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Module performance degradation

During the simulation, the degradation will be applied by:

  • A degradation of the module performance
  • A mismatch increasing along the years.

These values are evaluated once according to the year number after commissioning, and stay constant for the whole year.

Module degradation

The module degradation is specified by the Pmpp degradation factor, and the degradation sharing between the Imp and Vmp.

A degradation sharing of 50% means that both Vmp and Imp degrade by a same amount. The degradation rate of each one (Imp or Vmp) is the square root of the Pmpp degradation rate.

Now several studies show that the degradation rate of the current is significantly higher than the degradation of the voltage. This behavior is specified by the Imp Degradation Sharing parameter. An Imp Degradation Sharing value of 80% indicates that 80% of the degradation loss is due to the Imp factor, and 20% is due to the Vmp factor. This is roughly the result of these studies, and the default value established in the version 8 of PVsyst.

The Imp Degradation Sharing parameter used in the simulation is specified in the Advanced parameters, topic "System design parameters, losses, shadings", variable "Long term Pmp degradation Current sharing".

For establishing the one-diode model of the PV module corresponding to a given degradation, we have to modify the usual STC values. We apply the Imp degradation factor to both Isc and Imp; and the Vmp factor to Voc and Vmp. Then PVsyst adjusts the Rshunt and Rserie values for obtaining a comparable low-light performance. It is sometimes necessary to increase the Voc value, but this is without consequences on the model behaviour.

Defining a degraded module

The elaboration of the degraded module is automatically performed at the beginning of the simulation, according to the degradation rate and the year specified in the "Detailed Losses" dialog. It stays valid for the whole year.

However it is possible to explicitly create a degraded module. In the PV module definition dialog, choose the page "Additional Data" > "Degradation". Here you can define a degraded module, analyze its characteristics, and save it as a *.PAN file.

This is useless for the usual simulations. But sometimes people have to mix a new installation with an old one. In this case you may use the already pre-degraded module for the old part of the PV system.