Which is better for radiant cooling -- Helium-3 or Helium-4?

[Green Xenon]

Hi:

 

I thinking of a radiant cooling device for houses and buildings in which the cooling -- in the direct sense -- involves only radiation. Sort of like a glass-ceramic radiant-stove-top in reverse. Indirectly, however, some amount of convection and conduction will be needed [liquid helium, cold metals]. The cooling panel is the ceiling and cools objects below it.

 

Quote from http://en.wikipedia.org/wiki/Glass-ceramic :

 

"A glass-ceramic stove uses radiant heating coils as the heating elements. The surface of the glass-ceramic cooktop above the burner heats up, but the adjacent surface remains cool because of the low heat conduction coefficient of the material."

 

Here is an example of a radiant stovetop switched on: http://www.istockphoto.com/file_thumbview_approve/4799702/2/istockphoto_4799702-glowing-ceramic-stove-top.jpg

 

My theoretical glass-ceramic radiant cooler is similar to the infrared radiant stovetop described in the wikipedia link, except:

 

1. It faces downward

2. The coils are hollow [as opposed to being solid all the way through], this hollow within the coils allows liquid helium to flow through them can cool them down to near absolute zero

3. Liquid helium -- not electricity -- flows through the coils.

4. Obviously, the coils get cold instead of hot.

 

Which is better to use -- Helium-3 or Helium-4? Which one would have a stronger cooling effect if both were at the same temperature, pressure, volume and concentration?

 

The radiant cooling panel is on the ceiling of the room it is supposed to cool. Those standing under it will feel cold.

 

Yes, heat absorbed into the radiant cooling panels is carried off using convection and conduction -- but this is not what the subject inside the room feels. The direct cooling effect on anything/anyone inside the room is radiant.

 

By direct radiant cooling, I mean that if you place your body at a noticeable distance from panel, you'll feel cold because the extreme cold of the coil will draw IR radiation away from your body.

 

I’m thinking of more intense versions of this hypothetical glass ceramic radiant infrared cooler to be used in refrigerators and freezers.

 

 

Thanks a bunch,

 

Green Xenon

[CaptainPanic]

Yes, heat absorbed into the radiant cooling panels is carried off using convection and conduction -- but this is not what the subject inside the room feels. The direct cooling effect on anything/anyone inside the room is radiant.

 

By direct radiant cooling, I mean that if you place your body at a noticeable distance from panel, you'll feel cold because the extreme cold of the coil will draw IR radiation away from your body.

 

Your body radiates heat... and a cold object will not radiate as much. So, indeed, the balance will be that you radiate more to the cooler than the cooler to you. The net effect will be that you cool.

 

However, you mention that people will be there. So, I assume there will be air too. At low temperatures, you can radiate as much as you want, but convective effects will be much greater.

 

Radiation is a 4th order function of the temperature. So, at low temperatures, this effect becomes negligible, and at high temperatures it becomes dominant. Other forms of heat transfer are often a function of the temperature difference (1st order) and will almost be as effective at low temperatures as at high ones (perhaps even more effective). You need a near perfect vacuum to reach low temperatures and have radiation as the dominant effect.

 

To answer your question: it seems that 3He is used for cryogenics, sometimes in combination with 4He.

 

But why do you want a cooling machine that can go to such low temperatures? There must be easier ways to cool your beer.

[insane_alien]

and why do you want a radiative cooling solution anyway?

 

convection and conduction are several orders of magnitude more effective AND you don't need to shell out for extremely expensive liquid helium(and thats just He-4, He-3 is worth more than gold) so unless you have the necessary equipment to capture the evapourating helium and recondense it(which in itself runs into the thousands) actually... this isn't really worth it at all.

[Green Xenon]

This radiant cooling is something that I am deeply interested in. I don't know why. To make applications of it is going to be very difficult because of the costs. However, I'm more interested in the theory of it rather than physically doing something with it.

 

On the other hand, if I had the money, time, and energy to put this radiant cooling to work, I'd definitely do it. Radiant cooling will feel to the object like "cold rays" just like radiant heating feels like "heat rays".

 

I know there is no such thing as "cold rays", it's simply heat radiating from my body to a colder object. My body is giving of heat rays causing it's temperature to lower, thereby giving me a perception of coldness.

