DunoAir: Electricity from Wind
About wind power
Wind power is one of the oldest forms of energy used by man. Windmills have been used for centuries to make mechanical work easier. In the 19th century, the first attempts were made to convert the mechanical energy into electrical energy. The advance of wind power began with the progress in technology, the increasing demand for energy and growing awareness for the environment.
Solar radiation heats the earth’s surface and the air masses above it to different degrees depending on the geographic region. Thus, the radiation at the equator is stronger than at the poles. Water masses heat up more slowly than land masses but also release their heat more slowly, as a result land masses warm up faster during the day but also cool down faster again at night compared to water masses. The rising air in very hot regions creates areas of low pressure while high pressure areas form in cooler regions. To compensate for these differing pressure areas, air flows from high to low pressure areas creating wind.
Wind, therefore, is nothing but moving air.
In the generation of electricity by wind power, the wind movement is converted into rotational energy by means of the rotor blades which transfer the wind’s energy into a rotary motion. The rotor transmits this rotational energy to a generator which produces electric current from it. The amount of electricity produced depends essentially on the diameter of the rotor, the generator’s output, the hub height, and the wind strength.
In the case of a wind turbine, therefore, the kinetic energy is converted into electrical energy.
When converting kinetic energy into rotational energy, all common types of turbine use the principle of aerodynamic lift. If an air flow has to flow around a convex-shaped rotor, a positive pressure forms under the wing, while a negative pressure prevails above the wing. These differences in pressure create an upward force which sets the rotor blades of the wind turbine in motion.
Wind power becomes usable for electricity generation above a wind speed of 3 to 4 m/s, depending on the turbine type. The rotor blades begin to turn above this wind speed. The output increases with the cube of the wind speed. If the wind speed doubles, the output increases eightfold. The generators achieve the rated power, that is the maximum energy yield, at 12 to 15 m/s, which they then keep constant even at higher wind speeds. The active blade pitch at different wind speeds optimises the energy yield of the pitch-controlled Enercon systems. This state-of-the-art, efficient control technology in which the power is adjusted to the wind speed by rotating the rotor blades (controlling the angle of incidence), guarantees operation of the wind turbines even in strong winds and storms (above 25 m/s). Only above a wind speed of 30 m/s (hurricane strength) are the Enercon wind turbines switched off automatically due to the wind being too strong.
When converting energy into the various energy forms, some of the energy utilised is released into the environment unused. Thus, purely physically, a wind turbine can only use 59% of the kinetic energy contained in the wind and convert it into mechanical energy. However, due to aerodynamic, mechanical and electrical losses, this theoretical maximum calculated by Albert Betz is not achieved in practice. Thus, aerodynamic losses arise due to friction and turbulence on the rotor blade and the swirl effect in the rotor’s wake. Mechanical losses are caused by friction in the bearing and the gearbox as well as in the generator itself. Heat generated by electrical resistances in the converter and cables cause electrical losses. Due to these conversion losses, our state-of-the-art Enercon wind turbines now achieve a yield of between 40 and 50%.