How to model rows on roof slope with tree next to roof

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davyboy
How to model rows on roof slope with tree next to roof

I have a PV model w/ 11 panels. They are laid on a sloping roof with a tree off to one side. The higher up the roof slope, the less shade. The panels and single inverter are already specified, so how to I model the Syetem Design page to reflect reality. The panels are laid 6 panels bottom row, 4 panels second row and 1 panel 3rd row. I tried to set up more than one subarray, but parameters are grayed out. Also, the modules per string that I set for the Always Enabled subarray won't let me change it for the second subarray.

I think I figured out a string has nothing to do with physical configuration of panels. So how do I indicate where the panels are in relation to the tree? Each row has a different number of panels.

Paul Gilman

Hello,

You are correct that the "modules per string" and "strings in parallel" inputs on the System Design page describe how the modules are wired electrically, not how they are physically laid out. If all of the modules in the array are connected to the same inverter and have the same orientation (tilt and azimuth angle), then you should model the array as a single subarray.

The number of modules along side and bottom of row inputs on the Shading and Snow page describe how the modules are arranged in rows for modeling self shading, which is caused by modules in neighboring rows casting shadows on each other. In your case, there is no self shading, so you can ignore those inputs.

To model the effect of shadows from the tree on the array, you need to provide a table of beam shading losses that indicate how much of the sunlight incident on the array is blocked for each hour of the year. You can either use a shade analysis tool to generate that data and import it into SAM, or you can use SAM's 3D shade calculator. Here is a link to a recording of a webinar where we summarized the different ways to model shading in SAM:

https://sam.nrel.gov/node/69098

In the 3D scene in your file, you can add three active surfaces: One for each row of the array. According to the data sheet for the REC290TP2 module, its dimensions are 65.9" x 39.25". So, for the bottom row, you should create an active surface with width = 39.25 * 6 and length = 65.9 for portrait orientation, or width = 65.9 * 6 and length = 39.25 for landscape orientation. Note that "width" is the length along the X axis when azimuth = 180, and "length" is the length along the Y axis when azimuth = 180. Similarly, create an active surface for the middle row with the length of 4 modules, and a third active surface with the dimensions of one module. Then, determine the X, Y, and Z coordinates to position the active surfaces along the roof, and make sure that subarray = 1 and string = 1 for all three rows. As you rotate the scene in 3D view, you should see shadows from the tree on the active surfaces. When you save and close the 3D shade calculator, SAM will calculate a set of time series shade losses that you can see by clicking the Edit Shading button on the Shading and Snow input page.

It looks like you have some extra trees in your scene that you should delete. (See the Shade Calculator's Help system for tips on how to create and manipulate objects in the scene.)

Let me know if you need more help getting things set up.

Best regards,
Paul.

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