Sign (negative) ignored for 1 axis tilt

[williamhobbs]

Describe the bug

If a user assigns a negative tilt for a 1 axis tracking system it appears to be treated as a positive tilt.

To Reproduce

Steps to reproduce the behavior:

1. Start a detailed PV model
2. In System Design, select "1 Axis" and assign a negative tilt value (e.g., -10)
3. Simulate
4. See the notice that describes the tilt as positive
5. Compare results (energy, surface tilt and surface azimuth) with a similar case using a positive tilt and see that results are identical.

Expected behavior

One of the following, or similar:

• Issue an error message about negative values being invalid
• Issue a notice message about negative values automatically being converted to positive
• Model the system with the described geometry but with a notice (e.g., model tilt = -10 and azimuth =180 as tilt = 10 and azimuth = 0)

Screenshots

Notice that occurs when assigning a negative tilt (note that tilt angle in the notice is positive):

Results with tilt = 10, azimuth = 180:

Results with tilt = -10, azimuth = 180 (same as above, not what I expected):

Results with tilt = 10, azimuth = 0 (what I expected -10, 180 to look like):

Inputs browser for reference:

Operating System and Version

• OS: Windows
• Version: SAM 2021.12.2 r2 [e.g. SAM 2020.2.29 r3]

Tagging @kanderso-nrel and @kurt-rhee, since they brought up this issue here pvlib/pvlib-python#1471.

[kandersolar]

Thanks @williamhobbs, I was just looking into this myself. I believe this is the relevant line of code in SSC: https://github.com/NREL/ssc/blob/1ed56222b4f4a5991eaef697dac2af7a929401c3/shared/lib_pv_io_manager.cpp#L339

        if (cm->is_assigned(prefix + "tilt")) tiltDegrees = fabs(cm->as_double(prefix + "tilt"));

As far as "expected behavior", I'd favor the first of @williamhobbs's suggestions (Issue an error message about negative values being invalid). This would also be consistent with the constraint "MIN=0,MAX=90" listed in _cm_vtab_pvsamv1 for subarrayN_tilt.

[williamhobbs]

The error message is also my preferred solution. Changing user inputs to give them either result, even with a notice, seems problematic.