MPPT and Shadowing

I just received my Morningstar MPPT controller and got to thinking, will using a higher voltage panel and an MPPT controller mean less loss due to shadowing. Unfortunately many of the places where we camp have trees and a good percentage of the day there will be shadowing on the solar panel.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,609 admin
    Re: MPPT and Shadowing

    If you place two or more panel in series (within specifications of MPPT controller)--a little bit of shadowing will cause and equivalent loss in production (cover 1 out of three panels, and you will lose 1/3rd the power generated).

    However--In the grand scheme of things--Put the panels in full sun. Filtered light through trees is going to cut pretty near 100% of the output of your panels. There is shading patterns that are worst than others--but that usually is only worth worrying about when you have a fixed obstruction (i.e., vent stack shade on roof mounted panels in winter). Otherwise, random shading is just "bad" and there is nothing else you can do except get more full sun on the panels.

    One thing you can do--put the panels in series (get Vmp as high as practical for your panel/controller combination--this reduces current and voltage drop). And buy a nice 100 foot heavy duty extension cord (12 or 10 AWG--or whatever is required for your current rating). Place the panels in sun (staked to prevent wind blowing over and to prevent them from walking away) and run it back to the RV+charge controller parked in the shade.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: MPPT and Shadowing

    the bottom line here is that no matter what way you do it that shading will kill it. you have to get them into the sun.
  • ShadowcatcherShadowcatcher Solar Expert Posts: 228 ✭✭✭
    Re: MPPT and Shadowing

    Being one of those that have to know "everything about something" what is the source of loss. Let me clarify, logic would say all things being equal, if voltage is maintained above battery/controller input needs a 20% shadowing would reduce amperage by 20%.
  • BB.BB. Super Moderators, Administrators Posts: 32,609 admin
    Re: MPPT and Shadowing

    Not quite--It depends on how the shadows fall on the array and how the array is wired...

    A cell in shade goes "high resistance"--so the entire string's current flow is blocked. However, there will probably be bypass diodes installed. For example, if you have three bypass diodes then 1/3rd of the array output will drop to ~zero current the bypass diode will pass current around the high resistance section of the panel.

    Now, if the shadow falls on only that 1/3rd of the cells in the one bypassed string--then you lose 1/3rd of the output voltage but maintain the current.

    However, if the 1/3rd shadow covers sections of all three bypassed elements, then you (roughly) would lose 100% of the output voltage and the entire panel would be bypassed (if there are 2 or more series panels in a string, and the Vmp-array-shaded voltage is still higher than > Vbatt-charging+Vsystem-drops).

    One person here actually mounted their panel (horizontally as I recall) so that winter shadows only affected the "bottom" 1/3rd contiguous bypass section of the panel.

    The bypass diodes certainly can help limit shading losses--but they are really there to prevent damage to shaded cells in an otherwise sunny array (high resistance equals high reverse bias voltage across "dark cells").

    In general, avoid shading whenever possible and assume a drastic decrease in power if a portion of a panel is shaded. Certainly, there are times of the day when shading is unavoidable--but working to make those times rare (especially during the middle of the day) is very important to having a well performing solar PV system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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