Wind energy is a form of solar energy produced by uneven heating of the Earth's surface. The sun radiates 100,000,000,000,000 kilowatt hours of energy to the earth per hour.  In other words, the earth receives 10 to the 17th power of watts of power. About 1 to 2 per cent of the energy coming from the sun is converted  into wind energy. That is about 50 to 100 times more than the energy converted into biomass by all plants on earth.

For several thousand years now, man has known how to extract energy from the wind by means of ships, sails or wind wheels, because the kinetic energy of wind is available more or less all over the world. Wind energy is environmentally attractive for many reasons. It produces no health-damaging air pollution, forest-destroying acid rain, climate-destabilizing carbon emissions, or dangerous radioactive waste.
Wind, as the primary energy source, costs nothing and can be used decentrally. There is no need for an extensive infrastructure such as that required for a power supply network or for the supply of oil or natural gas.

Wind has been used by humankind as a natural source of energy for tens of thousands of years. The use of wind energy dates back to the dawn of civilization when sailing vessels were powered by the wind. The first simple sailboats were set afloat in Egypt about 5,000 years ago. Around the year 700 AD, in what is Afghanistan today, the first wind machines rotating around a vertical axis were employed to grind grain. The famous fixed-tower windmills with sails provided irrigation for many parts of the Mediterranean island of Crete. Wind-driven gristmills were one of the greatest technical challenges of the Middle Ages. In the 14th century, the Dutch improved on the design that had spread throughout the Middle East and continued to use it for its primary purpose of grinding grain.

A wind powered water pump was introduced in the United States in 1854. It was the familiar fan type with many vanes around a wheel and a tail to keep it pointed into the wind. By 1940, over 6 million of these windmills were being used in the United States mainly for pumping water and generating electricity.  The “Wild West” was won at least in part with the help of these wind pumps that were used to supply water for the massive herds of cattle.

However, the 20th century soon brought an end to the widespread use of wind energy, which gave way to the “modern” energy resources, oil and electricity. It was not until after the oil crisis that wind energy options met with renewed interest. As a result of the drastic rises in oil prices at the beginning of the 1970s, energy planners have once again been turning their attention increasingly to the utilization of wind energy. State-sponsored research and development grants in many countries have provided a fresh stimulus to the development of technology for the utilization of wind energy. Efforts have been concentrated on developing wind energy converters for generating electricity, because in the industrialized countries the application of wind pumps is of minor importance.

The oil embargo of 1973 was the driving force behind wind turbine development programs in the United States. Westinghouse Electric developed first generation of 200 kW wind turbines, known as MOD-OAs. The largest of this series the 3,2 MW MOD-5B is operating in Oahu, Hawaii.  The Public Utilities Regulatory Policies Act (PURPA) of 1978 and a 25% tax credit for investors in turbines jump started commercial development of the United States wind industry and resulted in 6870 turbines being installed in California between 1981 and 1984. The tax credits expired on Dec. 31, 1985. None of the small wind turbine companies, however, were owned by large companies committed to long term market development, so when the federal tax credits expired and oil prices dropped to USD 10 a barrel, most of the small wind turbine industry once again disappeared. The companies that survived this “market adjustment” and are producing small wind turbines today are those whose machines were the most reliable and whose reputations were the best.

Nevertheless since the year 1998 the interest in wind energy is back again and installed capacity reached 25.170 MW in 2008 and USA became the top world producer of wind energy. Wind power represents around 1% of total U.S. consumption but varies in different regions. In 2008 Texas became the leading wind power state in the U.S. and due to the favorable condition there it is expected to rise further in the future.

Denmark's wind energy industry is a major commercial success story. From standing start in the 1980 to a turnover of 1 billion USD in 1998. Danish wind turbines dominate the global market. From a few hundred workers in 1981 the industry now employs 15.000 people. Its turnover is twice as large as the value of Denmark's North Sea gas production. Output , mainly for export around the world, has increased to 1216 MW of capacity in 1998. Now over half of the wind turbine capacity installed globally is of Danish origin.

The Danish government introduced support for renewable energy technology in 1979, covering 30% of capital cost. State aid encouraged the development of a highly successful wind turbine industry (it has also been used to promote the use of straw, biogas and solar projects).Danish wind turbine manufacturers were advised on ways of improving the performance and reducing costs of their machines  by experts based at the National Wind Turbine Test Centre at Riso. The grants for wind turbines were reduced to 15% in 1986 and finally phased out all together in 1989 as the industry became established. They have since been replaced by tax credits – the owners of wind turbines obtain a proportion of the income from the sale of electricity tax free.

