A modern wind generator converts air movement into electricity. It employs a turbine linked to a generator and is, in principle, not too dissimilar to the hydroelectric arrangement described in the previous section. The turbine usually takes the form of a three bladed propeller on large wind machines in which the turbine and generator are mounted on a common horizontal axis. Three blades provide optimum stability with the fewest number of elements. However, in small wind turbines, multi-element propellers with more than three blades are not uncommon. Turbines with blades, which in shape are rather reminiscent of curved sails rotating around a vertical axis, also exist, but their wind power to mechanical power conversion efficiency is as much as 50% lower than that of the equivalent horizontal axis machine. Consequently, they have a low likelihood of being adopted by the electric power industry unless there are good non-engineering reasons for this type of installation, which over-ride efficiency considerations.
The main determinant of the power capacity of a horizontal axis wind turbine is the diameter of the blades, although their cross-sectional shape is also important . The larger the diameter of the propeller the larger is the swept area through which the moving air passes. The theoretical power contained in the air stream is the kinetic energy per second passing through the swept area of the propeller. From the definition of kinetic energy given in Chap. 2, the theoretical power is equal in magnitude to the product of the air density, the swept area of the turbine blades, the air velocity cubed, all divided by two [12, 13]. However the German scientist Albert Betz (ca 1927) has shown that the maximum power that can be extracted from a laminar stream of air (i.e., the maximum conversion efficiency) is 16/27ths or 59% of the theoretical value. Modern aerodynamic wind turbine propellers operate with a conversion efficiency of nearer to 40%, with blade drag and air turbulence representing the main sources of this efficiency reduction. This seems low, but it is not too different from the conversion efficiency of steam turbines.
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