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  • Mismatched Panels Help

    It is my understanding that the higher voltage panels will either backfeed current into the lower voltage panels or the blocking diode on the lower voltage panels will prevent the backfeed current but also prevent the lower voltage panels from adding any current to the string. Is this assumption correct?

  • #2
    Pretty close. You do not want to mix panels. This is why you will always hear me say buy panels from manufactures whose name you know and solar is a small fraction of their biz like Sharp, BP Kyocera. These companies have been around a very long time, make excellent products, will be around tommorow for warranty replacements and future expansions with the same model you bought yesterday.

    Use a company whose only biz is RE products like Evergreen, and you can be pretty sure they will be bankrupt and out of biz in the very near future.

    About the only thing you can do is if you have a battery system and have two different panels is create two separate systems using a common battery bank. Unfortunately not many if any options for grid tied, unless you use an inverter that can be cascaded on the outputs and use two separate systems. Otherwise you are going to loose a very large amount of power generation. You would be better off selling them on ebay and starting over with quality panels.

    As the ole saying goes you get what you paid for.
    MSEE, PE

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    • #3
      Thanks for the reply. This was what I was thinking. The system was for a customer that already had panels and wanted to use them. I was trying to explain to them why I couldn't use the panels and wanted to make sure i was right. Its a mix of Canadian Solar, Sharp and some panels i cant find the literature for. What do you mean by cascading the inverter output? Do you mean running a separate inverter for each panel type and paralleling the outputs?

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      • #4
        Originally posted by SolarGuyTX View Post
        What do you mean by cascading the inverter output? Do you mean running a separate inverter for each panel type and paralleling the outputs?
        Exactly.

        To use extremes to demonstrate the point imagine we have two panels one is the Kyocera KD210 rated @ 210 watts, Vmp=26.6, Imp = 7.9 amps. The second panel is the Kyocera KC87 rated @ 87 watts, Vmp = 17.4 volts, and Imp = 5 amps. We should expect 297 watts right?

        We connect them in series so now we have a Vmp = 43.4 volts and a Imp = 5 amps. What is the power 43.4 volts x 5 amps = 217 watts. We should have 297 watts. What happened? Well when wired in series we limited the maximum current to the lowest value panel in this case was the 87 watt panel of 5 amps. You would be better off selling or chunking the 87 watt panel in the garbage.

        Us the same two panels in parallel and you get a Vmp = 17.4 and Imp = 12.9 amps and a power of 17.4 volts x 12.9 amps = 224 watts. Again toss the 87 watt panel in the trash heap.

        Seeing the pattern here?
        MSEE, PE

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        • #5
          This is what i expected. The only possibility was to some how come out with strings of each type of panels in parallel (I was not going to mismatch within the series string for the reason you mentioned above) with the exact same string voltage and the same voltage temperature change (Not gonna happen). I just needed to be sure

          Thanks for the help.

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          • #6
            Your welcome and your instincts were correct.
            MSEE, PE

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            • #7
              You can use individual MPPT charge controllers on each panel. Along with giving you full power from each panel, this will eliminate the need for blocking diodes.
              -
              Alexander Hayman
              Electrical Engineer
              [url]http://www.genasun.com[/url]

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              • #8
                I was thinking of using an enphase microinverter for the mismatched panels and just make a separate AC run. I have experience using them and they seem to work well.

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                • #9
                  Originally posted by SolarGuyTX View Post
                  I was thinking of using an enphase microinverter for the mismatched panels and just make a separate AC run. I have experience using them and they seem to work well.
                  Yes, using the mismatched panels in a compeltely separate string was what I would suggest, with a compeltely separate inverter. If you use batteries keep them separate too.
                  Lawrence Lile, PE

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                  • #10
                    Sunking's advice is naive and misguided.

                    There are very basic ways to combine mismatched panels.

                    A solar panel is a "constant voltage" current source. Pretty much as soon as sun is on the panel, Vout rises to Voc, and from there, you track the voltage at Vmpp for the environmental conditions, etc.

                    As a "constant voltage" current source, the smallest current panel is the bottleneck -- if Impp differs between two series panels, the string current is the smallest Impp value.

                    A panel will make close to Impp from Vmpp to 0 -- look at the values for Isc and Impp and you see they aren't very far apart, compared to the current from Voc to Vmpp. What that means is that if you have a "higher voltage" panel, it will either be pulled down to the other strings / panels Vmpp, or if it is large enough, will pull the other string / panel up to its voltage.

                    From this, you see that each string must have the same Vmpp.

                    The "extreme example" wasn't just an "extreme", it was about the worst case possible. Why? Because most panels are made in multiples of 12 nominal volts. The KC87 is a "12 volt nominal" panel, and the KD210 is a "high voltage grid-tie" panel, not a "24 volt nominal" panel.

                    In more typical cases of mixed panels, you =can= mix panels to create strings with matching Vmpp.
                    Julie in Texas

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                    • #11
                      Originally posted by greenHouse View Post
                      A solar panel is a "constant voltage" current source. string / panel up to its voltage.
                      Beg your pardon, solar panels are constant current sources.
                      MSEE, PE

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                      • #12
                        Originally posted by Sunking View Post
                        Beg your pardon, solar panels are constant current sources.
                        Excuse me? Constant =when=?

                        The current is constant from Vmpp to 0, more or less. In the voltage range from Voc to Vmpp, the current isn't constant (it goes from 0 to Impp). From this, you can deduce how to add mismatched panels to an array (the topic ...) in a way that minimizes losses.
                        Julie in Texas

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                        • #13
                          Originally posted by greenHouse View Post
                          The current is constant from Vmpp to 0, more or less. In the voltage range from Voc to Vmpp, the current isn't constant (it goes from 0 to Impp). From this, you can deduce how to add mismatched panels to an array (the topic ...) in a way that minimizes losses.
                          Yep and that is why pros should be doing that work. It is beyond DIY knowledge in most cases. As for me I like to KISS.
                          MSEE, PE

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                          • #14
                            Originally posted by Sunking View Post
                            Yep and that is why pros should be doing that work. It is beyond DIY knowledge in most cases. As for me I like to KISS.
                            The rules are simple enough that if explained properly, even a 6th grader can do the math. It's far from difficult, and "replace them all" is far from the only reasonable course of action.
                            Julie in Texas

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                            • #15
                              Originally posted by greenHouse View Post
                              "replace them all" is far from the only reasonable course of action.
                              Who said that? You did right? You forget most the people who come here are DIY and have no electrical knowledge what so ever and are not going to take the time to learn the math.

                              If you mix panels there will be consequences with some power losses ranging from minor to severe. In a battery system there are effective means to mix panels by segregating panels into separate groups and running them on a dedicated charge controller combing there output to a common battery. All that can be avoided by purchasing panels from companies like BP, Kyocera, and Sanyo who have been around a long time and will be around tomorrow to supply you with the same model panel for replacement and future expansions. That is the smart money approach.
                              MSEE, PE

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