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  • Shading

    Newbie here. Shopping for PV system, and trying to educate myself. Can I please be pointed in the direction of learning about the effects of shading on cells, pannels, and arrays? I'm @ Lat 43 26' 21" N, long 70 46' 29" W (So. Maine). I've been reading about diodes being used in the pannels, and micro-inverters used for each pannel as a means for mitigating shadings effects. My roof is bathed in sun between aboiut 9AM and 4PM, but as the sun begins its' decent for the winter, branches causing shading come into the equation.

    All help is appreciated.

    Kev
    Last edited by kevpat; April 14th, 2012, 14:50.

  • #2

    Re: Shading

    Re: Shading

    Solar and even the slightest of shade do not mix. You will lose a significant amount of solar production. I've heard say that amorophous panels are tolerant of this, but I wouldn't bank on it.

    Comment


    • #3

      Re: Shading

      Re: Shading

      I would say you are probably a good candidate for the SolarEdge distributed system. Check out their videos at solaredge.com
      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


      • #4

        Re: Shading

        Re: Shading

        Even one branch can easily cost you 50% to 100% of the panel/array's output, no matter the controller used.

        Yes--there are some things you can do with solar arrays to reduce the losses to possibly 10% of string (more or less)... But, in the end, you either need to accept the shading or remove/move the cause of the shade, or relocate the panels.

        One problem with "green shade" (bushes, trees, etc.)--Is that they usually only get worse as time goes on (greenery grows out/higher) either making the problem worse or forcing you to trim/remove the source of shading.

        More or less, solar panels output a (sort of) fixed voltage and variable current (related to amount of sunlight falling on array). If you shade one or more cells, it pretty much takes those out of the energy production. And reduces the output voltage (Vmp-array or Vmp-panel) by upwards of 12 volts or by 1/3 to 1/2 the output current of the panel.

        Which will happen to your panels/array is very difficult to predict. It depends on the solar array and how it is wired (with bypass diodes) and the connection with other panels/charge controllers.

        In some rare cases, people have "worked around" some shading issues... For one person, their panels had "one" string along the bottom of the panel and a second parallel string at the top of the panel (when mounted in "landscape" or "sideways"). This allowed the "top 1/2" of the panel to still produce while the bottom 1/2 was covered by snow... Still cost about 1/2 of the power production until the snow melted or was removed from the panel.

        If you have a large MPPT GT or Solar Charge controller with several (to 10 or more) panels in series)--It is possible to wire the string so that only one panel is shaded--which still leaves enough voltage for the controller to produce power while current is "bypassed" around the shaded panel. More useful with predictable shade (chimney, vent stack, 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


        • #5

          Re: Shading

          Re: Shading

          By 4pm in the winter you're not losing much valuable energy anyway. The sun has put most of it's energy into your system between 11am and 3pm, especially in the winter. Any shading after 4pm is nothing to worry about IMO.

          Ralph
          http://enlighten.enphaseenergy.com/p...stems/hY7W5536
          see the project buildhttp://www.greenpowertalk.org/showthread.php?t=11702

          Comment


          • #6

            Re: Shading

            Re: Shading

            Thanx for the replies. Its eye-opening to realize the extent shading has on a system. A prospective PV installer has suggested the pictured array for us, and that, being grid-tied to CMP (Maine) it would effectively supply all our yearly electric needs, with our costs at the minimal level to maintain a service connection to the grid. It has a Solectria String inverter, 250W Canadian Solar Monosilicon PV Panels, mounted on a 60 deg pitched roof oriented at 175 degs. The shading travels from the lower left to the lower right, while rthe upper panels are unaffected. I suppose ther's a way to wire the array to minimize the shadings' effect on the array. Of course, the salesman makes it sound exciting, but I'm agreing w/many of his conclusions regarding the cost/payback factors. Shading is one of the effects that weren't discussed, so I'm trying to get a grip on this factor.
            Again, thanx for the replies, and I am always open to more discussion/info on this and any other stuff.

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            Comment


            • #7

              Re: Shading

              Re: Shading

              Originally posted by kevpat View Post
              Thanx for the replies. Its eye-opening to realize the extent shading has on a system. A prospective PV installer has suggested the pictured array for us, and that, being grid-tied to CMP (Maine) it would effectively supply all our yearly electric needs, with our costs at the minimal level to maintain a service connection to the grid. It has a Solectria String inverter, 250W Canadian Solar Monosilicon PV Panels, mounted on a 60 deg pitched roof oriented at 175 degs. The shading travels from the lower left to the lower right, while rthe upper panels are unaffected. I suppose ther's a way to wire the array to minimize the shadings' effect on the array. Of course, the salesman makes it sound exciting, but I'm agreing w/many of his conclusions regarding the cost/payback factors. Shading is one of the effects that weren't discussed, so I'm trying to get a grip on this factor.
              Again, thanx for the replies, and I am always open to more discussion/info on this and any other stuff.

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              If shade is only an issue after 4PM in the winter, I wouldn't worry too much about it. Very little power is contained in those high-angle rays. Mounting those panels in landscape mode, as pictured, and putting the bottom panels on a separate string from the top panels will minimize the effects.

              One thing that you can do is set up a webcam to take pictures of your roof every half hour or so on a sunny day, and you can see how high the shadows get, and when they would shade your prospective array.

              PV-watts says that the system you describe will probably produce around 5,000 kWh a year on average (assuming Portland, ME).

