Using radiation model for simulation of flat plate solar collector
 

My research is about simulating flat plate solar collector with Ansys Fluent but I have some problems using radiation model.I will be so thankful if anyone could answer my questions. My model includes glass cover, absorber plate (copper), air gap (between absorber plate and glass) and tube (water).
First, I tried using Do (Discrete Ordinates) for radiation model but do-intensity did not converge (Do-intensity). Then, I tried using P1 model. This model works but only without gravity. Then, I tried using DTRM but it does not work since my model includes sliding meshes. Finally, I used only solar load (with solar calculator) without activating any radiation model. This option works only without gravity too. Actually, I am new in using radiation model in Ansys. I don't know which model is suitable for my case.
I want to know that how I can activate gravity option since my simulations with gravity and radiation model does not work at all. I need to activate gravity due to free convection in air passage between the absorber plate and glass cover.

Dear marjanbn, please what did you chose finally . I am working now in the same thing?