How to rewire GT PV Array for off grid?

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Comments

  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    OK, that's too risky. :grr

    What if I parallel 2 panels in 1 string with 3 panels in the other string? Will it "average" ( ( 50 x 2 = 100) + (50 x 3 = 150) ) / 2 = 125 voc?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How to rewire GT PV Array for off grid?

    Not really... The panels probably have a blocking diode so the "low voltage" string will not pull down the 3 string Voc (when cold)...And the controller may lock on the Vmp of the 2 string panel (when hot).

    If you have to use the panels... A small relay across one panel in each string. Connect a thermostat set to 32F (or whatever you like) such that it powers the relay "on" at <32F. When cold, the 2 panel string will have a good Vmp>62 volts or so... When >32F, the relay opens and Voc cold is no longer an issue.

    When you short out 1/3 of the panels when cold, you will lose 1/3 of the power output.

    If you measure the array temperature itself (glue probe to back of panel with a little insulation (so it measures panel temp)--Once the panels get a good dose of warm sun--they will probably quickly exceed your set point even if the ambient temperature remains cold.

    Automating the function would help prevent an "oops". It is OK to short the panels--that does not harm them.

    Other options could include a stack of series connected diodes (voltage drop of 5 volts of diodes may keep you out of trouble at the cost of 3% power loss. Take out/short out diodes when weather turns warm.

    Watch the specs for the controllers. For example (if I recall correctly) the XW MPPT will run up to 140 VDC. Over Voltage at 150V.

    You have to be careful...For example, the diodes will require a little bit of leakage current to draw the 5 volts across them. If the charge controller has very little leakage current, you would need to add a small power resistor to "load" the diodes...

    Or ignore me plus the overly complex solutions and run two panel strings when cold and 3 panel strings when >32F/panels are warmed up....

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    I like the simplicity of your last idea to use 2 panels in series in winter (<32F) and 3 in summer. I tried disconnecting / reconnecting the MC4 cables, and it was easy - no tool necessary. I've ordered a couple 15' #12 m/f MC4 extensions, so it should be a 5 minute job to disconnect the 9 panels in series and connecting the first 1 to the last (or in summer, connecting the first 2 panels to the last). Thanks.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How to rewire GT PV Array for off grid?

    See if there is an "easy way" to make a sane switching arrangement for groups of 6 panels... 3s*2p summer and 2p*s in winter. I don't want to see you lose power with the simple disconnect:
    [FONT=Fixedsys]
    =+=====+=====+======== + Bus
     |fuse |fuse |fuse
     |     |     |
    ---   ---    |
    |p|   |p|    |
    ---   ---    |
     |     |     |
     |     |     |
    ---   ---    |
    |p|   |p|    |
    ---   ---    |
     |     |     |
     +==+==+     |
        |        |
        *a       *b
        
        *c    *d
        |     |
       ---   ---
       |p|   |p|
       ---   ---
        |     |
        *g    |
              |
        *h    |
        |     |
    ====+=====+============= - Bus[/FONT]               
    
    3sx2p:
    a-c
    a-d
    g-h

    2sx3p:
    a-h
    c-b
    g-d

    Switch X:
    Pos1 a
    Com c
    Pos2 b

    Switch Y:
    Pos1 a
    Com d
    Pos2 g

    Switch Z:
    Pos1 g
    Com h
    Pos2 a

    All switches in POS-1 = 3 panels in series
    All switches in POS-2 = 2 panels in series

    Not sure that this makes sense (3 switches per 6 panel sets)... But I think it works.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    Re: How to rewire GT PV Array for off grid?

    Bill,

    Clever, thanks.