Design of a 21 m blade with Risø-A1 airfoils for active stall controlled wind turbines

This is the final report, from the project, "Design of a Rotor/Airfoil Family for Active Stall-regulated Wind Turbines by Use of Multi-point Optimization". It describes the full scale testing of a 21 m wind turbine blade specially designed for active stallregulation. Design objectives were increased ratio of produced energy to turbine loads and more stable power control characteristics.

Both were taken directly into account during the design of the blade using numerical optimization. The blade used theRisø-A1 airfoil family, which was specially designed for operation on wind turbine blades. The new blade was designed to replace the LM 21.0P blade. A measurement campaign was carried out simultaneously on two identical adjacent wind turbines where onehad the new blades and the other had LM 21.0P blades.

Power and loads including blade section moments for the new blades were measured to assess the characteristics of the new blade. Airfoil characteristics, power curve and fatigue loads were derived onbasis of the measurements. Most of the design criteria for the new blade were met. The new blade had a reduced weight of 4% reducing blade cost compared with LM 21.0P.

The measurements showed that the wind turbine with the new blades had the same energyproduction as the wind turbine with LM 21.0P blades but at the same time a 15% decrease in blade fatigue loads. However, the derived airfoil characteristics for the new blade were not in good agreement with the expected characteristics.

The new blade wasmore sensitive to roughness and imperfections at the leading edge than initially foreseen and a high relative thickness in the tip region caused an unexpected drop in the maximum lift coefficient. This lead to discrepancies between the initial expectedand actual pitch control characteristics.

It could be concluded that the new LM 21.0 ASR blade could replace the LM 21.0P leading to improved cost efficiency and that the Risø-A1 airfoils were well suited for active stall control. With the newestablished knowledge of the actual airfoil characteristics, a possible future blade design could be made also with improved power control characteristics.