NUMERICAL ANALYSIS OF EFFECTS OF SUPPORT STRUCTURE & GROUND PROXIMITY ON THE PERFORMANCE OF VERTICAL AXIS WIND TURBINE

Abstract

Energy crisis is a major concern all around the globe. With the global focus on exploring more efficient and cost effective wind energy based power generation techniques, vertical axis wind turbines (VAWT) have emerged as a promising option for urban roof top applications. A Computational Fluid Dynamics study was undertaken in order to investigate the flow configurations. On a fixed pitch straight bladed VAWT with three Darrieus H-type blades we also studied the effect of third dimensionality, support arms, central hub on the overall performance. The turbine under consideration has a diameter of 2.5 meters with expected output of approximately 1.5KW. Symmetric airfoils NACA0022 with a chord length of 0.2 meters have been employed as blades of the wind turbine. This type of small VAWT are expected to perform better on roof tops of the built-up urban area. Numerical analysis is performed out using sliding mesh concept in commercial CFD software ANSYS FLUENT 13. Accuracy of numerical simulations was validated by conducting grid independent study, time step sensitivity, spatial dependence and effects of turbulence modelling studies. Torque generated by one blade, overall torque, Tip Speed Ratio (TSR) and power coefficient (Cp) were the performance parameters considered to evaluate the VAWT. Initially, we performed the two dimensional analyses to compute performance parameters of the VAWT. Since, two dimensional results were limited in their practical application; therefore we performed finite unit length three dimensional analyses. We then compared these results with the results obtained from two dimensional studies. Later, we included the support arms, the central disc & shaft assembly in the finite unit length 7 computational model to determine their effect on the overall performance of the turbine. It was observed that the struts and central hub assembly induce additional drag, cause flow distractions and generate strong vortices causing a substantial decrease in the performance parameters of the turbine. It was also found that both cases show a similar trend of the torque ripple for any one blade for the upstream path. On the contrary the blades experience a drop in performance from 220o to 360o due to the added struts and central hub assembly. Finally, the simulations were run for complete turbine model (d=2.5m, c=0.2m, b=2.5m). The numerical accuracy was checked by generating two meshes in different meshing software’s. The results for the two cases overlap which further support accuracy of the numerical solution. The simulations were performed over the range of TSR values (1.5-4.5) for each case. An interesting finding was the prediction of the optimum turbine height from the ground level at which the turbine aerodynamic efficiency is not affected by the ground shear effects. It appeared that there would be no loss in performance due to ground shear forces if the turbine is put at a height of 1.5metres from the ground. At the heights of 0.2m, 0.5m or 0.85m from the surface of the ground than it will experience drop in the performance of about 13%, 10% and 6% respectively.