Remove diffuse_fraction calculation from infinite_sheds (#2451)

* get rid of `diffuse_fraction`

* adjust test

* whatsnew

* lint

Author: kandersolar

docs/sphinx/source/whatsnew/v0.12.1.rst

@@ -30,6 +30,8 @@ Enhancements
   :py:func:`~pvlib.iotools.get_nsrdb_psm4_full_disc`,
   :py:func:`~pvlib.iotools.read_nsrdb_psm4`, and
   :py:func:`~pvlib.iotools.parse_nsrdb_psm4`. (:issue:`2326`, :pull:`2378`)
+* :py:mod:`pvlib.bifacial.infinite_sheds` no longer emits "invalid value" warnings
+  when supplying irradiance arrays with nighttime zero values. (:issue:`2450`, :pull:`2451`)
 
 Documentation
 ~~~~~~~~~~~~~

pvlib/bifacial/infinite_sheds.py

@@ -9,21 +9,23 @@
 from pvlib.irradiance import beam_component, aoi, haydavies
 
 
-def _poa_ground_shadows(poa_ground, f_gnd_beam, df, vf_gnd_sky):
+def _poa_ground_shadows(ghi, dhi, albedo, f_gnd_beam, vf_gnd_sky):
     """
     Reduce ground-reflected irradiance to the tilted plane (poa_ground) to
     account for shadows on the ground.
 
     Parameters
     ----------
-    poa_ground : numeric
-        Ground reflected irradiance on the tilted surface, assuming full GHI
-        illumination on all of the ground. [W/m^2]
+    ghi : numeric
+        Global horizontal irradiance, with no adjustments. [W/m^2]
+    dhi : numeric
+        Diffuse horizontal irradiance, with no adjustments. [W/m^2]
+    albedo : numeric
+        Ground albedo, the ratio of reflected to incident irradiance of the
+        ground surface. [W/m^2]
     f_gnd_beam : numeric
         Fraction of the distance between rows that is illuminated (unshaded).
         [unitless]
-    df : numeric
-        Diffuse fraction, the ratio of DHI to GHI. [unitless]
     vf_gnd_sky : numeric
         View factor from the ground to the sky, integrated along the distance
         between rows. [unitless]
@@ -35,7 +37,7 @@ def _poa_ground_shadows(poa_ground, f_gnd_beam, df, vf_gnd_sky):
         ground. [W/m^2]
 
     """
-    return poa_ground * (f_gnd_beam*(1 - df) + df*vf_gnd_sky)
+    return albedo * (f_gnd_beam * (ghi - dhi) + vf_gnd_sky * dhi)
 
 
 def _poa_sky_diffuse_pv(dhi, gcr, surface_tilt):
@@ -339,18 +341,11 @@ def get_irradiance_poa(surface_tilt, surface_azimuth, solar_zenith,
     # and restricted views
     # this is a deviation from [1], because the row to ground view factor
     # is accounted for in a different manner
-    ground_diffuse = ghi * albedo
-
-    # diffuse fraction
-    diffuse_fraction = np.clip(dhi / ghi, 0., 1.)
-    # make diffuse fraction 0 when ghi is small
-    diffuse_fraction = np.where(ghi < 0.0001, 0., diffuse_fraction)
-
     # Reduce ground-reflected irradiance because other rows in the array
     # block irradiance from reaching the ground.
     # [2], Eq. 9
-    ground_diffuse = _poa_ground_shadows(
-        ground_diffuse, f_gnd_beam, diffuse_fraction, vf_gnd_sky)
+    ground_diffuse = _poa_ground_shadows(ghi, dhi, albedo, f_gnd_beam,
+                                         vf_gnd_sky)
 
     # Ground-reflected irradiance on the row surface accounting for
     # the view to the ground. This deviates from [1], Eq. 10, 11 and

tests/bifacial/test_infinite_sheds.py

@@ -42,19 +42,20 @@ def test_system():
 
 
 def test__poa_ground_shadows():
-    poa_ground, f_gnd_beam, df, vf_gnd_sky = (300., 0.5, 0.5, 0.2)
-    result = infinite_sheds._poa_ground_shadows(
-        poa_ground, f_gnd_beam, df, vf_gnd_sky)
-    expected = 300. * (0.5 * 0.5 + 0.5 * 0.2)
+    ghi, dhi, albedo, f_gnd_beam, vf_gnd_sky = (300., 100, 0.3, 0.5, 0.2)
+    result = infinite_sheds._poa_ground_shadows(ghi, dhi, albedo, f_gnd_beam,
+                                                vf_gnd_sky)
+
+    expected = 0.3 * (200 * 0.5 + 100 * 0.2)
     assert np.isclose(result, expected)
     # vector inputs
-    poa_ground = np.array([300., 300.])
-    f_gnd_beam = np.array([0.5, 0.5])
-    df = np.array([0.5, 0.])
-    vf_gnd_sky = np.array([0.2, 0.2])
-    result = infinite_sheds._poa_ground_shadows(
-        poa_ground, f_gnd_beam, df, vf_gnd_sky)
-    expected_vec = np.array([expected, 300. * 0.5])
+    ghi = np.array([ghi, ghi])
+    dhi = np.array([dhi, 0])
+    f_gnd_beam = np.array([f_gnd_beam, f_gnd_beam])
+    vf_gnd_sky = np.array([vf_gnd_sky, vf_gnd_sky])
+    result = infinite_sheds._poa_ground_shadows(ghi, dhi, albedo, f_gnd_beam,
+                                                vf_gnd_sky)
+    expected_vec = np.array([expected, 300. * 0.5 * 0.3])
     assert np.allclose(result, expected_vec)