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Titel:
Turbine Interaction in Large Offshore Wind Farms; Atmospheric Boundary Layer above a Wind Farm
 
Auteur(s):
 
Gepubliceerd door: Publicatie datum:
ECN Windenergie 1-8-2004
 
ECN publicatienummer: Publicatie type:
ECN-C--04-033 ECN rapport
 
Aantal pagina's: Volledige tekst:
55 Download PDF  (311kB)

Samenvatting:

Background and Aims

Large wind farms are to be expected in the coming decades. We are talking about farms of for example 20 times 20 turbines or larger. Of course it is of utmost importance to be able to predict the production of such farms accurately. However, little is known on cumulative effects of wind turbine wakes. Therefore this project studies the wind turbine interference and especially wake losses for larger offshore farms.

Model

The decrease of the wind speed in the wake of wind turbines can be compared to the frictional drag exerted to the planetary boundary layer by surface roughness. We calculated the effect to the boundary layer when the wind encounters a wind farm. The wind farm was implemented as a sudden change of the surface roughness. Our calculations showed that after an increase of the surface roughness, the wind speed was decreasing up to large distances downstream. Several wind farm models assume that equilibrium is reached after 5 rows of wind turbines, our model showed that it will take at least 10 rows before equilibrium is reached to some extent. This means that our classical models predict a too high power output for really large wind farms.

Experiment

In order to validate the results of the model we set up an experiment in the boundary layer tunnel of TNO, Apeldoorn. We designed and manufactured 30 wind turbines on a scale of 1:400. The turbines are 25 cm diameter and have a hub height of 25 cm. Our design philosophy has delivered rotors which are very similar to full scale rotors regarding the axial force and wake properties, however regarding the efficiency their performance is less. The power coefficient is about 0.30, while is can be between 0.45 and 0.50 for commercial rotors. In the wind tunnel the surface roughness (excluding that due to the wind turbines) was set to sea conditions: after scaling this was approximately 0.2 mm / 400. Sometimes the roughness was adapted to onshore circumstances as well. We installed several wind farms layouts and measured the wake losses and the effects on the boundary layer.

Results and Discussion

The validation suggests that the wind velocity has not reached its equilibrium value after 5 rows, which indicates that the numerical model can be used for global studies of atmospheric flows above (and behind) large wind farms. The words ?suggests? and ?indicates? were used since the situation in the tunnel was different from the real situation in several aspects. For several reasons the interpretation of the results was difficult. Especially the accurate measurement of the wind speed was a difficulty. A way out for this accuracy problem was the application of differential measurements: we finally set up two farms next to each other in the tunnel.

Conclusion

We can not rely on wind farm models that assume that the wind flow in a farm is stabilizedafter five rows of turbines. Many models will over predict production.

Recommendation

We recommend that much attention is to be paid to improvement of the accuracy in the wind tunnel and to the organization of good experiments with actual farms in the field. Both suggestions have been implemented in successive projects, which already have been started.


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