pvlib · ncroft-b4 · Oct 26, 2020 · Oct 30, 2020 · Oct 30, 2020 · Nov 16, 2020
diff --git a/docs/sphinx/source/api.rst b/docs/sphinx/source/api.rst
@@ -328,6 +328,7 @@ Pvsyst model
    temperature.pvsyst_cell
    pvsystem.calcparams_pvsyst
    pvsystem.singlediode
+   pvsystem.dc_ohms_from_percent
 
 PVWatts model
 ^^^^^^^^^^^^^
@@ -598,6 +599,7 @@ ModelChain properties that are aliases for your specific modeling functions.
    modelchain.ModelChain.aoi_model
    modelchain.ModelChain.spectral_model
    modelchain.ModelChain.temperature_model
+   modelchain.ModelChain.dc_ohmic_model
    modelchain.ModelChain.losses_model
    modelchain.ModelChain.effective_irradiance_model
 
@@ -628,6 +630,8 @@ ModelChain model definitions.
    modelchain.ModelChain.pvsyst_temp
    modelchain.ModelChain.faiman_temp
    modelchain.ModelChain.fuentes_temp
+   modelchain.ModelChain.dc_ohmic_model
+   modelchain.ModelChain.no_dc_ohmic_loss
    modelchain.ModelChain.pvwatts_losses
    modelchain.ModelChain.no_extra_losses
 

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
@@ -327,6 +327,11 @@ class ModelChain:
         The ModelChain instance will be passed as the first argument to a
         user-defined function.
 
+    dc_ohmic_model: None, str or function, default None
+        Valid strings are 'dc_ohms_from_percent', 'no_loss'. The ModelChain
+        instance will be passed as the first argument to a user-defined
+        function.
+
     losses_model: str or function, default 'no_loss'
         Valid strings are 'pvwatts', 'no_loss'. The ModelChain instance
         will be passed as the first argument to a user-defined function.
@@ -343,6 +348,7 @@ def __init__(self, system, location,
                  airmass_model='kastenyoung1989',
                  dc_model=None, ac_model=None, aoi_model=None,
                  spectral_model=None, temperature_model=None,
+                 dc_ohmic_model=None,
                  losses_model='no_loss', name=None, **kwargs):
 
         self.name = name
@@ -361,7 +367,10 @@ def __init__(self, system, location,
         self.spectral_model = spectral_model
         self.temperature_model = temperature_model
 
+        # losses
+        self.dc_ohmic_model = dc_ohmic_model
         self.losses_model = losses_model
+
         self.orientation_strategy = orientation_strategy
 
         self.weather = None
@@ -924,6 +933,41 @@ def fuentes_temp(self):
             self.weather['wind_speed'])
         return self
 
+    @property
+    def dc_ohmic_model(self):
+        return self._dc_ohmic_model
+
+    @dc_ohmic_model.setter
+    def dc_ohmic_model(self, model):
+        if model is None:
+            self._dc_ohmic_model = self.no_dc_ohmic_loss
+        elif isinstance(model, str):
+            model = model.lower()
+            if model == 'dc_ohms_from_percent':
+                self._dc_ohmic_model = self.dc_ohms_from_percent
+            elif model == 'no_loss':
+                self._dc_ohmic_model = self.no_dc_ohmic_loss
+            else:
+                raise ValueError(model + ' is not a valid losses model')
+        else:
+            self._dc_ohmic_model = partial(model, self)
+
+    def dc_ohms_from_percent(self):
+        """
+        Calculate time series of ohmic losses and apply those to the mpp power
+        output of the `dc_model` based on the pvsyst equivalent resistance
+        method. Uses a `dc_ohmic_percent` parameter in the `losses_parameters`
+        of the PVsystem.
+        """
+        Rw = self.system.dc_ohms_from_percent()
+        ohmic_loss_fraction = Rw * self.dc['i_mp'] / self.dc['v_mp']
+        self.dc_ohmic_losses = ohmic_loss_fraction * self.dc['p_mp']
+        self.dc['p_mp'] = self.dc['p_mp'] - self.dc_ohmic_losses
+        return self
+
+    def no_dc_ohmic_loss(self):
+        return self
+
     @property
     def losses_model(self):
         return self._losses_model
@@ -1374,6 +1418,7 @@ def _run_from_effective_irrad(self, data=None):
         """
         self._prepare_temperature(data)
         self.dc_model()
+        self.dc_ohmic_model()
         self.losses_model()
         self.ac_model()
 

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
@@ -838,6 +838,27 @@ def pvwatts_ac(self, pdc):
         return inverter.pvwatts(pdc, self.inverter_parameters['pdc0'],
                                 **kwargs)
 
+    def dc_ohms_from_percent(self):
+        """
+        Calculates the equivalent resistance of the wires using
+        :py:func:`pvlib.pvsystem.dc_ohms_from_percent`,
+        `self.losses_parameters["dc_ohmic_percent"]`,
+        `self.module_parameters["V_mp_ref"]`,
+        `self.module_parameters["I_mp_ref"]`,
+        `self.modules_per_string`, and `self.strings_per_inverter`.
+
+        See :py:func:`pvlib.pvsystem.dc_ohms_from_percent` for details.
+        """
+        kwargs = _build_kwargs(['dc_ohmic_percent'], self.losses_parameters)
+
+        kwargs.update(_build_kwargs(['V_mp_ref', 'I_mp_ref'],
+                                    self.module_parameters))
+
+        kwargs.update({'modules_per_string': self.modules_per_string,
+                       'strings_per_inverter': self.strings_per_inverter})
+
+        return dc_ohms_from_percent(**kwargs)
+
     @deprecated('0.8', alternative='PVSystem, Location, and ModelChain',
                 name='PVSystem.localize', removal='0.9')
     def localize(self, location=None, latitude=None, longitude=None,
@@ -2334,6 +2355,49 @@ def pvwatts_losses(soiling=2, shading=3, snow=0, mismatch=2, wiring=2,
     return losses
 
 
+def dc_ohms_from_percent(V_mp_ref, I_mp_ref, dc_ohmic_percent,
+                         modules_per_string=1,
+                         strings_per_inverter=1):
+    """
+    Calculates the equivalent resistance of the wires from a percent
+    ohmic loss at STC.
+
+    Equivalent resistance is calculated with the function:
+
+    .. math::
+
+        Rw = (L_{stc} / 100) * (Varray / Iarray)
+
+    :math:`L_{stc}` is the input dc loss as a percent, e.g. 1.5% loss is
+    input as 1.5
+
+    Parameters
+    ----------
+    V_mp_ref: numeric
+    I_mp_ref: numeric
+    dc_ohmic_percent: numeric, default 0
+    modules_per_string: numeric, default 1
+    strings_per_inverter: numeric, default 1
+
+    Returns
+    ----------
+    Rw: numeric
+        Equivalent resistance in ohms
+
+    References
+    ----------
+    -- [1] PVsyst 7 Help. "Array ohmic wiring loss".
+            https://www.pvsyst.com/help/ohmic_loss.htm
+    """
+    vmp = modules_per_string * V_mp_ref
+
+    imp = strings_per_inverter * I_mp_ref
+
+    Rw = (dc_ohmic_percent / 100) * (vmp / imp)
+
+    return Rw
+
+
 def combine_loss_factors(index, *losses, fill_method='ffill'):
     r"""
     Combines Series loss fractions while setting a common index.