Better handling of missing inputs in get_total_irradiance, get_sky_diffuse (#1225)

* edits to get_total_irradiance and get_sky_diffuse

* add tests

* whatsnew

* improvements from review

* spell Tucson

* overlooked comments

Author: cwhanse

docs/sphinx/source/whatsnew/v0.9.0.rst

@@ -125,6 +125,10 @@ Enhancements
 * Added :py:func:`~pvlib.ivtools.sdm.pvsyst_temperature_coeff` to calculate
   the temperature coefficient of power for the pvsyst module model.
    (:pull:`1190`)
+* :py:func:`~pvlib.irradiance.get_total_irradiance` and
+  :py:func:`~pvlib.irradiance.get_sky_diffuse` now fill in ``airmass``
+  if required and not provided. These functions now raise a ``ValueError``
+  if ``dni_extra`` is required and not provided. (:issue:`949`, :pull:`1225`)
 
 Bug fixes
 ~~~~~~~~~

pvlib/irradiance.py

@@ -14,7 +14,7 @@
 from pvlib import atmosphere, solarposition, tools
 
 
-# see References section of grounddiffuse function
+# see References section of get_ground_diffuse function
 SURFACE_ALBEDOS = {'urban': 0.18,
                    'grass': 0.20,
                    'fresh grass': 0.26,
@@ -326,38 +326,51 @@ def get_total_irradiance(surface_tilt, surface_azimuth,
     Parameters
     ----------
     surface_tilt : numeric
-        Panel tilt from horizontal.
+        Panel tilt from horizontal. [degree]
     surface_azimuth : numeric
-        Panel azimuth from north.
+        Panel azimuth from north. [degree]
     solar_zenith : numeric
-        Solar zenith angle.
+        Solar zenith angle. [degree]
     solar_azimuth : numeric
-        Solar azimuth angle.
+        Solar azimuth angle. [degree]
     dni : numeric
-        Direct Normal Irradiance
+        Direct Normal Irradiance. [W/m2]
     ghi : numeric
-        Global horizontal irradiance
+        Global horizontal irradiance. [W/m2]
     dhi : numeric
-        Diffuse horizontal irradiance
+        Diffuse horizontal irradiance. [W/m2]
     dni_extra : None or numeric, default None
-        Extraterrestrial direct normal irradiance
+        Extraterrestrial direct normal irradiance. [W/m2]
     airmass : None or numeric, default None
-        Airmass
+        Relative airmass (not adjusted for pressure). [unitless]
     albedo : numeric, default 0.25
-        Surface albedo
-    surface_type : None or String, default None
-        Surface type. See grounddiffuse.
-    model : String, default 'isotropic'
-        Irradiance model.
-    model_perez : String, default 'allsitescomposite1990'
-        Used only if model='perez'. See :py:func:`perez`.
+        Surface albedo. [unitless]
+    surface_type : None or str, default None
+        Surface type. See :py:func:`~pvlib.irradiance.get_ground_diffuse` for
+        the list of accepted values.
+    model : str, default 'isotropic'
+        Irradiance model. Can be one of ``'isotropic'``, ``'klucher'``,
+        ``'haydavies'``, ``'reindl'``, ``'king'``, ``'perez'``.
+    model_perez : str, default 'allsitescomposite1990'
+        Used only if ``model='perez'``. See :py:func:`~pvlib.irradiance.perez`.
 
     Returns
     -------
     total_irrad : OrderedDict or DataFrame
         Contains keys/columns ``'poa_global', 'poa_direct', 'poa_diffuse',
         'poa_sky_diffuse', 'poa_ground_diffuse'``.
+
+    Notes
+    -----
+    Models ``'haydavies'``, ``'reindl'``, or ``'perez'`` require
+    ``'dni_extra'``. Values can be calculated using
+    :py:func:`~pvlib.irradiance.get_extra_radiation`.
+
+    The ``'perez'`` model requires relative airmass (``airmass``) as input. If
+    ``airmass`` is not provided, it is calculated using the defaults in
