Selective Surfaces for Solar Thermal Conversion

[Carlos de Meo]

Hi Guys
I was reading about selective surfaces for solar thermal conversion and, according to the literature, an ideal material for that would have high absorptance in the 0.2-2.5 μm (due to the Planck distribution for a 5000 K black body, i guess) and also low emittance to suppress the losses due to reradiation. How can that be possible?
According to Kirchoff law, emissivity=Absorption in thermal equilibrium, so, a good absorber is also a good emitter (that´s how high emissivity coatings work)

 

[Bystander]

in thermal equilibrium

Is the system in thermal equilibrium?

 

[Carlos de Meo]

So the main idea behind of this kind of material is to control the nonradiative relaxation and preventing it to achieve thermal equilibrium?

 

[Bystander]

Thermal equilibrium with a "black body" at 5000 K is 5000 K. Emissivities at all wavelengths are near one at 5000 K; bright metallic surfaces have low emissivities at lower temperatures, closer to (our) ambient. The object is absorbing heat radiated from a high temperature black body, say 5000 K, and preventing emission below? What? 500 K?

 

[Carlos de Meo]

I was thinking about the solid structure and the interaction with . When a photon is absorbed promoting a system to a quantum excited state, it might emit a photon, undergo a nonradiative relaxation or a combination of both. For a selective surface, the key is to enhance the nonradiative relaxation process through chemical composition or other changes?

 

[Bystander]

For a selective surface, the key is to enhance the nonradiative relaxation process through chemical composition or other changes?

"Yes." Take advantage of extant physical properties, plus perhaps some observed behaviors; specular surfaces have lower emissivities than non-specular, ...

 

[Carlos de Meo]

Now that makes sense. Thank you very much Bystander

 

[Cutter Ketch]

I think this is all sounding too exotic. The spectrum being absorbed is from the 5000K black body called the sun. It's radiation is essentially all between 0.2 and 2.5 um. You want good absorption there. The absorber is NOT at 5000K and it doesn't emit 0.2 to 2.5um photons. It is probably around 300K and has its black body emission spectrum peaked at about 9.5 um. It doesn't violate any rules for a surface to be a good absorber at one wavelength and highly reflective in another. In the visible we call that color. In exactly the same way that red barn paint absorbs blue light but reflects red light you can choose a solar material that absorbs in the visible through NIR but reflects (and emits poorly) in the FIR. It's just color.

 

[Bystander]

I think this is all sounding too exotic.

See Rohsenow & Hartnett, Handbook of Heat Transfer, Ch. 15; emissivities/absorbances are as advertised, low for bright metals at ordinary temperatures (0.01 for polished), > 0.5 for other surfaces (all painted surfaces), and increasing generally from there with temperature to ~ 1.