Numerical method of solving three unkown parameters

I'm new to Matlab and I'm in the midst of determining three unkown parameters of three equations, but I so lost - hopefully someone can guide me of doing this.

This is for determinig some parameters of a PV system. My unkowns are Rsh, Rs and Vt and I have written two of my known equations, but I'm not sure how to do the last one because it is dependent in the derivative of power with voltage = 0.

My three equations are from the paper 'PV panel model based on datasheet values'(IEEE Xplore Full-Text PDF: (cvt.dk)) and are numbered (12), (18) and (19) - I have attached at photo in this post.

Hopeful for some guidance :-)

clc;
clear all;
close all;
%datasheet values
Isc=3.87;
Impp=3.56;
Vmpp=33.7;
Pmpp=120;
Voc=42.09;
ns=72;
%Vt=(A*k*T_STC)/q - junction thermal voltage

syms Rsh Rs Vt;

equ1=Impp==Isc-((Vmpp+Impp*Rs-Isc*Rs)/Rsh)-(Isc-(Voc-Isc*Rs)/Rsh)*exp((Vmpp+Impp*Rs-Voc)/ns*Vt);
equ2=
equ3=-1/Rsh==((-(Isc*Rsh-Voc+Isc*Rs)*exp((Isc*Rs-Voc)/ns*Vt))/ns*Vt*Rsh-1/Rsh)/(1+((Isc*Rsh-Voc+Isc*Rs)*exp((Isc*Rs-Voc)/ns*Vt))/(ns*Vt*Rsh)+Rs/Rsh),Rsh)

Answer 1

[Torsten]

As far as I understand the article, the left-hand side of equation(18) is 0 at I = I_mpp, V = V_mpp according to equation (6). But it's only a guess since I don't know anything about the subject.

And take care that you implemement divisions correctly. Note the difference

2/5*3

ans = 1.2000

2/(5*3)

ans = 0.1333

I think you often want to write the second version in fractional expressions, but you use the first.