Selective surface in Solar Radiation Model

[Anonymous]

I am trying to simulate a solar collector in Simulation CFD and I do not know hot to define a selective surface material.

A selective surface material such as the black coating used in solar thermal systems has a high solar absorbance (0.90) and low emissivity (0.2). However, simulation CFD does not take into account absorption and considers emissivity = absorptivity, which are quite different in selective surface.

Any method to properly simulate a thermal collector in Simulation CFD?

Thanks

[Royce_adsk · Accepted solution]

Just some information for the observer to this post.

Selective surfaces take advantage of the differing wavelengths of incident solar radiation and the emissive radiation from the absorbing surface:

• Solar radiation covers approximately the wavelengths 350 nm...4.000 nm; UV-A, visible and near infrared (NIR - or IR-A plus IR-B)).
• Thermal infrared radiation, from materials with temperatures approximately in the interval -40..100°C, covers approximately the wavelengths 4.000 nm...40.000 nm = 4 um...40 um; The thermal infrared radiation interval being named or covered by: MIR, LWIRor IR-C. (wikipedia)

In Simulation CFD we don't integrate absorption and emission across wavelengths. We assume "grey" body properties. So emissivity averaged over wavelengths and absorption averaged over wavelenghts. We also assume that the radiation is diffuse ( no angular dependance ). For instance we cannot do mirrors accurately because of this assumption.  Since the selective surface design works over different wavelength you are not going to be able to accurately model this in Simulation CFD

What are you to do?  I think the best approach for you would be to use a black collector approach setting the emissivity to 1 for the solar collector materials.  Did you come up with a better approach?

There are 3 areas that our radiation model does not address:

Angular dependence
Wave length dependence
Attenuating media effects (gas participating in absorbing radiation energy)

To make enhancement request please use this site!
http://forums.autodesk.com/t5/Simulation-CFD-IdeaStation/idb-p/82

Thanks,

Royce