Prevent v_oc to be negative in _lambertw

docs/sphinx/source/whatsnew/v0.10.0.rst

@@ -49,6 +49,8 @@ Enhancements
 Bug fixes
 ~~~~~~~~~
 
+* Prevent `v_oc` to be negative in `_lambertw` (:pull:`1782`)
+
 
 Testing
 ~~~~~~~
@@ -71,3 +73,4 @@ Contributors
 * Adam R. Jensen (:ghuser:`AdamRJensen`)
 * Echedey Luis (:ghuser:`echedey-ls`)
 * Cliff Hansen (:ghuser:`cwhanse`)
+* Cédric Leroy (:ghuser:`cedricleroy`)

pvlib/singlediode.py

@@ -794,6 +794,13 @@ def _lambertw(photocurrent, saturation_current, resistance_series,
     # Compute open circuit voltage
     v_oc = _lambertw_v_from_i(0., **params)
 
+    # Set elements <0 or close to 0 in v_oc to 0
+    if isinstance(v_oc, np.ndarray):
+        v_oc[(v_oc < 0) | ((v_oc > 0) & (v_oc < 1e-12))] = 0.
+    elif isinstance(v_oc, (float, int)):
+        if v_oc < 0 or 0 < v_oc < 1e-12:
+            v_oc = 0.
+
     # Find the voltage, v_mp, where the power is maximized.
     # Start the golden section search at v_oc * 1.14
     p_mp, v_mp = _golden_sect_DataFrame(params, 0., v_oc * 1.14, _pwr_optfcn)

pvlib/tests/test_singlediode.py

@@ -168,6 +168,21 @@ def test_singlediode_precision(method, precise_iv_curves):
     assert np.allclose(pc['i_xx'], outs['i_xx'], atol=1e-6, rtol=0)
 
 
+def test_singlediode_lambert_negative_voc():
+
+    x1 = np.array([0., 1.480501e-11, 0.178, 8000., 1.797559])
+    outs = pvsystem.singlediode(*x1, method='lambertw')
+    assert outs['v_oc'] == 0
+
+    x2 = np.array([0., 1.456894e-11, 0.178, 8000., 1.797048])
+    outs = pvsystem.singlediode(*x2, method='lambertw')
+    assert outs['v_oc'] == 0
+
+    x  = np.array([x1, x2]).T
+    outs = pvsystem.singlediode(*x, method='lambertw')
+    assert np.array_equal(outs['v_oc'], [0, 0])
+
+
 @pytest.mark.parametrize('method', ['lambertw'])
 def test_ivcurve_pnts_precision(method, precise_iv_curves):
     """