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.

[smeredith]

Resurrecting this thread since I've noticed the same thing happening in PVsyst v8.

The main difference between v7 and v8 seems to be that circumsolar is separated from the rear sky diffuse in v8, regardless of whether the user chooses the setting that separates it from the diffuse or includes it.

[Michele Oliosi]

Hi @smeredith sorry for the late response, and thanks for reviving this thread. I have made a ticket we will be looking into this possible issue soon.

[smeredith]

@Michele Oliosi Thanks for the response.

I'd like to also add some observations about how this factor changes with number of rows.  The fewer rows there are, the more "scattered" the factor is for both negative and positive PhiAng.  See below some graphs generated in PVsyst 7.4.8.  Though these were generated in v7, the scattering is also observing in v8.  I'd like to know what's causing the scattering and if this is intentional.