Figure 5.11 Rotors with Vertical Axes
Figure 5.11 Rotors with Vertical Axes opposite sides. Near the axis, the blades overlap so that the redirected wind can flow from one blade to the other. The rotor blades also utilize the lift principle so that the efficiency of the Savonius rotor is little better than that of simple drag devices. However, the efficiency is much worse than that of good lift devices, reaching maximum power coefficients of the order of 0.25 (Hau, 2000). Savonius rotors have the advantage that they can start at very low wind speeds. They are therefore used for ventilation purposes on buildings or utility vehicles. Besides poor efficiency, Savonius rotors have the disadvantage of a high material demand. Thus, they are not used in large systems.
The Darrieus rotor was developed by the Frenchman Georges Darrieus in 1929. The Darrieus rotor consists of two or three rotor blades that have the shape of a parabola. The profile of the rotor blades corresponds to lift devices so that the Darrieus rotor utilizes the lift principle. Due to its vertical axis the angle of attack at the Savonius rotor changes continuously. The efficiency of the Darrieus rotor is much above the efficiency of the Savonius rotor; however, it reaches efficiencies of only about 75 per cent of modern rotors with horizontal axes. A grave disadvantage of the Darrieus rotor is that is cannot start on its own: it always needs an auxiliary starting system that can be a drive motor or a coupled Savonius rotor.
A further development of the Darrieus rotor is the H rotor or H-Darrieus rotor. This rotor is also called the Heidelberg rotor after the company Heidelberg Motor. A permanent-magnet generator is directly integrated into the rotor structure and needs no gearbox. The rotor works in the same way as the Darrieus rotor as a lift device. The three rotor blades of the H rotor are attached vertically. Supports to the vertical axis help the rotor maintain its shape. The very robust H rotor was designed for the extreme weather conditions existing in high mountains or in Antarctica.
Wind power plants with vertical axes have some advantages. Their structure and their assembly are relatively simple. The electric generator and the gear as well as all electronic components can be placed on the ground. This simplifies the maintenance compared to rotors with horizontal axes. Rotors with vertical axes need not be oriented into the wind; therefore, they are perfectly suited for regions with very fast changes of wind direction.
However, these advantages have not resulted in a breakthrough for wind generators with vertical axes. Today, almost all wind power plants use rotors with horizontal axes; systems with vertical axes are only used for very special applications. The poorer efficiency and higher material demand of systems with vertical axes have been the deciding factors for the market dominance achieved by horizontal axis turbines to date.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.