SAM Isc Output not corresponding to Module wattage

Hi Tory,

The modules' rated short circuit current (Isc) and temperature coefficient of short circuit current contribute to the short circuit current reported under operating conditions. You might compare the module ratings (on the Module page) to see if they explain the effect you are observing.

If you would like me to investigate, please attach a copy of a .sam file that demonstrates these results.

Best regards,
Paul.

Hi Tory,

Thank you for attaching your .sam file.

In my comparison, I found that the maximum Isc for the 685 W case is 26.28 Amps, and for the 670 W case it is 25.6. This is consistent with the higher Isc rating at STC for the larger capacity module.

I found the maximum value of "Subarray 1 String short circuit DC current (A)" on the Statistics tab of the Results page.

I also compared the time series data, and, at least for the first day at least, it appears that the system with the 685 W module has a higher Isc under operating conditions than the 670 W system:

 

 

These results are from the latest version of SAM, SAM 2022.11.21 r3.

Best regards,
Paul.

Hi Tory,

Under the same operating conditions, the larger modules operate at slightly higher cell temperatures, causing a small reduction in short circuit current (Isc).

For an explanation, I will refer to Equations 10.30 and 10.37 in the SAM Photovoltaic Reference Manual, which you can download from the Publications page: sam.nrel.gov/photovoltaic/pv-publications.html

The equation for Isc in each time step (Equation 10.30) depends on the cell temperature and effective irradiance in that time step among other parameters that do not vary with time. The cell temperature (Equation 10.37) depends on the ambient temperature and module rated maximum power. For this comparison, the cell temperature is slightly different for the different modules because they have different maximum power ratings (rated Vmp and Imp are different for the different modules). The cell temperature difference causes a small difference in Isc under the same operating conditions.

The weather file I used was a TMY file for the same location from the NREL NSRDB as your TMY file from Vaisala. The NSRDB file uses a  a warmer month for January than the Vaisala file. The day I picked for my screenshot above (January 1) happens to be a colder data in the Vaisala data than the NSRDB data.

The relevant PV model outputs are:

• Subarray 1 string short circuit DC current (A)
• Weather file ambient temperature (C)
• Subarray 1 cell temperature (C)
• Subarray 1 POA total irradiance after reflection (IAM) (W/m2)

Best regards,
Paul.