Add PVSystem pvsyst_celltemp methods

docs/sphinx/source/whatsnew/v0.6.1.rst

@@ -60,6 +60,7 @@ Enhancements
 * Add option for :py:func:`pvlib.irradiance.disc` to use relative airmass
   by supplying `pressure=None`. (:issue:`449`)
 * Created :py:func:`pvlib.pvsystem.pvsyst_celltemp` to implement PVsyst's cell temperature model. (:issue:`552`)
+* Add `PVSystem` class method :py:func:`~pvlib.pvsystem.PVSystem.pvsyst_celltemp` (:issue:`633`)
 
 
 Bug fixes
@@ -73,6 +74,8 @@ Bug fixes
 * Fix documentation errors when using IPython >= 7.0.
 * Fix error in :func:`pvlib.modelchain.ModelChain.infer_spectral_model` (:issue:`619`)
 * Fix error in ``pvlib.spa`` when using Python 3.7 on some platforms.
+* Fix error in :func:`pvlib.irradiance._delta_kt_prime_dirint` (:issue:`637`). The error affects
+  the first and last values of DNI calculated by the function :func:`pvlib.irradiance.dirint`
 
 
 Testing

pvlib/irradiance.py

@@ -1558,14 +1558,20 @@ def _dirint_from_dni_ktprime(dni, kt_prime, solar_zenith, use_delta_kt_prime,
 
 def _delta_kt_prime_dirint(kt_prime, use_delta_kt_prime, times):
     """
-    Calculate delta kt prime (Perez eqn 2), or return a default value
+    Calculate delta_kt_prime (Perez eqn 2 and eqn 3), or return a default value
     for use with :py:func:`_dirint_bins`.
     """
     if use_delta_kt_prime:
         # Perez eqn 2
-        delta_kt_prime = 0.5*((kt_prime - kt_prime.shift(1)).abs().add(
-                              (kt_prime - kt_prime.shift(-1)).abs(),
-                              fill_value=0))
+        kt_next = kt_prime.shift(-1)
+        kt_previous = kt_prime.shift(1)
+        # replace nan with values that implement Perez Eq 3 for first and last
+        # positions. Use kt_previous and kt_next to handle series of length 1
+        kt_next.iloc[-1] = kt_previous.iloc[-1]
+        kt_previous.iloc[0] = kt_next.iloc[0]
+        delta_kt_prime = 0.5 * ((kt_prime - kt_next).abs().add(
+                                (kt_prime - kt_previous).abs(),
+                                fill_value=0))
     else:
         # do not change unless also modifying _dirint_bins
         delta_kt_prime = pd.Series(-1, index=times)

pvlib/pvsystem.py

@@ -46,6 +46,16 @@
 }
 
 
+TEMP_MODELS = {
+    'sapm': {'open_rack_cell_glassback': [-3.47, -.0594, 3],
+             'roof_mount_cell_glassback': [-2.98, -.0471, 1],
+             'open_rack_cell_polymerback': [-3.56, -.0750, 3],
+             'insulated_back_polymerback': [-2.81, -.0455, 0],
+             'open_rack_polymer_thinfilm_steel': [-3.58, -.113, 3],
+             '22x_concentrator_tracker': [-3.23, -.130, 13]},
+    'pvsyst': {'freestanding': (29.0, 0), 'insulated': (15.0, 0)}
+}
+
 # not sure if this belongs in the pvsystem module.
 # maybe something more like core.py? It may eventually grow to
 # import a lot more functionality from other modules.
@@ -511,6 +521,22 @@ def sapm_effective_irradiance(self, poa_direct, poa_diffuse,
             poa_direct, poa_diffuse, airmass_absolute, aoi,
             self.module_parameters, reference_irradiance=reference_irradiance)
 
+    def pvsyst_celltemp(self, poa_global, wind_speed, temp_air, eta_m=0.1,
+                        alpha_absorption=0.9):
+        """Uses :py:func:`pvsyst_celltemp` to calculate module temperatures
+        based on ``self.racking_model`` and the input parameters.
+
+        Parameters
+        ----------
+        See pvsystem.pvsyst_celltemp for details
+
+        Returns
+        -------
+        See pvsystem.pvsyst_celltemp for details
+        """
+        return pvsyst_celltemp(poa_global, wind_speed, temp_air, eta_m,
+                               alpha_absorption, temp_model=self.racking_model)
+
     def first_solar_spectral_loss(self, pw, airmass_absolute):
 
         """
@@ -1860,13 +1886,7 @@ def sapm_celltemp(poa_global, wind_speed, temp_air,
     sapm
     '''
 
-    temp_models = {'open_rack_cell_glassback': [-3.47, -.0594, 3],
-                   'roof_mount_cell_glassback': [-2.98, -.0471, 1],
-                   'open_rack_cell_polymerback': [-3.56, -.0750, 3],
-                   'insulated_back_polymerback': [-2.81, -.0455, 0],
-                   'open_rack_polymer_thinfilm_steel': [-3.58, -.113, 3],
-                   '22x_concentrator_tracker': [-3.23, -.130, 13]
-                   }
+    temp_models = TEMP_MODELS['sapm']
 
     if isinstance(model, str):
         model = temp_models[model.lower()]
@@ -1880,9 +1900,9 @@ def sapm_celltemp(poa_global, wind_speed, temp_air,
 