+    :py:func:`~pvlib.atmosphere.get_relative_airmass`.
     """
+
     poa_sky_diffuse = get_sky_diffuse(
         surface_tilt, surface_azimuth, solar_zenith, solar_azimuth,
         dni, ghi, dhi, dni_extra=dni_extra, airmass=airmass, model=model,
@@ -390,34 +403,56 @@ def get_sky_diffuse(surface_tilt, surface_azimuth,
     Parameters
     ----------
     surface_tilt : numeric
-        Panel tilt from horizontal.
+        Panel tilt from horizontal. [degree]
     surface_azimuth : numeric
-        Panel azimuth from north.
+        Panel azimuth from north. [degree]
     solar_zenith : numeric
-        Solar zenith angle.
+        Solar zenith angle. [degree]
     solar_azimuth : numeric
-        Solar azimuth angle.
+        Solar azimuth angle. [degree]
     dni : numeric
-        Direct Normal Irradiance
+        Direct Normal Irradiance. [W/m2]
     ghi : numeric
-        Global horizontal irradiance
+        Global horizontal irradiance. [W/m2]
     dhi : numeric
-        Diffuse horizontal irradiance
+        Diffuse horizontal irradiance. [W/m2]
     dni_extra : None or numeric, default None
-        Extraterrestrial direct normal irradiance
+        Extraterrestrial direct normal irradiance. [W/m2]
     airmass : None or numeric, default None
-        Airmass
-    model : String, default 'isotropic'
-        Irradiance model.
-    model_perez : String, default 'allsitescomposite1990'
-        See perez.
+        Relative airmass (not adjusted for pressure). [unitless]
+    model : str, default 'isotropic'
+        Irradiance model. Can be one of ``'isotropic'``, ``'klucher'``,
+        ``'haydavies'``, ``'reindl'``, ``'king'``, ``'perez'``.
+    model_perez : str, default 'allsitescomposite1990'
+        Used only if ``model='perez'``. See :py:func:`~pvlib.irradiance.perez`.
 
     Returns
     -------
     poa_sky_diffuse : numeric
+        Sky diffuse irradiance in the plane of array. [W/m2]
+
+    Raises
+    ------
+    ValueError
+        If model is one of ``'haydavies'``, ``'reindl'``, or ``'perez'`` and
+        ``dni_extra`` is ``None``.
+
+    Notes
+    -----
+    Models ``'haydavies'``, ``'reindl'``, and ``'perez``` require 'dni_extra'.
+    Values can be calculated using
+    :py:func:`~pvlib.irradiance.get_extra_radiation`.
+
+    The ``'perez'`` model requires relative airmass (``airmass``) as input. If
+    ``airmass`` is not provided, it is calculated using the defaults in
+    :py:func:`~pvlib.atmosphere.get_relative_airmass`.
     """
 
     model = model.lower()
+
+    if (model in {'haydavies', 'reindl', 'perez'}) and (dni_extra is None):
+        raise ValueError(f'dni_extra is required for model {model}')
+
     if model == 'isotropic':
         sky = isotropic(surface_tilt, dhi)
     elif model == 'klucher':
@@ -432,6 +467,8 @@ def get_sky_diffuse(surface_tilt, surface_azimuth,
     elif model == 'king':
         sky = king(surface_tilt, dhi, ghi, solar_zenith)
     elif model == 'perez':
+        if airmass is None:
+            airmass = atmosphere.get_relative_airmass(solar_zenith)
         sky = perez(surface_tilt, surface_azimuth, dhi, dni, dni_extra,
                     solar_zenith, solar_azimuth, airmass,
                     model=model_perez)
@@ -504,7 +541,7 @@ def poa_components(aoi, dni, poa_sky_diffuse, poa_ground_diffuse):
 def get_ground_diffuse(surface_tilt, ghi, albedo=.25, surface_type=None):
     '''
     Estimate diffuse irradiance from ground reflections given
-    irradiance, albedo, and surface tilt
+    irradiance, albedo, and surface tilt.
 
