<< Click to Display Table of Contents >> Navigation:  Physical models used > PV Module - Standard one-diode-model > Rseries and Rshunt determination procedure

The previous topic Model parameters besides Datasheets defines the  resistances involved in the one-diode model.

This explains that the Low-light performance (relative efficiency) is determined by the  RSerie, RShunt and RShunt(0) parameters.

When entering the page "Model parameters > Rshunt-Rserie" in the PV module definition dialog, you are advised to choose "Relative effic." for seing the graph of the Low-light behaviour:

You can easily check by playing with the parameters that this low-light efficiency behavior  depends on the 3 mentioned resistances:

Default values

The key point here is that the main default condition is based on the relative efficiency @ 200 W/m² = -3%. The RSeries is adjusted to obtain this condition, whatever the Rshunt and Rshunt(0) values. Therefore usually, when manipulating these values you don't notice that the low-light behavior is also depending on them.

Constraints between these values

These variables are strongly interdependent.  

Therefore, when the desired Relative Efficiency cannot be reached, it is necessary to increase the Rshunt in order to increase the RSerieMax.

Vmp/Voc ratio limitation

The RSerieMax is also limited by the Vmp/Voc ratio at STC.

The next graph shows the evolution of RSMax, which is linearly but quickly decreasing with the Vmp/Voc ratio. You can notice that we have a decrease of -50% for an increase of only 3% of the Vmp/Voc ratio.  

In practice, Vmp/Voc ratios greater than around 84% should be avoided.

Some manufacturers artificially decrease their Voc value, probably in order to increase the Fill Factor. We have sometimes observed values greater than 84% on the datasheet specifications, when the detailed measurements provided by an independent laboratory showed values below 82 or 83%.  

The next graph shows examples of the possible low-light efficiencies as a function of Rshunt and Vmp/Voc.

Additional constraint: low temperature behavior

When Rserie is close to  RserieMax, the establishment of the model leads to very low  diode saturation current (IoRef) as well as a low diode ideality factor (Gamma, up to 0.92).

Now the diode saturation current Io is highly varying with the temperature: i.e. by a factor of the order of 10'000  between  + 25°C and -10°C !  Due to numeric problems with the exponential, PVsyst has to put a lower limit to the  temperature-dependent Io value.  This limit is extremely low (0.01 pA), but when it is reached the temperature behavior of the model is altered, the muPmpp thermal coefficient changes sign.

When establishing the default, PVsyst tries to push this limit below -10°C, by increasing the RserieMax value so that the suited default Rserie is not so close. This use the same arguments as previously: increasing Rshunt, and possibly Voc is this is not sufficient.

Establishing the default values for  RSerie and RShunt

When RSerie and RShunt are not defined in the PAN file (most of the cases), PVsyst has to determine default values.

As mentioned previously, this default is characterized by the Low-light relative efficiency = -3% at 200 W/m².

The procedure is as follows:

This procedure is applied when reading the PAN file from the database, when RShunt and RSerie are not defined. You can also execute it by clicking the default cases for Rshunt and RSerie.