Huge wind power development In Denmark was mainly based on activity of local people organized in co-operatives. Here is one example from Bryrup Wind turbine Co-operative (Jutland), 110 km from the West-coast and 50 km from the Eastern coastline. This co-operative has 70 partners owning three wind turbines installed between 1986 and ‘89. The effects is as follows: one 95 kW producing 184 000 kWh a year and two 150 kW each producing 275,000 kWh. Thus average total production amounts to 734.000 kWh annually.

Total price for all three turbines including foundation and connection to the public grid amounted to 2,5 million DKr (1 USD equals 6.2 DKr). This investment is split up in 734 “shares!’, each related to a production (and a consumption) of 1000 kWh, at a cost of 3,400 DKr. This equals half a month salary after tax for an unskilled Danish worker. Each partner can buy “shares” in proportion to his annual consumption of electricity plus 30%. If for instance annual consumption is 10 000 kWh you may add 3 000 kWh and thus be able to acquire maximum 13 “shares”. This restriction is applied because the profit for co-operative partners is tax- free, and the Danish legislators did not wanted this profit to be unreasonable. The partners have bought an amount of “shares” at numbers between 1 and 28. At the democratic general assemblies each partner has one vote despite numbers of “shares”. The reason for putting shares in quotation marks is related to the fact that these “shares” can not be traded like normal shares. By coming sales, buyers must  apply to the rules referring to electricity consumption. The economy of this co-operative is good. They distribute every year - after putting aside a reasonable amount for maintenance and renewals - 510 DKr per “share”, which gives a tax-free Interest rate of 15% what is more than banks can offer for your money. Today installation of wind turbines is a bit more costly. A share will amount to 4000 DKr, thus reducing interest rate to 12,75%. The Danish governmental support for wind power has caused that every tenth Danish family is member of a wind turbine co-operative or single owner of a wind turbine.

In contrast to the situation in Denmark or California, where a large number of wind generators were installed early on, the revival in Germany was relatively late in coming. In 1989, the German Federal Government initiated a promotion programme which called for the installation of wind generators with a total capacity of 250 MW over the next seven years. German legislation supports the installation of wind turbines through the feed-in tariff system. Investors receive fix rates for their wind electricity for several years. The feed-in tariffs were set so that they are interesting for the investors (large and small). Wind-generated electricity is supplied to the public power mains by any operator of the grid who is obliged to buy the wind electricity from producers. This programme has led to a rapid increase in the number of installations and today Germany is ranked second in the world in installed wind power capacity (23.903 MW in 2008).

Wind power has retained its status as the fastest growing energy source in the world. Globally the wind installed capacity reached 121.188 MW in 2008. Capacity in EU has hit 65.255 MW in end of this year. USA, Spain, Germany, China and India are leading markets. Leader of the new installation in 2008 were USA with almost 9000 MW. China and India are expected to be the largest wind energy market in the future. The global rate of growth fell from 26 % in the year 2003 down to 21 %. This is the result of slowing down of traditional markets in Denmark, USA and, to a lesser extend, Germany.

Hundreds more megawatts of energy capacity are scheduled to be built in years to come, encouraged by a tariff systems in Europe and tax breaks in other parts of the world. World wind production capacity more than quadrupled between 2000 and 2006, doubling about every three years. Wind energy now accounts almost 20 percent of national electricity production in Denmark. European and the U.S. success for wind energy development is just the beginning. It is estimated that this source, if the appropriate policies are put in place.


Installed capacity in MW 
December 31, 2007

Installed capacity in MW 
December 31, 2008




























During 2003 and 2008 the average growth rate in new wind power installations has been 27,6 percent a year and the forecasts until 2013 expect growth around 15-16 percent per year.



The cost of wind power continued to decline through advancements in design, siting practices and the cost of capital from around 14 US cents per kWh in 1986 to below 5 cents per kWh in 1999. Wind power is now cost-competitive in many electric power applications and that is why it is experiencing rapidly growing deployment.

Over the past two years wind energy capacity has been expanding at an annual rate of more than 30%. In contrast, the nuclear industry is growing at a rate of less than 1% whilst coal has not grown at all in the 1990’s.  Europe is the centre of this young and high-tech industry. 90% of the world's manufacturers of medium and large wind turbines are European. The average size of turbine increased by 150 kW to 900 kW.

Wind power potential is much greater than current world energy consumption. There are several studies which found[update] that this potential (land and off-shore) is around 72 TW (72 million MW) per year. This is five times more than the world's total energy production from all fuel sources. This potential covers only areas with mean annual wind speeds less than 6.9 m/s at 80 m. 

