Thin Film Modules

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Paul Gilman
Thin Film Modules

I was wondering if you could help me understand how SAM models the efficiency improvement under low-light conditions for thin film-modules? I'm interested in the user-entered specification module selection. Under "cell-type", the drop-down menu does not toggle energy at all between technologies. However my understanding is that thin-film technology operates better under low-light conditions, and was wondering if that's included in my modeling at all?

Is there a better way to model a comparison between CIS and c-Si modules? If I provide you with module data, would you be able to create a CEC performance model for a specific module? It is not inthe SAM database at the moment.

With SAM's "CEC Module Model with User Entered Specifications," you to enter a set of manufacturer specifications for a module, and SAM calculates the set of coefficients required to model the module with the CEC module model. The CEC model in SAM is an implementation of the California Energy Commission PV Calculator, which is based on the five-parameter module model developed at the University of Wisconsin.

SAM's coefficient calculator is described in the following article:

The role of the cell type options is described in Section 4.3, "Empirically Guided Guess Values." To summarize, NREL analyzed parameter data of differerent types of modules over the entire CEC database to determine a set of relationships that SAM uses to determine the value of the non-ideality factor a, and series and shunt resistances (Rs and Rsh) initial values for each of the six cell types (Mono-Si, Multi-Si, Amorphous, CdTe, CIGS, CIS).
SAM uses this information to determine the value of the CEC 6-parameter model coefficients, which characterize the module under reference conditions.
From what I understand, how well the models characterize module performance under low-light conditions is not well understood by the photovoltaic modeling community. However, our comparisons of SAM results to data measured from operational systems shows that the results are reasonable for the kind of techno-ecnomic analysis for which SAM is designed.
Here is one article that touches on the subject:
Stein, J., Sutterlueti, J., Ransome, S., Hansen, C., King, B. "Outdoor PV Performance Evaluation of Three Different Models: Single-diode, SAPM, and Loss Factor Model."  Sandia National Laboratories. (PDF 583 KB)

If you'd like to investigate this further in SAM, you could compare results for the CEC Performance Model with User Entered Specifications using the same specifications and different cell types. Because the Sandia PV Array Performance Model uses parameters from emprical analysis of data from field measurements, differences in results between different cell types may be more representative of the module's actual performance in low-light level conditions. For example, you could find a thin-film module in the Sandia library, and then model it using the Sandia model, CEC model with database parameters, and CEC model with user-specfied parameters and compare the array output during hours of low light.

Best regards,

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