Add 2025 coefficients for Huld model (#2486)

* Enhance Huld model with EU JRC coefficients

- Introduced a new function to infer updated coefficients for the Huld model based on EU JRC research.
- Added a parameter to the existing Huld function to optionally use these updated coefficients.
- Updated documentation to reflect the new functionality and included references to the EU JRC paper.
- Added tests to verify the implementation and ensure compatibility with existing functionality.

(cherry picked from commit f0ba338)

* Refactor Huld model tests for EU JRC coefficients

- Updated the test for the Huld model to use non-reference values for irradiance and temperature.
- Enhanced the test to verify that results differ for all supported cell types when using EU JRC coefficients.
- Added checks to ensure all cell types are supported and that a KeyError is raised for invalid cell types.

(cherry picked from commit 7c0feba)

* Fix Flake8 linter errors

remove trailing whitespace and break long lines

* Fix Flake8 errors:

remove whitespace from blank lines and break long lines

- Removed trailing whitespace from blank lines in pvlib/pvarray.py
- Broke long lines in function definitions and assertions to comply with line length limits
- Reformatted comments and code in tests/test_pvarray.py for Flake8 compliance

* restructure lookup and test

* fix test and formatting

* adjust error message

* add whatsnew

* Apply suggestions from code review

Co-authored-by: Anton Driesse <anton.driesse@pvperformancelabs.com>

* Update pvlib/pvarray.py

Co-authored-by: Echedey Luis <80125792+echedey-ls@users.noreply.github.com>

* Update pvlib/pvarray.py

* Update pvlib/pvarray.py

* correct coefficients for 2025 and references

* fix docstring

* change parameter key to pvgis5, etc.

* error message

* lint

* fix test

* Apply suggestions from code review

---------

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
Co-authored-by: Anton Driesse <anton.driesse@pvperformancelabs.com>
Co-authored-by: Echedey Luis <80125792+echedey-ls@users.noreply.github.com>
Co-authored-by: Kevin Anderson <kevin.anderso@gmail.com>

Author: 5 people

docs/sphinx/source/whatsnew/v0.13.1.rst

@@ -21,6 +21,7 @@ Bug fixes
 
 Enhancements
 ~~~~~~~~~~~~
+* Add new parameters for the Huld PV array mode :py:func:`~pvlib.pvarray.huld` (:issue:`2461`, :pull:`2486`)
 * Add k coefficient in :py:func:`~pvlib.temperature.ross`
   (:issue:`2506`, :pull:`2521`)
 * Add iotools functions to retrieve irradiance and weather data from Meteonorm:
@@ -73,6 +74,9 @@ Maintenance
 Contributors
 ~~~~~~~~~~~~
 * Elijah Passmore (:ghuser:`eljpsm`)
+* Omar Bahamida (:ghuser:`OmarBahamida`)
+* Cliff Hansen (:ghuser:`cwhanse`)
+
 * Ioannis Sifnaios (:ghuser:`IoannisSifnaios`)
 * Rajiv Daxini (:ghuser:`RDaxini`)
 * Omar Bahamida (:ghuser:`OmarBahamida`)

pvlib/pvarray.py

@@ -37,7 +37,7 @@ def pvefficiency_adr(effective_irradiance, temp_cell,
         the reference conditions. [unitless]
 
     k_d : numeric, negative
-        “Dark irradiance” or diode coefficient which influences the voltage
+        "Dark irradiance" or diode coefficient which influences the voltage
         increase with irradiance. [unitless]
 
     tc_d : numeric
@@ -225,24 +225,55 @@ def adr_wrapper(xdata, *params):
         return popt
 
