Radiation Heat Transfer, Material Question 1

[kevindurette]

A few years ago I read somewhere that a material can have two different emissivity coefficients for absorbing and radiating energy, even orders of magnitude apart. For a device designed to absorb energy, for example, this would result in less loss due to re-emission of the radiant energy. Where can I research these materials online?

Thank you very much.

Kevin Durette

 

[tkordyban]

What you are thinking of is called a "selective surface". An example is something we are all familiar with: white paint. It has two different values of emmisivity (and absorptivity) because they are different for different wavelenghts of radiation. Solar radiation is mostly short wavelength radiation. White paint is highly reflective in that that wavelength range, which means it reflects about 80% and absorbs about 20% of sunlight hitting it. For sunlight the absorptivity is 0.2.

But at longer wavelengths, white paint emits and aborbs differently. At long wavelengths, the emissivity is about 0.9. Objects at normal temperatures (0 to 100 deg C) emit most of their thermal radiation at long wavelengths. So a white painted surface is bad at absorbing sunlight, but good at emitting long-wave thermal radiation. That is why if you want an object to stay cool when exposed to the sun, you should paint it white.

Another example is window glass. It is transparent to short wavelengths like visible light, but opaque to long wavelengths, like thermal radiation. That is how a greenhouse traps heat from the sun. Sunlight passes in, but heat does not pass back out.

Any textbook on heat transfer will discuss this in the chapters on radiation. If you want to know about specific materials, I'd look at texts on solar engineering. This is not an exotic new technology.

 

[IRstuff]

The key is to minimize the blackbody temperature, which puts Wien's displacement peak as far to the right as possible. For example, a typical room temperature blackbody would have its peak power emission around 10 um, while glass can only transmit up to about 2 um.

TTFN

FAQ731-376

 

[kevindurette]

You're missing the point. If my goal is to capture as much energy as possible, there are materials that will absorb energy at a greater rate than they will emit it back out.

I'm sure you've seen this...

q flux = epsilon * sigma * T^4

Well, what is commonly done is to assume that energy goes into the object at the same efficiency with which it goes out. You're assuming a constant epsilon.

total q flux = epsilon * sigma * (T_object^4 - T_surroundings^4)

I've seen materials that don't do this. They look more like this...

total q flux / sigma = epsilon_out*T_object^4 - epsilon_in*T_surroundings^4

It still maintains equilibrium. The power in is still the power out minus the rate of energy storage, but now you can maintain a higher efficiency if your goal is to run either hotter...
epsilon_out < epsilon_in
or colder...
epsilon_out > epsilon_in

 

[kevindurette]

Minimizing the black body temperature is not an option because that would kill my goals elsewhere.

 

[kevindurette]

I think I'll just forget about this in the interest of cost and use more conventional materials. I'll have to make up the difference in system size.

 

[IRstuff]

The fact that we were talking about differing spectral reflectivities indicates that we were not assuming constant emissivity. However, what you are talking about is, in general, not possible, UNLESS, there is spectral selectivity, hence, the suggestion of glass. At any given wavelength, emissivity = 1 - transmission - reflectivity, there is no way around that.

Glass transmits below 2 um, and absorbs above. Since sunlight peaks at about 500 nm, the interior temperature must be less than 6000K, or its Wien peak will also be at 500 nm. Luckily, you can't realistically get anything that hot, so that's not a major problem.

However, you've already alluded to the basic issue, the energy storage. If the energy is stored in the absorber, it will radiate away, therefore, you must move the collected energy elsewhere, thereby bringing in cooler material, which reduces the blackbody temperature, which reduces the emission. This is the foundation of almost all solar collectors. After all, keeping the energy at the collection point provides no useful work.

TTFN

FAQ731-376

 

[davefitz]

This topic is well developed in the design of space vehicles. Refer to the NASA textbook "thermal radiation heat transfer" by Siegel + Howell.

The coatings or surface prep can be designed to custom specify the emmissivity , absorptivitiy and reflectivity as a function of wavelength .