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Good Idea? 15 panels facing SSE, 3 panels facing WSW. String Inverter.

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  • Good Idea? 15 panels facing SSE, 3 panels facing WSW. String Inverter.

    Planning a strategy for a new install, in the 18 or so panel range, for this roof:

    SMA Secure Power Supply Two Roof Face MPPT.JPG

    If 3 panels are on the West side, will there be enough string voltage for the inverter to start?
    Will the installer need to include panels from both faces in the same string?

    The probable inverter is a Sunny Boy, because of the included grid down power outlet.
    This is in an area with rapid shutdown requirements from the 2017 NEC.

    Sunny Boy seems to say cross facing is "not a problem" and losses should be in the single digits. Should I be concerned?
    Polystring Operation – Optimization of Self-Consumption vs. Mismatch Loss – Dr. Thomas Straub, Joachim Laschinsk

    Would the new SMA "shade fix" feature find the right MPPT?

    The goals by the way are: Flatten the duck curve
    The area has morning fog, but usually clear afternoon and evenings
    To lengthen the time of day the grid down power outlet is useful, flattening the production curve even at some small loss to total production.
    Use the longest lasting most reliable equipment allowed by the NEC
    Last edited by brycenesbitt; 06-05-2020, 09:58 PM.

  • #2
    You have misread the first SMA paper, I think. They show that if you parallel two otherwise identical strings with different facing directions into one MPPT input there are almost no losses compared to individual MPPT inputs. But they certainly do not recommend mixing facing directions in one string.
    And finding the right MPPT for a partially shaded string amounts to not generating less power than the same string with the shaded panels removed, not getting any additional power from adding the different facing panels.
    And a single 3 panel string is almost certainly not going to have a high enough voltage for its own MPPT input.
    If you were able to put up two equal length strings with different facing directions, you would definitely flatten the curve and extend the production hours. To the extent that inverter cost is high compared to panel cost this would be a useful configuration. Or if, as you state, your goal is specifically to extend the useful time of the Secure Power outlet of the SMA.
    But if you are in an area where the Electrical Code requires Rapid Shutdown, you will be hard pressed to make use of the Secure Power outlet in the first place
    SunnyBoy 3000 US, 18 BP Solar 175B panels.


    • #3
      At least 6 panels are needed for each string on a Sunnyboy inverter (although this varies a bit depending on which one. Check out the string sizer program at Putting in rapid shutdown capability prevents the Secure Power outlet feature.
      Looks like your do not have enough room for southwest facing panels unless you do some tilt up panels on the back roof. If you are on the 2017 NEC code you also have to leave 3' of emergency access around the perimeter of the array.
      (Do you get the feeling the safety first folks that write the code could care less about clean, renewable electric generation?)
      BSEE, R11, NABCEP, Chevy BoltEV, >3000kW installed


      • #4
        @inetdog good point on the matched strings.

        SMA seems to have a number of workarrounds for the Secure Power Outlet issues, even with rapid shutdown.
        Regardless the codes seem to have damaged one of the selling point of a string inverter: simplicity and lack of heat degrading electronic components on the roof.

        Is cantilevering the West Southwest panels possible under the code, or likely under an exception by ajh?

        Solar PV roof layout cantilever.jpg

        Has anyone tried proposing pre-building ventilation shafts for the fire department, as an alternative to leaving the 18" or 36" gaps?

        Note for the above hip roof it's just one 36" pathway, not "all the way around"

        (IFC 605.
        c. Hips and Valleys: Modules should be located no closer than one and one half (1.5) feet to a hip or a
        valley if modules are to be placed on both sides of a hip or valley. If the modules are to be located on
        only one side of a hip or valley that is of equal length then the modules may be placed directly adjacent
        to the hip or valley.

        Comments on the code by the way, go to:

        Last edited by brycenesbitt; 06-06-2020, 10:11 PM.