Utilization of wind energy has increased spectacularly in recent years, with annual increases in installed capacity of around 20% in recent years. In 2014 political and regulatory uncertainty especially in the EU caused the rate of wind farm installations to drop markedly there as well as in some other countries. Investments were deterred by uncertainties and changes to renewable energy policies in several previously large wind markets.
Wind turbines of up to 6 MWe are now functioning in many countries, though most new ones are 1-3 MWe. The power output is a function of the cube of the wind speed, so doubling the wind speed gives eight times the energy potential. In operation such turbines require a wind in the range 4 to 25 meters per second (14-90 km/hr), with maximum output being at 12-25 m/s (the excess energy being spilled above 25 m/s). While relatively few areas have significant prevailing winds in this range, many have enough to be harnessed effectively and to give better than a 25% capacity utilization.
Where there is an economic back-up which can be called upon at very short notice (eg hydro), a significant proportion of electricity can be provided from wind. The most economical and practical size of commercial wind turbines is now about 2 MWe, grouped into wind farms up to 200 MWe. Depending on site, most turbines operate at about 25% load factor over the course of a year (European average), but some reach 33%. There is a distinct difference between onshore and offshore sites, though the latter are more expensive to set up and run. For the UK, in 2013 onshore wind averaged 28.7% capacity, and offshore 37.1%, following a similar spread in 2012.
China leads the field with 115 GWe installed, USA has 66 GWe, Germany has 39 GWe, Spain has 23 GWe, India 22 GWe, and UK with 12 GWe at the end of 2014. World total then was 370 GWe. (Data taken from Global Wind Statistics 2014 published by the Global Wind Energy Council.
Wind turbines have a high steel tower to mount the generator nacelle, and have rotors with three blades up to 50m long. Foundations require a substantial mass of reinforced concrete. Hence the energy inputs to manufacture are not insignificant. Also siting is important in getting a net gain from them. In the UK the Carbon Trust found that small wind turbines on houses in urban areas often caused more carbon emissions in their construction and fitting than they saved in electrical output (CT 7/8/08).
Bird kills, especially of raptor species, are an environmental impact of wind farms. In the USA half a million birds are killed each year, including 83,000 raptors (hawks, eagles, falcons etc) according to reports of a peer-reviewed published estimate in Wildlife Society Bulletin. A similar estimate comes from the US Fish & Wildlife Service. Other figures are based on 2.1 fatalities per turbine per year. There is particular concern regarding birds covered by the US Migratory Bird Treaty Act and the Bald and Golden Eagle Protection Act, which make bird fatalities illegal.
New wind farms are increasingly offshore, in shallow seas. The UK has 3300 MWe wind capacity offshore, more than the rest of the world combined as of early 2013. The London Array, 20 km offshore Kent, has 175 turbines of 3.6 MWe, total 630 MWe, on a 245 km2 site and claims to be the world’s largest offshore wind farm.