Issue#144-fixed the azimuth calculation of rotated PV panel

docs/sphinx/source/whatsnew/v0.3.1.txt

@@ -15,8 +15,18 @@ Enhancements
 Bug fixes
 ~~~~~~~~~
 
-
-
+* Fixes night tare issue in snlinverter. When the DC input power (p_dc)
+  to an inverter is below the inversion startup power (Ps0), the model
+  should set the AC output (ac_power) to the night tare value (Pnt). 
+  The night tare power indicates the power consumed by the inverter to
+  sense PV array voltage. The model was erroneously comparing Ps0 with
+  the AC output power (ac_power), rather than the DC input power (p_dc).
+
+* Fixed the azimuth calculation of rotated PV panel in function
+  pvlib.tracking.singleaxis(...) so that the results are consistent
+  with PVsyst
 Contributors
 ~~~~~~~~~~~~
 
+* ejmiller2
+* Yudong Ma

pvlib/pvsystem.py

@@ -1662,7 +1662,7 @@ def snlinverter(inverter, v_dc, p_dc):
 
     ac_power = (Paco/(A-B) - C*(A-B)) * (p_dc-B) + C*((p_dc-B)**2)
     ac_power[ac_power > Paco] = Paco
-    ac_power[ac_power < Pso] = - 1.0 * abs(Pnt)
+    ac_power[p_dc < Pso] = - 1.0 * abs(Pnt)
 
     if len(ac_power) == 1:
         ac_power = ac_power.ix[0]

pvlib/test/test_pvsystem.py

@@ -384,6 +384,17 @@ def test_snlinverter_float():
     assert_almost_equals(pacs, 132.004278, 5)
 
 
+def test_snlinverter_Pnt_micro():
+    inverters = sam_data['cecinverter']
+    testinv = 'Enphase_Energy__M250_60_2LL_S2x___ZC____NA__208V_208V__CEC_2013_'
+    vdcs = pd.Series(np.linspace(0,50,3))
+    idcs = pd.Series(np.linspace(0,11,3))
+    pdcs = idcs * vdcs
+
+    pacs = pvsystem.snlinverter(inverters[testinv], vdcs, pdcs)
+    assert_series_equal(pacs, pd.Series([-0.043000, 132.545914746, 240.000000]))
+
+
 def test_PVSystem_creation():
     pv_system = pvsystem.PVSystem(module='blah', inverter='blarg')
 

pvlib/test/test_tracking.py

@@ -107,7 +107,7 @@ def test_axis_tilt():
                                        max_angle=90, backtrack=True, 
                                        gcr=2.0/7.0)
 
-    expect = pd.DataFrame({'aoi': 7.286245, 'surface_azimuth': 37.3427,
+    expect = pd.DataFrame({'aoi': 7.286245, 'surface_azimuth': 142.65730,
                            'surface_tilt': 35.98741, 'tracker_theta': -20.88121},
                            index=[0], dtype=np.float64)
 
@@ -118,7 +118,7 @@ def test_axis_tilt():
                                        max_angle=90, backtrack=True, 
                                        gcr=2.0/7.0)
 
-    expect = pd.DataFrame({'aoi': 47.6632, 'surface_azimuth': 129.0303,
+    expect = pd.DataFrame({'aoi': 47.6632, 'surface_azimuth': 50.96969,
                            'surface_tilt': 42.5152, 'tracker_theta': 31.6655},
                            index=[0], dtype=np.float64)
 
@@ -188,12 +188,32 @@ def test_SingleAxisTracker_tracking():
 
     tracker_data = system.singleaxis(apparent_zenith, apparent_azimuth)
 
-    expect = pd.DataFrame({'aoi': 7.286245, 'surface_azimuth': 37.3427,
+    expect = pd.DataFrame({'aoi': 7.286245, 'surface_azimuth': 142.65730 ,
                            'surface_tilt': 35.98741, 'tracker_theta': -20.88121},
                            index=[0], dtype=np.float64)
 
     assert_frame_equal(expect, tracker_data)
 
+    ### results calculated using PVsyst
+    pvsyst_solar_azimuth = 7.1609
+    pvsyst_solar_height = 27.315
+    pvsyst_axis_tilt = 20.
+    pvsyst_axis_azimuth = 20.
+    pvsyst_system = tracking.SingleAxisTracker(max_angle=60.,
+                                               axis_tilt=pvsyst_axis_tilt,
+                                               axis_azimuth=180+pvsyst_axis_azimuth,
+                                               backtrack=False)
+    # the definition of azimuth is different from PYsyst
+    apparent_azimuth = pd.Series([180+pvsyst_solar_azimuth])
+    apparent_zenith = pd.Series([90-pvsyst_solar_height])
+    tracker_data = pvsyst_system.singleaxis(apparent_zenith, apparent_azimuth)
+    expect = pd.DataFrame({'aoi': 41.07852 , 'surface_azimuth': 180-18.432 ,
+                           'surface_tilt': 24.92122 , 'tracker_theta': -15.18391},
+                           index=[0], dtype=np.float64)
+
+    assert_frame_equal(expect, tracker_data)
+
+
 
 def test_LocalizedSingleAxisTracker_creation():
     localized_system = tracking.LocalizedSingleAxisTracker(latitude=32,
@@ -254,7 +274,7 @@ def test_get_irradiance():
                                        surface_azimuth=tracker_data['surface_azimuth'])
 
     expected = pd.DataFrame(data=np.array(
-        [[ 142.71652464,   87.50125991,   55.21526473,   44.68768982,
+        [[ 961.80070,   815.94490,   145.85580,   135.32820,
           10.52757492],
        [          nan,           nan,           nan,           nan,
                   nan]]),

pvlib/tracking.py

@@ -485,7 +485,7 @@ def singleaxis(apparent_zenith, apparent_azimuth,
 
     # 4. Rotate 0 reference from panel's x axis to it's y axis and
     #    then back to North.
-    surface_azimuth += 90 + axis_azimuth
+    surface_azimuth = 90-surface_azimuth + axis_azimuth
 
     # 5. Map azimuth into [0,360) domain.
     surface_azimuth[surface_azimuth<0] += 360