# Approach to determine the value of Rs,max in Pvsyst

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Dear Professor Mermoud A , recently I read your article--'MERMOUD, André. Conception et dimensionnement de Systèmes Photovoltaïques : introduction des

Modules PV en couches minces dans le logiciel PVsyst. 2005' https://archive-ouverte.unige.ch/unige:38786
In the P37(Détermination à partir des données du fabricant ) , you said  '
PVsyst détermine par approximations successives la valeur maximale de résistance Rs qui permette encore à notre paramétrisation de passer par les 3 points. ....................De fait, PVsyst ne propose pas de solution toute faite à ce problème. Par défaut, il choisit Rs correspondant à un facteur Gamma de 1.3, valeur qui semble raisonnable pour les modules au Si-cristallin. Mais il laisse à l'utilisateur la possibilité de modifier Rs, par exemple s'il veut obtenir un coefficient μVco donné.'

I also read the help document in PVsyst--Rseries and Rshunt determination procedure, but I don't know the limited condition (saturation) https://www.pvsyst.com/help/index.html?pvmodule_rserie_rshunt_determ.htm

When I attempt to search the Rs,max value , I find there are no clear norm to judge whether it is the biggest Rs , I find it is feasible to use Rs greater than Rs,max given in PVsyst to solve for the remaining five parameters , I can also use the calculated five parameters to plot IV curves which can also through the three point(Isc,Impp,Vmpp.Voc).

As show below , the Rs,max given in PVsyst is 0.23 , but I can use larger Rs(0.3,0.4) to plot , when Rs is too big(0.5) , the image is indeed distorted , but now the value is far ahead of Rs,max.

So what is the correct method to limit the Rs,max ?

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You are referencing a document which I have written 18 years ago, which was the final report of my first experiments about the PV module modelling.

Things have obviously evolved since  this date, and the criteria for choosing the Rserie has been highly refined. We have recognized since over 12 years that the Rserie is related to the low-light efficiency, and this is now the basis for defining this value  if we avail of "official" measured data. In absence of such data, PVsyst chooses a reasonable default value corresponding to a low-light relative efficiency   of -3% @ 200 W/m2.

However the way of defining RSMax has not changed: it is still the limit Rserie value which allows to solve the 3 equations at ((0,Isc), (Vmp, Imp), (Voc, 0)  at STc when fixing Rshunt. We don't propose an explicit equation for that.

The model and its "modern" declinations is fully described in the help of PVsyst  "Physical models used > "Standard one-diode-model"

NB: In your plot for Rserie = 0.5 ohm, I don't know how you have determined the values of IoRef, IphRef, and diode ideality factor. These are obviously not correct as your curve doesn't match the voltage condition  at the  (Voc, 0) specified point.

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