# Night Sky Cooler

1. Jul 20, 2007

### BernieM

I have a basic question regarding black body cooling. Suppose a person at sea level on a clear night, were to use a hot water solar panel which is a good black body, and contain it in a box in a vacuum with a 'window' that was very good at transmission of long wave ir 'looking' only at the sky, could one get any significant cooling effect say for example on fluids circulated throught the panel?

2. Jul 20, 2007

### xez

Sure, it'd lose heat via radiation.

The question is what is a "significant" cooling effect to you?

Certainly it seems plausible that it'd lose additional heat
to the environment more quickly (if the water was
hotter than ambient air temperature) if there
wasn't a vacuum seal and both radiant cooling and
evaporative/convective/conductive cooling processes were
allowed to occur.

I'm sure you could find the emissivity figures for water
somewhere to give you an idea of how many watts
of radiation might be produced per cubic meter of water.

BTW low vacuum levels aren't all that good at insulation,
though they're better than nothing. Really good insulation
values start to occur at pressures well below 0.5mm Hg.

I believe insulated double pane windows are often filled
with argon or xenon in the gap between panes since it
apparently is a better insulator than whatever slight vacuum level could be produced therein.

3. Jul 20, 2007

### marcus

there are two limitations to think about

radiation from the atmosphere
the shortness of the night which is usually only on the order of 10 hours of real darkness

if you would try this on a planet with no atmosphere which as tidal locked so the same side always dark
then you could radiate heat away down to very low temp----theoretically 2.75 kelvin

but it would take a very long time because of the Stef Boltz fourth power law. the colder the radiator got, the slower it would radiate (the fewer watts)
==================

maybe you want to do a calculation to find out how low a temperature you could get, in one night, with a pure square meter radiator

I think you have to assume some definite infrared brightness of the atmosphere because that is always radiating heat down towards the earth surface.

I guess you can assume the radiater is perfectly insulated from the ground

I expect there is a differential equation for radiative cooling and someone here can supply it. I can't. I can only discuss it in general terms. Hope someone helps.

4. Jul 20, 2007

### mgb_phys

There is a definite cooling effect on the black vanes holding the secondary mirror of a large telescope from radiation. It's enough to cause turbulence around them and we have looked at heating them back up to the surrounding air temperature.
You can see a similair effect even in the UK in winter with a car parked overnight. In the morning there will be a lot more ice on the window with an open view of the sky than one facing a house.

In thought experiment terms, if you had a perfectly insulated box with a long pass IR filter then the panel (and the interior wall of the box) would cool to the temperature of the upper atmosphere ( -50deg C). It wouldn't cool quite to 2.7K because the atmosphere has some optical thickness in the far IR.

5. Jul 20, 2007

### BernieM

My needs are not -2.7K or even -50C ... -5 or -10C would be sufficient. I am going to be building a solar panel array and want to cool a fluid during the night and heat the fluid and circulate it through a different circuit during the daytime.

6. Jul 20, 2007

### mgb_phys

Shouldn't be too difficult you need to get enough insulation and choose the correct fluid (antifreeze if you want to go below 0c) depending on your location the main problem might be condensation on the outside of the window.
If you are in a desert I can see this giving you a fair degree of cooling.

7. Jul 20, 2007

### BernieM

I have had good luck in the past using a vacuum for insulation, as I have pulled 10^-9 Torr and greater vacuums. This kind of hard vacuum is what I was planning to use, much like a dewar, where the panel would sit inside the evacuated box resting on only a few points of cork to keep it out of physical contact with the box itself. Oh, and I am in the Mohave Desert in Arizona. The liquid I had been planning to circulate in the system is mineral oil.

Last edited: Jul 20, 2007
8. Jul 20, 2007

### Integral

Staff Emeritus
It is not a matter of thermal conductivity here, vacumn would be the best insultor but it would cost to much to build a double frame window that could hold a high vacumn. Very nearly as good a insultor is nearly any gas at 1 atm. this of course is much easier to seal, the key is not the gas but that it is dry gas. My bet, as to why they use Argon, is that it is cheap, and that it sounds sexier then Air.

9. Jul 20, 2007

### mgb_phys

Argon also has lower convection because it is relatively heavy.
You use argon as insulation in drysuits because of this, conversely if you want good themal conductivity in a gas you use helium - being light it has the best conductivity.
But I agree dry Nitrogen would probably work as well in a window