PV single-axis tracking, backtracking, and self-shading (SAM 2017/Windows)

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SolH
PV single-axis tracking, backtracking, and self-shading (SAM 2017/Windows)

Hi Paul,

When I enable backtracking for single-axis tracker, I notice I can l can no longer enable "self-shading for fixed subarrays and one-axis trackers".

When I run the simulation without backtracking, there is shading loss but the moment I enable backtracking, there is no shading loss reported regardless of what I set the gcr value (even if it's very high). I would think that self-shading losses should depend on gcr values and the tracker rotation limit even with backtracking.

Should I use the 3D shade calculator to enable self-shading in the case of a single-axis tracker with backtracking?

Thanks,
Sol

Paul Gilman

Hi Sol,

SAM doesn't run the self-shading model when you enable backtracking. Instead, it calculates the shaded fraction of the array, and adjusts the tracker rotation angle to minimize it. That's why you don't see any self-shading data in the results. Unfortunately, SAM does not report the shaded fraction in the results, so there is not a direct way to see the impact of the backtracking algorithm.

If you want to compare results with and without backtracking, I would recommend creating two cases that are identical except for the following (click Duplicate on the Case menu to create a copy of a case):

1. The "with backtracking" case has backtracking enabled on the System Design page.

2. The "no backtracking" case has backtracking disabled, and self shading enabled on the Shading and Snow page. (You will have to specify numbers of modules along side and bottom of row.)

Then, you can compare the "nominal" and "after shading only" POA total irradiance data in the results for the two cases to see the effect of backtracking. You can also compare the subarray "Axis rotation for 1 axis tracking" variables: "Ideal" is the rotation angle without backtracking. On the Time Series tab of the Results page, zoom in to see a few days and plot those variables to do the comparisons visually. Use the two check boxes for each variable to plot data on separate top and bottom graphs.

Also, if you haven't seen it already, you might find this post about comparing results from a system with backtracking to those from one with no backtracking helpful:

https://sam.nrel.gov/node/70188

Best regards,
Paul.

Paul Gilman

Hi Sol,

SAM doesn't run the self-shading model when you enable backtracking. Instead, it calculates the shaded fraction of the array, and adjusts the tracker rotation angle to minimize it. That's why you don't see any self-shading data in the results. Unfortunately, SAM does not report the shaded fraction in the results, so there is not a direct way to see the impact of the backtracking algorithm.

If you want to compare results with and without backtracking, I would recommend creating two cases that are identical except for the following (click Duplicate on the Case menu to create a copy of a case):

1. The "with backtracking" case has backtracking enabled on the System Design page.

2. The "no backtracking" case has backtracking disabled, and self shading enabled on the Shading and Snow page. (You will have to specify numbers of modules along side and bottom of row.)

Then, you can compare the "nominal" and "after shading only" POA total irradiance data in the results for the two cases to see the effect of backtracking. You can also compare the subarray "Axis rotation for 1 axis tracking" variables: "Ideal" is the rotation angle without backtracking. On the Time Series tab of the Results page, zoom in to see a few days and plot those variables to do the comparisons visually. Use the two check boxes for each variable to plot data on separate top and bottom graphs.

Also, if you haven't seen it already, you might find this post about comparing results from a system with backtracking to those from one with no backtracking helpful:

https://sam.nrel.gov/node/70188

Best regards,
Paul.

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