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Solar Shading Study Comparison

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  • Solar Shading Study Comparison

    I've got a situation where two parties, with opposing interests, may be arguing over the degree to which a solar system will be shaded, on an annual basis, by a not-yet-built neighboring structure. Now, there is no such thing as a standardized solar shading study (as far as I can tell), and the complexity of the situation is such that, if absolute accuracy is the determining factor, it might well devolve to who has the deeper pockets: that is, who can afford more convincing engineering. My first approach has resulted in a somewhat complicated approach to comparing often-very-different studies. While I believe it to be reasonable, I am trying to simplify the situation.
    I am wondering if anyone has any thoughts about ultra-simple shading analyses that would give some reasonably reliable indication of shading. For example: percentage of system surface shaded at noon on the equinox. If one boiled the situation down to that, maybe there's less wriggle-room to game the system.
    I would be particularly interested if anyone wants to mock up some situations and run the numbers with some shading software. I realize this won't be precisely "accurate": there will be differences based on whether the modules are mounted vertically or horizontally, for example, and the precise shading pattern and timing. The question is, can one develop a sufficiently simple metric that functions to compare situations without demanding a thorough performance assessment. I suppose one simplifying element is that I have no concern whatsoever, in this scenario, over actual performance, only in percent performance reduction or something.
    Anyway, I welcome any help thinking about the situation.

  • #2
    Taking you at your word that you welcome any thinking and leaving it to you to judge its helpfulness:

    What's your stake in this ? Are you part of a governmental or authoritative body who may have some jurisdiction over the outcome ?
    Are you legal counsel to any of the interested parties ?
    Who have you contacted about this that may know something about what's involved ?
    Can the structures and their relative positions be dimensionally modeled ?

    You seem to recognize there will never be absolute accuracy in a shading model. How much uncertainty is acceptable to all parties ?

    In modeling available solar irradiance, like other areas where things are primarily of a stochastic nature, models can rather easily be done if the geometry can be specified, but their accuracy as a predictive tool still falls back on weather vs. climate, atmospheric clearness or other conditions and other weather variables of a probabilistic nature. Bottom line: Absolute accuracy, in a strict definition, does not exist anywhere except as a mathematical concept.

    Shading of one structure by another, or more accurately, how that shading may impact another structure or a device's performance is, and because of the random nature of weather, including clouds, will always be, an approximation.

    Still, with some relaxation of the absolute accuracy requirements as a realistic accommodation to the vagaries of weather, it's possible to rather easily model when shade from a known structure will fall on the location of another known structure, just not how much, or for how long or the temporal distribution of that insolation. Software is available and doing the code is rather easy for a DIYer once solar geometry plug/chug cookbook stuff that's available in the open literature is utilized.

    And/Or, consult a solar modeling program called "SAM", available free from NREL. That model will do a shading analysis as will a number of other models. Other models are available whose results are about as reliable as currently available.

    Spend a few bucks and get a hired gun.

    Add: What state are you in ? if CA, there is solar rights legislation and case law that may be useful for the situation you describe.
    Last edited by J.P.M.; 10-20-2018, 11:27 AM.


    • #3
      There is no ultra simple system that will give a good result. Aurorasolar with a decent operator or as the company to do it for you will hive a very accurate result if you have the information or drawings on the structure to be built that will cause the shadow. It can even give monthly differences very easily. We have used it many times to verify systems that were as built differently than designed and a few systems that were built by others and shown it to be very accurate. The company has lots of documant sto its accuracy as well.
      OutBack FP1 w/ CS6P-250P


      • #4
        Originally posted by GreenCodeInspector View Post
        I am wondering if anyone has any thoughts about ultra-simple shading analyses that would give some reasonably reliable indication of shading.
        Erect two poles (schedule 40 PVC or something similarly cheap) at the corners of the proposed structure, of the height of the finished structure. Then use one of the commonly available site analysis tools across the array to determine when and how much it will shade. I use a Solar Pathfinder which is decidedly old-school, but the modern ones that use a cellphone and a lens will work fine as well.

        It's not very elegant, but it is super simple.


        • #5
          +1 on Aurorasolar, there isn't really a simple way, but this is a good tool