different module orientation in same string

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System
System Posts: 2,511 admin
in Advanced Solar Electric Technical Forum #1
I know this is a no-no. But is there a way that an installer can estimate string production?
Here is my problem. I have a customer located at 30 degrees latitude who wants a 6kw system but has limited south(130 degrees) and west(210 degrees). I can install enough panels on the 130 degree facing roof plus the North facing roof(310 degrees) and get 81.5% of optimal production using PV watts 1.
I want to use Evergreen 210s-no shade-and a SMA 6000. How damaging would it be to put 12 panels on the 210 degree roof and 5 panels on the 130 degree roof in a single string? Would it not derate down to the 130 degree module production?

I know I could use a higher voltage module and fit 12 on a string.
Any ideas?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: different module orientation in same string

    If I understand your question--splitting a series string across two roofs--one east, another west facing.

    My concern is that Solar Cells are, for the most part, current mode power sources... When you split a string across the two roofs, the maximum series string output current will always be limited by the cells with the limited direct sun light--essentially, the "sum" of the worst of all possible orientations over the entire day for the shared faces string.

    I could imagine creating a spread sheet model of the different panel orientations of the east/west strings (hourly output from PV Watts), convert watts into amps, take the lowest current of the east/west stings, then get a new power output based on P=Imin*Vtyp).

    You should be much better off to keep all west panels on west strings, and all east panels on east facing strings.

    Play with panel and string configurations, along with inverter sizes so that, if you cannot match Vmp across the roofs, that you may look at two inverters... One "large" for the west roof, and one small for the east roof (perhaps, a few of those Enphase modules for the east roof--although, mixing brands and using Enphase means that your data collection will be a pain and you will need different hardware for the two differing brands of systems--unless you chose Enphase for everything--which may be expensive in itself for larger systems).

    My 2 cents worth.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Ecnerwal
    Ecnerwal Solar Expert Posts: 101 ✭✭
    #3
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    Re: different module orientation in same string
    Will wrote: »
    I know this is a no-no.

    ...so pay attention to what you know. As Bill says, you are basically limiting output of the whole string to the output of the part with the worst lighting.

    Not A Good Plan.

    [please--we don't know the details or roles of anyone involved. Making assumptions does not help anyone here. -Bill B. Mod]
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  • System2
    System2 Posts: 6,290 admin
    #4
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    Re: different module orientation in same string

    Thanks for the info. I thought about two inverters but from what I read there is not a significant loss by putting to strings at different orientations into the same inverter.

    As for the Enphase inverters I am located in central Texas and worry that the roof temperature will be too hot in the summer. Average temp is 96 F with some days in the 100s.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
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    Re: different module orientation in same string
    Will wrote: »
    Thanks for the info. I thought about two inverters but from what I read there is not a significant loss by putting to strings at different orientations into the same inverter.

    For the above to work well... Each of the strings must be an identical number of solar panels (same Vmp for each string), and each string must be on the same orientation (all panels in a string--point the same way).

    Basically, the voltage of a solar panel reaches near Vmp as soon as there is enough light to generate any useful power. The panel current is pretty much related to the amount of light hitting the panel. If you put a "dark" panel (west facing) in series with a "bright" panel (east facing)--the dark panel will be the one that sets the overall current for the string (very little current--therefore little power).

    With West and East strings (no series panel string with panels on both roofs) sharing a single MPPT controller works because the Vmp is about the same for both panel sets. And the parallel strings both add current that is based on the amount of light hitting them... West (dark) string, not much light, not much current). East (bright) string, lots of light, lots of current.

    The loss, according to one white paper from Fronus (another thread here) was ~1-2% overall sharing one inverter on multiple east/west strings (still usually better to have all panels face south--but that is not always possible).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Malasombra
    Malasombra Solar Expert Posts: 24 ✭
    #6
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    Re: different module orientation in same string

    Diferent facing ,diferent string. So ask SMA about which inverter with multi-string capability ( HC ) is better for you.
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