Modelling Tools for Wind Farm Upgrading

Planning of modifications of existing wind farms by adding or replacing turbines makes new demands on wind farm modelling. Emphasis shifts from the calculation of the mean efficiency of the farm to that of individual turbines. Additionally, modelling has to deal with different turbine types and hub heights combined in one farm. Full scale measurements at two wind farms in Northern Germany provide test cases for investigating the capability of wind farm models to predict the power output of individual turbines. Calculations are performed with a kinematic and an eddy-viscosity model. Both procedures result in systematic deviations from the measured single turbine performance. These are believed to be due to the increase in turbulence intensity in wakes, which is not modelled adequately. Simple modifications are proposed to enhance the accuracy of the models. The analysis of the wake losses in the case of turbines with different hub heights shows that both models give results within the limit of the measurement uncertainties.