According to the study Wind Force 12 – a blueprint to achieve 12% of the world's electricity from wind power by 2020 - there are no technical, economic or resource limitations to achieve this goal. By 2020 the industry is capable of installing 1,260,000 MW of wind power throughout the world. According to the study the cost of generating electricity with wind turbines is expected to drop to 2.5 US cents/kWh by 2020, compared to the current 4.0 US cents/kWh.

Wind Force 12, by 2020 the wind industry can deliver:

12% of global electricity demand, assuming that global demand doubles by 2020.

installed capacity of 1,261,000 MW, generating 3,093 terawatt hours (TWh), equivalent to the current electricity use of all Europe.

Cumulative CO2 savings of 11,768 million tones.

Creation of 1.475 million jobs.

Renewable energy has become an important employer. There are over 110.000 jobs in the manufacture, installation and maintenance of renewable energy technologies in the European Union.  Wind energy accounts for around 20% of this. Most of the 700 companies involved are small and medium sized enterprises. As the industry grows, so more jobs are created. At the end of 1999 more than 20.000 Europeans were estimated to be employed in wind energy, and this figure is projected to grow to 40.000 by the year 2005 and to more than 1,4 mil. in 2020.


Wind power systems are being built all over the world. They are ideally suited to the needs of developing countries, which urgently need new capacity. They can be brought on line relatively cheaply and quickly in comparison with large power stations, which need major electrical infrastructure and grid systems to transmit their power. Developed countries are also a key growth area as they turn to wind power for environmental and economic reasons. Wind energy can be integrated into existing electrical systems, reducing the amount of power which needs to be generated by burning fossil fuels.

Wind resources are best along coastlines and on hills, but usable wind resources can be found in most other areas as well. As a power source wind energy is less predictable than solar energy, but it is also typically available for more hours in a given day. Wind resources are influenced by the ground surface and obstacles at altitudes up to 100 meters. The wind energy is thus much more site specific than solar energy. In hilly terrain, for example, two places are likely to have the exact same solar resource. But it is quite possible that  wind resource can be different at both places because of site condition and different  exposure to the prevailing wind direction. In this regard, wind turbines planning must be considered more carefully than solar technology. Wind energy follows seasonal patterns that provide the best performance in the winter months and the lowest performance in the summer months. This is just the opposite of solar energy. For a Denmark conditions a PV plant has a production per month varying between 18% in January and 100% in July. The wind power plant produces 55% in July and 100% in January. For this reason small wind and solar systems work well together in hybrid systems. These hybrid systems provide a more consistent year-round output than either wind-only or PV-only systems.
It is important to know that the amount of wind power generated is proportional to the density of air, area swept by the rotor blades of the wind turbine, and to the cube of the wind speed.

Blades of the wind generator rotate because air mass is moving them. The more air can move the blades, the faster the blades will rotate, and the more electricity the wind generator will produce. From the physics comes out that the kinetic energy of a moving body (e.g. air) is proportional to its mass (or weight) so the energy in the wind depends on the density of the air. Density refers to the amount of molecules in unit volume of air. At normal atmospheric pressure and at 15° Celsius air weighs some 1,225 kg per cubic meter, but the density decreases slightly with increasing humidity. Air is more dense in winter than in the summer. Therefore, a wind generator will produce more power in winter than in summer at the same wind speed. At high altitudes, (in mountains) the air pressure is lower, and the air is less dense. It is obvious that the density of air is variable that we can't do anything about.

The rotor of the wind turbine “captures” the power in the mass of the air that are passing through. It is clear that the larger area covered by a rotor means, the more electricity it can produce. The rotor area determines how much energy a wind turbine is able to use from the wind. Since the rotor area increases with the square of the rotor diameter, a turbine which is twice as large will receive four times as much energy. But increasing rotor area is not as simple as putting bigger blades on a wind generator. At first glance, this appears to be a very easy way to increase the amount of energy that a wind generator can capture. But by increasing the swept area we have also increased all of the stresses on the wind system at any given wind speed. In order to compensate for this change and let the wind system survive, it is important  to make all of the mechanical components stronger. Obviously this approach is going to get very expensive.

