Numerical Modeling And Design Of A Multi-Bladed Savonius Vertical Axis Wind Turbine

Among the vertical axis wind turbines, we find two main families: the Darrieus (H or V rotor) and the Savonius (S rotor) which work respectively on lift and drag. The first family has a low starting torque and a high efficiency and the second has a high starting torque and a low efficiency. The trend is to find a cheap way to increase the starting torque and the efficiency of these turbines. Mostly, designers either increase the number of blades or work on their geometry. Some designs include both rotors on the same axis to form a mixed lift-drag turbine. Although Savonius turbines have a lower aerodynamic efficiency, they have a high starting torque. Consequently, they start producing power under lower winds. Moreover, they continue producing power even under very strong winds. As a result, Savonius turbines make a very good use of the wind resource available because they have a broader working interval; their capacity to transform wind into usable energy is higher than horizontal axis rotors of the same swept area. Besides, Savonius rotors overall costs remain low; these rotors are also easy to manufacture and to operate. Despite their low efficiency, they are suitable turbines for populated areas where the wind speed is at minimum.The efficiency of Savonius turbines is low because of the"negative drag"exerted on the convex part of the blades, this drag results in a negative torque lowering the efficiency.The best way to improve the Savonius rotor efficiency is:(1) Preventing the wind from striking the convex parts: it can be done by hiding the convex part of the blades behind a shield or a vane. (2) Increasing the number of blades and finding the related optimum geometry. The aim of the present study is to design a cheap high efficient vertical axis drag wind turbine. The concept chosen is a six-bladed Savonius rotor. Among all the drag turbines available on the market, no model with wind shield has been found. Therefore, the type of turbine designed in the present study is considered new. The full turbine has been designed by CAD and manufactured; its aerodynamic behavior tested by CFD. The CAD design was modified according to the CFD computations results. The present thesis mainly shows the results of the two-dimensional CFD computations and also some observations made during operation of the prototype. In theory this new design is a success because the power coefficient improves significantly. However, in application it is not perfect yet because it also brings some drawbacks: orientation of the shield towards the wind, friction and stability. The principal issue of adding external devices is that: if the inlet is not maintained at its optimum position, the efficiency would drop immediately. The rotor alone (without shielding) is a simple but effective configuration, it can start at very low wind speeds with wind blowing from any direction. As a reason, the future work should be focused on improving the design of the rotor (geometric parameters of the blades, number of blades, dimension ratios).This study is a success because it opens the way to a new type of cheap and effective wind turbines.