Tentative fix for #125 (#128)

docs/sphinx/whatsnew/v1.5.2.rst

@@ -16,9 +16,11 @@ Fixes
   output moving forward, the pvlib dependency is updated from ``pvlib>=0.7.0,<0.9.0`` to
   ``pvlib>=0.9.0,<0.10.0``.  This will likely change the results of irradiance simulations.
   According to the
-  [pvlib release notes](https://pvlib-python.readthedocs.io/en/v0.9.0/whatsnew.html#bug-fixes),
+  `pvlib release notes <https://pvlib-python.readthedocs.io/en/v0.9.0/whatsnew.html#bug-fixes>`_,
   the differences are "expected to be small and primarily occur at low irradiance conditions".
   (:ghpull:`121`)
+* Fixed a bug that affected some irradiance simulations when `surface_tilt` is exactly zero.
+  See `GH #125 <https://github.com/SunPower/pvfactors/issues/125>`_ for details. (:ghpull:`128`)
 
 
 Contributors

pvfactors/geometry/base.py

@@ -122,7 +122,7 @@ def _get_rotation_from_tilt_azimuth(surface_azimuth, axis_azimuth, tilt):
     # Calculate rotation of PV row (signed tilt angle)
     is_pointing_right = ((surface_azimuth - axis_azimuth) % 360.) > 180.
     rotation = np.where(is_pointing_right, tilt, -tilt)
-
+    rotation[tilt == 0] = -0.0  # GH 125
     return rotation
 
 

pvfactors/tests/test_engine.py

@@ -562,6 +562,47 @@ def test_check_direct_shading_continuity():
     np.testing.assert_allclose(out, expected_out)
 
 
+def test_check_tilt_zero_discontinuity():
+    """
+    Before version 1.5.2, surface_tilt=0 with certain combinations of
+    surface_azimuth and axis_azimuth showed anomolous behavior where
+    the irradiance at zero tilt was significantly different from the
+    irradiance at very small but nonzero tilts.  Additionally, the
+    calculated VF matrix could have values outside [0, 1].  See GH #125
+    """
+    # expected value calculated for surface_tilt=0.001, so should
+    # not be significantly different from result for surface_tilt=0
+    rear_qinc_expected = 76.10
+
+    timestamps = np.array([dt.datetime(2019, 6, 1, 10)])
+    solar_azimuth = np.array([135])
+    solar_zenith = np.array([45])
+    dni = np.array([200])
+    dhi = np.array([400])
+    albedo = np.array([0.2])
+    surface_tilt = np.array([0.0])
+
+    # the discontinuity did not occur for all combinations of
+    # (surface_azimuth, axis_azimuth), so test all four "primary" pairs:
+    for surface_azimuth in [90, 270]:
+        surface_azimuth = np.array([surface_azimuth])
+        for axis_azimuth in [0, 180]:
+            params = dict(n_pvrows=3, axis_azimuth=axis_azimuth,
+                          pvrow_height=2, pvrow_width=1, gcr=0.4)
+
+            pvarray = OrderedPVArray.init_from_dict(params)
+            eng = PVEngine(pvarray)
+            eng.fit(timestamps, dni, dhi, solar_zenith, solar_azimuth,
+                    surface_tilt, surface_azimuth, albedo)
+
+            # Run simulation and get output
+            eng.run_full_mode()
+            out = pvarray.ts_pvrows[1].back.get_param_weighted('qinc')
+            assert np.all(pvarray.ts_vf_matrix >= 0)
+            assert np.all(pvarray.ts_vf_matrix <= 1)
+            assert rear_qinc_expected == pytest.approx(out[0], abs=1e-2)
+
+
 def test_create_engine_with_rho_init(params, pvmodule_canadian):
     """Check that can create PV engine with rho initialization
     from faoi functions"""