externally forced current

[Chen]

Dear pvsyst team：

   I am using this software to simulate the temperature and power when a cell is local shading. I find that “externally forced current= 4.4A” is all the same for the different solar modules. What does it mean？ thanks！

   

[Chen]

as we can see the three values on the figure， the V (-5.2V）,I（7.93A） and P（128W），Is there a formula that can explain their relationship，thanks！

[André Mermoud]

This tool shows what happens for a specified current  (i.e. imposed by the rest of the string). You can drag the red point of this forced current for analysing what happens at different values.

The voltage of this point is the result of the reverse characterisics model: 

You can find this formula in the help  https://www.pvsyst.com/help/physical-models-used/pv-module-standard-one-diode-model/reverse-characteristic-of-a-cell.html

The temperature is obtained by applying the U-value on this cell, taking the dissipated power as input parameter instead of GlobInc. 

[Chen]

Excuse me, Prof.Andre Mermoud

is I the reverse current in this formula，so a more accurate representation would be   IRev = Iph +aRev *（V+Rs * IRev）   ；and the externally forced current is IRev？ thanks for reply.

 

[André Mermoud]

In PVsyst, the model for the reverse characteristics is this quadratic expression. This corresponds rather well to the reality, but this is a rough approximation.

By the way the values of this model have a quasi null impact on the PVsyst simulation results. 

Now the term  (Rserie *  I)  is indeed a little contribution  (perhaps it should not be taken into account, we will check). You can forget it.  

[Chen]

OK

 IRev = Iph +aRev *V，for aRev=800mA/V^2,

800/1000*（-5.2）*（-5.2）= 21.632 (A) ；the  externally forced current is 7.93A； how to calculate  IRev？ thanks

[André Mermoud]

IRev is calculated according to the expression mentioned above, by neglecting Rserie (put Rs = 0).

This expression gives the  IRev as a function of a specified voltage.  Iph is the short circuit current for this shaded cell.

[Chen]

On 5/19/2025 at 6:37 PM, André Mermoud said:

IRev is calculated according to the expression mentioned above, by neglecting Rserie (put Rs = 0).

This expression gives the  IRev as a function of a specified voltage.  Iph is the short circuit current for this shaded cell.

Will the temperature increase and power decrease when local solar cells of photovoltaic modules experience hot spots be considered in the power generation simulation？thanks

[André Mermoud]

No, due to by-pass diodes, the hot spot situation is not supposed to arise in normal operation. 

[Chen]

OK，thanks.