The wind speed is most important factor influencing the amount of energy a wind turbine can convert to electricity. Increasing wind velocity increases the amount of air mass passing the rotor, so increasing wind speed will also have an effect on the power output of the wind system. The energy content of the wind varies with the cube (the third power) of the average wind speed. Thus, if wind speed doubles, the kinetic power gained by the rotor increases eight times. From the following table you can estimate the power of the wind for standard conditions (dry air, density 1.225 kg/m3, at sea level pressure). The formula for the power in Watts per m2  = 0.5 * 1.225 * v3, where v is the wind speed in m/s (according to Danish Wind Turbine Manufacturers Association).





















Nature provide us with a different wind conditions and wind speed is continuously changing. Wind turbines are specially build to make use of wind which range in speed between 3 to 30 m/s. Higher wind speed can damage the turbine so large turbines are equipped with the brakes. Smaller turbines can make use of wind speeds lower than 3 m/s.

Wind speed scale:

Wind speed m/s

Type of wind














Strong gale

more than 43


Earth surface with its vegetation and buildings is the main factor reducing the wind speed. This is sometimes described as roughness of the terrain. As you move away from the earth's surface, roughness decreases and the laminar flow of air increases. Expressed another way, increased height means greater wind speeds. High above ground level, at a height of about 1 kilometer, the wind is hardly influenced by the surface of the earth at all. In the lower layers of the atmosphere, however, wind speeds are affected by the friction against the surface of the earth. For the wind power utilization it means the higher the roughness of the earth's surface, the more the wind will be slowed down. Wind speed is slowed down considerably by forests and large cities, while plains like water surfaces or airports will only slow the wind down a little. Buildings, forests and other obstacles are not only reducing the wind speed but they often create turbulence in their neighborhood. The lowest influence on the wind speed have the water surfaces. When people in the wind industry evaluate wind conditions in a landscape they describe it by roughness class. Higher roughness class means more obstacles in terrain and larger wind speed reduction. Sea surface is described as roughness class 0.

Roughness Class and Landscape Type:
0 = Water surface
0.5 = Completely open terrain with a smooth surface, e.g. runways in airports, mowed grass, etc.
1 = Open agricultural area without fences and hedgerows and very scattered buildings. Only softly rounded hills
1.5 = Agricultural land with some houses and 8 meter tall sheltering hedgerows with a distance of approx. 1250 meters
2 = Agricultural land with some houses and 8 meter tall sheltering hedgerows with a distance of approx. 500 meters
2.5  = Agricultural land with many houses, shrubs and plants, or 8 meter tall sheltering hedgerows with a distance of approx. 250 meters
3 = Villages, small towns, agricultural land with many or tall sheltering hedgerows, forests and very rough and uneven terrain
3.5 = Larger cities with tall buildings
4 = Very large cities with tall buildings and skyscrapers

In the industry also the term wind shear is used. It describes the fact that the wind profile is twisted towards a lower speed as we move closer to ground level. Wind shear may also be important when designing wind turbines. Here large rotor diameter and only a few meter higher tower could mean that the wind is blowing with higher speed when the tip of the blade is in its uppermost position, and wit much lower speed when the tip is in the bottom position.

Wind turbines are moved by the wind and convert this kinetic energy directly into electricity by spinning a generator. Usually they use blades like the wing of an plane to turn a central hub which is connected through a series of gears (transmission) to an electrical generator. The generator is similar in construction to the generators used in traditional fossil fuel power plants. The variety of machines that has been devised or proposed to harness wind energy is considerable and includes many unusual devices. Nevertheless modern wind turbines come in two basic configurations:
Horizontal axis turbines (HAT) are the most common type seen siting on top of towers with two or three blades. The orientation of the drive shaft, the part of the turbine connecting the blades to the generator, is what decides the axis of a machine. Horizontal axis turbines have a horizontal drive shaft. The blades may be facing into the wind, upwind turbine, or the wind may hit the supporting tower first, downwind turbine. Horizontal axis wind turbines generally have either one, two or three blades or else a large number of blades. Wind turbines with large numbers of blades have what appears to be virtually a solid disc covered by solid blades and are described as high-solidity devices. These include the multi-blades wind turbines used for water pumping. In contrast, the swept area of wind turbines with few blades is largely void and only a very small fraction appears to be ‘solid’. These are referred to as low-solidity devices.
Extracting energy from the wind as efficiently as possible means that the blades have to interact with as much as possible of the wind passing through the swept area of rotor. The blades of a high-solidity, multi-blade wind turbine interact with all the wind at a very low tip speed ratio, whereas the blades of a low-solidity turbine have to travel much faster to virtually fill up the swept area, in order to interact with all the wind passing through. Theoretically, the more blades a wind turbine rotor has, the more efficient it is. However, large numbers of blades interfere with each other, so high-solidity wind turbines tend to be less efficient overall than low-solidity turbines.
The pumps that are used with water pumping wind turbines require a high starting torque to function. Multi-bladed turbines are therefore generally used for water pumping because of their low tip speed ratios and resulting high torque characteristics.

