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Solar report + adding slighly different rated panels in parallel?

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  • Solar report + adding slighly different rated panels in parallel?

    Hi guys, I used this forum years ago in setting up my solar system. Much appreciation. I have very happy with how it is working I have 8x128 watt panels(24volt each) arranged in a 48 volt 1028 watts system. I use 4 walmart marine batteries and ebay chinese inverter and mppt. Sorry if I am stepping on some toes here, but I have been succesful with this sytem and do recommend the unisolar panels I have been using, as well as the mppt and walmart batteries. The whole system was created for ~ $1400 with patient shopping.

    Now I had 4x68 watt unisolar panels on my van which I removed and wish to add to my existing system without changing anything else. I live in Oregon and in the winter I could use the extra power to keep the batteries topped off. The 2x128watt array tested at 82V while the 4x68watt array tested at 76V. Now I have read this: Mixing solar panels with different electrical characteristics is not recommended if you use an MPPT charge controller. Different wattages make impossible for the controller to find the optimal operating voltage and current, since they are different for each panel type.
    Is this true?
    I don't mind the small loss in power when my 4x68 brings down the voltage slightly...that is as long as I am not damaging the panels or decreasing their lifespan...but if the mppt (which can accept the current) won't adjust itself propertly then I will go ahead and get another inverter. Thanks in advance for any insights you can share

    Here are the panel specs from a screenshot:

    Screenshot - 07082017 - 01:09:41 PM.png




  • #2
    Try it. The 82V array will drop to 76V and you loose some wattage there. But, you do gain at least 200W. And if your current charge controller can handle an extra 200W, great.
    Powerfab top of pole PV mount (2) | 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

    Comment


    • #3
      Originally posted by Mike90250 View Post
      Try it. The 82V array will drop to 76V and you loose some wattage there. But, you do gain at least 200W. And if your current charge controller can handle an extra 200W, great.
      You might not even notice the mismatch loss. The mppt voltage would be in between 82 and 76, with neither losing much power at all. No trouble for any decent controller.
      CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

      Comment


      • #4
        But look at the Vmp values for each panel ...

        15.4 Vmp for the 68 Watt Panel
        30.8 Vmp for the 128 Watt Panel

        The two "mismatched" strings ...

        Vmp String #1 = 61.6 Volts = ( 15.4 Volts x 4 ) = 68 W + 68 W + 68 W + 68 W
        Vmp String #2 = 61.6 Volts = ( 30.8 Volts x 2 ) = 128 W + 128 W

        The two strings should have the exact same voltage at Max Power Point.

        Assuming strong sun to operate panels at Vmp and Imp at NOCT ...

        Existing 4 strings of 2 x 128 watt panels, with each string running at 200 Watts = 61.6 Vmp x 3.24 Imp
        800 Watts existing = 4 strings x 200 Watts per string

        Add 5th string of 4 x 68 watt panels running at 211 Watts = 61.6 Vmp x 3.42 Imp
        211 Watts added.

        1,011 Watts total = 800 Watts (existing) + 211 Watts (added)

        The MPPT logic will not have any issue with the added 5th string of 4 x 68 W panels.

        I wonder how much the Voc varies for each of the eight 128 Watt Panels that are already in use?
        Last edited by NEOH; 07-08-2017, 08:46 PM.

        Comment


        • #5
          Originally posted by NEOH View Post
          But look at the Vmp values for each panel ...

          The two strings should have the exact same voltage at Max Power Point.
          You don't need to be that close. The power either side of exact MPP doesn't drop off much. I just make sure
          each string has the same number of series cells, never mind the published numbers. Works well. Bruce Roe

          Comment


          • #6
            In general, parallel strings do not need to be exact same Vmp, but in this case the strings are exact.
            These 5 strings, in this thread, should have ( will have? ) the exact same 61.6 Vmp per the NOCT specs given.
            Regardless of the measured Voc, there no mismatch here.

            I find it much easier to compare the NOCT Vmp's from the labels vs counting the cells in series.

            These 5 parallel strings ( as discussed in this thread ) ...
            a) Should have ( will have ? ) the exact same 61.6 Vmp NOCT rating (+/-) per actual insolation.
            b) Will not have any "lost power".
            c) Will not be a "mismatch".
            d) Will not be operating between 76 volts and 82 volts, unless there is very little or no load ( ie Float Mode )

            Parallel string mismatch example: 200 Watt PV Panel @ 35 Volt Vmp
            Lowering the operating voltage by 5 volts below 35 Vmp, lowers the power to 160 watts = 20% Loss
            Raising the operating voltage by 5 volts above 35 Vmp, lowers the power to 100 watts = 50% Loss <<< !!!
            The power curve is not symmetrical.
            The power curve drops off more dramatically (5 times faster) above Vmp, than below Vmp.
            The string with the lowest Vmp will suffer significant power reduction compared to the other strings with higher Vmp.
            You mileage may (will) vary.

