Four strings of Panels. MPPT or PWM controller?

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BilljustBill
BilljustBill Solar Expert Posts: 219 ✭✭✭
in Off Grid Solar & Battery Systems #1
If I have equal voltages on each 12 volt panel, and the four panels have same wattage for 48v strings. When the array is completed I'll have: One 4-panel string of 75 watt panels, 90 watt, 100 watt, and 120 watt panels.

I'll run each series string to a combiner box and fuse/breaker protect each string. There, they are combined in parallel, and the volts and amps run to the controller and 48 volt battery bank.

Would a single MPPT controller be better than the PWM controller when using different string wattages?

Your insight and experience will surely help,

Bill
Bill
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Four strings of Panels. MPPT or PWM controller?

    The main advantage to MPPT is downconverting higher array Voltage to system Voltage. Since your arrays are all '48 Volt' and your system is '48 Volt' there probably is no advantage here.

    You really want to look at the Vmp of the different panels to be sure they're close enough to be tied together. Too much difference and you'll lose 'potential harvest'.

    You'll have 385 Watts @ '48 Volts' (The real Vmp is likely to be ~60 Volts)? - about 6-7 Amps. That's not very much current. (Unless I've just done the math 100% wrong - it's been one of those days).
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  • BilljustBill
    BilljustBill Solar Expert Posts: 219 ✭✭✭
    #3
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    Re: Four strings of Panels. MPPT or PWM controller?
    Cariboocoot wrote: »
    The main advantage to MPPT is downconverting higher array Voltage to system Voltage. Since your arrays are all '48 Volt' and your system is '48 Volt' there probably is no advantage here.

    You really want to look at the Vmp of the different panels to be sure they're close enough to be tied together. Too much difference and you'll lose 'potential harvest'.

    You'll have 385 Watts @ '48 Volts' (The real Vmp is likely to be ~60 Volts)? - about 6-7 Amps. That's not very much current. (Unless I've just done the math 100% wrong - it's been one of those days).

    One 4-panel string of 75 watt panels with 4.55amps, 90 watt 5.21amps, 100 watt with 5.71amps, and 120 watt (older panels 5. 30amps) panels.

    So if the strings are wired in parallel in the combiner box, wouldn't you add all the AMPS and total: 20.77 X 48 volts= 996.96 watts... in theory....

    So, would a MPPT do this array any good?


    Thanks,
    Bill
    Bill
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: Four strings of Panels. MPPT or PWM controller?

    Okay, that's twice today I've missed a key multiplier in a post and come up with a totally wrong answer as a result. Maybe I should have taken a nap this afternoon. :blush: For some reason I was reading the per panel Wattage as the string Wattage! Talk about getting it wrong!

    4 @ 75W = 300 Watts
    4 @ 90W = 360 Watts
    4 @ 100W = 400 Watts
    4 @ 120W = 480 Watts

    Grand Total: 1540 Watts. 1540 Watts / 60 Volts = 25 Amps or so charging.

    Or is it:
    4 @ 75W = 300 Watts, plus one 90W, one 100W, and one 120W?
    That would be 610 Watts.

    Or am I still reading it wrong?

    In any case, the Wattage is the total of all panels added together, regardless of how it's configured. The question is: how do you get each of these strings to be the same Voltage with different amount of panels (regardless of Wattage) per string? This is what I'm having trouble grasping here.

    With parallel connections you've got to check the Vmp on the panels to be sure they're all 'close' (within 10% arbitrarily). With series connections it's the Imp that needs to be 'within tolerance'.

    In any case, MPPT would really only do you good if you were using higher array than system Voltage. There is little to be gained from 'mismatched' panels (although others may disagree, and I'm not sure at this point if I agree with me, given today's performance!)

    Best case scenario: separate PWM controllers for each different set of panels. This could in fact work out to be cheaper than an MPPT and maximize the potential harvest.
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  • BilljustBill
    BilljustBill Solar Expert Posts: 219 ✭✭✭
    #5
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    Re: Four strings of Panels. MPPT or PWM controller?
    Cariboocoot wrote: »
    Okay, that's twice today I've missed a key multiplier in a post and come up with a totally wrong answer as a result. Maybe I should have taken a nap this afternoon. :blush: For some reason I was reading the per panel Wattage as the string Wattage! Talk about getting it wrong!

