pwm vs. mppt for 12V array.

I'm building a 12V system with 4 KC130's and a 400AH battery bank. ( I may add 2 more PV panels in the near future)

Because of unavoidable shading I am wiring all panels in parallel so only one panel at a time will be non-producing.

In this configuration does an MPPT controller have enough advantage over a PWM controller to justify the large difference in cost?
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Comments

  • mike95490mike95490 Solar Expert Posts: 8,579 ✭✭✭✭✭
    Re: pwm vs. mppt for 12V array.

    If you are using 18V Pmax panels, no. Light intensity adds amps, once the panels are turned on with enough light. Losses and stuff, you have no gain.

    BUT if you use 30 or 40V panels, in parallel, you can use much smaller wires, and then the MPPT can down-convert that to your 12V. So you save on wire costs, and if you are in a cold climate, you can pick up a few % more power in the winter with MPPT.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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  • Lefty WrightLefty Wright Solar Expert Posts: 111 ✭✭
    Re: pwm vs. mppt for 12V array.

    Thanks for the reply, mike. But here is the situation;

    My panels are mounted on top of my 16' tower, vertically oriented in a horizontal row.
    Because I live in the woods my panels are subject to shading at various times during the day. Usually, only one panel is shaded at a time.

    With my panels wired in parallel I only lose the production from that one shaded panel. If they were wired in series, for the advantages you mentioned, I would lose output from the whole array when one panel was shaded.

    Since I'm only going to get 17V or 18V from my array I wonder if the current that can be squeezed out of those extra 3 volts is worth the price of an MPPT controller.
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: pwm vs. mppt for 12V array.

    I would go with a PWM and add more panels.
  • Lefty WrightLefty Wright Solar Expert Posts: 111 ✭✭
    Re: pwm vs. mppt for 12V array.

    Yeah, Ken. I think you're right. The price difference will buy half a panel.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: pwm vs. mppt for 12V array.

    "With my panels wired in parallel I only lose the production from that one shaded panel. If they were wired in series, for the advantages you mentioned, I would lose output from the whole array when one panel was shaded."

    actually, that is not entirely correct as seriesed pvs with bypass diodes in them (most pvs have them) will simply bypass the shaded area and if the controller is mppt with downconverting ability it will adjust for the change on the input voltage. the higher voltage will afford lowered wire resistive losses.
    it is cheaper to parallel and use pwm though.
  • Lefty WrightLefty Wright Solar Expert Posts: 111 ✭✭
    Re: pwm vs. mppt for 12V array.

    I'll have to check that out , Niel.

    I haven't checked the output in some time. I'll put two panels in series and see if the current drops to zero when I shade one panel.
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,994 ✭✭✭✭✭
    Re: pwm vs. mppt for 12V array.
    I'll have to check that out , Niel.

    I haven't checked the output in some time. I'll put two panels in series and see if the current drops to zero when I shade one panel.

    And to do the test correctly, you must load the array!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
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  • Lefty WrightLefty Wright Solar Expert Posts: 111 ✭✭
    Re: pwm vs. mppt for 12V array.

    Won't it work if I just short the output with my ampmeter?
  • dmillerdmiller Solar Expert Posts: 68 ✭✭✭✭
    Re: pwm vs. mppt for 12V array.

    The biggest advantage of MPPT with a 12v bank is when the best $ per watt is on high voltage panels.

    Extrapolating data from variably cloudy conditions suggests MPPT may even be worse than straight PWM when erratic shading is an issue. I'm no expert on this, but when in doubt I would always put money towards PV rather than MPPT.

    But when 40v panels are $1.70/watt and 12v panels are $4.00/ watt MPPT can become a pretty clear choice for a larger 12v system. Good design is always about the whole system, not just one part.
  • BB.BB. Super Moderators, Administrators Posts: 29,994 admin
    Re: pwm vs. mppt for 12V array.

    Regarding how to test shade performance--Remember that we are after Pmp=Vmp*Imp (mp=Maximum Power)

    Shading will reduce Vmp and can affect Imp... When you short the output of a panel with an Amp Meter, you have effectively set Voutput=Zero Volts. In our power equation, that would be zero watts of useful power.

    So, the MPPT controller is attempting to find the Vmp*Imp maximum value (by adjusting I-current and measuring V-voltage--then doing the math). When you shade, a MPPT controller needs to "rescan" the array to find the new Vmp*Imp peak (note, different MPPT controllers have different algorithms to find Pmp. Some will take 5-15 minutes between scans.

    Dynamic MPPT is not a trivial problem for charge controller designers.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Lefty WrightLefty Wright Solar Expert Posts: 111 ✭✭
    Re: pwm vs. mppt for 12V array.

    I'm thinking of building a shack at the base of my tower for the batts and just running AC to the house. That will enable me to use PWM on my low voltage panels without worrying about line losses. I think my intermittent shading is too much for MPPT.
  • PhilSPhilS Solar Expert Posts: 370 ✭✭✭
    Re: pwm vs. mppt for 12V array.

    IF you build the shack and IF you bury your AC lines to the house, I'd suggest putting in at least two separate lines for your AC and another cable for future use.

    My arrangement is similar except that the panels aren't on a tower but beside the shack. I now run the house on two inverters (once circuit for lights/entertainment/AC, and the other for appliances). I put a six conductor cable in the conduit and use two of the wires for remote generator start and another two for a digital voltmeter in the house. Still have two for some future need.

    As much effort is involved in burying conduit, my opinion is it's way better to put in too much than bury more later.

    Phil
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