The success of wind power is usually measured by the growth in installed capacity. This capacity, however, is peak power: the maximum power at optimum wind speed. The average output of a wind turbine is always lower.
The capacity factor of a wind turbine expresses the ratio of average power output to peak power. Many national and European targets assume a capacity factor of around 30%, while the world’s average capacity factor in 2005 was only 19.6%.
The capacity factor of a wind turbine is determined by:
1) Operation at less than maximum output. Most wind turbines have their maximum output power at wind speeds between 12-15 m/s and 25 m/s. Below that range, the power output decreases by the third power of the wind velocity. In other words, at half the optimal wind speed (7.5 m/s), power output is only one eighth of peak power.
2) Shut down due to excessive or inadequate wind velocity. In general, wind turbines shut down when wind speeds drop below 3-4 m/s or rise above 25 m/s.
3) Other shut downs. These may occur due to scheduled maintenance, equipment failure, or for safety reasons during a grid incident. These same events also determine the capacity factor of conventional fossil fuel power plants, which varies roughly between 50% and 90%.
The average capacity factor differs significantly between countries. Countries with well exploited wind resources tend to have a lower capacity factor. Germany, for instance, has a capacity factor of only 16.9%. That is because the best sites get developed first, and subsequent development goes onto sites with poorer wind characteristics, thus reducing the average capacity factor. The U.S. have a large installed capacity, but a high capacity factor (28.8%), meaning that it still has a large wind development potential left to exploit.
Given this perspective, the target of the European Wind Association seems rather unrealistic. It aims to reach the figure of 180 GW installed capacity in Europe with an average capacity factor of 31.7% by 2010. It is argued that a large part of the growth in the European wind sector in the next two years will be achieved by off-shore wind parks, which are believed to have higher capacity factors. However, figures from the UK from 2005 indicate that this is not necessarily true. The UK on-shore wind park (1,651 MW) has an average capacity factor of 27.4% and the off-shore wind park (304 MW) a capacity factor of 27.2%. Wind characteristics tend to be better for off-shore turbines, but off-shore wind turbines also require more maintenance. This could explain why the UK capacity factor turns out to be similar than that of the on-shore turbines.