Wind energy
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Description
At first sight, it would seem that wind energy is the ideal form of renewable energy. It is easily harnessed and, in many places in the world, it is reasonably constant. However, it can never be the panacea for the energy crisis.
Detailed description
Historically, wind energy has been used for many centuries for milling grains and for pumping water. The variability of the wind is not a major issue for these applications, because it is possible to do the work when the wind conditions are suitable. This is not necessarily the case for generating electricity, where the consumers require a 24/7 service.
An aeolian generator is typically a mast-mounted generator, driven by a (usually) three-bladed impeller. For commercial wind sites, each generator has a peak capacity of one to six megawatts. The sweep of the blades may have a diameter of up to 100 m. Typically, full capacity is reached only when the average wind speed reaches about 15 or 16 m/s. To prevent damage to the installation, it is necessary to stop the rotation and feather the blades at windspeeds exceeding about 24 m/s. At windspeeds below 15 m/s, the electrical output drops non-proportionally. The graph shows this non-linearity quite clearly with the output at 10 m/s down to about 40% of peak output, and virtually no output below 4 m/s.
The second graph shows the wind speed with respect to time at a favourable location (north of Scotland). The wind speed axis is the annual average. By combining the data of the two graphs, it can be seen that the turbine will give practically zero output for about 23% of the time, will work at its full rated output for 6% of the time and will give about 50% of its output for half the time. However, this is not the full picture because the wind speed is never constant but is always gusty and the turbine/generator combination has considerable inertia. In fact, these two factors reduce the efficiency somewhat, but this can be partially overcome by a wind farm over a considerable area. Another point to be taken into consideration is the diurnal variation of wind speed. If the average speed is higher at night, when the energy demand is least, then it may be less economical to implement than if the velocity is higher during the day. This is a factor to consider particularly in coastal locations, with land and sea breezes.
In practice, the annual average output from a generator at a favourable location would rarely exceed 35% of its peak rated output. In most suitable locations, figures of 10 to 25% would be normal. A wind farm with, say, 10 generators, each of 3 MW capacity, would rarely average more than the equivalent of 8 MW over the year. This effective reduction of capacity is sometimes ignored when wind farms are planned. The corollary is that conventional electricity generation capacity is required to make up the shortfall, especially when there is no wind at all.
