Do not infer First Solar spectral coefficients for 'Thin Film' modules

docs/sphinx/source/whatsnew/v0.16.2.rst

@@ -14,6 +14,12 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+* :py:meth:`pvlib.pvsystem.PVSystem.first_solar_spectral_loss` no longer
+  infers spectral coefficients for modules whose CEC ``Technology`` is
+  ``"Thin Film"``: the technology is ambiguous (it may be CdTe, CIGS, or
+  another thin-film type), so it now raises a ``ValueError`` directing the
+  user to set the actual cell type or supply
+  ``first_solar_spectral_coefficients`` explicitly. (:issue:`1984`)
 * Correct the Series name in ``ModelChain.results.dc_ohmic_losses`` from
   ``i_mp`` to ``p_dc_ohmic_loss`` when using ``dc_ohms_from_percent``.
   (:issue:`1959`, :pull:`2912`)

pvlib/pvsystem.py

@@ -745,6 +745,12 @@ def first_solar_spectral_loss(self, pw, airmass_absolute):
         - 'Material' - a string describing the cell type, can be read from
           the Sandia module database.
 
+        A CEC ``Technology`` of ``'Thin Film'`` is not inferred: it is
+        ambiguous (it may be CdTe, CIGS, or another thin-film type), so
+        ``first_solar_spectral_loss`` raises a ``ValueError`` unless
+        user-supplied coefficients are provided or the ``Technology`` is
+        corrected to the actual cell type (e.g. ``'CdTe'``).
+
         Parameters
         ----------
         pw : array-like
@@ -771,6 +777,18 @@ def _spectral_correction(array, pw):
                         'first_solar_spectral_coefficients'
                     ]
                 module_type = None
+            elif array.module_parameters.get('Technology') == 'Thin Film':
+                # GH 1984: 'Thin Film' is ambiguous -- it may be CdTe,
+                # CIGS, or another thin-film type -- so do not infer
+                # coefficients from it. The user must specify the actual
+                # cell type or supply coefficients explicitly.
+                raise ValueError(
+                    'Cannot infer First Solar spectral coefficients for '
+                    'a module with CEC Technology "Thin Film": the '
+                    'technology is ambiguous. Set '
+                    'module_parameters["Technology"] to the actual cell '
+                    'type (e.g. "CdTe" for First Solar modules) or supply '
+                    '"first_solar_spectral_coefficients" explicitly.')
             else:
                 module_type = array._infer_cell_type()
                 coefficients = None

tests/test_pvsystem.py

@@ -366,6 +366,35 @@ def test_PVSystem_first_solar_spectral_loss(module_parameters, module_type,
     assert_allclose(out, 1, atol=0.5)
 
 
+def test_PVSystem_first_solar_spectral_loss_thin_film():
+    # GH 1984: a CEC Technology of 'Thin Film' is ambiguous (it may be
+    # CdTe, CIGS, or another thin-film type), so coefficients must not
+    # be inferred from it.
+    module_parameters = {'Technology': 'Thin Film'}
+    system = pvsystem.PVSystem(module_parameters=module_parameters)
+    with pytest.raises(ValueError, match='Thin Film'):
+        system.first_solar_spectral_loss(3, 3)
+
+    # the two remedies keep working: correct the Technology, or supply
+    # coefficients explicitly
+    module_parameters_cdte = {'Technology': 'CdTe'}
+    system = pvsystem.PVSystem(module_parameters=module_parameters_cdte)
+    out_cdte = system.first_solar_spectral_loss(3, 3)
+    assert_allclose(out_cdte,
+                    spectrum.spectral_factor_firstsolar(3, 3, 'cdte'))
+
+    module_parameters_coeffs = {
+        'Technology': 'Thin Film',
+        'first_solar_spectral_coefficients': (
+            0.84, -0.03, -0.008, 0.14, 0.04, -0.002)}
+    system = pvsystem.PVSystem(module_parameters=module_parameters_coeffs)
+    out_coeffs = system.first_solar_spectral_loss(3, 3)
+    assert_allclose(out_coeffs,
+                    spectrum.spectral_factor_firstsolar(
+                        3, 3, coefficients=(0.84, -0.03, -0.008, 0.14,
+                                            0.04, -0.002)))
+
+
 def test_PVSystem_multi_array_first_solar_spectral_loss():
     system = pvsystem.PVSystem(
         arrays=[