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  • PV output question

    Hello,

    I just turned on my system: 12 CS5P-230 panels and 3 BP 150W panels in two strings to a IG Plus 3.0-1
    Even though I go out and see 9 panels fully illuminated, with a theoretical 1950+ watts, the inverter says 1050W. I know you have
    to take off 10% for manufacturer lies, but still means I should be seeing 1700 W anyway.

    3-4 panels have some shading on them right now.

    Both strings show current draw (through 0.1 ohm shunt resistors), just no where near what they should be putting out.

    ???

    TIA

    Glen
    Moultrie, GA

  • #2

    Re: PV output question

    Re: PV output question

    How is your array configured (series/parallel/what panels). What are your panel's Vmp/Imp specs. And what voltage is your inverter/voltmeter reporting? And is it a hot or cool day right now (and what local time did you make the measurements).

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

    Comment


    • #3

      Re: PV output question

      Re: PV output question

      Bill,

      Sorry, two strings in series, one at 370 Voc and one at 380Voc. All CS5P are 5 amp 47 V max power rating, and BP are 27V 5.5 amp max power rating.
      bright clear day, 1pm EDT, panels still at winter angle of 45 degrees facing due south.

      Thought internal diodes were supposed to help with partial shading. Most panels in full sun right now.

      Take care

      Glen

      Comment


      • #4

        Re: PV output question

        Re: PV output question

        Normally we'd expect to see 77% of manufacturers' Watt rating as peak output.
        Shading, even a little bit, kills panel output and there's nothing you can do about it but remove the shading. Blocking diodes have no effect on this, but the panels' built-in bypass diodes will help keep reduction to a minimum, providing the shading only hits one string of cells within a panel. If it's over the whole thing you lose the whole thing.
        You seem to have mismatched panels too.

        If your array is 1950 Watts total then in a best-case scenario you should expect about 1500 Watts output.

        If I read your posts right you have nine panels with a Vmp of 47 and Imp of 5 which would be 235 Watts each for a grand total of 2115 Watts. That alone ought to supply 1.6 kW peak in a best-case scenario. If you have the three BP panels one per string (the Imp is close enough for this) with the 235 Watt units that adds 450 Watts for 2565 Watts with a peak output of 1975 Watts.

        I'll admit up front that I'm not clear as to the actual layout of your array. But it does seem you should be getting more than 1 kW from them, except for the shading issue. Three or four panels with shading is about 25% of the total, and will have a significant effect on the output.
        1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

        Ohm's Law: Amps = Volts / Ohms
        Power Formula: Watts = Volts * Amps

        Comment


        • #5

          Re: PV output question

          Re: PV output question

          What does fully illuminated mean for you? I notice your post was at 11:01 - so the sun was not at peak incidence to your array yet. The manufacturer STC rating is for ideal conditions of sun angle, temperature, and atmospheric clarity. You will likely never see the STC rating from your array. Maybe on a cool, clear day in April. What is your array's tilt angle? Just because the array is "fully illuminated" doesn't mean it is seeing the full power of the sun. Measure it at noon at least. Also, depending on how the array is wired, the shaded modules could really be affecting you.
          2.7kW Trina/Xantrex GT, 3.7kW Trina/SolarEdge, 3kW CSI/SMA GT, Solar well pump on 6, 25yr old Holeck 48W modules. Toyota SR5 converted to 108V EV. Prius w/Enginer PHEV conversion. BSEE, R11-residential, NABCEP, SunnyPro, >800kW installed
          "Faced with the choice between changing one's mind and proving that there is no need to do so, almost everyone gets busy on the proof." - John Kenneth Galbraith

          Comment


          • #6

            Re: PV output question

            Re: PV output question

            Well, I did not list all the panels just the one with full sun on them.

            There are 7each 47 volt panels at 5 amps in one string.
            There are 5each 47 volt panels + 3each 27V 5.5 amp panels in the other string.

            I already had been told to take 10% off the panel ratings for our local temps here. I think the California rating on the CS5P is 211 not 235.
            I do not know what the CA rating for the BP panel is. Most I saw were at least 10% below their claimed power output.

            The next step I think is to start by isolating one string, then take one panel at a time out of the string to see if I have a bad panel.

            I suppose I could stick pins in the USE-2 cable and measure the voltage on each one.

            Comment


            • #7

              Re: PV output question

              Re: PV output question

              That's a very strange way to configure an array. I wonder why the strings aren't six of the 235/230 Watt panels and one 150 Watt each. That would make each string 1530 Watts with a Vmp of 309 (which might be too low for the Fronius). Then you'd have a 3060 Watts and could expect something like 2350 Watts output peak (providing no shading or too high temps). Maybe this odd arrangement was done to try an compensate for the shading on one string? If so, I doubt it would work and it doesn't appear to be very effective.

