fix celltemp keys. make output a df. closes #54

Author: wholmgren

docs/sphinx/source/whatsnew/v0.2.0.txt

@@ -11,7 +11,9 @@ We recommend that all users upgrade to this version.
 API changes
 ~~~~~~~~~~~
 
-* Change variable names to conform with new `Variables and style rules wiki <https://github.com/pvlib/pvlib-python/wiki/Variables-and-style-rules>`_ (:issue:`37`, :issue:`54`).
+* Change variable names to conform with new
+  `Variables and style rules wiki <https://github.com/pvlib/pvlib-python/wiki/Variables-and-style-rules>`_ (:issue:`37`, :issue:`54`).
+* Make output of ``pvsystem.sapm_celltemp`` a DataFrame (:issue:`54`)
 
 Enhancements
 ~~~~~~~~~~~~

pvlib/pvsystem.py

@@ -768,7 +768,8 @@ def sapm_celltemp(irrad, wind, temp, model='open_rack_cell_glassback'):
 
     Returns
     --------
-    dict with keys tcell and tmodule. Values in degrees C.
+    DataFrame with columns 'temp_cell' and 'temp_module'.
+    Values in degrees C.
 
     References
     ----------
@@ -799,11 +800,11 @@ def sapm_celltemp(irrad, wind, temp, model='open_rack_cell_glassback'):
 
     E0 = 1000. # Reference irradiance
 
-    tmodule = irrad*np.exp(a + b*wind) + temp
+    temp_module = pd.Series(irrad*np.exp(a + b*wind) + temp)
 
-    tcell = tmodule + (irrad / E0)*(deltaT)
+    temp_cell = temp_module + (irrad / E0)*(deltaT)
 
-    return {'tcell':tcell, 'tmodule':tmodule}
+    return pd.DataFrame({'temp_cell': temp_cell, 'temp_module': temp_module})
 
 
 def singlediode(module, IL, I0, Rs, Rsh, nNsVth, **kwargs):

pvlib/test/test_pvsystem.py

@@ -9,6 +9,7 @@
 import pandas as pd
 
 from nose.tools import assert_equals, assert_almost_equals
+from pandas.util.testing import assert_frame_equal
 
 from pvlib import tmy
 from pvlib import pvsystem
@@ -135,10 +136,25 @@ def test_singlediode():
 
 def test_sapm_celltemp():
     default = pvsystem.sapm_celltemp(900, 5, 20)
-    assert_almost_equals(43.509, default['tcell'], 3)
-    assert_almost_equals(40.809, default['tmodule'], 3)
-    assert_equals(default, pvsystem.sapm_celltemp(900, 5, 20,
-                                                  [-3.47, -.0594, 3]))
+    assert_almost_equals(43.509, default.ix[0, 'temp_cell'], 3)
+    assert_almost_equals(40.809, default.ix[0, 'temp_module'], 3)
+    assert_frame_equal(default, pvsystem.sapm_celltemp(900, 5, 20,
+                                                       [-3.47, -.0594, 3]))
+
+
+def test_sapm_celltemp_with_index():
+    times = pd.DatetimeIndex(start='2015-01-01', end='2015-01-02', freq='12H')
+    temps = pd.Series([0, 10, 5], index=times)
+    irrads = pd.Series([0, 500, 0], index=times)
+    winds = pd.Series([10, 5, 0], index=times)
+
+    pvtemps = pvsystem.sapm_celltemp(irrads, winds, temps)
+    
+    expected = pd.DataFrame({'temp_cell':[0., 23.06066166, 5.],
+                             'temp_module':[0., 21.56066166, 5.]},
+                            index=times)
+    
+    assert_frame_equal(expected, pvtemps)
 
 
 def test_snlinverter():