Connecting modules on different roofs

[SpitfireW'sale]

Spitfire, your assumptions are not quite right in the partially shaded example. For the shaded string, consider the non-shaded panels. If for whatever reason the current through the string is reduced whilst full illumination continues, then the voltage across each of the unshaded panels in the shaded string will increase along the path in the curve on the data sheet you cited. You need a more complex model than you are using. I believe it very unlikely that more shading in the way you describe will increase the output.
Regards
Bruce

Bruce, I know that it's not such an easy calculation because I have not taken into account the coefficients of the panel and in addition everything is calculated based on STC conditions. Actually, using that simple maths, I tried to show the difference between the partial and fully shading of a panel in two strings connected to one MPPT.

I believe it very unlikely that more shading in the way you describe will increase the output.Bruce

I have not mentioned something like that. I just said that two uneven strings may have less losses than two even strings with a shaded panel.

I hope that this makes sense to you.

 

[BruceB]

Then I have misunderstood you, but your words say that completely shading one panel " = 12% Less impact than partial shading!!!"

 

[SpitfireW'sale]

Partial shading results in ca. 20% losses of power.
Completely shading of one panel results in ca. 12% losses of power.

So, it's more efficient to loss one panel instead of partial shading of that.

 

[BruceB]

So you are saying that increasing the shading from partial to full on a panel will increase the overall output of the array.

My point is that is almost certainly wrong, because of the wrong assumptions you are making about voltages in your example.

Regards
Bruce

 

[solarcat]

If both sets of panels have the same direction and tilt, then yes, no problem.
But beware of long DC cable runs. The longer the DC cable, tho more power you loose.

 

[SpitfireW'sale]

The thing is if we take into account all the factors (coefficients, etc), they impact both cases equally. the fluctuation of the irradiance affects only the current, not the voltage. So, the MPPT tries to maximize the power based on the current of each string, not the voltage. The full shading does not increase the overall output of the array, it does only result in less losses than partial shading.

What's your opinion in terms of full and partial shading? In which case you have got less losses?

 

[BruceB]

You are saying that partially shade a panel and you have 20% losses, then fully shade it and you have 12% losses. OK that is less losses as you say, but in the conventional use of English you have increased the amount of shading from partial to full and the output of the array has increased from 80% of max to 88% of max. As I have said, I do not believe it.

If your example assumptions had been accurate I might have believed it, but they are not. For instance, you are assuming the voltage generated by the unshaded string determines the voltage across both strings. It does not. They interact, but settle at a single value of course. You assume the voltage across each of the panels in the partially shaded string stays the same when the current is reduced. It will not. The voltages on the fully illuminated panels will increase along the line of the appropriate curve in the data sheet. That is after all how mppt tracking works. That means that the power reduction is not as large as you calculate.

If your example were true, that less energy in (ie more shading) gave more energy out then you have the basis of a perpetual motion machine that could make you very rich.

Regards
Bruce