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Thread: Shading

  1. #1
    Join Date
    Apr 2012
    Location
    Southern Maine
    Posts
    7

    Default 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 at 14:50 PDT.

  2. #2
    Join Date
    May 2011
    Location
    Tonopah, Az.
    Posts
    102

    Default 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.

  3. #3

    Default 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, 2kW 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, C11-residential, NABCEP, SunnyPro >700kW 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

  4. #4
    Join Date
    Mar 2006
    Location
    SF Bay Area (California)
    Posts
    20,993

    Default 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.

  5. #5
    Join Date
    Jul 2007
    Location
    Ontario Canada
    Posts
    815

    Default 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

  6. #6
    Join Date
    Apr 2012
    Location
    Southern Maine
    Posts
    7

    Default 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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  7. #7
    Join Date
    Jan 2011
    Location
    Baltimore, MD
    Posts
    157

    Default Re: Shading

    Quote 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.

    Name:  House Array.jpg
Views: 273
Size:  70.2 KB
    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).

  8. #8
    Join Date
    Apr 2012
    Location
    Southern Maine
    Posts
    7

    Default 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

  9. #9
    Join Date
    Nov 2011
    Location
    central Vermont
    Posts
    2,610

    Default Re: Shading

    Quote 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

  10. #10
    Join Date
    Apr 2010
    Location
    Austin, TX, USA
    Posts
    1,852

    Default Re: Shading

    Quote 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.

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