Separate string on same roof w/different pitch

Island Mon
Island Mon Solar Expert Posts: 29 ✭
in Advanced Solar Electric Technical Forum #1
I am planning an off-grid system in Maine for a summer home. I have a 10' x 19' roof with a 17 degree pitch, which is optimal for the summer months.
I am planning on using evergreen 210 watt panels which are roughly 5'x3'.

I can fit 12 panels on that section of roof, however, I would like to add more panels. Will putting another string of panels on a 40 degree pitch roof affect anything.

I am aware that the efficiency of that string will be lower, is there a way to calculate how much less energy this string will collect on the new 40 degree pitch?

The 40 degree roof is south facing and unshaded.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #2
    Re: Separate string on same roof w/different pitch

    For maximizing gain,, consider two different charge controllers. Each one controlling one string to maximum effect. Or wire the array in a series/parallel array in such a way to minimize the effect of the less efficient orientation.

    On the other hand,, the difference between 17 and 40degrees probably won't have much net effect on the two strings,, assuming that they are in the same orientation relative to south.

    Try running PV watts for the two orientations and see the difference,

    http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version2/

    Tony
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  • Island Mon
    Island Mon Solar Expert Posts: 29 ✭
    #3
    Re: Separate string on same roof w/different pitch

    I am planning on having 4 strings of 4 evergreen panels in parallel.

    So 4 panels wired in series to get 48V X 4 strings = 48 V 3360 watts (with 210 watt evergreen).

    3 strings will be on the 17 degree roof, and the remaining string will be on the 40 degree roof.


    Then combine the strings on the roof and run the wire to a xantrex xw-6048 inverter, charge controller.

    Will this one charge controller suffice, especially since the panels are wired in series and parallel?
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #4
    Re: Separate string on same roof w/different pitch

    Assuming that the sum total output from all the strings is within spec of the inverter controller,, your description will certainly work. The advantage of using a separate controller on the off angle string is that it will allow BOTH strings to operate more efficiently. It is my (limited) understanding that the MPP of the entire string will be regulated by the weakest sub-string,, the net effect of which is pull down the out put of the entire array. As I said before,,I don't think that the net/net will be all that much different with the two angles,,, but you should do a calculation to find out if that is correct. I'm sure Solar Guppy or Bill will have a better real life experience with this kind of set up than I.

    Tony
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