            Comment


            • #7
              I agree that looking at behavior under likely operating conditions to estimate mismatch loss is better than relying on differences in Voc and Isc. Although these particular panels aren't in the database, the CEC performance model gives us a tool that can estimate how an mppt controller would resolve the mismatch under any conditions fed into it. These panels are too close to the same to be worth looking at, but what happens if we put a 60 cell and 72 cell panel in parallel?

              Here are the operating curves created by the model under a realistic set of summertime conditions:
              60 vs 72.JPG


              Individually, under these conditions:
              the 60 cell panel has a Vmp of 28.87 V, an Imp of 8.29 A, and a Pmp of 239.3 W
              the 72 cell panel has a Vmp of 34.82 V, an Imp of 8.43 A, and a Pmp of 293.4 W
              If there were no mismatch, they would combine to 532.8 W

              If we add a new line to the chart to show what the combined power would be ass the controller sweeps across the voltage between the Vmp of the 72 cell panel and the Vmp of the 60 cell panel, we see:
              60 and 72 in parallel.JPG




              The peak is sharper than the individual power curves, and as NEOH suggests, it is skewed toward the lower voltage panel because the penalty for increasing voltage is higher than the penalty for decreasing voltage.

              Here, the maximum combined power occurs at Vmp = 30.12 V, with Imp's at 7.79 and 8.89 A, and Pmp's at 234.7 W and 267.7 W (502.4 W combined).
              The 60 cell panel took a 2% loss, the 72 cell panel takes a 9% loss, and the combined power has about 5.7% loss.

              The effect of mis-match over a year would need to be evaluated at many more conditions throughout the day and year to be estimated, but in the scale of these things, 5-6% total loss for relatively severe mismatch isn't really all that much.
              CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

              Comment


              • #8

                BIG THANKS for the feedback. Although this is over my head...it seems from the discussion that I don't have much to loose other than the power potential of the panels? Hoping that there is an agreement that my MPPT ...(this model here: https://www.ebay.com/i/321711597731?chn=ps&dispItem=1) will be less confused than I am....no panel degradation or other risks before I dive in and hook up the 5th string...?


                Originally posted by bcroe View Post

                You don't need to be that close. The power either side of exact MPP doesn't drop off much. I just make sure
                each string has the same number of series cells, never mind the published numbers. Works well. Bruce Roe

                I am not sure what you mean about the same number of cells...I don't think the 5th string does. How will the mppt act if it won't work properly?

                Comment


                • #9
                  Originally posted by preposterous4 View Post
                  I am not sure what you mean about the same number of cells...
                  I don't think the 5th string does. How will the mppt act if it won't work properly?
                  Here 10 series panels of 72 cells or 12 panels of 60 cells each total 720 cells in series, so these strings
                  parallel well. Hardly a concern for the MPPT.

                  Comment


                  • #10
                    [QUOTE=bcroe;n354595]
                    I just make sure
                    each string has the same number of series cells

                    The panels in question are Uni-Solar, thin film. I believe the 128 watt are twice as long as the 68 watt, 22 and 11 cells, respectively. Some thin film panels are quite vague as to number of cells.
                    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

                    Comment


                    • #11
                      The commonly quoted rule of thumb, based on the slope and curvature of the power curve at and near the MPP, is that if you can match the Vmp of two strings within 5% the difference in power between that mismatch and two exactly matched strings will be on the order of only 1%.
                      What you do need to be very careful of is paralleling two strings which will be experiencing partial shading at different times or to different extents. That sort of problem can change the MPP voltage by more like the full voltage of one or more panels, and can even effectively cost you the whole output of one of the strings.
                      That situation is where microinverters and optimizers come into play.
                      Or, of course, one MPPT input per string on your string inverter.
                      SunnyBoy 3000 US, 18 BP Solar 175B panels.

                      Comment


                      • #12
                        Just a last follow up. I connected the 4x68watt string and my voltage dropped 3 volts from 81 to 78. The MPPT didn't miss a beat.

                        I am not sure the true story about why unisolar went out of business, but when I was shopping for these panels to put on my van I was discouraged from doing so in forums..just saying that these panels had a bad rep. Quite frankly I now think that propaganda was probably sponsored by the shills for the grid. I was able to peel these panels off my van in the hot sun by pulling very hard on them. They still are working nicely. The coating is bullet proof. I guess the test of time with be in a decade. I had a pic to upload but it's not working atm, thanks to all for the help.

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