FarmFlow

The FarmFlow program is developed to calculate the wind turbine wake effects of offshore wind farms. The reduction of the wind speed behind wind turbines and the added turbulence is calculated. This makes FarmFlow an excellent tool for the optimization of the wind farm lay-out and, in conjunction with an aero-elastic code, performing design calculation on wind turbines which have to be placed in wind farms.

The problem

Nowadays most wind power is produced in wind farms, in which a number of wind turbines are grouped together. Especially offshore wind energy is developed in large farms. Under these circumstances there is a mutual influence of wind turbines on each other due to aerodynamic wake effects: The flow at a downstream wind turbine is affected by the wind turbine(s) which are placed upwind. For the design of offshore wind turbines, as well as for optimization of the wind farm lay-out, it is important to take these wake effects into account, because:

• The energy yield is decreased. This is a result of the lower mean wind speed in the wake;
• The loads on a turbine are increased. This is a result of the changed wind speed profile and the increased turbulence intensity in the wake.

Wake effects can be taken into account with a turbulence model. However, turbulence modelling is one of the most difficult problems in physics which makes the accurate prediction of wake effects a very difficult task.

The approach

The approach to predict the wind farm energy yield is essentially different from the approach which is followed to calculate the loads on a wind turbine in a wind farm. In order to predict the energy yield of a farm it is sufficient to calculate the rotor averaged wind speeds at the downstream turbines and to interpolate in the power curve. In order to predict the loads, the spatial variation of the time averaged wind speed must be known at the downstream turbines, together with the wind speed fluctuations in the wake. This information should be fed into programs which model the aero-elastic behaviour of a turbine in a detailed way.

The core of the problem is the prediction of the incoming wind field in front of a turbine in a wake. World-wide, a wide range of models have been developed to solve this problem, varying from very simple empirical formula to very complex and time consuming flow solvers. The ECN program FarmFlow is a compromise between these two extremes. Farmflow uses a parabolized Navier Stokes solver with a k-e turbulence model, in combination with a free vortex wake model to calculate the pressure grandients in the near wake. This approach delivers state of the art accuracy with reasonable calculation time. It calculates both the mean wind field as well as the fluctuations in the wind.

In order to use the FarmFlow model for optimization of 100+MW wind farms, a special version is created that is very fast with equal accuracies as the full model.

The results

The FarmFlow program is a validated tool to calculate the wake effects of large offshore wind farms. The FarmFlow tool allows the user to accurately optimise the lay-out of the wind farm with respect to power output and, in conjunction with an aero-elastic code, the possibility to perform design calculations on wind turbines that are placed in wind farms. The program combines a relatively short calculation time with an excellent accuracy. Multiple wake effects and wind farms which consist of different turbine types can be modelled in a straightforward way.