Add reference

Author: SabineHaas

windpowerlib/power_curves.py

@@ -77,18 +77,32 @@ def smooth_power_curve(power_curve_wind_speeds, power_curve_values,
     distribution of wind speed. This way of smoothing power curves is also used
     in [2]_ and [3]_.
 
+    The standard deviation :math:`\sigma` of the above equation can be
+    calculated by the following methods.
+
+    'turbulence_intensity' [2]_:
+    .. math:: \sigma = v_\text{std} \sigma_\text{n} = v_\text{std} TI
+
+    with:
+        TI: turbulence intensity
+
+    'Staffell_Pfenninger' [4]_:
+    .. math:: \sigma = 0.6 \cdot 0.2 \cdot v_\text{std}
+
     References
     ----------
     .. [1]  Knorr, K.: "Modellierung von raum-zeitlichen Eigenschaften der
              Windenergieeinspeisung für wetterdatenbasierte
              Windleistungssimulationen". Universität Kassel, Diss., 2016,
              p. 106
     .. [2]  Nørgaard, P. and Holttinen, H.: "A Multi-Turbine and Power Curve
-             Approach". Nordic Wind Power Conference, 1.–2.3.2004, 2000
+             Approach". Nordic Wind Power Conference, 1.–2.3.2004, 2000, p. 5
     .. [3]  Kohler, S. and Agricola, A.-Cl. and Seidl, H.:
              "dena-Netzstudie II. Integration erneuerbarer Energien in die
              deutsche Stromversorgung im Zeitraum 2015 – 2020 mit Ausblick
              2025". Technical report, 2010.
+    .. [4]  Staffell, I. and Pfenninger, S.: "Using Bias-Corrected Reanalysis
+              to Simulate Current and Future Wind Power Output". 2005, p. 11
 
     """
     # Specify normalized standard deviation