NB: The Maximum Power under STC (or the
nominal efficiency) may be slightly different between the 3
concurrent definitions:
The
Nameplate value Pnom is
the nominal (commercial denomination) of the module, which is the
basis of the definition of the installed "Nominal Power" of the system.
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The nameplate value is used for the estimation of the
PR (performance ratio). |
| The Impp*Vmpp |
as specified by the manufacturer
at STC may be different of PNom. |
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This is a problem in the
Manufacturer's specifications. Recently, some manufacturers begun
to include the positive sorting in this STC specifications. That
is, for a given nameplate module, they define STC Vmpp*Impp values
superior by 2-3%. This of course distorts the interpretation of the
PR: it produces an artificial increase of the PR, which is not
acceptable. |
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In the PVsyst database,
we now check that the
Imp*Vmp product matches the PNom value within 0.2%. If
not we modify either the Vmpp or the Impp or both for getting PNom.
This may be a reason why the PVsyst data don't exactly correspond
to the datasheets. |
| The Modelled Pmpp |
calculated by the one-diode model (see for example
"internal
model results tool") may be higher than the specified Impp*Vmpp. The reason is that the
model is established for passing the I/V (or P/V) curve through the
3 specified points (0,Isc), (Vmp, Imp), (Voc,0). Now, nothing
ensures that the specified (Vmp, Imp) point is the true
maximum of this P/V modelled curve. If the true maximum is
different, it will of course be higher ! |
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The modelled Pmpp is used as
starting point for the evaluation of the array losses in the PVsyst
loss diagram. Therefore the PR calculated from the array loss
diagram may be slightly different from the PR shown on the results.
This last effect may be considered as a weakness of the one-diode
model: the model doesn't reproduce the exact I/V curve shape of the
manufacturer. But this can also indicate that the manufacturer's
specifications are not quite correct. |
