Prevent v_oc to be negative in _lambertw (#1782)

* Prevent `v_oc` to be negative in `_lambertw`

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

Co-authored-by: Cliff Hansen <[email protected]>

* Target small negative elements only

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

---------

Co-authored-by: Cliff Hansen <[email protected]>
Co-authored-by: Kevin Anderson <[email protected]>

Author: 3 people

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

@@ -49,6 +49,9 @@ Enhancements
 Bug fixes
 ~~~~~~~~~
 
+* Prevent small negative values of `v_oc` in :py:func:`pvlib.singlediode._lambertw`
+  which result from accumulated roundoff error. (:issue:`1780`, :issue:`1673`, :pull:`1782`)
+
 
 Testing
 ~~~~~~~
@@ -71,3 +74,5 @@ Contributors
 * Adam R. Jensen (:ghuser:`AdamRJensen`)
 * Echedey Luis (:ghuser:`echedey-ls`)
 * Cliff Hansen (:ghuser:`cwhanse`)
+* Cédric Leroy (:ghuser:`cedricleroy`)
+* Jean-Baptiste Pasquier (:ghuser:`pasquierjb`)

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 small elements <0 in v_oc to 0
+    if isinstance(v_oc, np.ndarray):
+        v_oc[(v_oc < 0) & (v_oc > -1e-12)] = 0.
+    elif isinstance(v_oc, (float, int)):
+        if v_oc < 0 and 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 +168,19 @@ 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():
+
+    # Those values result in a negative v_oc out of `_lambertw_v_from_i`
+    x = np.array([0., 1.480501e-11, 0.178, 8000., 1.797559])
+    outs = pvsystem.singlediode(*x, method='lambertw')
+    assert outs['v_oc'] == 0
+
+    # Testing for an array
+    x  = np.array([x, x]).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):
     """