Need advice on best panel wiring in partial shade

[supersignman]

Thanks in advance for any solar gurus taking time to give advice! The knowledge on this forum has been invaluable to me in my solar journey and I'll be eternally grateful.

I have a limited amount of space in my back yard due to several constraints including tree shading from the neighbors but I'd like to try to squeeze out the absolute maximum solar production that I can. I have two options for wiring my panels shown in the diagram below. You can see in the diagram where I get shade on the right side until about noon every day. I'll be using three Victron 250/100 SCC's with a total of 36 Hyperion 395W bifacial panels. The panel are 37V OC so I can put 6 panels in series for a total of 222V which should give me about 15% safety margin on my OC voltage. I live in south Alabama so it's almost never below freezing during the day in the winter. I'll be putting two strings in parallel per SCC.

My question is which option would be best for maximum output? I don't really understand how the MPPT maximizes output so I don't know if it would be better to have have three strings with partial output like option 1 or have 4 strings with full output, 1 string with partial output and 1 string with very little output like option 2. Also, if option 2 is best, how should I parallel the strings? String 1&2, 3&4, 5&6 together or something else?

 

[dopeassjackson]

Parallel wireing works the best for shaded pannels. How you work that into your setup with an odd number of CC and pannels in the shade can be a bit tough. Your going to loose some either way.

 

[sunshine_eggo]

I have a limited amount of space in my back yard due to several constraints including tree shading from the neighbors but I'd like to try to squeeze out the absolute maximum solar production that I can. I have two options for wiring my panels shown in the diagram below.

No diagram

You can see in the diagram where I get shade on the right side until about noon every day.

No I can't.

I'll be using three Victron 250/100 SCC's with a total of 36 Hyperion 395W bifacial panels. The panel are 37V OC so I can put 6 panels in series for a total of 222V which should give me about 15% safety margin on my OC voltage. I live in south Alabama so it's almost never below freezing during the day in the winter. I'll be putting two strings in parallel per SCC.

My question is which option would be best for maximum output? I don't really understand how the MPPT maximizes output so I don't know if it would be better to have have three strings with partial output like option 1 or have 4 strings with full output, 1 string with partial output and 1 string with very little output like option 2. Also, if option 2 is best, how should I parallel the strings? String 1&2, 3&4, 5&6 together or something else?

Portrait/landscape orientation of panels also matters.

Since I can't see the diagram or options, I'll just blab a bit...

Order of effectiveness:

1. Remove shade.
2. Favor parallel over series.
3. Use optimizers on shaded panels (though Victron says don't).

Shade is devastating. Shade a single cell in any panel, and you cut it's production by 30-50%. Shade a single row or column, depending on orientation, and you can take production down to 0.

There is absolutely NOTHING you can do to get anywhere near rated power from a shaded array.

Performance of shaded systems also depends heavily on the robustness of the MPPT controller's algorithm. Victron is generally regarded as good. They regularly do "sweeps" through a large voltage range to try and find the max power point. I've witnessed this many times on VRM.

It also depends heavily on how well the bypass diodes in the panels perform.

"almost never" below freezing isn't "NEVER EVER DOES IT EVER GET BELOW FREEZING". You plan for the record low temperature. Your 6S proposal will be good down to about 20°F for "conservative" Voc temp coefficient.

Your MPPT are limited to 70A Isc PV input current. That would limit you to 5P.

 

[supersignman]

Thanks sunshine_eggo! The file in my initial post wasn't displaying right so I had to edit several times and you may have been viewing it during that time. I think it's showing correctly now.

 

[supersignman]

As an added note, I realize the shade cuts production tremendously on those panels. If it was like that all day I wouldn't be wasting my time, but after noontime I get full sun on those panels so it's worth it to me to put them there.

 

[sunshine_eggo]

Off the top of my head, option 2 - IMHO better to kill 2 strings than compromise 3.

I may be wrong. One may be better than the other, but I can't say conclusively either way.

If shade moves horizontally, portrait panel orientation will help exploit bypass diode function.

 

[supersignman]

Thanks SUNSHINE! Also, thanks for calculating my safe low temp on the OC voltage. I assumed it was somewhere slightly below freezing but I wasn't sure how to figure it. I've read about how smoke will come out of your SCC if you go over the voltage limit - and I definitely don't want that - so I will probably just shut off the panels on any mornings it might even be a possibility. We have gotten down to the mid teens at night before so there is a slight possibility I could get full SUNSHINE while still in the 20's. Better safe than sorry - especially if it's only a few hours once or twice a year.

 

[sunshine_eggo]

Voltage limit is definitely a "never exceed" value.

I figured that based on -0.4%/°C. If your panels are less negative (and most are), e.g., -0.36%°C, you will have even more safety margin. Their datasheet should provide you with a Voc temperature coefficient.