Implement CEC module performance model in place of desoto

[cwhanse]

The Desoto module performance model and the CEC model differ in that the CEC model has one additional parameter (an adjustment to temperature coefficients for Isc and Voc). pvlib-python implements only the Desoto model, but supplies parameters that are fit for the CEC model. Replacing the Desoto model with the CEC model would remove this inconsistency. The differences between the two model's results are quite minor.

[adriesse]

Are you talking about the so-called "adjust" parameter, whose origins I had trouble tracing a year or more back?

[cwhanse]

Yes

[adriesse]

I retained the impression that it was used to influence the fitting process rather than being a model parameter.

[cwhanse]

Adjust enters the IV curve calculation via the equation for light current: IL = G * ( IL0 + alphaprime (Tc - T0)), and alphaprime = alpha ( 1 - Adjust/100). As far as I know this is only detailed in A. Dobos paper describing the 6 parameter model coefficient calculator. The procedure to determine the value for Adjust is still obscure to me despite Aron's paper.

[adriesse]

The way I view this is that your diode model parameters may end up with a new, adjusted alpha (compared to the datasheet value) but that doesn't really constitute an additional model parameter.

There are a number of things that worry me about that paper, including the sanity check that allows alpha and beta to be adjusted by +/- 100%.

[wholmgren]

Can we add a CEC model while keeping desoto? Perhaps replace was in reference to the ModelChain logic?

I think I'd also want to add a corresponding model option in the ModelChain initialization. The model option could be automatically determined based on the presence or absence of the "Adjust" parameter in the supplied PVSystem.module_parameters.

[cwhanse]

We can (and I agree that we should) add the CEC model and keep the Desoto functions. However, strictly speaking we have CEC model parameters, rather than Desoto parameters. The values in SAM are determined simultaneously with the Adjust parameter (in the CEC bot not the Desoto model) and thus there is some (certainly minor) influence of the Adjust value on the other parameters.

[markcampanelli]

FWIW there is https://www.nrel.gov/docs/fy18osti/67399.pdf, esp. section 10.4. There are many reasons why “the” reported Isc temperature coefficient could be need adjustment, such as its spectral dependence during measurement. Why the same adjustment would be used simultaneously on Isc and Voc (presumably in data fitting) I cannot say. I too would be suspect if the adjustment were large, and is it true then that an IV curve’s Voc could be very different from the “adjusted” value? If yes, then what Voc value would pvlib report?

[cwhanse]

The Adjust parameter acts to bring the model's temperature coefficients for power and Voc into line with the datasheet values. The temperature coefficients of power and Voc are model outputs rather than model parameter. The CEC model implements Adjust to provide this matching, PVSyst uses a temperature coefficient for the diode factor. In either case, the effect is to add a model parameter for fitting purposes.

[wholmgren]

Are we in agreement that:

1. CEC model should be added to pvsystem.py as a function with a corresponding PVSystem wrapper method.
2. ModelChain should use CEC model if it detects that PVSystem.module_parameters comes from the NREL database (possibly via the presence/absence of an Adjust parameter).
3. Documentation should recommend using CEC model if module parameters come from the NREL database. Perhaps will be needed in a few places.
4. Documentation examples that currently use desoto should be modified to use CEC (they all come from the NREL database)

[cwhanse]

#1 - 4 : I agree.

[adriesse]

I'm not sure. I think part of my unease comes from the frequent use of the word model in this discussion without knowing exactly what it refers to in each instance. I think there are three separate "calculation steps" (for lack of a better word): the one-diode I-V curve equation (which hasn't changed in a long time and isn't associated with any particular name); the process of determining its 5 parameters at reference conditions from various measured quantities; and the translation of its parameters to represent different operating conditions. The third step is governed by additional measured quantities, such as alpha, or additional parameters produced in the second step, such as alphaprime/adjust.

Is there an easy way to keep these distinctions clear in the code structure etc?