Definitions for tilt with 1-axis trackers may be ambiguous

[williamhobbs]

Describe the bug

From the help docs for both PVWatts and Detailed PV Model, it is not clear how tilt for 1-axis tracking systems is defined. Based on testing and assumptions, tilt appears to be defined as the angle measured sloping down towards the user-entered azimuth.

For example, if you wanted to model a North-South 1-axis tracker sloping down to the North (not ideal in the northern hemisphere, but maybe that's the natural slope of the land), I think you would have to enter an azimuth of 0 and then a tilt > 0. In that case, will the sign for tracker rotation angle in outputs be reversed? I think so, but only based on testing.

Expected behavior

More clear docs would be helpful.

Additional context

See pvlib/pvlib-python#1976 for some related discussion with pvlib.

[janinefreeman]

Hi @williamhobbs ! Based on my understanding of the code, your suspicions are correct that a North-South axis sloping to the north would be azimuth=0 and tilt>0. We can certainly update the docs to this effect (@cpaulgilman ). And yes, the tracker rotation angles output would be reversed, which I agree is confusing. Inside the code, we have to keep these tilt/azimuth/rotation conventions to make the math in the functions work out. Maybe there could be a warning that pops up in the (hopefully rare?) instance that a tracker in the Northern hemisphere is oriented between 181 and 89 degrees azimuth and has a tilt, explaining that tracker rotation angles would be reversed...? @cpaulgilman thoughts on that? Open to suggestions on that front.

[williamhobbs]

@janinefreeman, a rotation angle convention warning like you suggested makes sense to me. There's already a warning for non-zero tilt, I think.

Clear documentation seems like the key, here and for pvlib. And then maybe a nice description on the PVPMC website as well.

Thanks!