Changing row type or box count for ground irradiance

I don't have any experience yet in cracking open the SAM code and making changes, but I wanted to know if the following is possible.

I'm trying to model different PV systems on canals, both overhead and in the form of vertical systems along the banks. I want to compare both generation and the impact on irradiance striking the water. Unfortunately, even when set to 1 or 2 rows, SAM treats these as interior rows to larger system with "phantom rows" on either side that impact ground shading. While I could simply set the pitch length to be excessively large and minimize this impact, that would make the 10 boxes that SAM divides the spacing into too large to line up with the canal boundaries (we want box size to be around 1.3m to separate out shore overhang from the actual start of the water edge).

I know that Bifacialvf has a variable called "Row Type" that can be set to first, last, interior, or single, and am wondering if SAM has some similar internal variable that could be altered. Alternatively, I'd like to be able to change it so that rather than dividing the row spacing into 10 boxes, it can be divided into 100 boxes or any arbitrarily large number.

Hi Jeremiah,

Can you tell me more about which SAM results are relevant to your analysis and what method are you using to determine the irradiance striking the water?

Modifying SAM's source code is an option, but it takes quite a bit of effort to set up the development environment to get things running.

Best regards,
Paul.
 

I'm interested in the "Spatial Irradiance incident on the ground between rows (W/m2)", which SAM currently splits into 10 equally sized boxes. I have been adjusting GCR to produce a desired pitch length and corresponding box size so that the box edges align with the edges of the canal, and averaging the boxes that correspond to the water.

For example, if the first box matches the horizontal overhang between the leading edge of the panel and start of the canal, the end of box 6 aligns with far end of the canal, and boxes 7 - 10 correspond to the far shore, then boxes 1 and 7-10 are discarded from the analysis and only boxes 2-6 are kept.

I can set this much up, but SAM behaves as if there are additional rows in front of the single real row I'm examining, blocking some of the light would strike the first few boxes underneath the array at low sun elevations. I discovered this when looking at different canal widths (and thus different array side lengths, since the array scales with canal width). As I've been trying to keep the box size (and thus the pitch) the same in each case so the first box matches the overhang, this means adjusting the GCR in each case. So the same pitch, but with different array side lengths. The narrower width cases allow more light underneath the leading edge compared to wider cases in shoulder months, even though they should have identical geometry if we were only looking at single rows. The wider cases end up more crowded by the phantom rows and the irradiance drops off in the shoulder months.

I've attached some slides that hopefully lay this out more clearly. For the moment I'm going ahead with trying to switch to Bifacialvf for the ground irradiance part, where I can use 100 boxes instead of 10,   and working out how to feed it the weather files I want to use.

Hi Jeremiah,

Thank you for the clarification and diagram.

If the only SAM output you are interested in is the ground irradiance data that results when you choose the Use monthly spatial albedo values option on the Location and Resource page, and you are set up to use Python, I think the bifacialvf Python package is a good option because, as you note, it gives you more control over the bifacial model than SAM.

If you want to use SAM to get power generation and financial model output data, you should be able to model a subarray with a single row of modules by choosing values for Number of modules along side of row and Number of modules along bottom of row on the Shading and Layout page so that their product equals the total number of modules in the subarray. (The total number of modules in the subarray is determined by Modules per string and Strings in parallel on the System Design page.

SAM divides the ground area between rows (or behind a single row) into 10 strips (or boxes). For now, it is not possible to change the number of strips as you can in BifacialVF. It is also not possible to change the BifacialVF parameter Rowtype in SAM.

Best regards,
Paul.