Adjust and document dni/ beam irradiation

feedinlib/models/geometric_solar.py

@@ -134,6 +134,8 @@ def geometric_radiation(data_weather,
     Parameters
     ----------
     data_weather : :pandas:`pandas.DataFrame<dataframe>`
+        Has to contain time stamps (including time zone) as index,
+        and irradiation data ("dhi" and one of "ghi"/"dni")
     collector_slope : numeric
         collector tilt in degree
     surface_azimuth : numeric
@@ -152,6 +154,13 @@ def geometric_radiation(data_weather,
     -------
     :pandas:`pandas.DataFrame<dataframe>`
     containing the total radiation on the sloped surface
+
+    Internally, beam irradiation (bi, direct radiation to a horizontal surface)
+    is used. However, typically, either direct normal irradiation (dni)
+    or global horizontal irradiation (ghi) is given. So we use
+    .. math:: \mathrm{ghi} = \mathrm{bi} + \mathrm{dhi}
+                           = \mathrm{dni} * \cos(\theta) + \mathrm{dhi}
+    to calculate the beam irradiation.
     """
     angle_of_incidence, solar_zenith_angle = solar_angles(
         data_weather.index, collector_slope, surface_azimuth,
@@ -174,7 +183,7 @@ def geometric_radiation(data_weather,
         irradiation_beam = irradiation_direct_horizontal/np.cos(
             angle_of_incidence)
     else:
-        irradiation_beam = data_weather['dni']
+        irradiation_beam = data_weather['dni'] * np.cos(angle_of_incidence)
 
     # beam radiation correction factor
     beam_corr_factor = np.array(angle_of_incidence / solar_zenith_angle)

test/test_models.py

@@ -197,6 +197,7 @@ def test_geometric_angles(self):
         assert incidence_a == pytest.approx(
             incidence_b, 1e-5)
 
+    def test_geometric_radiation(self):
         # For calculation of radiation, direct radiation is blocked at night.
         # So if there is neither reflection (albedo) nor diffuse radiation,
         # total radiation should be 0.
@@ -233,11 +234,11 @@ def test_geometric_angles(self):
                                              freq="h", tz='UTC'))
 
         assert (plant4.geometric_radiation(data_weather_test)[0]
-                == pytest.approx(345.424687, 1e-5))
+                == pytest.approx(302.86103, 1e-5))
 
         # extra test for feedin
         assert (plant4.feedin(data_weather_test)[0]
-                == pytest.approx(89.23768, 1e-5))
+                == pytest.approx(78.67677, 1e-5))
 
         # check giving same weather with temperature in Kelvin
         data_weather_kelvin = pd.DataFrame(data={'wind_speed': [0],