Panels in Series - effects of 1 dirty panel

[thescoutranch]

This might be a simple question, but if you don’t know the answer (like I don’t) it’s very complicated.

I understand that when the panels are in series the voltage is Cumulative and the current is Constant. What I am not sure of is what happens if one of the panels in the middle of a string is under performing (due to dirt/bird crap/deterioration/kids with a paintball gun, etc - doesn’t matter why, but pretend 1/4 of the panel is covered).

3 identical Clean 400W panels in series
Measured output downstream of #1
should be 40vdc & 10 amps.
Measured output downstream of #2
should be 80vdc & 10 amps.
Measured output downstream of #3
should be 120vdc & 10 amps.

3 identical 400W panels in series - 1 clean / 1 dirty / 1 clean
Measured output downstream of #1
should be 40vdc & 10 amps.
Measured output downstream of dirty (loss of 25%, 300W output) #2
should be ?vdc & ? amps.
Measured output downstream of #3
should be ?vdc & ?amps.

If the number 2 panel is under performing when connected in series, does it just contribute less amperage but the voltage output would be the same ie 40vdc, or would the amperage stay the same and the #2 would drag down the other voltages in the series? Or some other combination?

I understand, that if #2 dies completely the whole string would not put out voltage downstream of it (ie open circuit/ old style Christmas lights).

This is part of a bigger question that I will need to ask, but I need to understand this part first, to be able to phrase the next question properly.

Thanks for any input.
Epp

Edited to correct the question. Thanks @squowse

 

[squowse]

You're getting series and parallel mixed up.
For your 3 panels in series, the total string voltage will be 120V.
The current through each panel will be the same - say 10A.
If one panel is dirty and would only produce say 5A, then the other two will also be limited to 5A, as the current has to be the same through all of them. There is nowhere else for it to go.

 

[thescoutranch]

You're getting series and parallel mixed up.
For your 3 panels in series, the total string voltage will be 120V.
The current through each panel will be the same - say 10A.
If one panel is dirty and would only produce say 5A, then the other two will also be limited to 5A, as the current has to be the same through all of them. There is nowhere else for it to go.

Crap crap crap, I completely mistyped that, and knew better. Thank you for pointing this out, I will edit the original thread. Also thank you for your answer.

 

[ericfx1984]

So I have some real world samples of this.. something to keep in mind is a dirty panel is basically similar to a shaded panel

It just so happens that the shading is directly on the glass of the panel

So a while back I had four t250 Santan solar panels in series... I had about a 20 to 30% shade on one of the panels... And I was outputting about 600w

Then I also had another series string on a different charge controller but it was only three panels with no shading and that one was putting out close to 700 Watts

So this gave me a pretty good indication of the overall performance degradation of that particular set of panels one panel shaded to 25% yielded an overall system performance of about 60%

So as a result three panels with no shade definitely outperformed four panels with some shade

 

[thescoutranch]

So is a solar panel designed to put out a constant voltage and the amps vary with the panel’s wattage generation?

Ie a 400W panel that is capable of 40 V and 10 A in a “perfect world, if you shade part of it, and it’s only generating 200 W of power, The voltage should still always be 40 V and amperage would drop to 5 A?

Thanks
Epp

 

[squowse]

So is a solar panel designed to put out a constant voltage and the amps vary with the panel’s wattage generation?

Ie a 400W panel that is capable of 40 V and 10 A in a “perfect world, if you shade part of it, and it’s only generating 200 W of power, The voltage should still always be 40 V and amperage would drop to 5 A?

Thanks

Pretty much. The volts appear first, as soon as light hits the panel. But if the MPPT "opens the tap" (MOSFET) to get current (amps) then the voltage will disappear quickly. Once there is stronger light the voltage can be used to support more and more current with only a small voltage drop.

 

[squowse]

So is a solar panel designed to put out a constant voltage and the amps vary with the panel’s wattage generation?

Ie a 400W panel that is capable of 40 V and 10 A in a “perfect world, if you shade part of it, and it’s only generating 200 W of power, The voltage should still always be 40 V and amperage would drop to 5 A?

Thanks
Epp

Consider the Voc and the Vmpp stated in the panel specification