Thanks for your replies. As I indicated a solid material and a porosity of 0.8, there should be 20% of solid material. I added the absorption coefficient to the solid material corresponding to the Porous media. In this way I would think that the Porous media accounts for the light absorption.
How would you suggest to change the 'appropriate variables' to make this work?
The problem: it seams that the heat flux at the bottom of the porous media is larger than on top, so could it be that the radiation is not absorbed, but enhanced by the Porous media?

Thank you again in advance!