Ideal MPPT PV Configuration

Gino
Gino Registered Users Posts: 11 ✭
in Off Grid Solar & Battery Systems #1
Hello, just got a MPPT Morningstar 60Amp controller

So i feel that my current PV setup which is all parallel PV's isn't ideal anymore.

I currently have 4x 21.2V (9.9Amp) 150W PV's and 3x 22.2V 130W (Ipmax 7.39A) PV's (different brand)
and 1x unknown PV that is hooked up, its of similar size to the rest but some unknown brand so I'm not sure what its specs are.. might unplug it...

From what research I've done I'm thinking of hooking all these PV's in series.. since they are a few years old I'm pretty sure I'll never hit the
150V max on the MPPT Controller.. But I think having them all in parallel I might hit the 60Amp max on a sunny day so I'm more worried about that.
I've also heard that all series might be more efficient..

*My battery bank is 12V

So I think getting the voltage as high as possible would be the ideal choice.
All in a series would be V(oc) 151.4V the max on the controller is 150V
With all in parallel (my current setup) is an I(sc) of 62.17 Amps... I see about 18V on a sunny day right now (in winter)..
What do you all think? Do I need to add special diodes to put all them in a series?

So I suppose my question is, do you think all series is a sound like a good idea??
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Ideal MPPT PV Configuration

    All in series is not a good idea for two reasons:

    1). The Voltage of the array will be much higher than the nominal system Voltage, thus reducing controller efficiency.

    2). The current of the array will be limited to that of the panel with the lowest Imp.

    In short it is bound to be less efficient.

    As a rule you don't want your array nominal Voltage to be more than 2X your system nominal unless you need higher Voltage to overcome wiring resistance over long distances.

    Here's a thread on array configuration using all the same panel type: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

    In your case you have four of one type, three of another, and one of yet another. Trying to mix them into any pattern that includes all and has some in series is going to cost power.

    For example if you put a 150 Watt panels with its Imp of 9.9 in series with a 130 Watt panel with its Imp of 7.3 the power from the 150 Watt will drop to 110 Watts.

    If you put four panels with a Voc of 21.2 in series you get 84.8 Volts, and add to that three with Voc of 22.2 (66.6) you get 151.4 Volts which is over the limit for most MPPT controllers even without factoring in Voltage increase due to cold temps.

    In short, you're stuck with what you've got unless you spend some money to get all the same panels or sort them out into two separate arrays on two separate controllers.

    BTW, the numbers you quote for the specs of the panels look like Voc and Isc. It is important to know which specs are used for what purposes. Power is Vmp * Imp, and these are the numbers that matter for operating Voltage and current (including current limiting). Voc is vital to determining the maximum Voltage fed to a controller and not exceeding its input limit.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #3
    Re: Ideal MPPT PV Configuration

    did you have a PWM controller before, you could use it for the odd panel(s), in parallel to the MPPT.
     
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  • Gino
    Gino Registered Users Posts: 11 ✭
    #4
    Re: Ideal MPPT PV Configuration
    westbranch wrote: »
    did you have a PWM controller before, you could use it for the odd panel(s), in parallel to the MPPT.

    I do still have the old PWM controller, its a Xantrex C35

    I never considered that.. hmm so you think I should take my 4x 21.2Voc 10Amp (Isc) panels and put them on the MPPT (in series or parallel?)
    then take the rest the 3x 21.9(Voc) 8.02A(Isc) and put them on the old Xantrex? (in series or parallel?)

    I thought the behavior of PWM vs MPPT was so different that mixing them would be a bad idea..

    We are a all communal house, it took a while to save up the money for the morningstar (which is working amazingly well compared to the old PWM BTW)
    I'm not sure if an all new solar panel arrangement is really realistic.. Generator backup power is the next thing on the list..

    Thanks!
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  • Gino
    Gino Registered Users Posts: 11 ✭
    #5
    Re: Ideal MPPT PV Configuration

    Hey Cariboocoot thanks for the great response,

    yes I'm quoting you mostly Voc and Isc.. Since they were the highest ratings listed on the charts for the panels I figured that was the safest to use to calculate limits.
    I do realize Vmp*Imp is my actual power input..

    It seems you feel I should stick with parallel for now.. I can try to scrap the 3x Isc 8.02A units and get some similar to my others, but I don't think its likely that i will find them, they all seem pretty old.

    My initial interest in switching to all in series is that I heard it is more ideal for MPPT tracking, The Morningstar website seems to imply that tracking improves at higher voltages as well.
    I'm concerned that on some winter days I may fail to reach absorption voltage in the winter.. (14.5V)

    Thanks again
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Ideal MPPT PV Configuration

    You could put the four 150 Watt panels on the MPPT as a serial/parallel arrangement: two parallel strings of two in series. This will raise the array Voltage without going too high above system Voltage.

    Then put the three 130's on the PWM all in parallel. The oddball panel too, if the Voltage specs are close enough.

    There should not be any serious consequences running the Morningstar 60 MPPT in parallel with the C35. Don't expect them to agree exactly on battery Voltage or set points. Let the MPPT controller take priority over the PWM for settings and charge stage: the C35 will merely help out with current during Bulk.

    You should see about 38 Amps max from the 600 Watts on the MPPT and about 22 Amps from the 390 Watts on the PWM.
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  • Gino
    Gino Registered Users Posts: 11 ✭
    #7
    Re: Ideal MPPT PV Configuration

    Cariboocoot,

    Great thanks allot, I also have a TS-60 Morning star laying around here but we got, so I think i'll put the 3x 130W on that.
    This is great!

    Just off hand, Do you think this two controller setup will be a significant improvement over my current all parallel one, or just a modest improvement?
    Just curious,

    Thanks again.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Ideal MPPT PV Configuration

    Probably no great improvement as the panels you have are very close in Voltage so when they are all in parallel the current adds up about as you'd expect. You might see 3-4 Amps more from the MPPT on the larger panels. Despite some manufacturer's assertions, under normal operating condition the MPPT usually provides only about 10% boost if any. The "up to 30% more power" claim is based on having higher Voltage from cold panels, not typical conditions.
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