Sky Diffuse VF for Rear Side

[smeredith]

I noticed something about bifacial simulations for trackers that I think is an error.

If you calculate the sky diffuse view factor on the rear side, you'll notice that the view factor is different depending on whether the module has a positive or negative tilt (PhiAng).

Below is a graph showing the view factor as a function of tilt for GCR = 0.5.  As you can see, the view factor is always higher for negative tilts compared to positive tilts.  This behavior seems to be consistent across all GCRs.  I was wondering if this was intended behavior or if this is an error - to me, it doesn't make sense that the view factor would be different depending on if the tracker was facing east or west.

[smeredith]

Does anyone from PVsyst have thoughts on this?  Is this a bug or intended behavior?

[Bruno Wittmer]

This is not intended behavior. Can you explain how exactly you created this plot? Is it a simulation result, or were the values obtained with the tools in the bifacial model window?

[smeredith]

This was calculated from the 8760 CSV.

Before running the simulation, I changed the circumsolar calculation to be included in the diffuse component in order to get a more accurate calculation of the rear sky diffuse factor, as I understand that it is by default removed from the diffuse component and added to the direct component.

Then I calculated the rear sky diffuse factor as DifSBak/DiffHor, and plotted the view factors for both positive and negative PhiAng.  The above graph is the result of that.