Confusion Regarding Irradiance Calculation

Hi Michel,

By default, there is a 5% soiling loss on the Losses input page that applies to the POA irradiance. Do you see that difference if you set the soiling loss to zero?

Best regards,
Paul.

Hi Paul:
Yes, I set all soiling monthly values to zero to eliminate any potential 'background noise'.  frac values calculated by SAM should be the same as the value calculated manually.
You could verify it with any model of your choice as the manual calc is straightforward.

 

Hi Michel and Paul,

So how to solve this unequal result problem? I try to calculate it by hand as well and face the same Problem. I use the same equation for S_dss as Michel wrote before.
Regards,
Daniel

From Daniel's email, it seems that this discrepancy persists.  Did NREL look into the code to find out why?

Hi Michel,

The sky diffuse factor for non-linear self shading has a relatively small effect on the output of the array, and the discrepancy you have pointed out between SAM's results and values calculated using the equations in Seciton 9.2 of the PV reference manual is also small, so we haven't made sorting this out a priority.

The equations for these calculations are in the sssky_diffuse_table() function, which is defined here in the SSC source code:

github.com/NREL/ssc/blob/f6cf8d33f9f8bc859d174b7c41e8a07eda0d019b/shared/lib_pvshade.cpp#L300

I created an issue for this in the SAM GitHub repository: github.com/NREL/SAM/issues/1705 .

Best regards,
Paul.