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Thread: Musings on snow on panels and production estimating

  1. #1
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    Musings on snow on panels and production estimating

    Do the values in PV watts take into account snow covering the modules for significant periods during the winter? Anyone heard of an analysis of different panel tilts and its effect on snow shedding?

    Ideal fixed tilt here near Albany NY is 34 degrees. 22 degrees only results in 2% less annual production - in theory. But how much quicker will they shed snow? Dec Jan and Feb together are 17% of annual production. This gets back to the first question, but I assume that assumes no snow cover. WAG, 1/4 of the time covered with snow at 34 degrees, 3/4 of the time at 22 degrees?
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  2. #2
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    Very old, but interesting. In particular, see page 11:

    https://www.osti.gov/scitech/servlets/purl/5232456


    A brief primer on a model in SAM:

    https://sam.nrel.gov/sites/default/f...Snow_Model.pdf
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  3. #3
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    Keep in mind that snow reflectance from the ground can increase irradiance and thus increase winter output beyond the expected. A higher tilt is better for that, too. I can't point to any kind of robust analysis but my intuition says to go with the higher tilt, all else being equal.

  4. #4
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    Quote Originally Posted by electrofelon View Post
    Do the values in PV watts take into account snow covering the modules for significant periods during the winter? Anyone heard of an analysis of different panel tilts and its effect on snow shedding?

    Ideal fixed tilt here near Albany NY is 34 degrees. 22 degrees only results in 2% less annual production - in theory. But how much quicker will they shed snow? Dec Jan and Feb together are 17% of annual production. This gets back to the first question, but I assume that assumes no snow cover. WAG, 1/4 of the time covered with snow at 34 degrees, 3/4 of the time at 22 degrees?
    When you measure tilt that way, are you talking about raising the top from horizontal, or raising the bottom from vertical?

  5. #5
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    From horizontal.

  6. #6
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    Quote Originally Posted by jaggedben View Post
    From horizontal.
    That didn't seem to make sense, so I went here and confirmed my hunch that it is from vertical, as in pin the top of the panel like it's the rear door of a hatch-back and lift from the bottom. 90 degrees would be parallel to the ground, 0 degrees is perpendicular to the ground. According to the calculator the ideal sun angle for Albany, NY varies from 24 in December to 70 in June.

  7. #7
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    Quote Originally Posted by jaggedben View Post
    Keep in mind that snow reflectance from the ground can increase irradiance and thus increase winter output beyond the expected. A higher tilt is better for that, too. I can't point to any kind of robust analysis but my intuition says to go with the higher tilt, all else being equal.
    Yeah good point.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  8. #8
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    Quote Originally Posted by gadfly56 View Post
    That didn't seem to make sense, so I went here and confirmed my hunch that it is from vertical, as in pin the top of the panel like it's the rear door of a hatch-back and lift from the bottom. 90 degrees would be parallel to the ground, 0 degrees is perpendicular to the ground. According to the calculator the ideal sun angle for Albany, NY varies from 24 in December to 70 in June.
    This site has the equations:

    http://www.solarpaneltilt.com/

    The common theory of "set them to your latitude" is incorrect.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  9. #9
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    Quote Originally Posted by gadfly56 View Post
    That didn't seem to make sense, so I went here and confirmed my hunch that it is from vertical.
    You found a site using that convention. This thread has been using the other convention, as does the site that electrofelon mentioned, as does the (non-optimal) rule of thumb "tilt angle = latitude".

    Cheers, Wayne

  10. #10
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    Industry standard, at least in this country, is tilt is from horizontal. In the decade I've been doing this I've never seen or heard anyone measuring from vertical. See Solmetric tools, for example. Or PVwatts. Or CSI.

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