 

In theory, is there a way to make radiant cooling the dominant force without using a vacuum?

[insane_alien]

not unless the temperature is a few million kelvin.

[John Cuthber]

In zero gravity there's no convection; if you use a gas with a poor conductivity radiation might win out.

Incidentally, monatomic gases do a really bad job of radiating energy unless they are hot enough to be electronicly excited- say 10000 degrees or so.

 

If you wait for a cloudless night and go out to look at the stars you will observe the effect of radiant cooling- it gets cold.

Nothing "magical" I'm afraid.

 

(space is very cold and the stars don't add a lot of heat, so radiant cooling is a major contributor in this case.)

[CaptainPanic]

In zero gravity there's no convection; if you use a gas with a poor conductivity radiation might win out.

Incidentally, monatomic gases do a really bad job of radiating energy unless they are hot enough to be electronicly excited- say 10000 degrees or so.

 

If you wait for a cloudless night and go out to look at the stars you will observe the effect of radiant cooling- it gets cold.

Nothing "magical" I'm afraid.

 

(space is very cold and the stars don't add a lot of heat, so radiant cooling is a major contributor in this case.)

 

If space were really cold, you'd expect it to be very efficient to cool the earth. But while it is quite cold (when the sun isn't shining), the main issue is that there is just nothing there... And therefore it cannot take up heat. Radiation is the only way to transfer heat through vacuum, as was mentioned before. Logically therefore that radiation is dominant. Radiation is also dominant at daytime. For the earth's energy balance, radiation is the only type of energy coming in and going out (unless you want to include solar flares, spaceships and the odd meteor).

In theory, is there a way to make radiant cooling the dominant force without using a vacuum?

You can calculate the radiation from any object.

You start off with the Stefan-Boltzmann Law, and you should include the emissivity factor (all explained on wikipedia).

You then also realize that you should calculate this radiation for the object that is cooling, but also for the object that is (net) receiving the radiation.

Then you should compare that to (convective) heat transport (which I admit is a pain in the ass to calculate).

 

For a perfect black body, the radiation at 298K is:

[math]j^{\star} = \sigma T^{4}. [/math]

with: [math]\sigma=\frac{2\pi^5 k^4}{15c^2h^3}= 5.670 400 \times 10^{-8} \textrm{J\,s}^{-1}\textrm{m}^{-2}\textrm{K}^{-4} [/math]

[math]j^{\star}=5.6704\cdot{10^-8\cdot{298^4}}=447 W/m2 [/math]

 

which is most likely smaller than heat transport.

Liquid-liquid heat transport generally proceeds at 1000 W/m2K (1000 W/m2 per Kelvin). So, with 10 K difference, it's already 10,000 W/m2

Gas-liquid or gas-solid heat transfer will be an order of magnitude smaller, but will still be larger than radiation.

 

The exact value where radiation will be dominant is hard to calculate... but you can be sure that it won't suddenly become dominant at lower temperatures. Only at higher temperatures.

[John Cuthber]

The microwave background radiation corresponds to a temperature of about 2.8K.

If the sun and stars were not there that's how cold the world would end up.

Many of us consider that to be quite cold.

[J.C.MacSwell]

Radiant cooling requires a large percentage of the background to be effective.

 

Having a near absolute zero radiation sink of a small subtended area will not allow much cooling.

[harry]

In theory, is there a way to make radiant cooling the dominant force without using a vacuum?

 

Yes there is!

It was discovered more than 200 years ago that radiant cold could be reflected and focused like radiant heat using parabolic mirrors.

See the1985 paper in the American Journal Physics for some history behind this simple and important experiment:

 

http://www2.ups.edu/...0experiment.pdf

 

The experiment is hardly known today, and has never been analzyed and explained in a QUANTITATIVE fashion

using modern heat theory. While a qualitative explanation in terms of modern heat theory is given at the end of this paper

I have a hunch the observed RATE of cooling is faster than you would expect with modern heat theory.

 

I also suggest you read the book _Inventing Temperature_ by Chang c. 2004.

 

Available as a printed book from Amazon or as a pdf file:

 

http://bib.tiera.ru/...29%28286%29.pdf

 

Harry