A wind turbine’s ‘rated’ capacity is its maximum power at an optimum wind speed. This is the value commonly quoted for a wind farm’s ‘installed capacity’. How does this compare with the wind farm’s average power output? That is, what is the capacity factor of wind power?
The capacity factor of a power plant is the ratio of the electrical energy produced in a given period of time to the electrical energy that could have been produced at continuous maximum power operation during the same period. For a conventional fossil-fuel power station, the capacity factor is determined by planned maintenance downtime, unplanned equipment failure, and by shutdowns when the station’s electricity is not needed. For wind and solar energy, power output is also determined by the availability of wind and sunlight. The maximum power output, or ‘installed capacity’, is a rather theoretical value that is rarely reached. It would be clearer to quote the mean power for solar and wind energy, but because peak power is more commonly quoted, it’s important to know the capacity factor as well, to make sense of the peak numbers.
In an earlier post I looked at the difference between peak power and mean power for solar panels. Here I’ll repeat that process for wind energy.
A wind turbine’s power output depends on the wind speed:
Most wind turbines start generating electricity at wind speeds of around 3–4 metres per second (m/s), (8 miles per hour); generate maximum ‘rated’ power at around 15 m/s (30mph); and shut down to prevent storm damage at 25 m/s or above (50mph).
The turbine’s power output varies as the third power of the wind velocity. That is, if the wind velocity doubles, the power output rises eight-fold .
Power output depends critically on the location of the wind farm. A site with a steady 30 mph wind would be ideal. The ratio between the ideal maximum power output and the average power output achieved under real operating conditions is the capacity factor.
Table 1 shows installed capacity, mean power and capacity factor for wind energy in several countries, and it shows the global average. All the data are from key primary sources.
Table 1. Installed capacity and capacity factor for wind energy installed capacity mean power capacity factor MWe MWe Morocco Amogdoul wind farm  60 23 38.1% U.S. (total)  (a) 6740 1941 28.8% U.K. (off-shore)  (b) 304 74 28.7% U.K. (on-shore)  1651 408 27.2% Denmark (total)   (c) 3128 755 24.1% Spain (total)  11615 2534 21.8% Portugal (total)   (d) 1022 202 19.8% Netherlands (total)   (e) 1219 236 19.3% Germany (total)  20622 3482 16.9% India (total)   (f) 4430 704 15.9% Italy (total)   (g) 1718 268 15.6% Poland (total)  153 22 14.6% France (total)   (h) 757 109 14.5% World (total)   (i) 59051 11559 19.6% Table is sorted by decreasing load factor, with World figures at the bottom; Data are for 2006 except where noted. Notes: (a) U.S. data are for 2004; (b) U.K. load factor is for average installed capacity for 2006, not the end-of-year capacity; (c) Denmark data are for 2005; installed capacity from ref ; mean power from ref ; (d) Portugal data are for 2005; installed capacity from ref ; mean power from ref ; (e) Netherlands data are for 2005; installed capacity from ref ; mean power from ref ; (f) India data are for 2005; installed capacity from ref ; mean power from ref ; (g) Italy data are for 2005; installed capacity from ref ; mean power from ref ; (h) France data are for 2005; installed capacity from ref ; mean power from ref ; (i) World data are for 2005; installed capacity from ref ; mean power from ref . Some Conclusions
The global average capacity factor for wind farms is just under 20%. Wind farms on the very best sites, such as those in the North African desert, can achieve capacity factors approaching 40%. France has an exceptionally poor wind resource.
The countries with well exploited wind resources, such as Germany and Spain, tend to have lower capacity factors. That’s because the best sites get developed first, and subsequent wind farm development goes onto progressively poorer sites, thus reducing the average capacity factor. The U.S. has a large installed capacity, yet it has a high capacity factor too, indicating that it has used only the very best sites so far, and still has a very large wind resource left to exploit.
The European Wind Energy Association has set a target for European wind energy for 2010 of 180 GW installed capacity and 500 TWh/yr (57.1 GW) output , corresponding to a 31.7% capacity factor. This is greater than the capacity factor of Europe’s existing installed capacity, and much greater than the global average. Unless they’ve saved the best sites till later, this seems to be a rather optimistic assumption about the future capacity factor.
The total energy contribution from wind power remains very small. The total global wind power output in 2005 was 11.6 GW . This is only as much as four large coal-fired power stations. For example, the UK’s Drax coal-fired power station delivered 2.9 GW of average power in 2006 .
- The capacity credit of wind power
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