler than ambient temperature if it was stored in a dark room?

If the ambient temperature were (somehow) warm but the walls of the room were as cold as outer space, sure.

How?

By emitting more radiation toward the cold walls than they emit toward it.

If not, how would shining light(from the sun or wherever) on it make it lose energy?

No, it's not the sun's hot light shining on it that makes it cold; it's its exposure to the cold sky that makes it cold.

 

[Roboramma]

Okay, wait, I think I get it, and it sounds really cool.

Everything (including the air)is constantly being warmed by the sun. Everything is also radiating some of this heat back (eventually into outer space).
So the idea is that while the air will be kept at it's ambiant temperature in part by the heat of the sun, this material would not be absorbing much of that heat, but would be radiating a lot. So as the air heats the material, a lot of that heat is radiated out into space?
It's like if you could put a stone into a fire that was somehow uneffected by the heat from the flames - it would absorb some heat from the other material there, but since it wasn't absorbing heat from the flames, but was still giving it off to the air, or through radiation, as quickly as everything else, it would be colder than the surrounding material.

Did I come anywhere close to getting it?

If so, neat!

 

[Soapy Sam]

My girlfriend's feet are frequently colder than the ambient air temperature, despite the other evidence that she is actually still alive.

Not absolutely sure if this the kind of thing you had in mind...

 

[.13.]

No. For one thing the surface temp of objects often drops many degrees below ambient at night. The surface temp would be in equilibrium with conductive/convective loss to air vs net radiated and absorbed light energy.

Are you sure?

If there were condensated water on the surface of the object at night it might get cooler as the water evaporates. But eventually it will be the same temperature as surrounding air. I don't think the objects will get many degrees cooler than air though.

As Mosquito pointed out, there are no free lunches.

 

[Hellbound]

Even assuming this works, the energy for these changes is coming from sunlight, right?

So how would this compare to a standard solar cell? I doubt the efficeincy would compare very well to a solar panel, if this is a possible idea. But, I dunno. I'm just tossing out somewhat educated guesses

 

[Dr Adequate]

Not to kill your dreams of a perpetuum mobile, but your surface would be AT air temperature, not below it, as it would be heated by the physical contact with the air. And this assumes that its total capacity for radiating heat is, at air temperature, higher than its capacity for capturing solar radiation.

I've had the same idea myself, but it would mean free lunch, and that ain't happening anywhere near here, I tell ya'.

You're saying it's impossible 'cos of the Second Law Of Thermodynamics, aren't you?

Stand still while I burn you at the stake.

The idea does work. At its simplest, take two bricks. Paint one black and one white. Leave them out in the sun. The black brick will get hotter than the white brick. There's your energy potential. Now provide a conduit for heat to flow from one brick to the other. This can be driven by the black brick (thanks to the Second Law) and you can tap into this energy flow to do work. And, of course, the result is a net increase of entropy.

Lunch is on the Sun.

 

[marting]

Even assuming this works, the energy for these changes is coming from sunlight, right?

So how would this compare to a standard solar cell? I doubt the efficeincy would compare very well to a solar panel, if this is a possible idea. But, I dunno. I'm just tossing out somewhat educated guesses

As to the efficiency, it would depend on the spectrum of radiated energy at the temperature of the "cooool white" object and spectrum of the solar energy. The atmosphere can be thought of as a sort of smoggy filter that absorbs and re-transmits energy bi-directionally. Depending on frequency, it varies from fairly clear to essentially opaque with water vapor and CO2 playing large roles. Discounting the effects of convection/conduction (which can be controlled and are distinct), the temperature could drop perhaps 10 degrees C or so if one has a "perfect" reflector below 3000nm or so where almost all the sun's direct energy is concentrated. A very small departure from 100% reflection would be a problem though. My guess is that equilibrium would be somewhere around 90% reflectance - a white that is whiter than most white paper. Even to achieve that one needs the "white" to extend pretty far into the IR.

It should be fairly trivial to demonstrate an object that cools below ambient under a blue sky shaded from direct sunlight with a small circular shade at a good distance. One wouldn't need a perfect white to get a negative temp diff.

marty