Higher voltage PV arrays

wespo
wespo Registered Users Posts: 20
Hi,
Is it more efficient to run a higher voltage array through an MPPT to a lower voltage battery, or run the same voltage as the battery? I am adding 2 more panels (180 watt evergreen SL) to my array and want to increase the voltage going into the FM60 from the current 48volts (2x 180W /24V) to 72 volts(3xpanels) to charge a 24 volt AGM battery bank. Thanks.
Bill

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Higher voltage PV arrays

    There is a trade off between the copper losses of lower voltage/higher amps, and the inverter gets a wee bit less efficient, the more it has to convert from high voltage to low. I suspect, that doing the math, the higher voltage is overall, more efficient. In your case, you have no other choice, unless it's to add even more panels, to get another string going.

    The thing to watch for, is cold morning start-up voltage, that the Vmax of all the panels, does not overvoltage the CC as it starts up cold.

    And it's only a couple of percent more loss I believe, barely measurable?

    Solar Guppy or Jim likely know the exact #'s to use for the math.
    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 ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: Higher voltage PV arrays

    Page 62 of the outback flexmax manual shows the graphs of efficiency vs. voltage. Looks like you'd lose about 1% efficiency going from 48V to 72V.
  • wespo
    wespo Registered Users Posts: 20
    Re: Higher voltage PV arrays

    Thanks. I need to get better aquainted with that manual...
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Higher voltage PV arrays

    It's a delicate balancing act. My view is a solution lies in a "systems approach" that considers all of the pro's and con's that drives a best fit answer. However, what's best for requirement "A" may not be the best for requirement "B".

    HTH,
    Jim / crewzer
  • wespo
    wespo Registered Users Posts: 20
    Re: Higher voltage PV arrays

    Hi Crewzer,
    I see that you have a very similiar arrangement of voltages as my current system. I have 4x180W evergreen SL (25.9Vmax) wired in 2 strings of ~48volts on #10AWG MC3 contacts wire. short runs of <10 feet. 440AH 24 volt bank. Off-grid. I want to add 2 more panels. It would be easier to add 1 panel to each string (~72volt output) and change the settings on the FM60, than to get more MC3 connectors/cable and drill another hole in the 5 inch thick concrete. I am not worried about the 150 volt max limit, as I live in the tropics and it rarely gets below 70. I have plenty of room to add the 2 new panels as a seperate string if you think that would be a better systems approach. We use our batteries fairly heavily and discharge about 25-30% every day.
    Thanks again.
    Bill
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Higher voltage PV arrays

    Wespo,

    It's probably a toss up between the two possible solutions. Adding a third string would avoid reducing controller efficiency, but there would be loss in the new wiring, and it sounds like it would be a lot of work!

    Adding a third module in series with each existing string would slightly decrease controller efficiency. However, by increasing the string voltage and not its current, the power from each new module would get a free ride on the existing cabling, so any additional cabling loss would be avoided.

    Probably OK to go with adding the additional module in series with the existing strings. FWIW, that's what I did when I increased my array size and went from 36 V nominal to 48 V nominal. 8)

    HTH, and good luck!

    Jim / crewzer