Sustainable wind turbine design for tropical wind conditions

• Main Author: Sundar, Dhivya
• Other Authors: Yang Yaowen
• Format: Thesis-Master by Research
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering::Motors, engines and turbines; Engineering::Civil engineering::Structures and design
• Online Access: https://hdl.handle.net/10356/152446
• Institution: Nanyang Technological University

One of the greatest challenges in designing wind turbines for low speed wind regions is surviving the sudden and excessive loads caused by tropical cyclones and typhoons in these areas. The severe turbulence and the abrupt change in the wind velocities leads to large responses in terms of rotor thrust and generator torque due to the blade pitch angle error in the conventional pitching systems used for load alleviation. This, in turn leads to complete failure of the wind turbine system, possibly resulting in fatal accidents. Hence, passive load alleviation techniques, such as bend to twist coupling of anisotropic composite laminates, are increasingly looking to be adopted. When a composite material is subjected to bending loads, additional twist is induced together with bending. This helps reduce excessive loads in the structure. Bend-twist coupling can be incorporated into the blades of stalled wind turbines to weather extreme loads. Since the spar of a blade is the main load-carrying structure, this technique can be mainly applied on the design of the spar. In this study, a MATLAB program is used to analyse five single-ply and five two-ply laminates to calculate their bending-twisting coupling coefficients and stress-strain distributions under an applied moment. For the given loading condition, it is observed that the incorporation of the 250 ply is the most favourable in terms of stress reduction. A cantilever model and a closed (D section) spar are also analysed using ANSYS to determine their bending-twisting coupling characteristics, deformation, rotation, strains and stresses under an applied load. The deflection and rotation values increase with the increase in the ply orientation for the cantilever beam but decrease with the increase in the ply orientation for the d-spar structure. The twist angle is highest for the [45/25]s degree orientation and decreases for higher orientation angles in the beam and is minimal in the case of d-spar, but decreases the deflection and rotation by about 18.7% as compared to the unidirectional ply architecture. Once the spar is designed, the requisite aerodynamic shape of the blade needs to be provided by the blade skin/shell. To improve sustainability in the design of wind turbines, a preliminary study is conducted on flax fibre reinforced composites. Tensile testing is carried out on flax fibre plain weave and twill weave reinforced epoxy composite specimens. It is observed that these composites are significantly weaker than glass fibre reinforced epoxy composites (GFRPs). Hence they can be used to build the non-load-bearing members of a wind turbine blade that do not demand the level of stiffness requirements provided by glass or carbon fibre reinforced composites, like the blade skin/shell. Poor fibre-resin adhesion and the presence of voids are the potential reasons for the reduced stiffness of the tested specimens. The final step is to integrate the skin and spar into a blade structure and study the wind turbine model as a whole. Cp-Max is a tool designed by the Technical University of Munich, developed for high-speed wind turbines. Rethinking the turbine design in terms of material and geometry, focussing on the skin and the spar - the two most important elements of a turbine blade, should be the starting point of working towards solving these challenges. A pitchable 3-bladed rotor of diameter 21 metres and rated power 100 kW is developed with different material options (Carbon/Glass/natural fibre composites) at a rated wind speed of 9 m/s and a rated rpm of 72.7. Both aerodynamic and structural optimisations are carried out for the load case DLC 1_1. Since the cost model has been developed for larger machines, it is not valid for a 100 kW machine. Hence, the simulations have been modified to optimise for minimum blade mass. A desired chord distribution was achieved by carrying out Eigen frequency analysis and the model is structurally optimised. The blade design contains structural reinforcements in the form of pressure side and suction side spar caps and front and rear webs. The blade structure presented is for a blade model consisting of shell skin made of flax fibre reinforced composites of E = 2454 MPa and the sparcaps and webs made of carbon fibre reinforced polymer.