use find_minimum in lambertw for mpp

Author: cwhanse

pvlib/singlediode.py

@@ -913,8 +913,28 @@ def _lambertw(photocurrent, saturation_current, resistance_series,
             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)
+    # use scipy.elementwise if available
+    use_gs = False
+    try:
+        from scipy.optimize.elementwise import find_minimum
+        init = (0., 0.8*v_oc, 1.01*v_oc)
+        res = find_minimum(_vmp_opt, init,
+                           args=(params['photocurrent'],
+                                 params['saturation_current'],
+                                 params['resistance_series'],
+                                 params['resistance_shunt'],
+                                 params['nNsVth'],))
+        if res.success.all():
+            v_mp = res.x
+            p_mp = -1.*res.f_x
+        else:
+            use_gs = True
+    except ModuleNotFoundError:
+        use_gs = True
+
+    if use_gs:
+        # gracefully switch to old golden section method
+        p_mp, v_mp = _golden_sect_DataFrame(params, 0., v_oc * 1.14, _pwr_optfcn)
 
     # Find Imp using Lambert W
     i_mp = _lambertw_i_from_v(v_mp, **params)
@@ -938,6 +958,15 @@ def _lambertw(photocurrent, saturation_current, resistance_series,
     return out
 
 
+def _vmp_opt(v, iph, io, rs, rsh, nNsVth):
+    '''
+    Function to find power from ``i_from_v``.
+    '''
+    current = _lambertw_i_from_v(v, iph, io, rs, rsh, nNsVth)
+
+    return -v * current
+
+
 def _pwr_optfcn(df, loc):
     '''
     Function to find power from ``i_from_v``.