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PostPosted: Wed Jul 16, 2014 11:09 am 
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Joined: Mon Apr 16, 2012 7:29 pm
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In his documentation and advertising, a manufacturer presents PVsyst simulation comparisons with modules of other manufacturers, and claims that according to the PVsyst simulations, their modules behave better than those of competitors.

PVsyst SA cannot endorse this claim!

The one-diode model as implemented in PVsyst is well established, and confirmed by long-term outdoor measurements.
However the parameters used as input to the model are not part of the official specifications of the modules, and they are not always known or set with perfect accuracy. Therefore it is not possible to perform meaningful comparisons of PV module performances with the PVsyst simulation, without first making sure that the sensitive parameters (Rserie, Rshunt, IAM) have been established with the same methodology.
When taking module data directly from the database, such an assumption cannot be made straight away.
It is not legitimate and unfair to use PVsyst for comparing and publishing explicitly the performances of competitor's products without showing that the input parameters for the models have the same degree of accuracy. The default values that PVsyst uses in the case of lacking measured parameters, are deliberately conservative, and they tend to underestimate the real performance of the modules.

Namely, for the particular case in question, we note:

For some modules of the comparison, the Rserie value has not been specified by the manufacturer, so that the default value of PVsyst was used.
This default Rs value was particularly conservative in the version 5. It was improved in the version 6, and tends to usual manufacturer's low-light data since the version 6.26 (see the"possible yield differences betwenn version":

Special IAM definition
In this particular case, the definition of the IAM behavior has been significantly enhanced for the modules of this manufacturerin the PVsyst database, which gives an advantage of the order of 1.3 to 1.9% in yield with respect to the IAM default function (ASHRAE with bo = 0.05) as used for all the other modules.

The IAM behavior has been measured for 2 modules of this manufacturer, by an independent laboratory, following the new IEC 61853-2 standard (draft). This measurement gives results which are much higher than the ASHRAE standard model used as default by PVsyst, especially in the range 45° to 70° of incidence.

This raises several questions:
- Is the glass used in these modules so special, that it can explain such an enhanced behavior ?
According to this manufacturer, it is an anti-reflecting coating. deposited on the exterior face before or after the glass tempering operation (depending on the glass manufacturer).
- If so, are all the modules of this manufacturer equipped with this special glass?
According to this manufacturer, recent poly modules with Pnom >= 250 Wp are equipped with this glass. But some other modules in lower ranges have also been specified with this IAM correction in the database.
- Is the standard ASHRAE model really too conservative for the commonly used glasses? (The few measurements we know of are rather contradictory).
In this case the PVsyst default should perhaps be revised.
- Which kind of glass is used in the other modules for which the IAM has not been measured? Are they significantly different?

See also How to establish the "PAN files ?.

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