|Power has been extracted from the wind over hundreds of years with historic designs, known as windmills, constructed from wood, cloth and stone for the purpose of pumping water or grinding corn. Historic designs, typically large, heavy and inefficient, were replaced in the 19th century by fossil fuel engines and the implementation of a nationally distributed power network. Energy is essential to human civilization development. With progress of economics and socialization, there is an expanding demand on renewable energy resources to secure energy supply, such as solar power, wind power, tide and wave power etc. As a clean renewable resource, wind power plays a more and more important role in modern life. Power in the wind comes from the transformation of the air that is driven by the heat of the sun, which is abundant, clean and renewable. Wind turbine technology is one of the effective means to implement this renewable resource in order to produce environmentally friendly electrical energy. As it is an intricate system it depends upon the unification of multiple engineering disciplines, which comprises of structures, aerodynamics, controls and electrical engineering.
A wind turbine converts kinetic energy into mechanical power through a rotor, and then converts the mechanical power into electric power through a generator which is linked to the rotor with and without a gearbox. Various types of wind turbines are designed to take advantage of wind power based on the principle of aerodynamics. Depending on the wind turbine rotor orientation, there are two types of wind turbine , horizontal axis wind turbine(HAWT) and vertical axis wind turbine(VAWT).When considering installation sites, there are onshore(free standing or building mounted) and offshore wind turbines. Based on the operation scheme, wind turbine can be divided into stall regulated (fixed pitch) wind turbine and pitch controlled (variable pitch) wind turbines. According to the relative flow direction of the wind turbine rotor, horizontal axis wind turbine are either upwind or downwind turbines.