The extraction of energy from the wind has evolved over thousands of years . Earliest recorded applications of wind energy were in the milling of grains, the pumping of water and other mechanical applications. The first recorded reference in England was from 1185 AD, where a windmill in the village of Weedley in Yorkshire is mentioned . However, the initial use of wind energy for electricity generation as opposed to mechanical applications only came to the fore with the successful commercial development of small wind generators in the nineteenth century. Since then there has been a worldwide resurgence in the wind energy industry, with installed capacity increasing five-fold. 
The consideration of wind energy as an alternative source of energy developed because of the emerging awareness of the finiteness of the earth’s fossil fuel reserve, the need to combat global warming, and the considerable energy density of wind in some locations. It has been estimated that roughly 10 million MW of energy are continuously available in the earth’s wind . The application of wind to the generation of electricity grew during the second half of the twentieth century when increased oil prices prompted intense interest in wind as a fuel-free, renewable energy source.  Wind power was extensively exploited in the USA in the 1980s [2, 4]; small turbines rated at about 100–150 kW were used. The growth of the modern wind industry can be traced from these events and has resulted in 1 – 5 MW turbine products, with sophisticated aerodynamics, structural dynamics, micrometeorology and variable speed controls being deployed onshore and offshore in a number of countries including Australia, Denmark, France, Germany, Spain and the UK (see Table 1).
In 2000, about 80% of the worldwide wind capacity was installed in only five countries: Germany, USA, Denmark, India and Spain. Thus, a large amount of the wind-energy knowledge is based in these countries . In 2006, in the UK, there were 1672 wind turbines rated at 1742 MW installed onshore and offshore, operating at a total installed generating capacity of 82,000 MW. Therefore wind represents about 2.5% of the installed capacity. 
Table 1: Contributions of wind power to electrical power generation in Europe EU15, 2002 (extract from European Wind Energy Association, 2005) 
|Electricity consumption (TW h)||Generated from wind (TW h)||Generated from wind (%)||Fraction of potential harnessed (%)|
Aside from being a clean and renewable source of energy, wind power also contributes significantly to the reduction of CO2. There are different ways of assessing the CO2 emissions that will be reduced by electricity generation from the wind. This depends on the assumptions made about the fuels which will be displaced by introducing wind into the energy mix; for instance a mix which clearly varies across the EU member states. The input being displaced by the wind will therefore be a combination of more ﬂexible loads provided by a mix of fossil-fuel power stations. For the European Union as a whole, it is assumed that each kilowatt hour (kWh) produced by wind power is displacing a kWh deﬁned by the energy mix of gas, oil and coal at the time of production.  Lately, the International Energy Agency (IEA) projected that all fossil fuels used for power generation in the EU, for example, coal, oil and gas, will be producing 1,827 terawatt hours (TWh) of electricity by 2010, and about 53 % of this comes from coal-fired power stations, 39 % from gas and 8 % from oil. In the process, these three fuel sources will emit 1,466 million tones of CO2 .
Despite the growth and development potential, wind power technology faces significant challenges in the quest to produce a long-term reliable and cost-effective renewable source of generating electricity on a significant scale. Some of the challenges are: the reduction of the capital cost of wind turbines and their installation, improving the technical and economic ways for the more effective penetration of variable and intermittent wind power into existing and future electrical systems, including the consideration of small-scale wind installations . Furthermore, land must be purchased, numerous public hearings must be held, electrical impact studies and even environmental (noise, bird kills by spinning blades and aesthetic impact problems) impact studies must all be run. These can take several years and require significant cooperation .
Although the wind turbine and its conversion equipment are complex products, they can be adapted for mass production. This would lead to improvements in their quality, reliability and cost-effectiveness . The aim of this work is therefore to present a review of the technology of wind power in order to highlight the theory of its operation, the material issues relating to its design, the major industrial players, the current state of the technology, as well as the future development of the technology.
MODE OF OPERATION
Horizontal axis wind turbines use airfoils to transform the kinetic energy in the wind to useful energy. Airfoils are structures with specific geometric shapes that are used to generate mechanical forces due to their relative motion with the surrounding fluid . Air flow over an airfoil produces a distribution of forces on its surface. The flow velocity over the airfoil increases around the convex surface resulting in lower average pressure on the suction side compared with the concave or pressure side of the airfoil. The viscous friction between the air and the airfoil surface slows the air flow next to the surface to some extent. The result of the pressure and friction forces is usually resolved into two forces – the lift which is perpendicular to the direction of the oncoming air flow and is a result of the unequal pressure on the upper and lower surfaces of the airfoil. The drag which is parallel to the direction of the oncoming airflow is a result of the viscous forces at the surface of the airfoil and unequal pressure on the airfoil surface.
