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Partial shading estimation...

Discussion in 'Photovoltaics' started by Dave, Nov 26, 2003.

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  1. Dave

    Dave Guest

    Picture a large fully grown tree in the middle of summer with all of its
    leaves in full array.

    Now picture the same tree in winter with all of its leaves blown off and
    only the branches remaining.

    The question is what is the net shading effect on solar panels in the winter
    when the sun is low and the tree is bare vs the summer when the tree is full
    (essentially opaque to the panels) but the sun is higher in the sky so there
    is no shade on the panels?

    A solar pathfinder analysis assumes the shading is complete and does not
    account for the no leaves in winter condition which allows a lot of sun
    light to penetrate the tree structure and fall on the panels.

    Is there any data regarding this situation or how to properly take it into
    account?

    Dave
     
  2. I'm not an expert and have no data to share. That said, I believe
    that the design of the array and how the power it's generated is
    used will vastly change it's performance under partial shading
    conditions. In the worst case, I believe that even a tiny bit of
    shade on just the wrong part of an array could reduce the output
    of the entire thing to zero.

    Anthony
     
  3. This is a case where the operating voltage of the array has a large effect.

    The basic fact is that the current generated by a PV module is determined
    by the cell with the lowest irradiance on it. Totally shade a single cell and
    lacking any bypass diodes, the output of a module would be zero. Bypass
    diodes are used across sections of PV modules to provide a current path
    around a shaded cell, but the voltage of the bypassed part of the module
    is lost and may result in no output from the remainder of the module if the
    resulting voltage is too low to use for charging a battery or feeding an
    inverter (to list a few possible designs).

    Shade from a branch that is spaced even a few meters or yards away
    from the PV module is not total, there is the diffuse light from clouds
    and/or blue sky and this amounts to some 30% of the total irradiance.
    Worst case is a 70-80% loss, not 100%.

    If you have an array for charging a 12-volt battery system, each PV module
    will have an output determined by the cell with the lowest irradiance on it.
    Modules without shading will have full output for the irradiance level.
    If you have a higher voltage array with PV modules in series, each series
    string will have a current determined by the cell with the lowest irradiance
    on it anywhere in the series string. If there are bypass diodes, the output
    voltage of the series string will be lowered by the voltage of the bypassed
    section of the module.

    It is not generally possible to calculate the effect of a tree on a PV array,
    but you can measure the current once the situation develops. If you have
    not yet built the array, you can use a single module as a sensor and move
    it across the intended mounting area while measuring the output current
    into a battery (or less accurately the short circuit current).


    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
     
  4. Bill Shuler

    Bill Shuler Guest

    From Joel Davidson's "The New Solar Electric Home" book, he states that
    winter branch shading will sacrifice at least ten percent of the potential
    power
    harnessed with a pv module or array. There is a definate reduction in power,
    for when I first began experimenting with a solar panel, a Solec s-050, I
    had it on my back deck, aimed to the South with some oak trees that way.
    Using an amp meter, I watched the deviation of current as the trees became
    obsticles of the sun light. As a result, I now have the pv array elevated
    and placed
    further North from the woods to overcome the loss during the period from
    October through February.

    Hope this helps, and I hope you don't have to fell many trees to get
    good results, though now, the burning logs feel nice in the woodstove!
    Bill (South Carolina)
     
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