Humans have been using wind power for over 5000 years, initially to propel sail craft, and later for farming applications such as irrigation and the milling of grain. Today, wind generated energy is one of the fastest growing source of renewable energy, and this growth is expected to continue well into the twenty-first century.
Wind is generated by the uneven heating of the earth by the sun. This differential heating drives a global atmospheric convection system reaching from the Earth’s surface up to the stratosphere. Wind energy is the conversion of the kinetic energy stored in wind into a useful form of energy such as electricity. It is estimated that the total amount of economically extractable energy available from wind is considerably more than present human energy consumption. In comparison to conventional fossil fuels, wind energy is renewable and clean, widely distributed, and produces no greenhouse gas emissions during operation.
Wind turbines can rotate about either a horizontal or a vertical axis. The vast majority of large scale commercial wind turbines are horizontal-axis wind turbines (HAWT), as presented in the schematic1, showing the main components and their percentage cost relative to the wind turbine cost.
Wind Turbine Cost Breakdown
In HAWTs, the main rotor shaft, hub and electrical generator are mounted at the top of a tower in the nacelle. In upwind turbines, where the rotor blade is located upwind of the tower, the nacelle is mechanically oriented into the wind, in order to optimise the extraction of wind energy.
Most HAWTs have a gearbox, which converts the slow rotation of the rotor blades into a rotational speed that is more suitable to drive an electrical generator. Direct drive designs are also available, as shown in the Figure. The most common configuration for large scale commercial turbines is a three-bladed rotor, where the length of a single blade can exceed 60m. The blades rotate at speeds in the region of 10-20 revolutions per minute, with the upper limit being constrained by the blade tip velocity and noise requirements.
Although older HAWTs generally operated at constant speed, most modern examples are variable-speed turbines using a solid-state power converter to interface with the transmission system.
In order to economically generate wind energy, wind turbines are usually assembled into a wind farm. A wind farm is a group of wind turbines in the same location used for production of electric power. A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, although the land between the turbines may still be used for agricultural or other purposes. A wind farm may also be located offshore, as shown below.
Today, there are thousands of wind turbines operating worldwide, with a total nameplate capacity of 194,400 MW. World wind generation capacity more than quadrupled between 2000 and 2006, doubling about every three years. The United States pioneered wind farms and led the world in installed capacity in the 1980s and into the 1990s. In 1997 German installed capacity surpassed the United States, a position it held until passed by the United States in 2008. Today China is rapidly expanding its wind installations and in 2010, became the world leader.
