This paper gives an overview of the design and wind tunnel test results of the wind turbine dedicated airfoils developed by Delft University of Technology (DUT). The DU-airfoils range in maximum relative thickness from 15% to 40% chord. The first designs were made with the XFOIL code. The computer program RFOIL, which is a modified version of XFOIL featuring an improved prediction around the maximum lift coefficient and the capability of predicting the effect of rotation on airfoil characteristics, has been used to design the airfoils since 1995. The measured effect of Gurney flaps, trailing edge wedges, vortex generators (vg) and trip wires on the airfoil characteristics of various DU-airfoils is presented. Furthermore, a relation between the thickness of the airfoil leading edge and the angle-of-attack for leading edge separation is given.

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In modern wind turbine blades, airfoils of more than 25% thickness can be found at mid-span and inboard locations. At mid-span, aerodynamic requirements dominate, demanding a high lift-to-drag ratio, moderate to high lift and low roughness sensitivity. Towards the root, structural requirements become more important. In this paper, the performance for the airfoil series DU FFA, S8xx, AH, Risø and NACA are reviewed. For the 25% and 30% thick airfoils, the best performing airfoils can be recognized by a restricted upper-surface thickness and an S-shaped lower surface for aft-loading. Differences in performance of the DU 91-W2-250 (25%), S814 (24%) and Risø-A1-24 (24%) airfoils are small. For a 30% thickness, the DU 97-W-300 meets the requirements best. Reduction of roughness sensitivity can be achieved both by proper design and by application of vortex generators on the upper surface of the airfoil. Maximum lift and lift-to-drag ratio are, in general, enhanced for the rough configuration when vortex generators are used. At inboard locations, 2-D wind tunnel tests do not represent the performance characteristics well because the influence of rotation is not included. The RFOIL code is believed to be capable of approximating the rotational effect. Results from this code indicate that rotational effects dramatically reduce roughness sensitivity effects at inboard locations. In particular, the change in lift characteristics in the case of leading edge roughness for the 35% and 40% thick DU airfoils, DU 00-W-350 and DU 00-W-401, respectively, is remarkable. As a result of the strong reduction of roughness sensitivity, the design for inboard airfoils can primarily focus on high lift and structural demands.

This paper gives an overview of the design and wind tunnel test results of the wind turbine dedicated airfoils developed by Delft University of Technology (DUT). The DU-airfoils range in maximum relative thickness from 15% to 40% chord. The first designs were made with XFOIL. Since 1995 RFOIL was used, a modified version of XFOIL, featuring an improved prediction around the maximum lift coefficient and capabilities of predicting the effect of rotation on airfoil characteristics. The measured effect of Gurney flaps, trailing edge wedges, vortex generators and trip wires on the airfoil characteristics of various DU-airfoils is presented. Furthermore, a relation between the thickness of the airfoil leading edge and the angle-of-attack for leading edge separation is given.

Abstract-The objectives of this paper are to investigate the feasibility of a 10 MW generator for a direct-drive wind turbine and to compare the generator systems for pitch control and for active speed stall control. The idea behind the active speed stall control concept is to make a rotor that is as simple as possible, and therefore very robust and suitable for offshore wind turbines. This is done by removing the pitch control of the blades. Above rated wind speed, the power is not controlled by controlling the pitch, but by controlling the rotor speed: the rotor speed is so much reduced that the aerodynamic power is limited to the rated value. A rough 10 MW permanent-magnet directdrive generator design is presented, indicating that such a generator is feasible. It is shown that for a thorough evaluation of active speed stall control, more knowledge is required about changes in the wind speed. However, a considerable increase in generator system cost is necessary to enable active speed stall control.