Choice of PV Power

TexasDanTexasDan Registered Users Posts: 10
Wiring (4) Sharp 240 watt (VPM 29.3 Volts IPM 8.19 Amps) PV panels in series will send about 117 Volts at 8 amps to my Midnight classic 200 MPPT charge controller which will feed a 24 volt battery bank and a 24VDC/120VAC inverter. Parallel/Series wiring the panels would send the controller 58 volts at 16 amps. As the classic CC can easily handle either power input, which would you choose and why? Awassume a short PV to CC wire distance with no voltage drop. Thanks.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Choice of PV Power

    No Voltage drop issues? Go with two parallel strings of two in series. The closer the array Vmp is to the system Voltage the more efficiently the charge controller will function. If it has to convert higher Voltage to lower Voltage some power is lost to heat in the controller. We're only talking about a couple of percent, but it's still a loss.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Choice of PV Power

    Check the Midnite manual... But the typical MPPT charge controller is most efficient at about 2x Vbatt-charging voltage (charge at 28 volts, then Vmp-arra ~ 56 volts would be most efficient.

    However, the difference it usually only 1-2% or so... I would not go out of my way to go lower voltage because of it. (although, heat is the enemy of electronics and anything you can do to reduce heat is a good idea).

    Some things to think about--You would need much heavier array copper cable at lower voltage. If you add more panels, 3 or more strings need a combiner box with fuse/breakers.

    On the other hand, are you going to add more panels later? If you do 2 in series, then you can add two panels at a time (with combiner box). If you do 4 in series, you can only add 4 panels at a time (and breaker box if needed) without rewiring the array back to 2 or three panels in series.

    Just do a paper design both ways, and see hour your wiring/combiner box/expansion plans would be affected.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RybrenRybren Solar Expert Posts: 351 ✭✭
    Re: Choice of PV Power

    How would shading affect a 4x single series string v a 2x2 series-parallel string? My gut tells me that the single series string would be more affected than the series-parallel setup.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Choice of PV Power
    Rybren wrote: »
    How would shading affect a 4x single series string v a 2x2 series-parallel string? My gut tells me that the single series string would be more affected than the series-parallel setup.

    Yes. This sort of thing has been discussed a lot around here, and it keeps coming back to shading one panel in a series string is worse than shading one panel in a parallel connection. Of course "shading" has about a million variations on it too. ;)
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Choice of PV Power

    Shading kills production of one (or more) panels. If there is anything you can do to reduce shading (trimming trees/plants, move vent stacks, avoid overhead power line shadows, move antenna, etc.), do it.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • NorthGuyNorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Choice of PV Power
    Rybren wrote: »
    How would shading affect a 4x single series string v a 2x2 series-parallel string? My gut tells me that the single series string would be more affected than the series-parallel setup.

    If you shade completely one panel:

    - with 4 series of 1 panel the rest of the panels are generally unaffected
    - with 2 series of 2 panels the whole series containing a bad panel will be eliminated from production
    - with 1 series of 4 panels the rest of the panels are generally unaffected, but the voltage drops and may become too low for your controller

    The problem is that the shade doesn't match panel boundaries, so you get a shaded cell here, a shaded cell there ... When these effects combine over the array, the results may be devastating - you may lose most of your production because some small shades here and there.

    Shading is very bad. I would do everything I can to eliminate it.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Choice of PV Power
    NorthGuy wrote: »
    - with 4 series of 1 panel the rest of the panels are generally unaffected
    - with 2 series of 2 panels the whole series containing a bad panel will be eliminated from production
    - with 1 series of 4 panels the rest of the panels are generally unaffected, but the voltage drops and may become too low for your controller

    This description is confusing.
    I think you mean:

    4 panels in parallel with 1 panel shaded
    2 parallel strings of 2 in series with 1 panel shaded
    1 string of 4 panels in series with 1 panel shaded

    In the last case the change in the shaded panel's ability to pass current (as it goes through the bypass diodes instead of the PV cells) can also have a negative effect.

    A lot depends on the particular panels too, as some have all cells in series and some have two paralleled segments, et cetera. It's a bit of a crap shoot and the only sure thing is that shading is bad.
  • NorthGuyNorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Choice of PV Power
    This description is confusing.
    I think you mean:

    4 panels in parallel with 1 panel shaded
    2 parallel strings of 2 in series with 1 panel shaded
    1 string of 4 panels in series with 1 panel shaded.

    Thanks Coot. I never can express my thoughts well :blush: I'll try harder next time.
    It's a bit of a crap shoot and the only sure thing is that shading is bad.

    That's what I was trying to say :D
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Choice of PV Power

    I've been trying to figure out an easier way of describing array layouts for years now. No can do. :blush:

    Yes, I know; draw us some pictures! :p
  • RybrenRybren Solar Expert Posts: 351 ✭✭
    Re: Choice of PV Power
    Yes, I know; draw us some pictures! :p


    Speaking of which, we're still waiting for those grounding diagrams :-)

    All: Thanks for the words on shading. The explanation pretty much matched my understanding. Luckily, for the most part, I don't have any serious issues - I only get a bit late in the summer/early fall and only after about 5pm.

    My neighbour, on the other hand, gets intermittent shading on a good chunk of his 2KW array all day long. I keep telling him to clear the brush in front of the array, but so far it's been falling on deaf ears.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Choice of PV Power
    Rybren wrote: »
    Speaking of which, we're still waiting for those grounding diagrams :-)

    We ran into legal problems with the grounding diagrams. ;)
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