Serial vs parallel Panels

[derekisastro]

For a situation where at least one cell in every volume is completely shaded, there is little difference between parallel and series.

In parallel, you’ll get 100% output from the unshaded panel and close to 0% from the shaded panel, or an average of 50% from the string (current of ~Imp @ voltage of Vmp).

In series, as I explained earlier, string voltage will drop from 2xVmp to 1x Vmp - 3 x 0.6V.

The unshaded panel will still be putting out Isc a full Vmp, but the shaded panel will actually be putting out Isc at minus 3 x 0.6V = -1.8V (since three bypass diodes have been activated to supply Isc by bypassing the tree sun-strings of the shaded panel).

So total power is (100% + ~-5%) / 2 = ~47.5% (95% of the power from a parallel string).

I already explained how things change if only 2/3 of the shaded panel is covered (meaning at least one cell fully shaded in 2 out of 3 2-column bypass sections). In that case, the parallel string still puts out 100% while the shaded panel puts out 0% for a string average output of 50% while a series string will activate 2 bypass diodes on the shaded panel to deliver full Isc @ Vmp/3 resulting in (100% + 33%) / 2 = 67.% output for the string (33% higher output than the parallel string), [precise power needs account for the same -1.2V drop from 2 bypass diodes being activated, so actual output of the shaded panel will be closer to 30% for full string output of ~65%]

And I’ll use your photo to tell you the impact of half-cut panels can have;

That same shade you’ve photographed on two half-cut panels in series will have string output of over 50% from both panels for string output of 50% (actually a bit higher since voltage of the unshaded panel will increase from Vmp to Voc for a ~10% output gain and string output closer to 55%), so similar to a series string of conventional panel but actually 15-16% higher once you dig into the details.

More importantly, in a parallel string of half-cut panels, the unshaded panel will deliver exactly 100% power while the shaded panel will deliver exactly 50% power for a string output of 75% (over 36% better than the same half-cut panels in a series string and a whopping 58% better than conventional panels in a similar series string).

I’m facing a severe morning shading issue with all 3 panels covered by shade in the way you’ve indicated (but in portrait orientation and only across the bottom half of the panels).

I get 50% output during that shaded period from a 3P parallel string where I’d get 0% output from the entire array as long as there are no 2-column bypass-diode sections free of belong blocked by shade…

Thanks. I get what you're saying.

I guess the only thing that still confuses me a little is that you're suggesting I may get some amount of power out of the partially shaded panel (say 25%-50%?), in a series connection with the second non-shaded panel, whereas when connected in parallel to the second non-shaded panel, I'd likely get 0 from it?

This is even knowing that when tested in isolation, as an individual panel, the VoC of that shaded panel in the picture is 33.4V and the Isc is 0.32 amps ... like 10W ... (compared to the non-shaded getting VoC of 33.3V and Isc of 7.05 amps). Somehow by being in series with the second unshaded panel, that 10W would possibly be more?

Is my orientation the worse orientation to take advantage of the bypass diodes of my panels in a series connection?

 

[Hedges]

I guess the only thing that still confuses me a little is that you're suggesting I may get some amount of power out of the partially shaded panel (say 25%-50%?), in a series connection with the second non-shaded panel, whereas when connected in parallel to the second non-shaded panel, I'd likely get 0 from it?

Nope, the shaded panel is introducing a couple of diode drops. Used in series with the other panel, it is slightly reducing voltage which would reach a SCC.

In parallel, you would get that fraction of an amp it can produce.

Is my orientation the worse orientation to take advantage of the bypass diodes of my panels in a series connection?

Yes. The shade shown hit all groups of diode-bypassed panels. Rotated 90 degrees, it might only shade one group. In that configuration, parallel wouldn't get much from it, but series would harvest power from the unshaded sections.

 

[derekisastro]

Nope, the shaded panel is introducing a couple of diode drops. Used in series with the other panel, it is slightly reducing voltage which would reach a SCC.

In parallel, you would get that fraction of an amp it can produce.



Yes. The shade shown hit all groups of diode-bypassed panels. Rotated 90 degrees, it might only shade one group. In that configuration, parallel wouldn't get much from it, but series would harvest power from the unshaded sections.

Is there a way to confirm the locations of these bypass diodes?

Are they always oriented in such a way that the panels are better off in portrait orientation for my example?

Are the numbers of diodes in a panel relatively consistnt? Do some panels have 12 and others maybe only 3? Ideally, it would be great to know exactly how the groups of cells on my panels are arranged with respect to the bypass diodes to maximize their orientation.

Thanks for all the info!

 

[fafrd]

Is there a way to confirm the locations of these bypass diodes?

Are they always oriented in such a way that the panels are better off in portrait orientation for my example?

Are the numbers of diodes in a panel relatively consistnt? Do some panels have 12 and others maybe only 3? Ideally, it would be great to know exactly how the groups of cells on my panels are arranged with respect to the bypass diodes to maximize their orientation.

Thanks for all the info!

There are three of them and you can generally see them as little black boxes at the back of your panel, one often hidden within each cable output and the third ‘cable-less’ box in the middle.

Standard panels are always 6 columns of cells and the bypass diodes are always positioned between columns 2 and 3 (to bypass columns 1 and 2), between columns 4 and 5 (to bypass columns 3 and 4), and after column 6 (to bypass columns 5 and 6).

They work effectively for vertical shading in portrait orientation (but are ineffective for horizontal shading).

And they work well for horizontal shading but only in landscape orientation (poor for vertical shading on landscape orientation).

 

[Hedges]

Is there a way to confirm the locations of these bypass diodes?

You can also test with shading patterns.
Isc would remain unchanged until you shaded at least one cell per diode-bypassed group.
Sliding a board over the panel, starting at one edge and again starting at one end, should show that.

Voc and Isc don't necessarily show impact on Vmp and Imp, as you have noted.
You can watch SCC display while shading.
You can connect a resistive load (I used a couple oil-filled electric radiator heaters) to get Vload and I load.

 

[fafrd]

You can also test with shading patterns.
Isc would remain unchanged until you shaded at least one cell per diode-bypassed group.
Sliding a board over the panel, starting at one edge and again starting at one end, should show that.

Voc and Isc don't necessarily show impact on Vmp and Imp, as you have noted.
You can watch SCC display while shading.
You can connect a resistive load (I used a couple oil-filled electric radiator heaters) to get Vload and I load.

Yes, as I stated earlier, picking up a 40Ohm or 50Ohm 50W power resistor would provide an easy way to confirm that maximum power point shifts from roughly Vmp when panel is unshaded to ~2/3 Vmp when any single cell is completely shaded, to ~1/3 Vmp when 2 cells in any 2 of the 2-column bypass zones (columns 1+2, 3+4 and 5+6) or completely blocked by shade.

An electric heating element also works well if you happen to have a spare one lying around…