Based on the two forces described above, wind energy converters can be divided into lift machines (also known as horizontal axis wind turbines) and drag machines. A lift machine uses lift forces to generate power, while a drag machine uses drag forces. The lift machine is considered in this work because it makes more economic sense.
The basic process of converting kinetic energy from wind into mechanical energy by the rotor can be described in two ways, namely from the point of view of the rotor (the driving force) and from the point of view of the wind stream (loss of flow energy). Both view points are connected by the laws of conservation of mass, momentum and energy.
MAJOR INDUSTRIAL PLAYERS
Manufacturers of wind turbines are graded according to the megawatts installed and market share. The leading suppliers of wind equipment based on these criteria are General Electric, Enercon, Vestas, Gamesa and Siemens Wind Power. Others include Suzlon, Clipper, Acciona and Ecotecnia. With the progress being made in wind technology it is expected that more manufacturing companies will join the leading suppliers in the market . The contributions of the top industrial players in wind technology are described below.
General Electric: The involvement of General Electric in wind technology spans two decades with about 13,500 wind turbine installations. It has assembly and manufacturing facilities in Germany, Spain, China, and the USA. In Canada, GE Wind Energy has contributed to establishing facilities in the Quebec province to enable up to 60% of wind turbine components to be manufactured and assembled locally. General Electric made the majority of the wind turbine supplies in the USA in 2008 with a total of 3,657 MW installed. The market share for new installations in Spain in the same year was 8.5% while their total online capacity in Italy was 4.17%.
Vestas Wind Systems: With about 28% market shares, Vestas is one of the major players in the wind turbine industry. It has about 39,000 installations in 63 countries and currently has production centers in Denmark, the USA, China and Spain.
In 2008, Vestas installed 1,120 MW of wind instruments in the USA. In the same year, it installed about 15% of the new capacity in Spain, adding 242.2 MW. Among wind manufacturers, Vestas still leads the Italian market with a share of total capacity close to 50% and more than 340 MW installed in 2008—one-third of the total capacity put into operation for the year.
Enercon: Founded in 1984, Enercon has become one of the leading wind turbine manufactures with about 15,000 turbines installed. Enercon currently has 17% market share of the total installations in Germany (with 15% of the total installed capacity), 12.67% market share of the total online capacity in Italy and 45% of the total capacity in Portugal. In 2008, Enercon supplied 10.6% of the installed capacity in Spain with a rather low accumulated capacity of 1.7%.
Gamesa: Gamesa is the leading wind turbine manufacturer in Spain with about 810 MW in new installations in 2008 leading to an accumulated supply of 9,480 MW. In Italy, Gamesa had a market share of 20.10% at the end of 2008 which placed it as the second largest manufacturer in the country. Its installed capacity is 13% in Portugal. It recently established a factory in the USA.
Siemens Wind Power: Siemens is the major wind turbine manufacturer in the UK with about 1000 installations and 40% of the UK’s generating capacity. In the USA, Siemens is the third largest contributor to the market for large scale turbines with 791MW installed in 2008. Siemens is also among the two leading manufacturers in Denmark and specialises in offshore turbines.
CURRENT STATE OF WIND TECHNOLOGY
Over the past five years, global wind power capacity has expanded at an average rate of 3.2%. In 2003, 8,133 MW of new capacity was added to the electricity grid worldwide. In 2003, 80% of the wind turbines sold worldwide came from Europe. With an annual global growth rate of 28%, wind energy has become the fastest growing energy source. Table 2 shows the newly installed wind turbines and their capacities globally from 1998 to 2010.
Table2: Newly Installed Capacities of Wind Turbines .
|Period||Installed Capacity (MW)|
The following section reports the current state of wind technology in different continents of the world and the future prospects inherent in the technology.
Present Scenarios and Future Prospects of Wind Energy in Different Continents.
Although Africa has huge potential, with the world’s best sites in the north and south of the continent, wind energy still plays a marginal role on the continent with 563 MW of total capacity. Most of the wind farms are located in North African countries like Morocco, Egypt or Tunisia. In 2010, substantial increases can be expected from projects which are already in the development stage. The positive signs of growth in the continent can be identified by the fact that companies from the region are showing an increasing interest and have started investing in the wind sector. This is confirmed by steps taken by the government of South Africa to introduce a feed-in tariff which would create a real market, enable independent operators to invest and thus play a key role in tackling the country’s power crisis.
