add orgill hollands decomposition

docs/sphinx/source/reference/irradiance/decomposition.rst

@@ -12,6 +12,7 @@ DNI estimation models
    irradiance.dirint
    irradiance.dirindex
    irradiance.erbs
+   irradiance.orgill_hollands
    irradiance.boland
    irradiance.campbell_norman
    irradiance.gti_dirint

pvlib/irradiance.py

@@ -2227,6 +2227,8 @@ def erbs(ghi, zenith, datetime_or_doy, min_cos_zenith=0.065, max_zenith=87):
     --------
     dirint
     disc
+    orgill_hollands
+    boland
     """
 
     dni_extra = get_extra_radiation(datetime_or_doy)
@@ -2265,6 +2267,93 @@ def erbs(ghi, zenith, datetime_or_doy, min_cos_zenith=0.065, max_zenith=87):
     return data
 
 
+def orgill_hollands(ghi, zenith, datetime_or_doy, dni_extra=None,
+                    min_cos_zenith=0.065, max_zenith=87):
+    """Estimate DNI and DHI from GHI using the Orgill and Hollands model.
+
+    The Orgill and Hollands model [1]_ estimates the diffuse fraction DF from
+    global horizontal irradiance through an empirical relationship between
+    hourly DF observations (in Toronto, Canada) and the ratio of GHI to
+    extraterrestrial irradiance, Kt.
+
+    Parameters
+    ----------
+    ghi: numeric
+        Global horizontal irradiance in W/m^2.
+    zenith: numeric
+        True (not refraction-corrected) zenith angles in decimal degrees.
+    datetime_or_doy : int, float, array, pd.DatetimeIndex
+        Day of year or array of days of year e.g.
+        pd.DatetimeIndex.dayofyear, or pd.DatetimeIndex.
+    dni_extra : None or numeric, default None
+        Extraterrestrial direct normal irradiance. [W/m2]
+    min_cos_zenith : numeric, default 0.065
+        Minimum value of cos(zenith) to allow when calculating global
+        clearness index `kt`. Equivalent to zenith = 86.273 degrees.
+    max_zenith : numeric, default 87
+        Maximum value of zenith to allow in DNI calculation. DNI will be
+        set to 0 for times with zenith values greater than `max_zenith`.
+
+    Returns
+    -------
+    data : OrderedDict or DataFrame
+        Contains the following keys/columns:
+
+            * ``dni``: the modeled direct normal irradiance in W/m^2.
+            * ``dhi``: the modeled diffuse horizontal irradiance in
+              W/m^2.
+            * ``kt``: Ratio of global to extraterrestrial irradiance
+              on a horizontal plane.
+
+    References
+    ----------
+    .. [1] Orgill, J.F., Hollands, K.G.T., Correlation equation for hourly
+      diffuse radiation on a horizontal surface, Solar Energy 19(4), pp 357–359,
+      1977. Eqs. 3(a), 3(b) and 3(c)
+      :doi:`10.1016/0038-092X(77)90006-8`
+
+    See Also
+    --------
+    dirint
+    disc
+    erbs
+    boland
+    """
+    if dni_extra is None:
+        dni_extra = get_extra_radiation(datetime_or_doy)
+
+    kt = clearness_index(ghi, zenith, dni_extra, min_cos_zenith=min_cos_zenith,
+                         max_clearness_index=1)
+
+    # For Kt < 0.35, set the diffuse fraction
+    df = 1 - 0.249*kt
+
+    # For Kt >= 0.35 and Kt <= 0.75, set the diffuse fraction
+    df = np.where((kt >= 0.35) & (kt <= 0.75),
+                  1.557 - 1.84*kt, df)
+
+    # For Kt > 0.75, set the diffuse fraction
+    df = np.where(kt > 0.75, 0.177, df)
+
+    dhi = df * ghi
+
+    dni = (ghi - dhi) / tools.cosd(zenith)
+    bad_values = (zenith > max_zenith) | (ghi < 0) | (dni < 0)
+    dni = np.where(bad_values, 0, dni)
+    # ensure that closure relationship remains valid
+    dhi = np.where(bad_values, ghi, dhi)
+
+    data = OrderedDict()
+    data['dni'] = dni
+    data['dhi'] = dhi
+    data['kt'] = kt
+
+    if isinstance(datetime_or_doy, pd.DatetimeIndex):
+        data = pd.DataFrame(data, index=datetime_or_doy)
+
+    return data
+
+
 def boland(ghi, solar_zenith, datetime_or_doy, a_coeff=8.645, b_coeff=0.613,
            min_cos_zenith=0.065, max_zenith=87):
     r"""
@@ -2326,6 +2415,7 @@ def boland(ghi, solar_zenith, datetime_or_doy, a_coeff=8.645, b_coeff=0.613,
     dirint
     disc
     erbs
+    orgill_hollands
 
     Notes
     -----

pvlib/tests/test_irradiance.py

@@ -821,6 +821,22 @@ def test_boland():
     assert np.allclose(out, expected)
 
 
+def test_orgill_hollands():
+    index = pd.DatetimeIndex(['20190101']*3 + ['20190620'])
+    ghi = pd.Series([0, 50, 1000, 1000], index=index)
+    zenith = pd.Series([120, 85, 10, 10], index=index)
+    expected = pd.DataFrame(np.array(
+        [[0.0,        0.0,        0.0],
+         [108.731366, 40.5234370, 0.405723511],
+         [776.155771, 235.635779, 0.718132729],
+         [835.696102, 177.000000, 0.768214312]]),
+        columns=['dni', 'dhi', 'kt'], index=index)
+
+    out = irradiance.orgill_hollands(ghi, zenith, index)
+
+    assert np.allclose(out, expected)
+
+
 def test_erbs_min_cos_zenith_max_zenith():
     # map out behavior under difficult conditions with various
     # limiting kwargs settings