Parallel strings of unmatched PVs

MarkCMarkC Solar Expert Posts: 155 ✭✭✭
Is it possible to configure two sets of unmatched PV panels in parallel operation and combine them to a single charge controller and/or inverter?  If so, what would be the expected solar (PV) production as a % independent operation?  Will diodes be recommended to prevent back-feed?  The two sets will be next to each other, however due to install angles and trees in the area, they will not always receive the exact same irradiance in early morning and late afternoon.  It is likely that the 345XLs will normally receive the highest levels (marginally).

PV-I => 14 SolarWorld 275 monos - Voc=39.4, Vmpp=31, Isc=9.58, Impp=8.94 @1000W/m2
PV-II => 12 SolarWorld 345XL monos - Voc=47.2, Vmpp=37.5, Isc=9.82, Impp=9.28 @1000W/m2

Note that this is not just an "academic" question - I need the re-configuration for test work on a high voltage charge controller project.  I do have the panels (most of them!) installed.



3850 watts - 14 - 275SW SolarWorld Panels, 4000 TL-US SMA Sunny Boy Grid tied inverter.  2760 Watts - 8 - 345XL Solar World Panels, 3000 TL-US SMA Sunny Boy GT inverter.   3000 watts SMA/SPS power.  PV "switchable" to MidNite Classic 250ks based charging of Golf cart + spare battery array of 8 - 155 AH 12V Trojans with an  APC SMT3000 - 48 volt DC=>120 Volt AC inverter for emergency off-grid.   Also, "PriUPS" backup generator with APC SURT6000/SURT003  => 192 volt DC/240 volt split phase AC inverter.  

Comments

  • BB.BB. Super Moderators Posts: 26,872 admin
    What voltage for the array do you need? (Vmp-array)

    The two panels have very similar Imp, and you should be able to put them in series for matched strings (if you want ~400 Volts Vmp-array for a high voltage MPPT charge controller).

    7x PV-1 panels in series with 6x PV-2 panels (they all should be next to each other, on the same plane). That would give you two parallel string with Vmp-array~442 volts. With only two panels left over--Although, you could probably put one each of those panels.

    If the arrays have different orientations, that is a bit more difficult--But if 1/2 faces SE and the other 1/2 faces SW, that could work OK.

    Remember the panel current (Imp) is directly proportional to the amount of sunlight. If 1/2 a string is facing different than other 1/2 of the string, you will (at best) only get the "full current" from the panels receiving the least amount of sunlight.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MarkCMarkC Solar Expert Posts: 155 ✭✭✭
    Thanks Bill - I see your logic of matching string voltages.  I do need ~ 400 volts minimum for my tests - which could be a problem with my suggested PV-1 array.  I assume the current would be limited by the PV-1 panels by mixing in series, and that would be the "hit" to the instantaneous power level?  The orientation is the same, the "plane" is a few degrees (~5) different (PV-2 has in increased tilt).  I've uploaded the new pic of my solar shed with both sets of panels.

    Out of curiosity, if the PV-1 and PV-2 panels (in series) where put in parallel as I had suggested, would that actually work and if so, would the energy production be more or less than mixed series arrangement?  Seems neither string would operated at MPP, but could traditional MPPT "logic" do a good job of optimizing?

    Appreciate it!
    Mark
    3850 watts - 14 - 275SW SolarWorld Panels, 4000 TL-US SMA Sunny Boy Grid tied inverter.  2760 Watts - 8 - 345XL Solar World Panels, 3000 TL-US SMA Sunny Boy GT inverter.   3000 watts SMA/SPS power.  PV "switchable" to MidNite Classic 250ks based charging of Golf cart + spare battery array of 8 - 155 AH 12V Trojans with an  APC SMT3000 - 48 volt DC=>120 Volt AC inverter for emergency off-grid.   Also, "PriUPS" backup generator with APC SURT6000/SURT003  => 192 volt DC/240 volt split phase AC inverter.  
  • BB.BB. Super Moderators Posts: 26,872 admin
    In parallel seems to work OK too. The Vmp for each string (14x31 volts vs 12x37.5 volts):

    PV-1 ~ 434 volts
    PV-2 ~ 450 volts

    If Vmp for strings in parallel or Imp for panels in series is withing 10%, it is usually close enough for good operation. Within 5% is even better.

    Because you will have two Vmp-string "peaks", your tracking will be a little less clear as you are doing the software. You will probably have three max power peaks (low, high from each string, and a third peak for the combined power of both strings). It will be a bit "interesting" doing the MPPT optimization routine. But it will probably be more robust after you are done.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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