Wind tunnel and rotation tests results of a flexible trailing edge for wind energy turbine blades

Abstract

In the last decades the dimensions of wind energy turbines have increased to allow higher power output. This results in a higher loading of the blades due to their own weight. Especially cyclic bending moments at the blade root caused by the weight of the blade, wind shear and gust loads are limiting the blade’s lifetime at larger scales. Lowering these loads could allow larger blades, a longer lifetime or a lighter construction. Load reduction can be achieved by using a trailing edge flap at the outer region of the blade, comparable to an aircraft’s control surface, which is capable of moving several times per blade revolution. The basic principle of the flap is to add an additional degree of freedom to influence the blade aerodynamics and there predominantly the lift coefficient. The advantage of the flap is its higher deflection velocity compared to the conventional blade pitching system which is already present on any modern wind energy turbine. In contrast to aircraft, sealing against environmental media, such as rain, dust, insects and so on is much more important to allow a high lifetime. Therefore, a flexible and gapless trailing edge has been designed within the SmartBlades projects for the mentioned purpose. The presentation will provide a short overview about the concept and the design of the trailing edge and the demonstrator used for the rotation and wind tunnel tests. It mainly focuses on the experiments and the test results. The rotation test took place at the rotating test rig for 2m span by 1m chord blade sections at the Danish Technical University Denmark (DTU) at their campus in Risø. This test was done to measure the flap polars in the free field with turbulent inflow and to investigate the load reduction potential of the trailing edge flap with simple control laws. Therefore, an angle of attack feed forward, a bending moment feedback and a rotor azimuth proportional control of the flap were investigated. The goal of these was to reduce the bending moment in the 10m span boom which was used to mount the rotor blade section to the rotating test rig. The focus of the wind tunnel test was to precisely measure the flap polars in laminar flow. In both tests, the functionality of the flap could be demonstrated as shown in the difference in the lift coefficient for the seven tested flap angles. Furthermore, a reduction of the flapwise bending moment in the mounting boom was successfully demonstrated in the rotating test. This reduction was achieved with the proportional angle of attack feedforward control of the flap deflection.