Designing and testing a flexible trailing edge for active load reduction on wind energy rotor blades

Abstract

Facing limited resources of fossil energy, an enduring supply of energy to sustain human civilization becomes challenging. Therefore, regenerative sources of energy are increasingly important. Especially for providing electrical power, wind energy is a promising choice. To allow the best use of the limited installation locations, wind energy turbines have grown to very large sizes utilizing the stronger wind in greater altitudes. Due to the square cube law enlarging technical structures increases the rigidity by the power of two, whereas the mass increases by the power of three. As a consequence, any structure will reach a limit, where further growth becomes impossible due to its own structural weight. For wind energy turbine blades, this limit is to be reached in the near future. A reduction of loads occurring at the rotor blade roots is a possibility to overcome this limitation and to allow a further growth of wind energy turbines and their blades. Since fatigue loads are the main design factor for long blades, reducing these loads is necessary. One solution therefore is the installation of a moving trailing edge to the outer part of the rotor blades comparable to a control surface on aircraft. By adjusting the trailing edge according to the inflow, wind gradients, tower shock and even gusts can be alleviated. Due to the necessity of wind energy turbines to work in harsh environments over long times without maintenance, only a completely sealed solution is feasible keeping water, dirt and insects out of the mechanism. Based on this, a flexible trailing edge has been designed, developed and tested at DLR within the SmartBlades projects. The presentation will provide an overview of the basic concept of the trailing edge, some design considerations and the modeling to derive the final design. The trailing edge itself consists of a glass fiber prepreg structure with elastomer covers to provide the sufficient strain for the movement and environmental sealing at the same time. For the experimental investigation, a demonstrator airfoil section is built. It has been tested in the DLR lab to compare the simulated structural behavior with the measurements. Furthermore, a rotating test has been undertaken at the Danish Technical University to obtain the aerodynamic polars in the relevant environment as well as to demonstrate the load reduction. Finally a wind tunnel test was done at the University in Oldenburg to investigate the lift polars for seven positions of the flexible trailing edge in detail in a more controlled airflow. In the presentation, some representative results of all measurement campaigns will be provided.