Voc Calculations for String Sizing

Hello SAM team, I am working in the utility scale solar space as the Engineer of Record and we have been using SAM to calculate the open circuit string voltage since its allowance in the NEC 2020 code cycle. We calculate these using the parametric funcionality to simulate Voc through 20years of weather data to find the absolute maximum voltage across the 20 years. Something we are coming across more and more now is the request to use the Sandia Calculation method but since the module database isn't actively updated and certainly not at the pace we see new equipment we are struggling to find a way to make this request work. I have been spending a few days crunching through the Sandia methodology and it appears to me that it would not be possible to utilize their calculation methods without the coefficients that Sandia would generate through empirical testing. 

What I am wondering if there would be a way to simulate the Effective Irradiance (Ee) and Cell Temperature across 20 years of data to be able to utilize the "Sandia Method" in our string sizing calculations. Below is our current understanding of the expanded equation.

 

Hi Matt,

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I'm not sure there is a way to come up with Sandia module model parameters without the empirically-derived coefficients. As you note, the Sandia module library has not been updated in many years, and I am not aware of any plans to update it.

If you did not already see it, this post may be of interest:  sam.nrel.gov/forum/forum-general/2257

Best regards,
Paul.

Thanks Paul,
I suppose my next question is if there is a way to use weather file data to approximate the Effective Irradiance and Cell Temperatures to approach a similar Voc as a full Sandia methodology. Through what I have found so far my thoughts are that the Effective Irradiance is the ratio of irradiance on the collector (both front and rear) over 1000W/m2 . This collector system can be modeled in a tracker simulation (we almost exclusively use SAT in our utility scale designs) that also takes into account the ground albedo, air mass, and other irradiance adders/detractors. The goal of this simulation is to have an output of an Effective Irradiance 8760 report for each of the 20years of historical data we would require for the site. Additionally the simulation would output the Cell Temp for each of the 8760 data series across the 20 years. Now to the fun part, we would shove each of these correlated datapoints (Ee & Tc) through the "Sandai" Voc calc to find each Voc for each 8760 entry across the 20 years. We would be looking for the maximum Voc in all entries.

I understand that we will be departing from the true Sandia Methodology and this would become something not quite aligned but I am curious what your thoughts are for what I am proposing.

We would need to cross check this calculation with current standard ways to calculate Voc to ensure our findings weren't drastically deviating until we are comfortable with this methodology.

Hi Matt,

You could use the CEC module model to see what Voc it calculates for your module. The CEC model reports the POA irradiance on the surface of the module (nominal), and the surface of the cell (after reflection).

Best regards,
Paul.

Hey Paul we are intimately familiar with both the 5 and 6 parameter models for the Voc calculations. I think it has become clear to us that we would need to develop a completely new calculation/methodology to accurately model the Voc with respect to site specific temperature / irradiance data.