Bifacial Diffuse IAM

[smeredith]

Hi,

I've been comparing the rear irradiance model published by Marion et al. ("A Practical Irradiance Model for
Bifacial PV Modules", 2017) to the PVsyst 2D bifacial model.  There are some differences, such the fact that PVsyst doesn't include irradiance reflected from the front of a row to the rear of another row, and the fact that the Marion model considers 1-degree slices of surface (ground, sky, or module row) instead of discrete ground points when calculating view factor and IAM factors.

One difference I'm trying to understand is the diffuse IAM correction factor as a function of ground point.  Marion prescribes an IAM correction factor curve as a function of one degree surface slice, shown below.

 

When using the values from this curve I obtain a view factor profile that is similar to, but noticeably different from, the profile shown in PVsyst.  Below is a PVsyst view factor profile for a system with zero degree tilt, 0.5 GCR, and 1 m height (blue curve), overlaid with one calculated using the Marion correction factors from the curve above (red dashed line).  The curves are similar but noticeably different.  I was wondering how PVsyst is calculating the IAM correction factors that are used, as they appear to be different from what was derived from Marion et al.

 

[HLubke]

Hi smeredith

i understand the question is rather academic than practical and focusses on the mathematical modelling.

I believe that for most practical design cases, the neglection of a second row's front side reflection on the rear side of the first row results in an extremely low error. The modules' IAM characteristics (i.e. high radiation absorption) avoids reflection as much as possible, after all. Maybe, with a N-S tracker with the sun at a low elevation (early, late daytime) and no backtracking some reflection on the rear side might occur.

Am I missing something with practical relevance?

Regards,

       Heiko