Sten Tronæs Frandsen

Survey of modelling methods for wind turbine wakes and wind farms

This article provides an overview and analysis of different wake-modelling methods which may be used as prediction and design tools for both wind turbines and wind farms. We also survey the available data concerning the measurement of wind magnitudes in both single wakes and wind farms, and of loading effects on wind turbines under single- and multiple-wake conditions. The relative merits of existing wake and wind farm models and their ability to reproduce experimental results are discussed. Conclusions are provided concerning the usefulness of the different modelling approaches examined, and difficult issues which have not yet been satisfactorily treated and which require further research are discussed. Copyright

Comparison of Wake Model Simulations with Offshore Wind Turbine Wake Profiles Measured by Sodar

Abstract This paper gives an evaluation of most of the commonly used models for predicting wind speed decrease (wake) downstream of a wind turbine. The evaluation is based on six experiments where free-stream and wake wind speed profiles were measured using a ship-mounted sodar at a small offshore wind farm. The experiments were conducted at varying distances between 1.7 and 7.4 rotor diameters downstream of the wind turbine. Evaluation of the models compares the predicted and observed velocity deficits at hub height. A new method of evaluation based on determining the cumulative momentum deficit over the profiles is described. Despite the apparent simplicity of the experiments, the models give a wide range of predictions. Overall, it is not possible to establish any of the models as having individually superior performance with respect to the measurements.

Quantifying the Impact of Wind Turbine Wakes on Power Output at Offshore Wind Farms

Abstract There is an urgent need to develop and optimize tools for designing large wind farm arrays for deployment offshore. This research is focused on improving the understanding of, and modeling of, wind turbine wakes in order to make more accurate power output predictions for large offshore wind farms. Detailed data ensembles of power losses due to wakes at the large wind farms at Nysted and Horns Rev are presented and analyzed. Differences in turbine spacing (10.5 versus 7 rotor diameters) are not differentiable in wake-related power losses from the two wind farms. This is partly due to the high variability in the data despite careful data screening. A number of ensemble averages are simulated with a range of wind farm and computational fluid dynamics models and compared to observed wake losses. All models were able to capture wake width to some degree, and some models also captured the decrease of power output moving through the wind farm. Root-mean-square errors indicate a generally better model pe...

On the wind speed reduction in the center of large clusters of wind turbines

Abstract The paper intends to probe the feasibility of a method to determine the wind speed reduction in the center of a large wind farm by means of simple boundary layer theory. Therefore, the simplest possible assumptions have been chosen. The paper presents an approach for determination of the reduction of the wind speed assuming the wind turbines effectively act as roughness elements. The model makes use of similarities to so-called canopy flows, where the surface drag and the drag on individual obstacles are added to form the total drag. Results are compared with existing models for reduction of efficiency within wind turbine clusters and good aggeement is found.

Modelling and measurements of wakes in large wind farms

The paper presents research conducted in the Flow workpackage of the EU funded UPWIND project which focuses on improving models of flow within and downwind of large wind farms in complex terrain and offshore. The main activity is modelling the behaviour of wind turbine wakes in order to improve power output predictions.

Ten Years of Meteorological Measurements for Offshore Wind Farms

Riso has been monitoring wind resources and power output from offshore wind farms since 1993. A considerable degree of expertise has been developed in optimizing measurements and in using these databases to develop and validate models for offshore environments. This paper describes the evolution of monitoring strategies to a fully automated satellite based retrieval that provides near-real time access to offshore data, even at remote stand-alone masts. An overview of wind speed and turbulence at offshore sites in Denmark is given. Finally, three methods of generating long-term wind resource estimates from short-term measurements are outlined.

Model for power spectra of the blade of a wind turbine measured from the moving frame of reference

Abstract While the static aerodynamic loads on wind turbine blades seem to a large extent predictable, little work has been done to clarify the dynamic loads due to atmospheric turbulence. A simple kinematical model is here derived in order to predict the power spectrum of the turbulence component of the wind along the mean wind direction, observed from a frame of reference that is moving with constant angular velocity along the circumference of a circle perpendicular to the direction of the mean wind velocity. The results deviate significantly from those obtained when disregarding the rotation. The model is based on the assumption that the turbulence is stationary, homogeneous, and isotropic. Predictions of power spectra seem to be in good agreement with experiments. In particular, comparison with a spectrum of structural response for the Gedser wind turbine is interesting, because it demonstrate the applicability of the overall concept of the model. A technique is outlined to obtain cross-spectra between turbulence wind components measured at different points in the moving frame of reference.

A CFD model of the wake of an offshore wind farm: using a prescribed wake inflow

An CFD model of the wake of an offshore wind farm, expanding existing measurements is proposed. The method is based on solving the Navier Stokes equation in a large domain downstream an offshore wind farm. The inflow of the domain is estimated using existing met mast measurements from both free stream and directly in-wake conditions. A comparison between the simulation results and measurements from a met mast are presented and the shortcomings of the methods are discussed.

Relevant Criteria for Testing the Quality of Models for Turbulent Wind Speed Fluctuations

Seeking relevant criteria for testing the quality of turbulence models, the scale of turbulence and the gust factor have been estimated from data and compared with predictions from first-order models of these two quantities. It is found that the mean of the measured length scales is approximately 10% smaller than the IEC model for wind turbine hub height levels. The mean is only marginally dependent on trends in time series. It is also found that the coefficient of variation of the measured length scales is about 50%. 3 s and 10 s preaveraging of wind speed data are relevant for megawatt-size wind turbines when seeking wind characteristics that correspond to one blade and the entire rotor, respectively. For heights exceeding 50-60 m, the gust factor increases with wind speed. For heights larger than 60-80 m, present assumptions on the value of the gust factor are significantly conservative, both for 3 s and 10 s preaverages. The usually applied value of k p ≈3 should probably be reduced.

Local Wind Climate within and Downwind of Large Offshore Wind Turbine Clusters

This Note reviews presentations made at a meeting for experts, held at the Risø Laboratory, concerning possible approaches to improving understanding of local wind climates within large offshore wind farms and particularly their impact on downwind wind farms. Such effects are detectable both within and beyond the boundaries of each windfarm, which typically has 10 to 100 large turbines of rotor diameter up to 80 m and hub-heights between 60 and 80 m.