     E0 = 1000.  # Reference irradiance
 
-    temp_module = pd.Series(poa_global*np.exp(a + b*wind_speed) + temp_air)
+    temp_module = pd.Series(poa_global * np.exp(a + b * wind_speed) + temp_air)
 
-    temp_cell = temp_module + (poa_global / E0)*(deltaT)
+    temp_cell = temp_module + (poa_global / E0) * (deltaT)
 
     return pd.DataFrame({'temp_cell': temp_cell, 'temp_module': temp_module})
 
@@ -1944,7 +1964,7 @@ def pvsyst_celltemp(poa_global, wind_speed, temp_air, eta_m=0.1,
     photovoltaic modules." Progress in Photovoltaics 16(4): 307-315.
     """
 
-    temp_models = {"freestanding": (29.0, 0), "insulated": (15.0, 0)}
+    temp_models = TEMP_MODELS['pvsyst']
 
     if isinstance(temp_model, str):
         natural_convenction_coeff, forced_convection_coeff = temp_models[

pvlib/test/test_irradiance.py

@@ -466,13 +466,13 @@ def test_dirint_value():
     pressure = 93193.
     dirint_data = irradiance.dirint(ghi, zenith, times, pressure=pressure)
     assert_almost_equal(dirint_data.values,
-                        np.array([ 888. ,  683.7]), 1)
+                        np.array([868.8,  699.7]), 1)
 
 
 def test_dirint_nans():
     times = pd.DatetimeIndex(start='2014-06-24T12-0700', periods=5, freq='6H')
     ghi = pd.Series([np.nan, 1038.62, 1038.62, 1038.62, 1038.62], index=times)
-    zenith = pd.Series([10.567, np.nan, 10.567, 10.567, 10.567,], index=times)
+    zenith = pd.Series([10.567, np.nan, 10.567, 10.567, 10.567], index=times)
     pressure = pd.Series([93193., 93193., np.nan, 93193., 93193.], index=times)
     temp_dew = pd.Series([10, 10, 10, np.nan, 10], index=times)
     dirint_data = irradiance.dirint(ghi, zenith, times, pressure=pressure,
@@ -489,7 +489,7 @@ def test_dirint_tdew():
     dirint_data = irradiance.dirint(ghi, zenith, times, pressure=pressure,
                                     temp_dew=10)
     assert_almost_equal(dirint_data.values,
-                        np.array([892.9,  636.5]), 1)
+                        np.array([882.1,  672.6]), 1)
 
 
 def test_dirint_no_delta_kt():
@@ -559,7 +559,7 @@ def test_gti_dirint():
     expected = pd.DataFrame(array(
         [[  21.05796198,    0.        ,   21.05796198],
          [ 288.22574368,   60.59964218,  245.37532576],
-         [ 930.85454521,  695.8504884 ,  276.96897609]]),
+         [ 931.04078010,  695.94965324,  277.06172442]]),
         columns=expected_col_order, index=times)
 
     assert_frame_equal(output, expected)
@@ -583,7 +583,7 @@ def test_gti_dirint():
     expected = pd.DataFrame(array(
         [[  21.05796198,    0.        ,   21.05796198],
          [ 289.81109139,   60.52460392,  247.01373353],
-         [ 932.22047435,  647.68716072,  323.59362885]]),
+         [ 932.46756378,  648.05001357,  323.49974813]]),
         columns=expected_col_order, index=times)
 
     assert_frame_equal(output, expected)
@@ -595,9 +595,9 @@ def test_gti_dirint():
         albedo=albedo)
 
     expected = pd.DataFrame(array(
-        [[  21.3592591 ,    0.        ,   21.3592591 ],
-         [ 292.5162373 ,   64.42628826,  246.95997198],
-         [ 941.47847463,  727.07261187,  258.25370648]]),
+        [[  21.3592591,    0.        ,   21.3592591 ],
+         [ 292.5162373,   64.42628826,  246.95997198],
+         [ 941.6753031,  727.16311901,  258.36548605]]),
         columns=expected_col_order, index=times)
 
     assert_frame_equal(output, expected)
@@ -611,7 +611,7 @@ def test_gti_dirint():
     expected = pd.DataFrame(array(
         [[  21.05796198,    0.        ,   21.05796198],
          [ 292.40468994,   36.79559287,  266.3862767 ],
-         [ 930.72198876,  712.36063132,  261.32196017]]),
+         [ 931.79627208,  689.81549269,  283.5817439]]),
         columns=expected_col_order, index=times)
 
     assert_frame_equal(output, expected)

pvlib/test/test_pvsystem.py

@@ -1118,6 +1118,23 @@ def test_pvsyst_celltemp_with_index():
     assert_series_equal(expected, pvtemps)
 
 
+def test_PVSystem_pvsyst_celltemp(mocker):
+    racking_model = 'insulated'
+    system = pvsystem.PVSystem(racking_model=racking_model)
+    mocker.spy(pvsystem, 'pvsyst_celltemp')
+    irrads = 1000
+    temps = 25
+    alpha_absorption = 0.9
+    winds = 1
+    eta_m = 0.1
+    out = system.pvsyst_celltemp(irrads, winds, temps)
+    pvsystem.pvsyst_celltemp.assert_called_once_with(irrads, winds, temps,
+                                                     eta_m, alpha_absorption,
+                                                     temp_model=racking_model)
+    assert isinstance(out, float)
+    assert out == 79.0
+
+
 def test_adrinverter(sam_data):
     inverters = sam_data['adrinverter']
     testinv = 'Ablerex_Electronics_Co___Ltd___' \