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Horizontal Axis Wind Turbine
Horizontal Axis Wind Turbine (HAWT), Onshore
Perhaps the most commonly recognizable icon of the renewable energy industry, the horizontal axis wind turbine (named because of the horizontal orientation of the central rotor shaft that gives mechanical power to the generator) has come a long way from its historic origins in Europe where the Dutch had perfected a design for use in milling grains and draining water. The first use of a windmill for generating electricity was a horizontal axis turbine based on the Dutch model. Such early wind turbines typically had at least four blades, and sometimes many more. The first megawatt capacity turbine, built in Vermont in 1942, had only two large blades. There are some interesting modern examples of single-blade turbines as well. Standard contemporary utility-scale models typically have three blades and can have capacities as high as 10 megawatts.

An interesting historical variation was the Éolienne Bollée turbine, named after the inventor who designed it in 1885. Similar to contemporary CWAT turbines, they included an outer ring and stationary blades that directed the wind into the rotor. They often had aesthetic embellishments in keeping with the architectural style of the time and were sold mostly to the French aristocracy.

The capacity of the modern HAWT is a function mostly of the overall outside diameter as measured from tip to tip, with some larger models exceeding 100 meters.

Smaller units can be designed to rotate with assistance of a vane on the downwind side. Larger models must be turned more slowly by computer guided gears.

Horizontal axis wind turbines typically harness between 40% and 60% of the kinetic energy of the wind that passes within the blade swept area. The use of concentrating hoods can increase this efficiency.