    4 @ 75W = 300 Watts
    4 @ 90W = 360 Watts
    4 @ 100W = 400 Watts
    4 @ 120W = 480 Watts

    Grand Total: 1540 Watts. 1540 Watts / 60 Volts = 25 Amps or so charging.

    Or is it:
    4 @ 75W = 300 Watts, plus one 90W, one 100W, and one 120W?
    That would be 610 Watts.

    Or am I still reading it wrong?

    In any case, the Wattage is the total of all panels added together, regardless of how it's configured. The question is: how do you get each of these strings to be the same Voltage with different amount of panels (regardless of Wattage) per string? This is what I'm having trouble grasping here.

    With parallel connections you've got to check the Vmp on the panels to be sure they're all 'close' (within 10% arbitrarily). With series connections it's the Imp that needs to be 'within tolerance'.

    In any case, MPPT would really only do you good if you were using higher array than system Voltage. There is little to be gained from 'mismatched' panels (although others may disagree, and I'm not sure at this point if I agree with me, given today's performance!)

    Best case scenario: separate PWM controllers for each different set of panels. This could in fact work out to be cheaper than an MPPT and maximize the potential harvest.

    There are 4 panels to each string. Because I have gathered these panels as I've found them at bargain prices, that's the reason for the different wattages for each string of four panels...
    Bill
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: Four strings of Panels. MPPT or PWM controller?

    Okay so it's 1540 Watts total @ nominal 48 Volts.

    The big question is: what's the Voltage at Maximum Power (Vmp) rating for the different panels? It will probably be close on these lower Wattage, '12 Volt' panels; around 17-18 Volts. The greater the difference the more potential power loss, but it isn't likely to be that high.

    To compare controllers, here's a couple of Morningstar units that should work:

    MPPT: http://store.solar-electric.com/motr45ampmps.html
    PWM: http://store.solar-electric.com/trts12vochco.html

    They are both 45 Amp units capable of 48 Volts, so there would be room to add some more PV in future.
    Note the price: $411 vs. $154

    Personally I don't think there's enough advantage in an MPPT controller in this application to justify the additional expense. You can always talk to the folks at NAWS for a real expert opinion; they won't just try to sell you the most expensive thing.
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  • BilljustBill
    BilljustBill Solar Expert Posts: 219 ✭✭✭
    #7
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    Re: Four strings of Panels. MPPT or PWM controller?

    Marc,

    Thank you for your thoughts and knowledge. As I continue gathering enough like-wattage panels to make 48 volt strings, I'm just trying to do this on a shoestring while learning what it takes to do it right. In a perfect world, having an array with matched panels would be my choice, but a mix of budget and the challenge of doing more with less keeps me gathering panels to make an array that will power a large part of my home. So, with my current collection of panels equaling $1.12 a watt, and with a TriStar 60 amp controller, wire, and 1,500 watt FSW 48v inverter, I've got about $1,500 in the componets, not counting the 30% tax credit.

    It would be easier to have a small grid-tie system, but the fees and liability risks with a company I do not trust make it trouble waiting to happen. Dealing with the local Co-Op has been a real pain, and I'd rather have my teeth pulled with Vise Grips than get tangled with their side-stepping, data skewing, white collar employees. Due to a need for 220v power for two deep wells, kitchen appliances, and some workshop power tools, I'll use them to fill in the gap, but only the gap.

    With the power from a 48 volt system, including a small DC wind generator, the solar panels should provide what I'll need with no penalty from the Co-Op for minimally purchased power.

    Any other experiences, suggested breaker/fuse ratings, or other information is always welcomed.

    Thank you and others on this forum,
    Bill
    Bill
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: Four strings of Panels. MPPT or PWM controller?

    I quite understand: solar is expensive. So much so that we usually recommend people who have grid power available should use it. But if it is your choice not to, for whatever reason, we try to supply the best options.

    Lots of people try to start small and build up. It's difficult to do. There's many strategies, including using multiple, independent systems - adding on an entire 1kW set up as it becomes financially feasible.

    Be glad you have some kind of tax credit: or moron government here in BC is about to shove HST down our throats, which will remove the current PST exemption from RE equipment. We will go from paying 5% sales tax on the already over-priced equipment to paying 12% tax on almost every good or service. :grr

    And this is a "green" government which will be upping its carbon tax at the same time.
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