Vertical axis turbines (VAT) have vertical drive shafts. The blades are long, curved and attached to the tower at the top and bottom. There is not so many manufacturers of such turbines in the world. Flowind is the most noted manufacturer of them. Vertical axis wind turbines have an axis of rotation that is vertical, and so, unlike their horizontal counterparts, they can harness winds from any direction without the need to reposition the rotor when the wind direction changes. The modern VAT evolved from the ideas of the French engineer G. Darrieus.

Despite the different appearances of HAT and VAT, the basic mechanics of the two systems are very similar. Wind passing over the blades is converted into mechanical power, which is fed through a transmission to an electrical generator. The transmission is used to keep the generator operating efficiently throughout a range of different wind speeds. The electricity generated can either be used directly, fed into a transmission grid or stored for later use.
Wind turbines can be built with two different forms of operation: pitch- or stall-regulation. Both systems have advantages and disadvantages. With pitch regulation, the blades can be pitched, which means better utilization of the wind and more energy from the wind turbine; on the other hand, the turbine has to be equipped with blade bearings, a blade-pitch regulation system, etc- parts which experience shows can give rise to operating problems. With stall regulation the blades are fixed and there is no pitch- adjusting system. A stall-regulated wind turbine is so to speak self-regulating and thus simpler, and it requires less maintenance and service; on other hand, one cannot utilize the wind quite as well as with pitch regulation.

Wind System Components
Modern wind turbine usually consists of  following components:

Blades are the part of a turbine that capture the wind. Advanced designs have led to higher energy capture. Two or three blades most often make up a rotor. Blades are made from fiber glass, polyester, or epoxy resins. Some have wood cores. These materials have the needed combination of strength and flexibility (and they don't interfere with television signals!). Blade diameters for commercial size turbines range from 25 to 50 meters and can weigh over 2000 pounds each.


The rotor is all the blades and the centre hub which the blades are attached to. The hub is attached to the drive shaft (or it is attached directly to a large gear in some systems). Upwind machines have their rotor in front of the tower (wind hits the rotor before the tower). Downwind machines are just the reverse arrangement.
Transmission and gears are important in order to transfer the rotating power through the spinning drive shaft to a generator.
The output from the transmission is then connected to an electric generator that produces electricity from motion.
Several control systems are all co-ordinated and monitored by a computer and can be accessed from a remote location. Pitch controls twist the blades to improve performance at different wind speeds. Yaw controls point the whole turbine into the wind.
Electronic controls keep the same voltage flowing from the generator as it changes speed. This variable speed generator is an important part of making wind turbines cost effective.

A wind turbine is a deceptively difficult product to develop and many of the early units were not very reliable. A PV module is inherently reliable because it has no moving parts and, in general, one PV module is as reliable as the next. A wind turbine, on the other hand, must have moving parts and the reliability of a specific machine is determined by the level of skill used in its engineering and design.
Modern wind turbines come in a wide range of sizes, from small 100 watt units designed to provide power for single homes or cottages, to huge turbines with blade diameters over 50 m, generating over 1 MW of electricity. The vast majority of wind turbines produced at the present time are horizontal axis turbines with three blades, 15 - 40 m in diameter, producing 50 - 600 kW of electricity. These turbines are often grouped together to form “wind farms” which provide power to an electrical grid. Modern large wind turbines generally produce electricity at 690 volts. A transformer located next to the turbine, or inside the turbine tower, converts the electricity to high voltage (usually 10-30 kilovolts).
Modern wind turbines costs around 800 USD/W what is sharp decline from 2500 USD/W for a turbine built in 1981.

Through the short history of the modern wind turbine, electric utilities have made it clear that they have held a preference for large scale wind turbines over smaller ones, which is why wind turbine builders through the years have made numerous attempts develop such machines - machines that would meet the technical, aesthetic and economic demands that a customer would require. Considerable effort was put into developing such wind turbines in the early 1980s. There was the U.S. Department of Energy's MOD 1-5 program, which ranged up to 3.2 MW, Denmark's Nibe A and B, 630 kW turbine and the 2 MW Tjaereborg machine, Sweden's Näsudden, 3 MW, and Germany's Growian, 3 MW. Most of these were dismal failures, though some did show the potential of MW technology.

