The capacity factor of wind power

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?

© BWEA

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).

Wind Turbine Technology, BWEA ^[1]

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 ^[1].

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 [2]	60	23	38.1%
U.S. (total) [3] (a)	6740	1941	28.8%
U.K. (off-shore) [4] (b)	304	74	28.7%
U.K. (on-shore) [4]	1651	408	27.2%
Denmark (total) [2] [6] (c)	3128	755	24.1%
Spain (total) [2]	11615	2534	21.8%
Portugal (total) [2] [7] (d)	1022	202	19.8%
Netherlands (total) [2] [8] (e)	1219	236	19.3%
Germany (total) [5]	20622	3482	16.9%
India (total) [2] [9] (f)	4430	704	15.9%
Italy (total) [2] [10] (g)	1718	268	15.6%
Poland (total) [2]	153	22	14.6%
France (total) [2] [11] (h)	757	109	14.5%
World (total) [2] [12] (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 [2]; mean power from ref [6]; (d) Portugal data are for 2005; installed capacity from ref [2]; mean power from ref [7]; (e) Netherlands data are for 2005; installed capacity from ref [2]; mean power from ref [8]; (f) India data are for 2005; installed capacity from ref [2]; mean power from ref [9]; (g) Italy data are for 2005; installed capacity from ref [2]; mean power from ref [10]; (h) France data are for 2005; installed capacity from ref [2]; mean power from ref [11]; (i) World data are for 2005; installed capacity from ref [2]; mean power from ref [12].

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 ^[13], 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 ^[12]. 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 ^[14].

Related Posts

• The capacity credit of wind power

References

1. Wind Turbine Technology, BWEA Briefing Sheet, British Wind Energy Association (2005)  (WebCite cache)
2. Global Wind 2006 Report, Global Wind Energy Council (2007)  (WebCite cache)
3. Wind Web Tutorial, American Wind Energy Association
4. Capacity of, and electricity generated from, renewable sources, Annual tables: Digest of UK Energy Statistics (DUKES), BERR (2007)  (WebCite cache)
5. Wind Power, Federal Ministry of Economics and Technology, Germany
6. Renewables and Waste in Denmark in 2005, International Energy Agency
7. Renewables and Waste in Portugal in 2005, International Energy Agency
8. Renewables and Waste in Netherlands in 2005, International Energy Agency
9. Renewables and Waste in India in 2005, International Energy Agency
10. Renewables and Waste in Italy in 2005, International Energy Agency
11. Renewables and Waste in France in 2005, International Energy Agency
12. Renewables and Waste in World in 2005, International Energy Agency
13. EWEC 2007 Review, European Wind Energy Association, (page 25) (2007)  (WebCite cache)
14. Principal performance indicators of 2006, 2006 Annual Report and Accounts, Drax Group plc (2007)  (WebCite cache)