A Numerical Study For Improving The Energy Capture Of Darrieus Wind Turbines

Wind energy is a green, non-pollution and renewable energy source which hasenormous potential for energy production. Darrieus type vertical axis wind turbineoffers some distinct advantages relative to horizontal axis turbines, including simplestructure, easy maintenance and no yaw system required for orientation into the wind.In this thesis, the aerodynamic performance of a straight-bladed Darrieus-typevertical axis wind turbine was numerically studied with the aid of an in-housedeveloped computational fluid dynamics (CFD) code. In addition, effectiveness oftwo proposed methods in improving energy capture of this type of wind turbine wasexamined. A summary of the work carried out in this study is presented as follows:1. An overview of recent research on Darrieus type VAWT is presented. Thesestudies support the main theme of our research which is focused on identifying newmethods for the improvement of aerodynamic performance of the Darrieus VAWT.In addition, the effect of rotor solidity and aspect ratio on the power coefficient ofthis type of VAWT is also studied using three-dimensional numerical simulations.2. The effectiveness of application of end plates for Darrieus VAWT performanceenhancement is examined using3D CFD analyses. Numerical results indicate thatthe turbine rotor blades with end plates give higher efficiency than those of withoutend plates. The improvement in rotor efficiency are assessed in terms of pressurecoefficient distribution on the blade surface and the vorticity field near the blade. It is found that the existence of end plates minimizes3D flow effects and reduces thetip loss at the end of each blade.3. A novel passive flow control method which uses a small-size cylinder in frontof the leading edge of blade to suppress flow separation is studied. The proposedmethod is applied to a Darrieus VAWT and its effectiveness is validated by means of2D computer simulations. The power coefficient of the turbine whose blades havesmall cylinders in front of their leading edges is apparently higher than that of aconventional Darrieus turbine with similar blade dimensions. Then, streamlines andvorticity contours around the turbine blade airfoil section are analyzed. The obtainedresults show that the size of separation bubble on the upper surface of the airfoil canbe considerably reduced due to installation of small cylinders in front of turbineblades.