CEC and SAM

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Anton Driesse
CEC and SAM

I am trying to understand the link(s) between CEC and SAM.

In SAM the one-diode model is referred to as the CEC model, but the CEC official calculator says it uses public domain algorithms published by Beckman. Is there something specific about the SAM implementation that requires it to be identified as the CEC model?

CEC publishes PTC module ratings. Are these used in SAM as inputs? Or can SAM generate them?

Paul Gilman

Dear Anton,

Here is a brief description of the model from SAM's Help system:

It is an implementation of the six-parameter single-diode equivalent circuit model used in the California Energy Commission New Solar Homes Partnership Calculator, and is an extension of the five-parameter originally developed model developed at the University of Wisconsin.

And a more detailed description, also from the Module page Help topic:

The California Energy Commission (CEC) Performance Model uses the University of Wisconsin-Madison Solar Energy Laboratory's five-parameter model with a database of module parameters for modules from the database of eligible photovoltaic modules maintained by the California Energy Commission (CEC) for the California Solar Initiative.
The five-parameter model calculates a module's current and voltage under a range of solar resource conditions (represented by an I-V curve) using an equivalent electrical circuit whose electrical properties can be determined from a set of five reference parameters. These five parameters, in turn, are determined from standard reference condition data provided by either the module manufacturer or an independent testing laboratory, such as the Arizona State University Photovoltaic Testing Laboratory.
Note. SAM's CEC module library contains parameters for the modules in the List of Eligible SB1 Guidelines Compliant Photovoltaic Modules at http://www.gosolarcalifornia.org/equipment/pv_modules.php. We try to keep the library as current as possible, but there may be periods when SAM's library is out of date. If the library appears to be out of date, you can check for updates by clicking the link on the Help menu to see if we have prepared a new module library.
If you are a module manufacturer and would like to add your module to the CEC database, you should contact the CEC directly. See http://www.gosolarcalifornia.ca.gov/equipment/add.php.
The five-parameter model is described in De Soto 2004, Improvement and Validation of a Model for Photovoltaic Array Performance, Master of Science Thesis, University of Wisconsin-Madison. http://minds.wisconsin.edu/handle/1793/7602.

The PTC rating are part of the CEC library of module parameters, but SAM does not use the values. In the next version of SAM, which we plan to release in a few weeks, the PTC values are shown in the table of modules on the Module page so that you can sort the list by PTC rating, and see what the value is for each module in the library

Best regards,
Paul.

Anton Driesse

Thanks for your detailed answer, Paul. But isn't there an inconsistency here? SAM's brief description says the CEC uses a six-parameter model, whereas the long description says the CEC uses a five-parameter model. And looking at the CEC Calculator, it appears to run TRNSYS Type 194, which is De Soto's implementation of the five-parameter model.

How should one interpret this?

Best regards,

Anton

Paul Gilman

Dear Anton,

The implementation of  the "CEC" model in both SAM and the TRNSYS type (used by the CECPV calculator and older versions of SAM) is an extension of the original "5-parameter" model described in DeSoto (2006) and elsewhere. (See the Performance Model Documentation page for document citations.)

The extension involves the addition of a sixth parameter called "adjust." The five parameters are parameters for the equation describing the single diode equivalent circuit (a, I_l, I_o, R_s, R_sh). The sixth parameter is used in the process that calculates the numerical solution to adjust the reference temperature coefficients as described in Dobos (2012) Section 1.2.

I will revise the descriptions in Help and in the PV reference manual to make this clearer.

Best regards,
Paul.

Anton Driesse

Hi Paul,

I see more clearly now. What really threw me off was that you refer to this "adjust" value as an additional /model/ parameter.

Is it possible to make Dobos(2012) available for download on your web site in some form?

Best regards,

Anton

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