              Lots of things affect panel output. Like solarix said, the STC rating is done in a lab under ideal conditions and the panels are "flashed" so they don't actually heat up. This is their "ideal" output rating and should only be used as a guideline. I don't know who suggested a 10% derating, but that's really optimistic. The 77% rating is based on typical operating temps with losses in wiring and MPPT function/conversion, and still is not an absolute. The sun angle makes a big difference (even a few degrees) as does temperature and that evil shading which we know is afflicting your array.

              You can check each panel's output individually by disconnecting them and testing the Isc in full sun, but such is a tedious process and it's difficult to be sure the panel actually is in full straight-on sun.

              I can't help but think the odd configuration is causing more trouble than it's solving.
              1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

              Ohm's Law: Amps = Volts / Ohms
              Power Formula: Watts = Volts * Amps

              Comment


              • #8

                Re: PV output question

                Re: PV output question

                Partial shading it a killer for paralleled strings of panels... It is very possible for partial shading on any one string to completely knock that particular string from production.

                Even a couple power lines across an entire array can cause 50% reduction in power output.

                Can you address the shading to any degree (trim foliage, cut/relocate vent pipes, etc.)?

                -Bill
                20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

                Comment


                • #9

                  Re: PV output question

                  Re: PV output question

                  Well, it should not make any difference what panel is hooked where if there operating current is close, which these are. I have 'balanced' the system to
                  get the use of those older BP panels...the funny thing is that the string with the BP panels has the higher output.

                  As they say " if it was easy, everyone would be doing it"

                  Comment


                  • #10

                    Re: PV output question

                    Re: PV output question

                    It has to be asked: which panels are being shaded?
                    I'm thinking a slightly smaller array that avoids the shading would actually result in a higher output.
                    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

                    Ohm's Law: Amps = Volts / Ohms
                    Power Formula: Watts = Volts * Amps

                    Comment


                    • #11

                      Re: PV output question

                      Re: PV output question

                      Glen-

                      A couple of days after a snowstorm here in Colorado, I noticed that my 14 panel (Sunpower 225 W each with Sunpower 3000 W inverter) grid-tied system was only putting out about 250 W when it should have been putting out about 2500 W based on previous experience. I went out to clear the snow off the panels, and discovered that each panel was covered on only the bottom 15% or 20%, the rest having melted. As it melted the snow slid down to collect on the bottom lip of the panel. So 15% or 20% shading was causing a 90% loss of power. This experience reminded me that solar cells are diodes, and they must act like low resistance paths when they are not illuminated, degrading the other cells. My shading was a different geometry than yours, but it makes me think that shading of some of the solar cells may have more than a linear effect in degrading the performance of the overall system.
                      Grid-tie PV system: 3.15 kW, 14 @ SunPower 225 W panels, SunPower SPR-3000m inverter.
                      Solar Thermal system: 1 @ Viessmann Vitosol 200 F flat plate collector 25 sq. ft. absorber area, Heat Flo HP 50-gallon stainless steel tank.
                      Residential Energy Laboratory in a net-zero source energy modified production house.

                      Comment


                      • #12

                        Re: PV output question

                        Re: PV output question

                        Originally posted by gfbankston View Post
                        Well, it should not make any difference what panel is hooked where if there operating current is close, which these are. I have 'balanced' the system to
                        get the use of those older BP panels...the funny thing is that the string with the BP panels has the higher output.

                        As they say " if it was easy, everyone would be doing it"
                        I wanted to give an update on this 'problem'.

                        I moved 9 of the CS5P-230 panels north, more out of the shade. I have checked the inverter output today, at peak sun angle. The inverter showed 2450 W at
                        one point in time, and was bouncing around 2300-2400 W output at the time.

                        Going back and using the PTC ratings of these panels I came up with 2770 W. Now, multiply that time 94% for inverter inefficiency give a little more than 2600 W.
                        So, 2450 W peak is not that far from the 2600 W I should expect. I will add another 400W worth of panels when I can to use the capability of the inverter.

                        Shading was my main problem. The shade will completely be gone in a couple months...

                        Comment


                        • #13

                          Re: PV output question

                          Re: PV output question

                          Originally posted by gfbankston View Post
                          I suppose I could stick pins in the USE-2 cable and measure the voltage on each one.
                          Don't do that. It probably wouldn't tell you anything useful and you would be opening a path to the conductor for water to get in. And then there's the safety issue; while the pin to pin voltage would not be all that much for a single module, the pin to ground potential could be considerable, depending on where in the string you would be taking your measurement.
                          Last edited by ggunn; March 26th, 2012, 9:25.

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