Backtracking

I have created a project using SAM (Version 3.14) using single axis tracking with a tilt of 0 deg. From my understanding I would expect, if “Backtracking” is enabled the annual energy would increase. Instead I am getting the opposite effect. I have run a number of simulations with and without backtracking enabled for different GCR’s. In all cases with Backtracking enabled (box ticked, both the Nominal POA (kWhr) and annual energy (kWhr) values drop, marginally when the GCR is closer to 0.3 but significantly when closer to 1 (approx. 24%). Is this a bug or is something else going on. Any thoughts appreciated.

Hello,
This is not a bug, but is a tricky aspect of comparing the different shading options in SAM.
When you check "backtracking" on the System Design page, SAM rotates the trackers to avoid any self shading of modules in neighboring rows. As you increase the GCR from 0.3 (rows further apart) to 0.9 (rows closer together), you should see both the annual energy and annual nominal total radiation decrease, showing the effect of backtracking on the system's output. For a meaningful comparison of a system with backtracking to one with no backtracking, for your no-backtracking scenario, you should enable self shading on the Shading and Snow input page so that the results show the effect of self shading. If you do that, you should see that the annual energy/nominal radiation decreases as GCR increases (there is more self shading as the rows move closer together). You should also see that the difference between the self shaded and backtracking scenarios is smaller as the GCR increases.
Here is a table showing how annual energy varies with GCR for self shading with no backtracking, backtracking (avoiding all self shading), and no self shading or back tracking. Note that for the backtracking case, as the GCR approaches zero, the annual energy approaches the ideal "no self shading or backtracking" scenario's value of 9,727 kWh:

On a related note, you can see the houlry tilt angle on the Time Series tab of the Results page to see how backtracking works. That angle is listed in the hourly results as "Subarray n Surface tilt (deg)."
Best regards,

Paul.

Thanks Paul for the response it was very helpful

I have one more question relating to the module length dimension used in the Self Shading page; it appears to be fixed at 1.7 m whereas the Help pages state the module length is 1.7 times its width. What is the rationale behind this when actual panel dimensions may differ greatly from this ratio? This will impact the calculation for row spacing. For example if the PV Module dimension is 2.16 x 1m and the GCR of 0.3 is used, SAM will give a row spacing of 5.67m instead of 7.2m if the actual module length is used.

The reason the module aspect ratio is fixed at 1.7 to 1 is that SAM does not have information about the module length and width. The CEC database that we currently use only provides the module area in square meters. The simplification makes it possible for SAM to estimate the effect of self shading without the length and width information.

On the Shading and Snow page, the "Length of side (m)" parameter is 1.7 meters if the module orientation is portrait, and module area / 1.7 if the module orientation is landscape.

For the next version of SAM, we are considering changes to the module database for the CEC model that will make length and width available so that the self-shading calculation can be based on the actual dimensions of the module.

Best regards,
Paul.