This paper presents the case study of possible design improvements for 20 MW Vertical Axis Wind Turbine (VAWT) rotors. Structural optimizations of a 3-bladed carbon-fiber H-rotor and Darrieus rotor are performed for different rotor sizes and heights. The results are used to construct rotor mass scaling trends for VAWT rotors. Furthermore, critical failure modes and their driving loads are identified. To mitigate fatigue and buckling in the blade, a non-constant chord distribution is recommended. Furthermore, further research on improving the buckling performance of the H-rotor strut is recommended. Nomenclature A Projected rotor area, m 2 a Curve coefficient of power curve fit b Curve exponent of power curve fit C p Power coefficient c ineq Curve exponent of power curve fit D Rotor diameter, m H Rotor height, m m Mass, kg R Rotor radius, m P Power capacity, W
This paper investigates the load alleviation capability of a camber morphing blade tip on multi megawatt scale HAWTs. A span-wise variation of camber at the outer blade region is proposed that blends seamlessly to the non-morphing part of the rotor blade. The NREL 5MW reference turbine is used as baseline design, where the outer 30% of the blade is linearly cambered with the maximum camber realized at the blade tip. The results obtained during simulations, following the IEC standard, indicate that the camber morphing blade tip is able to mitigate vibrational loads, such that reduction in fatigue loads range between 8%-37% for most wind turbine components. Furthermore, the controller is also capable of reducing ultimate loads due to extreme turbulence in the order of 30%-60%. Finally, a large reduction in peak-to-peak response, in the order of 43%-90%, is achieved for several turbine components under wind gust or extreme direction change.
A framework based on isogeometric analysis is presented for parametrizing a wind turbine rotor blade and evaluating its response. The framework consists of a multi-fidelity approach for wind turbine rotor analysis. The aeroelastic loads are determined using a low-fidelity model. The model is based on isogeometric approach to model both the structural and aerodynamic properties. The structural deformations are solved using an isogeometric formulation of geometrically exact 3D beam theory. The aerodynamic loads are calculated using a standard Blade Element Momentum(BEM) theory. Moreover, the aerodynamic loads calculated using BEM theory are modified to account for the change in the blade shape due to blade deformation. The aeroelastic loads are applied in finite element solver Nastran, and both the stress response and buckling response are extracted. Furthermore, the capabilities of Nastran are extended such that design dependent loads can be applied, resulting in correct aeroelastic sensitivities of Nastran responses, making this framework suitable for optimization. The framework is verified against results from the commercial codes FAST and GH Bladed, using the NREL 61.5m rotor blade as a baseline for comparison, showing good agreement.
Unlike Single-Rotor wind turbines, stability analysis of Multi-Rotor wind turbines is still in its initial stages. This paper presents the modal analysis of a Quad-Rotor wind turbine and identifies the new modes or possible instability modes that are otherwise not present on a Single-Rotor wind turbine. Multi-Blade Coordinate transformation scheme is adapted to a Quad-Rotor wind turbine to write the system’s equation of motion in fixed-reference frame followed by Eigenvalue analysis to determine the natural frequencies and mode shapes of the Quad-Rotor wind turbine. A Campbell diagram of the Quad-Rotor wind turbine is presented. Results indicate that the Quad-Rotor turbine is soft-soft to the first tower modes (fore-aft, side-side, and torsion). Furthermore, the modes with low natural frequency other than the tower modes are a combination of tower, boom, and blade modes. Therefore, due to the presence of blade modes, the modal frequency of these modes increases or decreases with increasing rotor speed due to centrifugal stiffening.
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