solar elevation and azimuth angles for PV systems

Hi,
I was comparing results results using SAM and PV_Lib (from the Sandia PVPMC website) PV systems and found some inconsistencies. Some of this appears due to the fact that for a given location (same weather file, TMY3 data file) the solar elevation and azimuth angles can differ by up to about 8 degrees (I am ignoring the "-999" entries for now).  I am surprised it is that big. I used both the ephemeris algorithm and the NREL solar position algorithm in PV_Lib and both agree to well under 1 degree. This is causing the angle of incidence to the PV panel and consequently the plane of array irradiance to be off when comparing modeling between SAM and PV_Lib.

I wanted to find out how these angles are being calculated in SAM and whether they have been compared with the algorithms I am using in PV_Lib. 

Thanks,

Ziggy

Hi Ziggy,

SAM's Detailed Photovoltaic model uses the Mikalsky (1988) algorithm for the sun position calculations. We hope to switch to the NREL SPA algorithm , and to make the SAM's sun position calculations for all of SAM's solar models consistent as described here:

github.com/NREL/ssc/issues/279

The Photovoltaic Reference Manual listed on the PV Publications page describes the PV model's sun position equations:

sam.nrel.gov/photovoltaic/pv-sub-page-2.html

Best regards,
Paul.