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The concept of a wind driven rotor is ancient, and electric motors have been widely disseminated, both domestically and commercially, in the latter half of the 20th century. Making a wind turbine may seem simple, but it is a big challenge to produce a wind turbine that:
- Meets specifications (frequency, voltage, harmonic content) for standard electricity generation with each unit operating as an unattended power station;
- Copes with the variability of the wind (mean wind speeds on exploitable sites range from 5 m/s to 11 m/s with severe turbulence in the earth’s boundary layer and extreme gusts up to 70 m/s); and
- Competes economically with other energy sources.
The traditional ‘Dutch’ windmill (Figure 3.1) had proliferated to the extent of about 100,000 machines throughout Europe at their peak usage in the late 19th century. These machines preceded electricity supply and were indeed wind-powered mills used for grinding grain. Use of the wind for water pumping also became common. The windmills were always attended, sometimes inhabited and largely manually controlled. They were also characterised by direct use of the mechanical energy generated on the spot. They were integrated within the community, designed for frequent replacement of certain components and efficiency was of little importance.
Figure 3.1: Traditional ‘Dutch’ Windmill
Source: Garrad Hassan
In contrast, the function of a modern power-generating wind turbine is to generate high quality, network frequency electricity. Each wind turbine must function as an automatically controlled independent ‘mini-power station’. It is unthinkable for a modern wind turbine to be permanently attended, and uneconomic for it to be frequently maintained. The development of the microprocessor has played a crucial role in enabling cost effective wind technology. A modern wind turbine is required to work unattended, with low maintenance, continuously for in excess of 20 years.
Although most of the largest wind turbines now employ active pitch control, in the recent history of wind turbine technology, the use of aerodynamic stall to limit power has been a unique feature of the technology. Most aerodynamic devices (aeroplanes and gas turbines, for example) avoid stall. Stall, from a functional standpoint, is the breakdown of the normally powerful lifting force when the angle of flow over an aerofoil (such as a wing section) becomes too steep. This is a potentially fatal event for a flying machine, whereas wind turbines can make purposeful use of stall as a means of limiting power and loads in high wind speeds.
The design requirements of stall regulation led to new aerofoil developments and also the use of devices, such as stall strips, vortex generators, fences, Gurney flaps, for fine tuning rotor blade performance. Even when not used to regulate power, stall still very much influences aerofoil selection for wind turbines. In an aircraft, a large margin in stall angle of attack, compared to the optimum cruising angle is very desirable. On a wind turbine, this may be undesirable and lead to higher extreme loads.
The power train components of a wind turbine are subject to highly irregular loading input from turbulent wind conditions, and the number of fatigue cycles experienced by the major structural components can be orders of magnitude greater than for other rotating machines. Consider that a modern wind turbine operates for about 13 years in a design life of 20 years and is almost always unattended. A motor vehicle, by comparison, is manned, frequently maintained and has a typical operational life of about 160,000 km, equivalent to 4 months of continuous operation.
Thus in the use rather than avoidance of stall and in the severity of the fatigue environment, wind technology has a unique technical identity and unique R&D demands.
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