Adding a real tracking algorithm to PVWatts

The web version of PVWatts doesn’t take into account the effect of one row of panels/trackers casting a shadow on another, and neither does it account for backtracking in the morning and evening to avoid shading. Essentially, you are modeling the performance of a single tracker.

NREL has a page devoted to derate factors here which provides a rough approximation for a shading derate based on GCR (ground coverage ratio), but in reality the affect of backtracking is dependent on location.

With this in mind, I modified the PVWatts source to calculate the correct incident angle (function incident2) for a horizontal single axis tracker given a GCR, and ran the simulation for a number of GCR’s and locations. Here is a sample result:

Derate factor versus GCR for 3 different locations

Derate factor versus GCR for 3 different locations

This shows the actual derate factor you can expect as a function of GCR for 3 different locations. Clearly, they are NOT all the same! At a GCR of 40%, there’s a 1.4% difference in energy lost between San Diego and Salt Lake City.

With small changes like this I’m getting closer to a model that accurately predicts real world performance.

5 thoughts on “Adding a real tracking algorithm to PVWatts

  1. Eric Alderman

    Hi Jason,

    We are developing software solutions for solar installers, and I was wondering if you can provide an updated pointer to the PVWatts v2 source code? The URL from your comment added last July is no longer valid.



  2. jalderman Post author

    It looks like someone at NREL decided not to share the source. I’m not sure I agree with that decision, but perhaps they’d provide you with a copy if you contacted them directly.

  3. Maury Markowitz

    They won’t, I asked.

    As I understand it, legally they are pretty much obliged to give it out. As a US government body any work they produce is generally in the public domain automatically. If they don’t want to release it, they have to add classification, which would fail in this case given it’s online!

    Perhaps a FOIA request would get it, but who wants to do that?!

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