Default Values for Single Diode Parameters

[smeredith]

I'd like to provide feedback about how PVsyst handles default values for the single diode parameters (Rshunt, Rseries, and Rshunt at g=0).

From what I've observed, PVsyst might (but does not always) choose different default values for these parameters depending on what version of PVsyst you're using. At first I thought this was just a difference between version 6.88 (the last version of 6) and all versions of 7. But looking at the previous versions of 7 that I have, it seems to vary unpredictably within 7 itself.

The following is an example of how different versions are choosing different values.

I have a PAN file with the following module characteristics:

Isc = 13.76 A

Voc = 49.20 V

Impp = 12.83 A

Vmpp = 41.35 V

muIsc = 6.7 mA/C or 0.049 %/C

Pmpp temp coeff = -0.390 %/C

Rshunt exponential = 5.5

Number of cells in series = 72

This PAN file has certain values for Rshunt, Rseries, and Rshunt at g=0. When I open this PAN file in different versions of PVsyst and enable the "Default" checkbox for Rshunt, I get a variety of changes to Rshunt and other single-diode parameters depending on the PVsyst I'm using. See the attached screenshot for the comparison.

I have a couple pieces of feedback.

1. Over the last couple years, too many changes have been made regarding how default values for these parameters are chosen. This is resulting in some modules behaving differently in different versions of PVsyst, if the PAN files were generated using one or more default values for single diode parameters. This is not a problem if the PAN files did not use default values.

2. Even though recent PVsyst help documentation has explained why these values are changing, it is not specific enough to describe how different versions are choosing the default values that they are.

In my opinion, PVsyst needs to either stick with a convention for choosing default values or get rid of default values entirely. My preference would be for default values to be eliminated - that way there is at least consistency in module performance between each version of PVsyst.

[André Mermoud]

Yes, the default values for the PAN file parameters are a difficult problem, and PVsyst has evolved about this question.

The objective is to find a set of parameters leading to the same behaviour in operating conditions (namely the low-light relative efficiency)

In PVsyst we have decided to specify the default values in order to obtain a relative efficiency of -3%. This may be achieved with different Rshunt values, but the Rserie is adjusted according to this criteria.

Please see the latest version 7.2.12, and especially the new help topic in this version: "Physical models used > PV Module - Standard one-diode-model > Rseries and Rshunt determination procedure"

[smeredith]

On 2/26/2022 at 3:29 AM, André Mermoud said:

Yes, the default values for the PAN file parameters are a difficult problem, and PVsyst has evolved about this question.

The objective is to find a set of parameters leading to the same behaviour in operating conditions (namely the low-light relative efficiency)

In PVsyst we have decided to specify the default values in order to obtain a relative efficiency of -3%. This may be achieved with different Rshunt values, but the Rserie is adjusted according to this criteria.

Please see the latest version 7.2.12, and especially the new help topic in this version: "Physical models used > PV Module - Standard one-diode-model > Rseries and Rshunt determination procedure"

I understand that your algorithm to determine default values has evolved.  But there is a difference between generating PAN files differently from version to version, and actually using the PAN files differently.

The problem is if default values are set, then PVsyst uses the PAN file differently depending on PVsyst version.  This leads to potentially different module temperature/irradiance losses than what was calculated in the version that the PAN file was generated in.

For example, the attached screenshot shows how the same PAN file can yield different temperature and irradiance losses.  The example PAN file used in this instance was generated in version 6.87.  The project run was a simple fixed tilt project with no shade scene defined.

Why is this a problem?  Manufacturers and third party labs expect that when they generate a PAN file that the performance of the module will be the same regardless of the version that it's used in.  But in PAN files where default values are used, that's not the case.

I strongly urge you to consider implementing a method in the next release such that PVsyst uses the same single-diode parameters that are defined in the PAN file, as opposed to re-calculating these parameters if they are defaults.

 

[André Mermoud]

The temperature behaviour is determined by the muPmpp value, which is always specified on the datasheets. I don't identify the origin of this very little discrepancy of 0.07-0.09% between versions.

I don't know which differences in input parameters are involved in lines #1 and #3. Even if the Rshunt value is different, the final default behaviour is determined  by the Rserie value, which is adjusted in order to fix the  Relative efficiency @ 200 W/m2 at -3%. The Rshunt value may change the curve shape, only in a marginal way. 

Now the differences in the results are in a fork of 0.08% in one case, or  0.14% in the other one. 

NB: If the parameters are specified within the PAN file, they are used as such. Only the default value may change.