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- Title
- Computational Techniques for Wind Turbine Power Prediction
- Creator
- Goyal, Udit
- Date
- 2011-11-28, 2011-12
- Description
-
Wind energy is expected to play an important role in meeting the ever- increasing energy requirements and reducing our dependence on...
Show moreWind energy is expected to play an important role in meeting the ever- increasing energy requirements and reducing our dependence on conventional sources of energy. Wind turbines are broadly classfied as horizontal-axis and vertical-axis depending upon the orientation of the rotor shaft relative to the wind direction. Considerable research has been carried out on horizontal-axis wind turbines, which today are sophisticated and efficient electro-mechanical systems. Continuous research and development in areas of electronics, controls and instrumentation aids in the advancements of this technology. From an aerodynamic point of view the Betz limit is known to impose theoretical limit on the power extraction of propellers. The momentum balance equations show that the maximum of 59.3 percent of free-stream energy can be extracted by propellers. This limit, however is not well de fined, particularly when considering diff user and nozzle-augmented wind turbines with local flow accelerations. In this study the actuator disk approach is used to model the momentum loss across a wind turbine rotor and simulate the Betz limit using Fluent software. This approach is subsequently applied to study the coefficient of performance expected from dif user and nozzle-augmented wind turbines. Vertical-axis wind turbines, on the other hand, are still not completely understood in terms of blade aerodynamics and are the focus of various research studies. Large variations in angle of attack and wake evolution downstream of the blade have a time-dependent e ffect on the blade forces, instantaneous torque and hence the coefficient of power of the turbine. Since Navier-Stokes solutions for vertical- axis wind turbines are expensive and complicated, various low-cost models have been developed based on momentum balance such as single, double and double multiple- streamtube formulations. These models, however, use static lift and drag data for the airfoils as inputs, neglecting the unsteady e ffects on aerodynamic coefficients. In the present study, an alternative approach based on the panel method is explored further for developing a low-cost computational method for simulating the aerodynamics of vertical-axis wind turbines. At each time step an airfoil is represented as a combination of source and vortex distributions which induce a potential in the flow fi eld. A time-stepping mechanism is implemented satisfying the Kutta and the Kelvin Helmholtz condition for the wake evolution behind the rotating blades. The e ffect of this vortex evolution on the aerodynamic forces on the airfoil is studied, focusing on the coefficient of performance (Cp) of the blade. Results show a decrease in Cp values till the wake attains a quasi steady state. A comparison study is performed with other computational models, showing the importance of the wake evolution in time. An optimization of the blade pitch angle is also performed by defi ning a composite variable pitch function in order to improve the torque and hence the instantaneous power from the blades.
M.S. in Mechanical and Aerospace Engineering, December 2011
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