Fix RuntimeWarning in singlediode when v_oc is zero

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

@@ -14,6 +14,9 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+* :py:func:`~pvlib.pvsystem.singlediode` with ``method='lambertw'`` no longer
+  emits ``RuntimeWarning: invalid value encountered in divide`` when the open
+  circuit voltage is zero, e.g. with zero photocurrent. (:issue:`2671`)
 
 
 Enhancements
@@ -42,4 +45,5 @@ Maintenance
 
 Contributors
 ~~~~~~~~~~~~
+* Jakub Krasuski (:ghuser:`Blizzeq`)
 

pvlib/singlediode.py

@@ -908,16 +908,22 @@ def _lambertw(photocurrent, saturation_current, resistance_series,
     # remove try/except when scipy>=1.15, and golden mean is retired
     try:
         from scipy.optimize.elementwise import find_minimum
-        # left negative to insure strict inequality
-        init = (-1., 0.8*v_oc, v_oc)
+        # Set up initial value x1 < x2 < x3 to find vmp.
+        # Set x2 to 0.8*v_oc. Set x1=-1, negative to insure strict inequality
+        # x1 < x2. Set right to 1.0 where v_oc = 0 to ensure
+        # x2 < x3, otherwise set x3 = np.abs(v_oc).
+        # Where v_oc = 0 the minimizer returns a tiny positive v_mp, so
+        # v_mp and p_mp are set to 0 there to keep v_mp <= v_oc (GH 2671).
+        voc_zero = v_oc == 0
+        init = (-1., 0.8*v_oc, np.where(voc_zero, 1., np.abs(v_oc)))
         res = find_minimum(_vmp_opt, init,
                            args=(params['photocurrent'],
                                  params['saturation_current'],
                                  params['resistance_series'],
                                  params['resistance_shunt'],
                                  params['nNsVth'],))
-        v_mp = res.x
-        p_mp = -1.*res.f_x
+        v_mp = np.where(voc_zero, 0., res.x)[()]
+        p_mp = np.where(voc_zero, 0., -1.*res.f_x)[()]
     except ModuleNotFoundError:
         # switch to old golden section method
         p_mp, v_mp = _golden_sect_DataFrame(params, 0., v_oc * 1.14,

tests/test_singlediode.py

@@ -2,6 +2,8 @@
 testing single-diode methods using JW Bishop 1988
 """
 
+import warnings
+
 import numpy as np
 import pandas as pd
 import scipy
@@ -191,6 +193,26 @@ def test_singlediode_lambert_negative_voc(mocker):
     assert_array_equal(outs["v_oc"], [0, 0])
 
 
+def test_singlediode_lambert_zero_voc():
+    """With v_oc = 0 the bracket passed to find_minimum collapsed to
+    x2 == x3 and scipy raised a RuntimeWarning. See issue #2671.
+    """
+    pytest.importorskip("scipy", minversion="1.15")
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        out = pvsystem.singlediode(0., 1e-12, 0.5, np.inf, 1.8,
+                                   method="lambertw")
+        outs = pvsystem.singlediode(np.array([0., 5.]), 1e-12, 0.5, np.inf,
+                                    1.8, method="lambertw")
+    assert out["v_oc"] == 0
+    assert out["v_mp"] == 0
+    assert out["p_mp"] == 0
+    assert outs["v_mp"][0] == 0
+    assert outs["p_mp"][0] == 0
+    assert 0 < outs["v_mp"][1] < outs["v_oc"][1]
+    assert outs["p_mp"][1] > 0
+
+
 @pytest.mark.parametrize('method', ['lambertw', 'brentq', 'newton',
                                     chandrupatla])
 def test_v_from_i_i_from_v_precision(method, precise_iv_curves):