Add minimum GHI limit check from Nollas et al. (2023)

docs/api.rst

@@ -47,6 +47,14 @@ Irradiance measurements can also be checked for consistency.
 
    quality.irradiance.check_irradiance_consistency_qcrad
 
+The validity of daytime GHI and DHI measurements can also be checked using
+a more restrictive lower limit provided in [2]_.
+
+.. autosummary::
+   :toctree: generated/
+
+   quality.irradiance.check_ghi_lower_limit_nollas
+
 GHI and POA irradiance can be validated against clearsky values to
 eliminate data that is unrealistically high.
 
@@ -196,6 +204,12 @@ the quality check.
 .. [1]  C. N. Long and Y. Shi, An Automated Quality Assessment and Control
         Algorithm for Surface Radiation Measurements, The Open Atmospheric
         Science Journal 2, pp. 23-37, 2008.
+        :doi:`10.2174/1874282300802010023`
+.. [2] F. M. Nollas, G. A. Salazar, and C. A. Gueymard, Quality control
+       procedure for 1-minute pyranometric measurements of global and
+       shadowband-based diffuse solar irradiance, Renewable Energy, 202,
+       pp. 40-55, 2023.
+       :doi:`10.1016/j.renene.2022.11.056`
 
 Features
 ========

docs/whatsnew/0.2.0.rst

@@ -15,7 +15,8 @@ Breaking Changes
 
 Enhancements
 ~~~~~~~~~~~~
-
+* Add QC function for checking GHI lower limit according to Nollas et al. (2023).
+  :py:func:`~pvanalytics.quality.irradiance.check_ghi_lower_limit_nollas` (:pull:`174`)
 
 Bug Fixes
 ~~~~~~~~~
@@ -44,3 +45,4 @@ Contributors
 * Kevin Anderson (:ghuser:`kanderso-nrel`)
 * Cliff Hansen (:ghuser:`cwhanse`)
 * Abhishek Parikh (:ghuser:`abhisheksparikh`)
+* Adam R. Jensen (:ghuser:`AdamRJensen`)

pvanalytics/quality/irradiance.py

@@ -357,6 +357,83 @@ def check_irradiance_consistency_qcrad(solar_zenith, ghi, dhi, dni,
     return consistent_components, diffuse_ratio_limit
 
 
+def _ghi_lower_limit_nollas(solar_zenith):
+    r"""Calculate lower limit for GHI according to Nollas et al.
+
+    See [1]_ for further information.
+
+    .. math::
+        ghi_min = (6.5331 - 0.065502 * solar\_zenith + 0.00018312 * solar\_zenith^{2}) /
+                   (1 + 0.01113*solar\_zenith)
+
+    Parameters
+    ----------
+    solar_zenith : Series
+        Solar zenith angle in degrees
+
+    Returns
+    -------
+    Series
+        Minimum possible GHI in :math:`W/m^2`
+
+    References
+    ----------
+    .. [1] F. M. Nollas, G. A. Salazar, and C. A. Gueymard, Quality control
+       procedure for 1-minute pyranometric measurements of global and
+       shadowband-based diffuse solar irradiance, Renewable Energy, 202,
+       pp. 40-55, 2023.
+       :doi:`10.1016/j.renene.2022.11.056`
+    """  # noqa: E501
+    ghi_min = ((6.5331-0.065502*solar_zenith + 0.00018312*solar_zenith**2) /
+               (1 + 0.01113*solar_zenith))
+
+    # Set limit to nan when the sun is below the horizon
+    ghi_min[solar_zenith >= 90] = np.nan
+
+    return ghi_min
+
+
+def check_ghi_lower_limit_nollas(ghi, solar_zenith):
+    r"""Test for lower limit on GHI using empirical limit from Nollas (2023).
+
+    Test is applied to each GHI value. A GHI value passes if value >
+    lower bound. The lower bound is from [1]_ and calculated as:
+
+    .. math::
+        lb = (6.5331 - 0.065502 * solar\_zenith + 0.00018312 * solar\_zenith^{2}) /
+             (1 + 0.01113*solar\_zenith)
+
+    Parameters
+    ----------
+    ghi : Series
+        Global horizontal irradiance in :math:`W/m^2`
+    solar_zenith : Series
+        Solar zenith angle in degrees
+
+    Returns
+    -------
+    Series
+        False where valuez are below the minimum limit.
+
+    References
+    ----------
+    .. [1] F. M. Nollas, G. A. Salazar, and C. A. Gueymard, Quality control
+       procedure for 1-minute pyranometric measurements of global and
+       shadowband-based diffuse solar irradiance, Renewable Energy, 202,
+       pp. 40-55, 2023.
+       :doi:`10.1016/j.renene.2022.11.056`
+    """  # noqa: E501
+    ghi_lb = _ghi_lower_limit_nollas(solar_zenith)
+
+    ghi_lower_limit_flag = quality.util.check_limits(
+        ghi, lower_bound=ghi_lb, inclusive_lower=False)
+
+    # Set flags to True when ghi_lb is nan (e.g., when solar_zenith>=90)
+    ghi_lower_limit_flag[np.isnan(ghi_lb)] = True
+
+    return ghi_lower_limit_flag
+
+
 def clearsky_limits(measured, clearsky, csi_max=1.1):
     """Identify irradiance values which do not exceed clearsky values.
 

pvanalytics/tests/quality/test_irradiance.py

@@ -179,6 +179,26 @@ def test_check_irradiance_consistency_qcrad(irradiance_qcrad):
                         check_names=False)
 
 
+def test_ghi_lower_limit_nollas(irradiance_qcrad):
+    """Test thet minimum GHI is correctly calculated using Nollas equation."""
+    data = irradiance_qcrad
+    actual_ghi = irradiance._ghi_lower_limit_nollas(data['solar_zenith'])
+    expected_ghi = pd.Series([3.548128, 3.548128, 3.548128, 6.5331, 4.7917837,
+                              1.9559971, 1.3039082, np.nan, 6.5331, 6.5331,
+                              1.9559971, 1.3039082, 1.3039082, np.nan])
+    assert_series_equal(actual_ghi, expected_ghi, check_names=False)
+
+
+def test_check_ghi_lower_limit_nollas(irradiance_qcrad):
+    """Test that min GHI is checked correctly."""
+    data = irradiance_qcrad
+    actual_flags = irradiance.check_ghi_lower_limit_nollas(
+        data['ghi'], data['solar_zenith'])
+    expected_flags = pd.Series([0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
+                               dtype=bool)
+    assert_series_equal(actual_flags, expected_flags, check_names=False)
+
+
 @pytest.fixture
 def times():
     """One hour of times at 10 minute frequency.