Wind turbines can convert wind into electrical energy. The design of these machines can be either a propeller style, with a horizontal axis, or an egg-beater style with a vertical axis. The majority of large-scale wind turbines operating worldwide are of the horizontal-axis type. The wind turbine system includes the rotor, made of wood epoxy or fiber-glass-reinforced polyester blades, which transforms the movement of air into rotational shaft energy; the enclosure, or nacelle, which holds a drive train and generator; the tower, which buttresses the rotor and nacelle; and equipment such as controls and cables to provide connectivity.
Rotor diameters for utility-scale turbines typically range from 50 meters to approximately 90 meters and are set on towers that are about the same size. While offshore turbines are designed for larger rotors, wind turbines for home use are considerably smaller, with rotors measuring eight meters or less. The world's largest wind turbine is being constructed by Sway, a Norwegian company. The 533-feet-tall, ten-megawatt titan, with a rotor diameter of 475 feet, will be able to provide energy for 2,000 houses.
The two factors that determine the output of a wind turbine are the size of the turbine and the wind speed. In particular, the wind speed is critical in determining where to build the system. The energy available in wind is triple the wind speed. Doubling the speed therefore increases the available power by a factor of eight. Even a small step-up in speed translates into a much greater output of electricity, and affects the cost of power produced. In general, small wind turbines require wind speeds of at least nine mph, while utility-scale wind farms depend on a minimum wind speed of 13 mph.
One measurement of the productivity of a wind turbine is the capacity factor, or the comparison of actual production versus what would have been produced at full capacity over the same period of time. For power plants that use fuel and operate without interruption, the capacity factor ranges from 40 to 80 percent. Because wind comes and goes, a wind plant's capacity factor is usually lower, ranging from 25 to 40 percent.
Availability revolves around the percentage of time that a wind plant can generate power. It measures the turbine's reliability. Modern machines show an availability factor of over 98 percent, which is higher than other types of power plants.