Bifacial vertical simulation difference between v7.4.8 and v8

[Erin T.]

Hi there,

I ran simulations in v7.4.8 before the new year, comparing modelled irradiance in PVSyst for a fixed-tilt vertical bifacial PV system to measurements of reference cells in a vertical bifacial test-site.  I was wondering if there are any key differences between v7.4.8 and the latest update with v8 which might impact the results of this comparison?

Thank you!

Erin

[Robin Vincent]

PVsyst V8 development was focused on improving the "orientations" management. This includes several modification that could impact the system you described. 

I cannot give you an exact list, since I don't know your exact project, but I think this one is very likely to generate changes in your results :

 

 

[Erin T.]

Hi, thanks for your quick reply.

So, for example, if I was simulating 1 row of modules in v7.4.8, then this old version was likely under-estimating the rear diffuse irradiance component, but if I was simulating 4 or more rows then this old version would be over-estimating rear diffuse irradiance?  This is because v7.4.8 always assumed 3 sheds?

[Robin Vincent]

After some archeology, here's a more complete answer :

If you were using Unlimited sheds or unlimited trackers, the fraction of unshaded and shaded rear side was correct

If you were using trackers on the 3D scene, the fraction of unshaded and shaded rear side was also correct (using the total number of sheds at least).

If you were using fixed tilt on the 3D scene, then fraction of shaded rear side was always 2/3, and in that case your analysis is correct. 

This behavior has been fixed in the 8.0, and improved in the 8.0.6 when you have multiple columns of sheds.

 

In the 8.0.6, you will be able to see and change the number of rows used for the bifacial calculation from the bifacial tool, which should let you try to reproduce the previous behavior if you want to isolate this specific change.