A number of R&D facilities in Europe decided to take advantage of these incentives and most received either partial to full financial support to develop prototype wind turbines. The first of these was completed and installed at the end of 1995. Today several have been installed and have been up and running for a years. One company, Nordex, has even been marketing one of these machines for more than a 3 years. Leading wind turbine manufacturers continue to up-scale their 500 kW machines. It appears the marketing strategy of most of these companies is to maintain a market hold with their proven turbines in the 600-800 kW class (39-50 meter) while expecting that commercial MW machines will be in greater demand in the near future.  The world's largest turbines are recently (2008) produced by German companies Enercon (E-126 with capacity of 6 MW) and REpower (REpower 5M with capacity of 5 MW). The overall height these turbines is around 198 m and has a diameter of 126 meters.

Installation of MW machines under all circumstances presents new challenges for meeting planning and siting requirements. In areas that have already been filled to near capacity with smaller turbines, it is going to be difficult find locations for MW turbines where they can be incorporated harmoniously with existing turbines. Studies have been conducted in Denmark which focus on the special siting considerations necessary for installing MW turbines in the "technical" landscape. Results of these studies indicate there is available space in areas such as harbors and industrial areas for about 200 units, or about 200-300 MW. Power production of such machines can be enormous. It has been showed that 1 MW turbine can annually produce more than 5 million kWh at average wind speed higher than 9 m/s. Turbine with 1,3 MW rated power can produce more than 7 million kWh per year under such conditions.

Important figure describing wind turbine is its rated power. This tells you how much e.g. kilowatt-hours (kWh) the wind turbine will produce when  running at its maximum performance. 500 kW turbine will produce 500 kilowatt hours (kWh) of energy per hour of operation at its maximum with wind speed say 15 meters per second (m/s). According to the experience large single turbines can generate a considerable amount of electricity. Usually 600 kW machine will generate about 500 000 kWh per year with an average wind speed of 4,5 m/s. With an average wind speed of 9 meters per second it will generate up to 2 000 000 kWh per year. The amount of energy produced can not be simply calculated by multiplying of capacity (here 600 kW) and average annual wind speed. Here we have to deal with the capacity factor what is another way of expressing the efficiency of power production by a turbine during the year in particular location. Capacity factor is actual annual energy output divided by the theoretical maximum output, if the machine were running at its rated (maximum) power during all of the 8766 hours of the year. For example if a 600 kW turbine produces 2 million kWh in a year, its capacity factor is = 2000000 : ( 365,25 * 24 * 600 ) = 2 000 000 :  5 259 600 = 0,38 = 38 %. Capacity factors may theoretically vary form 0 to 100 per cent, but in practice they will usually range from 20 to 70 %, and mostly be around 25-30 %.

A very important factor which influences the performance of the wind turbine is the location. In general, wind speeds increase with elevation. This is why most wind turbines are placed at the top of a tower. Because the higher you are above the top of the neighboring obstacles, the less wind shade. The wind shade, however, may extend to up to five times the height of the obstacle at a certain distance. If the obstacle is taller than half the turbine height, the results are more uncertain, because the detailed geometry of the obstacle will affect the result. Limitations in the strength of affordable materials has limited most towers to heights of approximately 30 m. On wind farms, turbines are most often spaced at intervals of 5 – 15 times the blade diameter. This is necessary to avoid turbulence from one turbine affecting the wind flow at others.


Wind power has no fuel cost but high proportion of capital cost. The estimated average cost of wind energy per unit incorporates the cost of investment and construction of the turbine, transmission facilities, estimated annual production, and other components averaged over the projected life of the equipment, which may be more than 20 years for typical wind power generators. Energy cost depends on these assumptions and they differ substantially. According to several reports average wind production cost for onshore wind power plants is around 5-6 US cents per kWh (2005). Cost per kWh produced (0,056 USD/kWh) was comparable to the cost of new coal (0,053 USD/kWh) and natural gas (0,052 USD/kWh) generating capacity in the USA in 2006.

