Add support for return_components to irradiance.isotropic

pvlib/irradiance.py

@@ -595,7 +595,7 @@ def get_ground_diffuse(surface_tilt, ghi, albedo=.25, surface_type=None):
     return diffuse_irrad
 
 
-def isotropic(surface_tilt, dhi):
+def isotropic(surface_tilt, dhi, return_components=False):
     r'''
     Determine diffuse irradiance from the sky on a tilted surface using
     the isotropic sky model.
@@ -619,11 +619,27 @@ def isotropic(surface_tilt, dhi):
     dhi : numeric
         Diffuse horizontal irradiance, must be >=0. See :term:`dhi`.
 
+    return_components : bool, default `False`
+        If `False`, ``sky_diffuse`` is returned.
+        If `True`, ``diffuse_components`` is returned.
+        For this model, return_components does not add more information,
+        but it is included for consistency with the other sky diffuse models.
+
     Returns
     -------
-    diffuse : numeric
+    numeric, Dict, or DataFrame
+        Return type controlled by ``return_components`` argument.
+        If `False`, ``sky_diffuse`` is returned.
+        If `True`, ``diffuse_components`` is returned.
+
+    sky_diffuse : numeric
         The sky diffuse component of the solar radiation. [Wm⁻²]
 
+    diffuse_components : Dict (array input) or DataFrame (Series input)
+        Keys/columns are:
+            * poa_sky_diffuse: Total sky diffuse
+            * poa_isotropic
+
     References
     ----------
     .. [1] Loutzenhiser P.G. et al. "Empirical validation of models to
@@ -638,7 +654,18 @@ def isotropic(surface_tilt, dhi):
     '''
     sky_diffuse = dhi * (1 + tools.cosd(surface_tilt)) * 0.5
 
-    return sky_diffuse
+    if return_components:
+        diffuse_components = {
+            'poa_sky_diffuse': sky_diffuse,
+            'poa_isotropic': sky_diffuse
+        }
+
+        if isinstance(sky_diffuse, pd.Series):
+            diffuse_components = pd.DataFrame(diffuse_components)
+
+        return diffuse_components
+    else:
+        return sky_diffuse
 
 
 def klucher(surface_tilt, surface_azimuth, dhi, ghi, solar_zenith,

tests/test_irradiance.py

@@ -168,6 +168,32 @@ def test_isotropic_series(irrad_data):
     assert_allclose(result, [0, 35.728402, 104.601328, 54.777191], atol=1e-4)
 
 
+def test_isotropic_components(irrad_data):
+    keys = ['poa_sky_diffuse', 'poa_isotropic']
+    expected = pd.DataFrame(np.array(
+        [[0, 35.728402, 104.601328, 54.777191],
+         [0, 35.728402, 104.601328, 54.777191]]).T,
+        columns=keys,
+        index=irrad_data.index
+    )
+    # pandas
+    result = irradiance.isotropic(
+        40, irrad_data['dhi'], return_components=True)
+    assert_frame_equal(result, expected, check_less_precise=4)
+    # numpy
+    result = irradiance.isotropic(
+        40, irrad_data['dhi'].values, return_components=True)
+    for key in keys:
+        assert_allclose(result[key], expected[key], atol=1e-4)
+    assert isinstance(result, dict)
+    # scalar
+    result = irradiance.isotropic(
+        40, irrad_data['dhi'].values[-1], return_components=True)
+    for key in keys:
+        assert_allclose(result[key], expected[key].iloc[-1], atol=1e-4)
+    assert isinstance(result, dict)
+
+
 def test_klucher_series_float():
     # klucher inputs
     surface_tilt, surface_azimuth = 40.0, 180.0