Unexpected Subarray Isc Behavior, 585W Module Producing Higher Current Than 605W

Howdy everyone,I'm currently troubleshooting an issue in NREL SAM and would appreciate insight from others who may have encountered similar behavior.I'm modeling two PV system configurations using NREL SAM, one with a 585W module and one with a 605W module. Both are from the same brand and product series, and both use identical subarray configurations:o   27 modules per stringo   8350 strings in parallelo   Identical temperature coefficients and tolerancesHowever, in the simulation results, the subarray short-circuit current for the 585W module configuration is higher than the one for the 605W module, which is counterintuitive, since the 605W module has the higher Isc on the datasheet.Parameter:           585W   605WIsc (A)                 14.01   14.25Imp (A)               13.29   13.57Voc (V)               52.7     53.5Vmp (V)             44.02   44.61Efficiency (%)    22.7     23.4Has anyone experienced module-level current anomalies in SAM under high irradiance or due to IAM or effective irradiance modeling?Any guidance or suggested testing approaches would be greatly appreciated.Thanks in advance!

Hi Ronnie,

There are a few different things that could be causing this behavior. For example, the subarray voltage might be clipped to the inverter maximum MPPT voltage if the number of modules per string is too high.

If you would like me to help troubleshoot, please attach a .sam files or files that demonstrate this behavior.

Best regards,
Paul.

Hi Ronnie,

Thank you for sending the files. SAM's inputs browser (File, Inputs browser) is useful for this kind of troubleshooting. (You can change the values of inputs directly in the table for most variables.)

The inverter's maximum MPPT voltage rating does cause some clipping, note the message at the bottom of the System Design page about Voc > Vmppt_high indicating that the string open circuit voltage at STC exceeds the inverter's maximum MPPT input rating. However, I don't think that is affecting the maximum short circuit current (Isc). You could set the maximum MPPT voltage on the system design page to 1500 to minimize clipping.

I found a couple of differences in inputs that you might not have noticed:

• Standoff height on the Module page.
• Tracker rotation limit on the System Design page.

The standoff height affects the cell temperature model, so that could have an effect on the maximum Isc.

On the module page, if you click Calculate and plot to generate the I-V curves for the two modules, you can see that the shape of the curve is slightly different for the two modules. Given that both modules have the same temperature coefficient of Isc, for slightly the smaller module (lower rated Pmp), the short circuit current increases at a faster rate as voltage decreases.

The difference in the resulting Isc is relatively small -- I would take that into consideration as you evaluate the results.

Best regards,
Paul.


 

I appreciate your review and feedback Paul!

My main question is, although the module values are aligned in terms of temperature coefficient, why does the I-V curve at STC still show slight differences?

 

Hi Ronnie,

The two modules have the same temperature coefficients but different voltage and current ratings. The interaction of these parameters in the I-V curve equations causes the shape of the I-V curves to be different.

Best regards,
Paul.