Quick question here. Is it possible that adding more panels, of the same spec, to a string could DECREASE the total kWh produced by that string? Say the string got no shade before but the new panels that were added now gets partial shade? Will the MPPT find an operating point that at least still provides the power of the panels that are still in the sun?
The current you get out of a string will be the smallest current from any panel. If one panel get shade that will reduce the current from that panel and so also from the entire sting.
So yes any shade will reduce the entire string, and so you can get less power from more panels.
These days the panels made from 2 parts of half cut wafer cells split your panel in 2 parts so you will get half the power if the one half of your panel is in the shade which helps.
I have added an image to help. Here you can see that each half cell is wired alonge the green or blue lines. Lets say that the very bottom left half cell are fully in the shade, then the entire blue line will give no power. If this was a 400W panel that would mean you only get the top half (200W) which limits the entire string.
That is not entirely correct - or more to the point - it’s not the whole story. But it does depend on the MPPT algorithm. Some will do a more thorough search for the peak power point, in this case it will search until the bypass diodes of the shaded panels conduct and you will essentially get the power from the old panels that are not shaded. Some MPPTs call this “shadow scan” or something like that and is a setting that is not on by default.
With regards to split panels - they are split by current. Which means that if one half has shade, then you half the current (unless you have the algorithm described above). So you halve the current of the entire string, which likely yields less current than the string without the shaded panel. With that said, split panels also usually have 3 bypass diodes, so you could potentially only lose 1/3rd of a panel if your MPPT can find that point.
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You would think the bypass diodes would eliminate the shading problem, but alas it does mess with the MPPT (on smaller strings) quite a bit. You can end up way off the Vmp or Imp.
If you do have known shading and want to use the maximum out of the modules you can look at optimisers (Tigo), you add them onto the modules with shading (only). They helped quite a bit on my system, but only where they are on the same orientation. Still trying to figure out the 2xorientation configuration the best.