Is this right? Solar Panels connection

JakachiraJakachira Solar Expert Posts: 47
Hello Guys

I got two more new panels from SolarWorld. The issue now is how to connect them but I have found what i think is best way to link them up. The specs differ so much that i need to device a good plan to maximise the output.

The solar panels specs are as follows (150W panels are high voltage ones but still 12V panels)

150Watts Vmp: 19.8V, Imp: 7.6A
150Watts Vmp: 19.8V, Imp: 7.6A
100Watts Vmp: 16.9V, Imp: 5.8A
500Watts Vmp: 16.8V, Imp: 2.8A

I decided to add an MPPT controller to this system and do a 36V system.
I will connect these strings (S1, S2 and S3) in series.

S1: 150Watts
S2; 150Watts
S3: 150Watts (100Watts + 50Watts in parallel)

Then I will have a Vmp=56..5V and Imp=7.6A connected to an MPPT controller. My batteries are 12V

What do you think guys. Any other suggestions?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Is this right? Solar Panels connection

    Let's start with the last panel on your list: it's 50 Watts, not 500 as first written. :D
    It's also pretty useless as the Vmp is too low for a 12 Volt system and the Imp is too low to connect in series with any of the other panels: on 12 Volts it won't produce enough Voltage for proper charging, and in series it will limit the current of the other panel to 2.8 Amps removing significant power.
    The second from the last panel (the 100 Watt) on the list also suffers from too low Vmp for 12 Volts. And again its current is too low to mix in series with the 150 Watt units.

    Basically those two panels aren't compatible with any of the others. You should see if you can't sell these for cheap to someone who wants to experiment; they aren't much good for anything else.

    The 150 Watt panels have a rather high Vmp for 12 Volts, so some of their power will be lost on that size system without an MPPT type controller. They'd actually run a peak power around 7.6 Amps * 12 Volts or 91 Watts through a PWM controller.

    You do not want to go to a 36 Volt system. For one thing, you don't have enough panel Vmp to support it. For another, 36 Volt equipment (controllers and inverters) is not common. The industry standards are for 12, 24, and 48.

    The two 150 Watt panels add up to 300 Watts, and could be used on a 12 Volt system with a Rogue charge controller. You could keep a set of golf cart batteries (6 Volt 220 Amp hour) charged with this. You'd have about 600 Watt hours AC to work with.

    Beyond that you'd have to get more of the 150 Watt panels (if possible) or start over completely. Another example of how hard it is to expand a system.
  • JakachiraJakachira Solar Expert Posts: 47
    Re: Is this right? Solar Panels connection

    Thanks

    The system is a 12V one that is to say the batteries are connected for 12V and the inverter is a 12V one as well. I think I made a mistake to assume that a 36V system is when you connect 3 x 12V panels in series.. The battery have to dictate the type of system i am using that is 12Volts. My apologies.

    So when I connect the 100Watts and 50Watts in parallel, technically i should get about Vmp 16.8V (21.1 Voc) and Imp 8amp. This will create a first string.

    I will connect this string to the other two panels all in series to solve the voltage difference problem. The voltage will add up to 55V. Current stays the same (about 7.6Amps .. all strings are 150Watts there wont be a mismatch in current, I assume).

    The MPPT will take in 55V and down converts to 12V.

    I do not see a problem with this set-up, do you?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Is this right? Solar Panels connection

    Apologies aren't necessary; it's all about getting the terms right so we all are talking about the same thing. We all make mistakes and get confused and pick the wrong words. Unfortunately sometimes the wrong word leads to some serious confusion.

    "System Voltage" is a reference to the nominal battery bank rating. "Array Voltage" is the nominal PV rating. They can be different, of course, but sometimes they're the same. Easy confusion there!

    So your plan is to put the 100 Watt and the 50 Watt in parallel, then those two in series with the two 150 Watts. You would get approximately Vmp 56.4 and Imp 7.6 or 428 Watts. The Imp will be limited by the lowest value in series, which in this scenario becomes the 7.6 Amps of the 150 Watt panels. If the connections go bad on one of the smaller panels it will "choke" the current to whichever is the lowest Imp. Otherwise it will work.

    The bad news is the 150 Watt panels probably have a Voc around 24-25 which when added to the Voc of about 21 for the other two brings the array Voc up to 71 which is definitely too high for the Rogue controller and the current will be too much for the Morningstar 15 MPPT. This means you have to ante up for one of the bigger controllers like the Morningstar 45 MPPT. It's a bit of a shame as the current output is likely to be less than 30 Amps on a 12 Volt system.

    I looked at specs on the Blue Sky and Stecca units I could find and they run into the same problem with Voc. Also they're not that much less money than the MS 45 MPPT.
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: Is this right? Solar Panels connection
    Jakachira wrote: »
    So when I connect the 100Watts and 50Watts in parallel, technically i should get about Vmp 16.8V (21.1 Voc) and Imp 8amp. This will create a first string.
    <snip>
    I do not see a problem with this set-up, do you?

    I see what you are trying to do, and I think it will work. The MPPT algorithms may not be optimal for mixed panels.

    I think you confuse the readers with your use of the word 'string'. A string usually refers to one series pathway in a parallel configuration. What you are proposing is to put the 100 watt and the 50 watt panel in parallel, and then putting those two parallel panels in series with the two 150 watt panels.

    Another option, not too expensive and potentially more efficient, would be to put the 100 watt and the 50 watt panel on their own PWM controller.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Is this right? Solar Panels connection

    And while we're on detail patrol :p I have to mention that the small panel (2.8 Amp) in that parallel configuration should have a fuse on it in case something goes wrong (lowered resistance) and the larger one tries to force 5.8 Amps into it. The value should be a bit above the small panel's Isc rating but below the larger panel's. Probably 5 Amps.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Is this right? Solar Panels connection
    vtmaps wrote: »
    Another option, not too expensive and potentially more efficient, would be to put the 100 watt and the 50 watt panel on their own PWM controller.

    A good idea since a 10 Amp controller can be had for < $50.
    The Vmp on these too is a tad low, though, so they may only contribute at lower charge Voltages.
    You could also run the 150 Watts through PWM and lose about 10% of the power (unusually high Vmp on them). You'd want a 20 Amp controller there, about $80. Total $130.
    This would be a fraction of the cost of the 45 Amp MPPT controller.
  • JakachiraJakachira Solar Expert Posts: 47
    Re: Is this right? Solar Panels connection

    Thanks, guys, for the suggestions.. I already have a Steca PR3030 PMW Contoller but I think I will go for this MPPT (Tristar 45A) because i will probably put more panels in future.. I am planning to install the new panels in September when i get down to my rural place (so enough time to look for it).. Haven't thought of installing a fuse on the smaller panel, thanks a lot for the advice, i will put it on my list.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Is this right? Solar Panels connection

    At least you already know what the problem is with "adding panels in the future":
    "What? They don't make those panels anymore?" :cry:
Sign In or Register to comment.