code: def mean_hub_height(self):
          r
          self.hub_height = np.exp(sum(
              np.log(wind_farm.hub_height) * wind_farm.get_installed_power() for
              wind_farm in self.wind_farms) / self.get_installed_power())
          return self
docs: r"""
          Calculates the mean hub height of the wind turbine cluster.
  
          The mean hub height of a wind turbine cluster is necessary for power
          output calculations with an aggregated wind turbine cluster power
          curve. Hub heights of wind farms with higher nominal power weigh more
          than others. Assigns the hub height to the turbine cluster object.
  
          Returns
          -------
          self
  
          Notes
          -----
          The following equation is used [1]_:
  
          .. math:: h_{WTC} = e^{\sum\limits_{k}{ln(h_{WF,k})}
                             \frac{P_{N,k}}{\sum\limits_{k}{P_{N,k}}}}
  
          with:
              :math:`h_{WTC}`: mean hub height of wind turbine cluster,
              :math:`h_{WF,k}`: hub height of the k-th wind farm of the cluster,
              :math:`P_{N,k}`: installed power of the k-th wind farm
  
          References
          ----------
          .. [1]  Knorr, K.: "Modellierung von raum-zeitlichen Eigenschaften der
                   Windenergieeinspeisung für wetterdatenbasierte
                   Windleistungssimulationen". Universität Kassel, Diss., 2016,
                   p. 35