              Comment


              • #8

                Re: Shading

                Re: Shading

                Again, much thanx to all for the replies. I've sent a request to the prospective installer for a discussion on our shading issue, and the info gleaned from you kind folks will help my understanding.

                Relating to strings: what are the determining factors for strings in an array? Isolating any shade from other panels/strings seems to be very important, but do strings have other factor; ie, do strings need to be equal in panel numbers (in our situation, 8 panels for two strings), or could we have the upper 12 panels on a string, and the other four on another string?

                Appoligies for my ignorance, and thanx for any repliues. This thread is very helpful.

                Kev

                Comment


                • #9

                  Re: Shading

                  Re: Shading

                  Originally posted by kevpat View Post
                  do strings need to be equal in panel numbers (in our situation, 8 panels for two strings), or could we have the upper 12 panels on a string, and the other four on another string?
                  Each parallel string should be equal in voltage to the other parallel strings.

                  Perhaps the best solution for you is to use microinverters. Each panel has its own inverter and shading of one panel has no effect on the others.

                  --vtMaps
                  4 x 235w Samsung, Outback fm60 & vfx3524 & mate, Midnite E-panel, four Interstate L16, Trimetric monitor, Honda eu2000

                  Comment


                  • #10

                    Re: Shading

                    Re: Shading

                    Originally posted by kevpat View Post
                    Relating to strings: what are the determining factors for strings in an array? Isolating any shade from other panels/strings seems to be very important, but do strings have other factor; ie, do strings need to be equal in panel numbers (in our situation, 8 panels for two strings), or could we have the upper 12 panels on a string, and the other four on another string?
                    For a string inverter, the strings must be of equal module count. The number of modules in a string determines the string voltage, and the voltage must be the same for parallel strings going into the same inverter.

                    Comment


                    • #11

                      Re: Shading

                      Re: Shading

                      Are the cameras that take a 360degree shot not commonly used in your area? Both contractors that came to my place were required to take a hemispherical photo which is analyzed for shading. I got the impression the state rebate depends on them ensuring there will not be more than a 20% drop from ideal.

                      Comment


                      • #12

                        Re: Shading

                        Re: Shading

                        Originally posted by kevpat View Post
                        Again, much thanx to all for the replies. I've sent a request to the prospective installer for a discussion on our shading issue, and the info gleaned from you kind folks will help my understanding.

                        Relating to strings: what are the determining factors for strings in an array? Isolating any shade from other panels/strings seems to be very important, but do strings have other factor; ie, do strings need to be equal in panel numbers (in our situation, 8 panels for two strings), or could we have the upper 12 panels on a string, and the other four on another string?

                        Appoligies for my ignorance, and thanx for any repliues. This thread is very helpful.

                        Kev
                        For a string inverter with the pictured panel layout and afternoon shading that goes from left to right, this is the best way to wire up the 2 strings:

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                        An 8-panel string seems a little low to me. Does the Solectria inverter have lower voltage requirements than a Sunny Boy?

                        Comment


                        • #13

                          Re: Shading

                          Re: Shading

                          Im informed that the modules, 240W Monosilicon Canadian Solar (CS6P-240M) have diodes which effectively create three, 20-cell sub-circuits, which can be bypassed independent of the other 40 cells in the module. This sounds like a good thing for the shading mitigation issue; micro-inverters were also touched upon, but didnt appear to be cost-effective. I will discuss this in more detail when the guy comes tomorrow AM.

                          The inverter is a Solectria PVI 4000 has 4 fused inputs, MPPT Range of 200-550 VDC, Max input of 600 VDC, Strike (?) Voltage of 235 VDC, Max Input current of 20 A. My ignorance-level regarding this aspect of the system is quite high, so I don't know what any of these specs mean, as I havent delved inti i nverters at all, other that to understand that they convert DC to AC.

                          Were going to some inclinometer surveys tomorrow to estimate shading from the trees. Camera surveys arent required for rebates, but it sounds like a really cool idea. In Maine, we need to show that the estimated payback period is less than 20 years. I think that an NREL type of estimate is acceptable for the rebate...not sure, tho.

                          You guys are showing patience and it is welcomed...Thanx...kv

                          Comment


                          • #14

                            Re: Shading

                            Re: Shading

                            The bypass diodes are really there to protect the cells from reverse voltage (reverse bias the solar cell "diode" by much more than 12 volts, it will blow the cell).

                            There is some advantage in series strings -- But remember those 20 cells + diode drop reduces the overall string voltage (about 12 volts in this case)--So the Vmp-array-shaded still needs to be higher than the Vmin of the inverter/solar charge controller for proper operation/to generate useful power.

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

                            Comment


                            • #15

                              Re: Shading

                              Re: Shading

                              Update:

                              So my guy came as promised with a solar pathfinder. After ascertaining all shading-on-the-modules probabilities, it was estimated that a loss of about 12% generation from one panel during the 4 months of shading would result if we kept our beloved elm tree. Otheer trees are not a factor, although a really nice Maple will have some effect in about 5 years, or so. This loss could be further mitigated by using Enphase 215 micro-inverters, at an added cost of about 10%. This sounds ok with us, as the string inverter was to be located on the exterior wall adjacent to a future extra bedroom, and the hum that I understand comes from these inverters could become an annoyance.

                              So, looks like were going with that system; its estimated to supply all our yearly electricity requirements w/o battery back-up, while well maintain our grid tie-in at the minimal cost of < $10/mo.

                              Thanks to the forum. If anybody needs info on testing submarine nuclear piping systems...Im your guy!!!

                              kev

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