Aerodynamic response of a twodimensional vertical axis wind turbine to turbulence

Abstract

The increasing globalwarming and climate change has boosted research on sustainable energy in the last few decades leading to a lot of research being carried out on wind energy. Wind turbines have played a major role in harvesting wind energy withHorizontal AxisWind Turbines (HAWTs) being the dominant design. However, for urban and offshore applications, Vertical AxisWind Turbines (VAWTs) have been under consideration for a while. VAWTs have some inherent advantages such as no need for a yawingmechanism, easy operation and maintenance, low noise emission, and better potential performance under skewed and unsteady wind conditions. But due to lack of research on VAWTs, they have not been commercialized on a scale which is equal to that of HAWTs. This study is one of such attempts for a better understanding of VAWTs. The primary aim of this study is to explore and analyse the behaviour of VAWTs under the influence of turbulent wind. This target has been achieved by analysing the loads on the turbine and their effect on its fatigue life under the influence of constant wind and turbulent wind. For this purpose, first one hour turbulent wind fields based on statistical models were generated using the turbulence simulator, ‘TURBSIM’ and two-dimensional turbulent wind fields were extracted from the generated wind fields. Next, the simulations of a two-dimensional vertical axis wind turbine using the generated turbulent wind fields were done in an aeroelastic code named ‘Unsteady Two-Dimensional Vorticity Aerodynamics (U2DiVA)model’. These simulationswere done between the cut-in (4 m/s) and cut-out (25 m/s) wind speeds of the turbine at intervals of 1 m/s. The results from these simulations were the streamwise force, cross-streamwise force and the moment about the quarter-chord points of the airfoils of the blades. A post-processing tool was then developed for the processing of these results. Using this tool and the results of the simulations, the equivalent tangential force, the equivalent radial force and the equivalent moment on the airfoilswere calculated. All calculationswere done for both types of wind. First the equivalent loads at each wind speed were calculated and then the total equivalent loads were calculated. The equivalent loads were calculated for various values of inverse slope (m) of the S-N curve of thematerial used to manufacture the blades of the turbine. The same procedure was repeated with constant wind. Finally,the changes in total equivalent loads under turbulent wind were calculated with respect to total equivalent loads under the constant wind. The variation in these changes with respect to the change in the value ofmwas then calculated. Additionally, the effect of phase angle (angle between the blades and the incoming windstream at the beginning of the simulation) on the equivalent loads was also calculated. The value of m used for this case was 10 as recommended in literature and the four phase angles chosen were 0o, 45o, 90o and , 135o.The equivalent loads at various wind speeds were calculated for the different values of m and various peaks were observed at various wind speeds. It was recommended that the peaks can be avoided using a suitable control scheme to control the rotational speed of the rotor. The changes in total equivalent loads under the influence of turbulent wind were found to be less than 0.2% which indicates that the effect of turbulence on the fatigue life of VAWTs is not so significant. The value of m at which the change was smallest was found to be around 3. Under phase effect, the variation of the radial force and the moment with wind speed seemed to follow Gaussian distribution at phase angles of 45o, 90o and 135o. The total equivalent loads were found out for all the phase angles and the change in these loads were found out with respect to 0o phase angle. These changes were quite high and indicate that the operation of a VAWT at phase with incoming wind can be detrimental for the fatigue life of the VAWT.