 
-def _infer_k_huld(cell_type, pdc0):
+def _infer_k_huld(cell_type, pdc0, k_version):
+    r"""
+    Get the EU JRC updated coefficients for the Huld model.
+
+    Parameters
+    ----------
+    cell_type : str
+        Must be one of 'csi', 'cis', or 'cdte'
+    pdc0 : numeric
+        Power of the modules at reference conditions [W]
+    k_version : str
+        Either 'pvgis5' or 'pvgis6'.
+
+    Returns
+    -------
+    tuple
+        The six coefficients (k1-k6) for the Huld model, scaled by pdc0
+    """
     # from PVGIS documentation, "PVGIS data sources & calculation methods",
     # Section 5.2.3, accessed 12/22/2023
     # The parameters in PVGIS' documentation are for a version of Huld's
     # equation that has factored Pdc0 out of the polynomial:
     #  P = G/1000 * Pdc0 * (1 + k1 log(Geff) + ...) so these parameters are
     # multiplied by pdc0
-    huld_params = {'csi': (-0.017237, -0.040465, -0.004702, 0.000149,
-                           0.000170, 0.000005),
-                   'cis': (-0.005554, -0.038724, -0.003723, -0.000905,
-                           -0.001256, 0.000001),
-                   'cdte': (-0.046689, -0.072844, -0.002262, 0.000276,
-                            0.000159, -0.000006)}
+    if k_version.lower() == 'pvgis5':
+        # coefficients from PVGIS webpage
+        huld_params = {'csi': (-0.017237, -0.040465, -0.004702, 0.000149,
+                               0.000170, 0.000005),
+                       'cis': (-0.005554, -0.038724, -0.003723, -0.000905,
+                               -0.001256, 0.000001),
+                       'cdte': (-0.046689, -0.072844, -0.002262, 0.000276,
+                                0.000159, -0.000006)}
+    elif k_version.lower() == 'pvgis6':
+        # Coefficients from EU JRC paper
+        huld_params = {'csi': (-0.0067560, -0.016444, -0.003015, -0.000045,
+                               -0.000043, 0.0),
+                       'cis': (-0.011001, -0.029734, -0.002887, 0.000217,
+                               -0.000163, 0.0),
+                       'cdte': (-0.020644, -0.035136, -0.003406, 0.000073,
+                                -0.000141, 0.000002)}
+    else:
+        raise ValueError(f'Invalid k_version={k_version}: must be either '
+                         '"pvgis5" or "pvgis6"')
     k = tuple([x*pdc0 for x in huld_params[cell_type.lower()]])
     return k
 
 
-def huld(effective_irradiance, temp_mod, pdc0, k=None, cell_type=None):
+def huld(effective_irradiance, temp_mod, pdc0, k=None, cell_type=None,
+         k_version='pvgis5'):
     r"""
     Power (DC) using the Huld model.
 
@@ -274,6 +305,11 @@ def huld(effective_irradiance, temp_mod, pdc0, k=None, cell_type=None):
     cell_type : str, optional
         If provided, must be one of ``'cSi'``, ``'CIS'``, or ``'CdTe'``.
         Used to look up default values for ``k`` if ``k`` is not specified.
+    k_version : str, optional
+        Either ``'pvgis5'`` (default) or ``'pvgis6'``. Selects values
+        for ``k`` if ``k`` is not specified. If ``'pvgis5'``, values are
+        from PVGIS documentation and are labeled in [2]_ as "current".
+        If ``'pvgis6'`` values are from [2]_ labeled as "updated".
 
     Returns
     -------
@@ -328,14 +364,19 @@ def huld(effective_irradiance, temp_mod, pdc0, k=None, cell_type=None):
 
     References
     ----------
-    .. [1] T. Huld, G. Friesen, A. Skoczek, R. Kenny, T. Sample, M. Field,
-           E. Dunlop. A power-rating model for crystalline silicon PV modules.
-           Solar Energy Materials and Solar Cells 95, (2011), pp. 3359-3369.
-           :doi:`10.1016/j.solmat.2011.07.026`.
+    .. [1] T. Huld, G. Friesen, A. Skoczek, R. Kenny, T. Sample, M. Field, and
+           E. Dunlop, "A power-rating model for crystalline silicon PV
+           modules," Solar Energy Materials and Solar Cells 95, (2011),
+           pp. 3359-3369. :doi:`10.1016/j.solmat.2011.07.026`.
+    .. [2] A. Chatzipanagi, N. Taylor, I. Suarez, A. Martinez, T. Lyubenova,
+           and E. Dunlop, "An Updated Simplified Energy Yield Model for Recent
+           Photovoltaic Module Technologies,"
+           Progress in Photovoltaics: Research and Applications 33,
+           no. 8 (2025): 905–917, :doi:`10.1002/pip.3926`.
     """
     if k is None:
         if cell_type is not None:
-            k = _infer_k_huld(cell_type, pdc0)
+            k = _infer_k_huld(cell_type, pdc0, k_version)
         else:
             raise ValueError('Either k or cell_type must be specified')
 
