Design and Aeroelastic Analysis of Flexible Wind Turbine Blades: Highly flexible materials in wind turbine blades to reduce maximum loading without compromising energy yield

A wind turbine is designed to produce a maximum amount of energy at minimum cost while it withstands any possible wind condition. In the wind conditions with the highest wind speeds the wind turbine has to cope with the most extreme loads. In these most extreme cases the wind turbine is parked, the blades do not rotate, and the blades deform elastically under these wind loads. An increase in the flexibility of the blades results in higher elastic blade deformations. The main objective of this project is to design blades in a way that the extreme loading on the blades reduces with more than 10 % without compromising the energy yield by making use of flexible materials and the corresponding increased deformations. The reason to focus on load reduction is that a wind turbine can be made cheaper if these loads are reduced. The research methodology is a three-step approach: Firstly a modelling tool is created to design and evaluate blades with new flexible materials at different locations in the blade. Secondly a verification procedure is performed to check the accuracy of the modelling tool. Thirdly this tool is used to design blades with highly flexible materials and to perform an iteration procedure to design the best flexible blade design. The modelling tool is based on the cross sectional software BECAS to design new flexible blades and on the aeroelastic software HAWC2 to analyse the behaviour. This BECAS-HAWC2 modelling tool is based on the existing XANT M-21 wind turbine of which only the blade materials are variable parameters, the rest of the wind turbine remains as it is. A verification procedure compares the modelling tool with two other models of the same blade. The minor differences between several modelled parameters increase the confidence in the BECASHAWC2 model. A material with unidirectional fibres and a highly flexible matrix material is stacked in different orientations to design different blade materials. These flexible materials are introduced in specific locations of the blade. The design exploration approach makes it possible to design and evaluate many different blades using different flexible materials at different locations. The current results show that the best option is to use the flexible material with fibres only in longitudinal and transverse direction in the skin of the blade. Not replacing the full skin but only the part of the root up to the middle of the blade results in the best flexible design. This best design has a reduction in maximum thrust force and maximum root bending moment of respectively 23 % and 26 % compared with the original blade, easily exceeding the predefined goal of 10 %. This significant load reduction is due to a significant blade twist rotation thereby reducing the area exposed to the wind. The annual energy yield is not compromised, it even shows a considerable 11 % increase due to a stall delay effect in the higher wind regimes which is also caused by an increase in blade twist. The best flexible design is a preliminary design that shows promising results. These results show that by using flexible materials in the blade skin a significant load reduction is combined with an increase in energy. This indicates an untapped potential for future wind energy which makes further research on this topic recommended