Blade sections close to the axis rotate more slowly and this results in reduced aerodynamic efficiency. However, it is evident that much of the blade surface is close to the axis. The blade shape is a so-called troposkein curve and is loaded only in tension, not in bending by the forces caused as the rotor spins. The Darreius design (Figure 3.3) is more efficient structurally. The vertical axis design also involves a lot of structure per unit of capacity, taking account of cross arms in the H type design (Figure 3.2). In addition, it was not found to be feasible to have the gearbox of large vertical axis turbines at ground level because of the weight and cost of the transmission shaft.įigure 3.2: H type vertical axis wind turbine, Source Garrad Hassan However, they are inherently less efficient (because of the variation in aerodynamic torque with wide range in angle of attack over a rotation of the rotor). Vertical axis designs were considered with expected advantages of omni-directionality, and having gears and generating equipment at tower base. Significant consolidation of design has taken place since the 1980s, although new types of electrical generators have also introduced further diversification.
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