@@ -346,7 +387,10 @@ def huld(effective_irradiance, temp_mod, pdc0, k=None, cell_type=None):
         logGprime = np.log(gprime, out=np.zeros_like(gprime),
                            where=np.array(gprime > 0))
     # Eq. 1 in [1]
-    pdc = gprime * (pdc0 + k[0] * logGprime + k[1] * logGprime**2 +
-                    k[2] * tprime + k[3] * tprime * logGprime +
-                    k[4] * tprime * logGprime**2 + k[5] * tprime**2)
+    pdc = gprime * (
+        pdc0 + k[0] * logGprime + k[1] * logGprime**2 +
+        k[2] * tprime + k[3] * tprime * logGprime +
+        k[4] * tprime * logGprime**2 +
+        k[5] * tprime**2
+    )
     return pdc

tests/test_pvarray.py

@@ -50,10 +50,12 @@ def test_pvefficiency_adr_round_trip():
 
 
 def test_huld():
+    # tests with default k_version='pvgis5'
     pdc0 = 100
     res = pvarray.huld(1000, 25, pdc0, cell_type='cSi')
     assert np.isclose(res, pdc0)
-    exp_sum = np.exp(1) * (np.sum(pvarray._infer_k_huld('cSi', pdc0)) + pdc0)
+    k = pvarray._infer_k_huld('cSi', pdc0, 'pvgis5')
+    exp_sum = np.exp(1) * (np.sum(k) + pdc0)
     res = pvarray.huld(1000*np.exp(1), 26, pdc0, cell_type='cSi')
     assert np.isclose(res, exp_sum)
     res = pvarray.huld(100, 30, pdc0, k=(1, 1, 1, 1, 1, 1))
@@ -67,5 +69,48 @@ def test_huld():
     res = pvarray.huld(eff_irr, tm, pdc0, k=(1, 1, 1, 1, 1, 1))
     assert_series_equal(res, expected)
     with pytest.raises(ValueError,
-                       match='Either k or cell_type must be specified'):
-        res = pvarray.huld(1000, 25, 100)
+                       match='Either k or cell_type must be specified'
+                       ):
+        pvarray.huld(1000, 25, 100)
+
+
+def test_huld_params():
+    """Test Huld with built-in coefficients."""
+    pdc0 = 100
+    # Use non-reference values so coefficients affect the result
+    eff_irr = 800  # W/m^2 (not 1000)
+    temp_mod = 35  # deg C (not 25)
+    # calculated by C. Hansen using Excel, 2025
+    expected = {'pvgis5': {'csi': 76.405089,
+                           'cis': 77.086016,
+                           'cdte': 78.642762
+                           },
+                'pvgis6': {'csi': 77.649421,
+                           'cis': 77.723110,
+                           'cdte': 77.500399
+                           }
+                }
+    # Test with PVGIS5 coefficients for all cell types
+    for yr in expected:
+        for cell_type in expected[yr]:
+            result = pvarray.huld(eff_irr, temp_mod, pdc0, cell_type=cell_type,
+                                  k_version=yr)
+            assert np.isclose(result, expected[yr][cell_type])
+
+
+def test_huld_errors():
+    # Check errors
+    pdc0 = 100
+    # Use non-reference values so coefficients affect the result
+    eff_irr = 800  # W/m^2 (not 1000)
+    temp_mod = 35  # deg C (not 25)
+    # provide both cell_type and k_version
+    with pytest.raises(KeyError):
+        pvarray.huld(
+            eff_irr, temp_mod, pdc0, cell_type='invalid', k_version='pvgis5'
+        )
+    # provide invalid k_version
+    with pytest.raises(ValueError, match='Invalid k_version=2021'):
+        pvarray.huld(
+            eff_irr, temp_mod, pdc0, cell_type='csi', k_version='2021'
+        )