fix _delta_kt_prime_dirint end values (#638)

* fix _delta_kt_prime_dirint end values

* syntax, test fixes:

* syntax and test fixes

* more test fixes

* more test fixes

* and more test fixes

* and still more test fixes

* add comments, update whatsnew

* delete space

Author: cwhanse

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

@@ -73,6 +73,8 @@ 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/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)