     Function to determine the portion of irradiance on a tilted surface
     due to ground reflections. Any of the inputs may be DataFrames or
@@ -518,7 +555,7 @@ def get_ground_diffuse(surface_tilt, ghi, albedo=.25, surface_type=None):
         (e.g. surface facing up = 0, surface facing horizon = 90).
 
     ghi : numeric
-        Global horizontal irradiance in W/m^2.
+        Global horizontal irradiance. [W/m^2]
 
     albedo : numeric, default 0.25
         Ground reflectance, typically 0.1-0.4 for surfaces on Earth
@@ -534,7 +571,7 @@ def get_ground_diffuse(surface_tilt, ghi, albedo=.25, surface_type=None):
     Returns
     -------
     grounddiffuse : numeric
-        Ground reflected irradiances in W/m^2.
+        Ground reflected irradiance. [W/m^2]
 
 
     References

pvlib/tests/test_irradiance.py

@@ -281,13 +281,35 @@ def test_sky_diffuse_zenith_close_to_90(model):
     assert sky_diffuse < 100
 
 
-def test_get_sky_diffuse_invalid():
+def test_get_sky_diffuse_model_invalid():
     with pytest.raises(ValueError):
         irradiance.get_sky_diffuse(
             30, 180, 0, 180, 1000, 1100, 100, dni_extra=1360, airmass=1,
             model='invalid')
 
 
+def test_get_sky_diffuse_missing_dni_extra():
+    msg = 'dni_extra is required'
+    with pytest.raises(ValueError, match=msg):
+        irradiance.get_sky_diffuse(
+            30, 180, 0, 180, 1000, 1100, 100, airmass=1,
+            model='haydavies')
+
+
+def test_get_sky_diffuse_missing_airmass(irrad_data, ephem_data, dni_et):
+    # test assumes location is Tucson, AZ
+    # calculated airmass should be the equivalent to fixture airmass
+    dni = irrad_data['dni'].copy()
+    dni.iloc[2] = np.nan
+    out = irradiance.get_sky_diffuse(
+        40, 180, ephem_data['apparent_zenith'], ephem_data['azimuth'], dni,
+        irrad_data['ghi'], irrad_data['dhi'], dni_et,  model='perez')
+    expected = pd.Series(np.array(
+        [0., 31.46046871, np.nan, 45.45539877]),
+        index=irrad_data.index)
+    assert_series_equal(out, expected, check_less_precise=2)
+
+
 def test_campbell_norman():
     expected = pd.DataFrame(np.array(
         [[863.859736967, 653.123094076, 220.65905025]]),
@@ -299,7 +321,8 @@ def test_campbell_norman():
     assert_frame_equal(out, expected)
 
 
-def test_get_total_irradiance(irrad_data, ephem_data, dni_et, relative_airmass):
+def test_get_total_irradiance(irrad_data, ephem_data, dni_et,
+                              relative_airmass):
     models = ['isotropic', 'klucher',
               'haydavies', 'reindl', 'king', 'perez']
 
@@ -337,6 +360,30 @@ def test_get_total_irradiance_scalars(model):
     assert np.isnan(np.array(list(total.values()))).sum() == 0
 
 
+def test_get_total_irradiance_missing_dni_extra():
+    msg = 'dni_extra is required'
+    with pytest.raises(ValueError, match=msg):
+        irradiance.get_total_irradiance(
+            32, 180,
+            10, 180,
+            dni=1000, ghi=1100,
+            dhi=100,
+            model='haydavies')
+
+
+def test_get_total_irradiance_missing_airmass():
+    total = irradiance.get_total_irradiance(
+        32, 180,
+        10, 180,
+        dni=1000, ghi=1100,
+        dhi=100,
+        dni_extra=1400,
+        model='perez')
+    assert list(total.keys()) == ['poa_global', 'poa_direct',
+                                  'poa_diffuse', 'poa_sky_diffuse',
+                                  'poa_ground_diffuse']
+
+
 def test_poa_components(irrad_data, ephem_data, dni_et, relative_airmass):
     aoi = irradiance.aoi(40, 180, ephem_data['apparent_zenith'],
                          ephem_data['azimuth'])