In Asia, the two leading countries in wind power technology are China and India, with a combined installed capacity 21800 MW as of January 2009. It can be expected that in the foreseeable future Chinese and Indian wind turbine manufacturers will be among the international top suppliers. The Indian market showed robust and stable growth in 2008. It has a well-established wind industry which already plays a significant and increasing role on the world markets. Furthermore, countries like South Korea (already with 45% growth rate in 2008) have shown significant investment on a larger scale in wind energy.
Australia and Oceania
The region has shown encouraging growth rates, with an installed capacity of 1,819 MW at the end of 2008. In lieu of the renewed commitments made by the Australian government at climate change mitigation, the expansion of renewable energies creates the expectation that the Australian wind energy market will show further robust growth in the coming years.
Europe has lost its dominant role as a new emerging market to the USA but has kept its leading position in terms of total installation capacity of 66,160 MW. Germany and Spain still boast the largest installed capacity in wind power technology after the USA. In 2008, the two most dynamic European markets in terms of additional installed capacity to their already existing ones were Ireland (adding 440 MW, 55 % growth) and Poland (196 MW added 71 % growth).
North America has demonstrated very strong growth in the installation of wind turbines. In 2008, its capacity more than doubled, attaining a value of 25,200MW . Further increase is expected by the current administration due to the introduction of new frameworks for wind power in the country. This is aimed at encouraging investors who have practically been excluded from the production tax credit scheme like farmers, small companies or community-based projects. Also, the Canadian provinces, Quebec and Ontario, are showing increasing commitment towards an accelerated deployment of wind energy technology.
GENERATION COSTS OF WIND ENERGY
Wind power costs are determined largely by the capital cost, variable cost and wind resources. In Europe, capital costs contribute about 80% of the total cost of a wind project over its entire lifetime and are presently in the range of 1,100 – 1,400 €/kW. In the USA and China, this cost is appreciably lower due to market factors and location. Capital costs comprise the production cost of the turbine, cost of grid connection, civil works and other costs like land costs, permits and licenses. Grid connection costs are about 12% of the total capital cost.
These are the operation and maintenance costs and they are low for wind electricity projects compared to electricity generation from fossil fuels. Variable costs are not easily predicted. However, studies carried out in Germany showed that the variable cost of a newly installed wind turbine is about 0.3 – 0.4 €cent/kW after the first two years of its lifetime and increases by 5% after six years.
Local wind resources determine to some reasonable extent the cost per kWh of wind projects. Full load hours vary from location to location. The average full load hours are 2342 h/year in Spain, 2300 h/year in Denmark and 2600 h/year in the UK. Therefore, in choosing the site of wind turbines, meteorological and topographical measurements are required for optimisation.
Estimated capital cost distribution of a wind project in Europe .
Table3: Electrical production cost in 2004 
|Energy source||€ cents/kWh|
Cost of wind generated electricity [16, 19]
There has been a significant drop in the cost of wind-generated electricity since 1980, from about 15-20 €cents/kWh to 3-7 €cents/kWh. In the future, the cost is expected to reduce with improvements to turbine design, optimisation of rotor blades and drive trains and improvement in materials. Table 4 shows the cost of the non-hydroelectric renewable energy in 2004.
Wind energy is the fastest growing source of power generation with an annual growth rate of 28%. It is estimated that it will contribute 1,500 GW of electrical power by the year 2020, which will account for 12% of the global energy demand. In Europe, capital costs constitute 80% of the total cost of a wind project over its entire life time. This cost is presently in the range of 1,100 – 1,400 €/kW. Between 1980 and 2004, there was a significant drop in the cost of electricity generated by wind power, from 15-20 cents/kwh to 3-7 cents/kwh. In comparison with other sources of energy, wind energy has the lowest cost savings. Furthermore, the intermittent nature of the wind power technology has necessitated its hybridisation with other renewable energy sources such as solar energy and hydro plants. But the proliferation of hybrid wind technologies has been limited due to its lack of proven track record. Analysis of the life cycle of a 4.5MW wind turbine has been estimated to bring about a savings in CO2 emissions of about 12*106 kg throughout its entire lifetime. This demonstrates wind power as an excellent source of clean energy for the future.
Univ. of Wisconsin 2007, 1st Annual IEEE Systems Conference, USA April 9-12, 2007