Morningstar string calculator

tabbycattabbycat Solar Expert Posts: 51 ✭✭✭
Using the Morningstar string calculator I notice that if you the 15 Amp MPPT controller with a 90 degree average high temperature it will say in some cases that the array performance is marginal. Given the same conditions with a PWM controller the calculator says you will get optimum performance. Does anybody have an explanation for this? I live in Florida so high temps are common.

Comments

  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Morningstar string calculator

    sounds like the heat lowered the voltage output of your pvs to the point a pwm cc would be better. what pvs are you using in series/parallel and list their vmp, voc, and imp.
  • mike95490mike95490 Solar Expert Posts: 8,380 ✭✭✭✭✭
    Re: Morningstar string calculator

    I'd think both a MPPT & PWM style controllers, would have about the same voltage drop across them, so this result seems odd, I too, agree with Niel, what are the panels you are using, and if you are using MPPT, why not up the voltage to take advantage of the downconvert function?
    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 ,

  • BB.BB. Super Moderators, Administrators Posts: 29,521 admin
    Re: Morningstar string calculator

    A bit of hand waving as there is nothing in their manual...
    1. PWM is ~100% efficient if Vpanel in = Vbatt charge (controller is just a closed switch)
    2. Depending on the exact down converter design--some need 1-2 volts across the controller to work correctly and if Vpanel-in=Vbatt-charge... It is not that if Vmp=Vbatt that the whole thing will stop moving current to the battery--It is just a diminishing return (Pmp is not a peak but a flat crest--so being 10% of Vmp is not the end of the world.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,380 ✭✭✭✭✭
    Re: Morningstar string calculator
    BB. wrote: »
    1. PWM is ~100% efficient if Vpanel in = Vbatt charge (controller is just a closed switch)
    -Bill

    There has to be some drop across a FET or something, unless they use a NC relay ! I'd figured the MPPT would have something about the same voltage drop too.
    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 ,

  • BB.BB. Super Moderators, Administrators Posts: 29,521 admin
    Re: Morningstar string calculator

    Some Buck Converter designs cannot run at 100% duty cycle... So there always has to be some down conversion (for good efficiency).

    I don't know what is in any of these controllers from a detailed engineering view to say what is their exact limitations by model number (and I am not the expert here anyway on switching supplies--I know just enough to be dangerous).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tabbycattabbycat Solar Expert Posts: 51 ✭✭✭
    Re: Morningstar string calculator

    Thanks for the replies. I don't have a system yet. I'm in the design phase. I was using a Kyocera 50C panel in the calculator with a low temp of 32 degrees F and a high of 90 degrees F. In the calculator (12 volt battery bank) using the 15 amp MPPT controller with only one panel the performance was marginal. If you have two panels in series then the performance was OK. The PWM controller was OK with either one or two panels (series). If I tried the calculator with a 24 volt battery bank it came up with the same type of solution. With the MPPT controller I needed three panels in series (36 volts) for the 24 volt bank. With the PWM controller only two panels in series (24 volts) were required for optimum performance with the 24 volt battery bank.

    The only explanation I can see right now is the possibility that the batteries may overheat because of too high of a current with the MPPt controller at the lower voltages assuming that the array wattage was the same in all cases. Thanks.
  • tabbycattabbycat Solar Expert Posts: 51 ✭✭✭
    Re: Morningstar string calculator

    After playing around with the calculator again it appears that an MPPT controller is not effective in hot temps if there is not a significant difference between the array voltage and the battery bank voltage. As an example, two fifty watt panels in series for 24 volts with a 12 volt battery bank would be better than two fifty watt panels in parallel even though the array wattage is the same in both cases. However the PWM controller is quite happy with the parallel configuration. The PWM controller would also let you use one 100 watt panel while the MPPT controller would require two 50 watt panels in series. Does this seem to be a fair conclusion? Thanks.
  • mike95490mike95490 Solar Expert Posts: 8,380 ✭✭✭✭✭
    Re: Morningstar string calculator
    tabbycat wrote: »
    The only explanation I can see right now is the possibility that the batteries may overheat because of too high of a current with the MPPt controller at the lower voltages assuming that the array wattage was the same in all cases. Thanks.

    That's not a good explanation (wrong)! Batteries overheat because of too much VOLTAGE, and the charge controllers job is to limit that, regardless of wattage it's fed. If the battry overheats, the charge controller has gone bad, or configured wrong. Apparently, the Morningstar MPPT controllers consume a bit more voltage in overhead, than the PWM ones do. Once you meet their minimum requirements, they generally outperform the PWM models.
    Selecting the proper panel, by voltage & wattage is important. A 90W, 30V panel is a poor choice for a 12V PWM system. It doesn't mean anything is wrong or bad, but just not properly matched for its job.
    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 ,

  • tabbycattabbycat Solar Expert Posts: 51 ✭✭✭
    Re: Morningstar string calculator

    You are right about my explanation being wrong. I was just trying to find a way to deal with the odd solutions the calculator was giving me. Florida's hot temps make the PWM controller a bit more acceptable (along with its' lower cost) as an alternative to the MPPT. Thanks.
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