The development of wind turbines started with small units for small applications, but as the turbines grew in size, they became less and less attractive as a source of electricity for individual or household consumption. Consequently, almost all of the electricity generated by such plants today is fed into the grid. The output of a wind turbine of typical size is already so high that it exceeds the capacity of the local electricity mains. This is precisely the case in areas along the coast with a good wind regime but often lacking electricity facilities, making it necessary to install new and higher-capacity mains facilities, with the related additional costs. Because the additional expense is not an economically viable venture in the case of individual units, there has been an increasing tendency to install several plants (at least five in most cases) in consolidated areas known as wind farms. The output of several turbines is combined and sold under contract to the utility company.
Starting in the early 1980’s, larger wind turbines were developed for “wind farms” that were being constructed in windy passes in California. In a wind farm a number of large wind turbines, now typically rated between 400-600 kW each, are installed on the same piece of property.
In the USA the wind farms are usually owned by private companies, not by the utilities. Although there were some problems with poorly designed wind turbines and overzealous salesmen at first, wind farms have emerged as the most cost effective way to produce electrical power from wind energy. There are now over 16,000 large wind turbines operating in the California and they produce enough electricity to supply a city the size of San Francisco. Large wind turbine prices are coming down steadily and even conservative utility industry planners project massive growth in wind farm development in the coming decade, most of it occurring outside California. One recent study actually called North Dakota the “Saudi Arabia of wind energy”.

Offshore Wind Turbines
The success story of onshore wind energy created an interest for the exploitation of wind energy at offshore sites since suitable locations on land are becoming scarce or do not have good enough wind conditions. On sea the wind blows harder and a large amount of space in shallow waters not too far from shore is available especially in most states of Northern Europe. Both aspects are essential for a future large scale development.  Firstly, a ten percents increase in the mean wind speed can result potentially in 30% more energy yield.  Secondly, it is generally believed that the continental shelf with water depth up to some 30 m and distance from shore of up to about 30 km offer considerable economic advantages. In the future technological progress, e.g. floating offshore wind farms or HVDC (High Voltage Direct Current) power transmission, may also enable exploitation of deeper water locations as typical for the Mediterranean and many sites outside Europe as well as more remote offshore sites. In a recent study carried out in the scope of the European non nuclear energy research programme JOULE the potential of offshore wind energy in the European Union has been estimated to be nearly two times the total consumption.

In the 1990s first promising steps were taken to develop the required technology and to gain experience. The general feasibility of offshore wind energy was demonstrated and together with the demand for environmentally green technology it was seen as a considerable and renewable contribution to the energy supply in Europe. Utilization of wind energy offshore has even less environmental constraints than on land due to large available space and relaxed noise limitations. Generally the prospects are assessed quite positively and investment in offshore wind energy today is a preparation for a big market tomorrow. Offshore wind energy is an extremely promising application of wind power, particularly in countries with high population density, and thus difficulties in finding suitable sites on land. Construction costs are much higher at sea, but energy production is also much higher. The Danish electricity companies have announced major plans for installation of up to 4000 MW of wind energy offshore in the years after the year 2000. The 4 000 MW of wind power is expected to produce some 13,5 TWh of electricity, equivalent to 40 % of  Danish electricity consumption. Four possible areas (ranging from 135 to 500 km2, water depths from 5 - 15 m) are designated suitable to erect turbines at sea, with only few conflicting interests (e.g. environment, landscape, fishing, defense, communication, transport and national monuments). Production prices of about USD 0,05/kWh (20 years loan, 5% discount rate) are estimated.

Offshore wind farm in the Netherlands.
In spring 1998, five offshore wind farms were realized in Denmark, The Netherlands and Sweden, respectively. These farms are demonstration projects, characterized by medium sized wind turbines of the 500 kW class, moderate farm capacity up to 5 MW, low water depths (less than 10 m) and small distance from shore (between 40 m and 6 km). The energy prices of the pilot plants are considerably higher than onshore wind farms at good coastal sites. Some e.g. the Danish ‘Plan of action for large scale offshore wind farms’, show that the cost of energy for large plants is competitive with onshore wind farms at average sites. Moreover the price of wind energy is close to or in the range of other energy sources.

The world's first offshore wind farm is located North of the island of Lolland in the Southern part of Denmark Vindeby.  The Vindeby wind farm in the Baltic Sea off the coast of  Denmark was built in 1991 by the utility company SEAS. The wind farm consists of eleven 450 kW wind turbines, and is located between 1,5 and 3 kilometers North of the coast of the island of Lolland near the village of Vindeby. The turbines were modified to allow room for high voltage transformers inside the turbine towers, and entrance doors are located at a higher level than normally. Two anemometer masts were placed at the site to study wind conditions, and turbulence, in particular. The park has been performing flawlessly. Electricity production is about 20 per cent higher than on comparable land sites, although production is somewhat diminished by the wind shade from the island of Lolland to the South.

Vindeby offshore wind farm in Denmark.

