Today, installed grid-connected wind turbine capacity worldwide totals roughly 4,000 megawatts.(10) Installed capacity includes intermediate-size turbines (100 to 400 kilowatts) and some small turbines (1 to 50 kilowatts). Small turbines have proven to be reliable in off-grid applications and now compete in markets for remote power supply worldwide. These machines usually deliver direct current (DC) power for battery charging, water pumping, refrigeration, and other uses.
There are two types of wind turbine design: the horizontal-axis wind turbine, which resembles a windmill, and the vertical-axis wind turbine, which resembles an upright eggbeater. Horizontal-axis wind turbines, the most commonly used, capture the wind's energy with a rotor, usually consisting of two or three blades mounted on a shaft (Figure 22). The rotating shaft is connected to a generator to produce electricity. New wind turbines incorporating incremental improvements in design and construction have continued to reduce the cost of wind energy. Among these features are improved blades, variable-speed generation, simplified mechanisms, state-of-the-art controls, and aerodynamic braking to protect turbines in high winds. The new designs offer improved performance in the form of better energy capture, reduced stress on machine components, and longer life for turbine drive train hardware.