The world's second offshore wind farm is located between the Jutland peninsula and the small island of Tunø in Denmark. The Tunø Knob offshore wind farm in the Kattegat Sea off the Coast of Denmark was built in 1995 by the utility company Midtkraft. The wind farm is situated in an area where the sea depth varies from 3-5 m. The Tunø Knob area is of considerable environmental interest, both as a resting area for birds and as a beautiful part of the coastline and landscape. Furthermore, a careful archaeological investigation of the site has been carried out as part of the off-shore wind farm planning process. The Wind farm consists of ten 500 kW wind turbines. Each turbine is a horizontal axis pitch regulated machine, orientated up-wind with a tubular tower, and a 3-bladed rotor of 39 m diameter. The turbines are mounted on specially-developed, reinforced concrete caisson foundations. The turbines are connected to the national grid via a 6 km submarine cable to the mainland of Jutland. Each turbine is controlled remotely. The production manager can monitor the performance and operation of the wind turbine from an operation centre in Hasle. The control system is continuously collecting all relevant data. The data are transmitted via a radio system from the individual data-collecting unit of each wind turbine to computers at Hasle. On-site maintenance is estimated to be needed only twice a year, when engineers will sail to the wind turbines and carry out the regular scheduled maintenance programme.

The turbines were modified for the marine environment, each turbine being equipped with an electrical crane to be able to replace major parts such as generators without the need for a floating crane. In addition, the gearboxes were modified to allow a 10 % higher rotational speed than on the onshore version of the turbine. This will give an additional electricity production of some 5 %. This modification could be carried out because noise emissions are not a concern with a wind park located 3 kilometers offshore from the island of Tunø, and 6 kilometers off the coast of the mainland Jutland peninsula. The park has been performing extremely well, and production results have been substantially higher than expected. In November 1995, its production was 1,3 GWh almost 40% more than originally estimated. The total production price/kWh is expected to be DKr 0,49 with an annual total production of 15 GWh. The entire costs of the off-shore farm are estimated to be about DKr 78 million.
The on-shore noise from the wind turbines has been calculated, at the nearest island of Tunø, to be less than someone whispering [15 dB(A)]. On the mainland it is inaudible.

Small wind energy systems can be used in connection with an electricity transmission and distribution system (called grid-connected systems), or in stand-alone applications that are not connected to the utility grid. A grid-connected wind turbine can reduce consumption of utility-supplied electricity for lighting, appliances, and electric heat. When the wind system produces more electricity than the household requires, the excess can be sold to the utility. With the inter-connections available today, switching takes place automatically.

Stand-alone wind energy systems can be appropriate for homes, farms, or even entire communities (a co-housing project, for example) that are far from the nearest utility lines. Either type of system can be practical if the following conditions exist.

Small wind generator sets for household electricity supply or water pumping represent the most interesting wind-energy applications in remote areas. Such generators can be very promising for the Third world countries as well where millions of rural households will be without grid connections for many years to come and will thus continue to depend on candles and kerosene lamps for lighting as well as batteries to operate radios or other appliances.
Wind turbines for domestic or rural applications range in size from a few watts to thousands of watts and can be applied economically for a variety of power demands.
In areas with adequate wind regimes (more than five meters per second annual average), simple wind generators with an output range of 100 to 500 W can be used to charge batteries and thus supply enough power to meet basic electricity needs. The families assign a very high priority to electricity and the range of services made possible by it (lighting, operation of radios and TVs). But relatively high investment costs of a complete wind-power system, which range from several hundred to a thousand US dollars or more, can be an obstacle for many households in developing countries.
In the past reliability of small wind turbines was a problem. Small turbines designed in the late 1970’s had a well deserved reputation for not being very reliable. Today's products, however, are technically advanced over these earlier units and they are substantially more reliable. Small turbines are now available that can operate 5 years or more, even at harsh sites, without need for maintenance or inspections. The reliability and cost of operation of these units is equal to that of photovoltaic systems.

Small wind mills are
sometimes better than diesel generators or extension of grid because they offer a number of other socio-economic benefits. Wind systems are smaller, modular and have a shorter lead-time than grid extension. In many countries for grid extension distances as short as one kilometer a wind system can be a lower cost alternative for small loads. While they cost more initially than diesels they are much better from the users point of view. Some donor agencies, for example in developing countries, typically supply diesels at no cost, but leave operational costs (fuel, maintenance and replacement) to the local people. This requires scarce hard currency and usually results in limited utilization and a shortened life of the diesel because of inadequate maintenance. Many countries must also import their fossil fuels, further magnifying the burden imposed by diesels. In such case small wind mills seems to be the better alternative.
The economies of scale in small wind turbines makes them particularly competitive in cost for sizes above 250 watts. For daily loads as small as one kilowatt-hour per day a wind turbine will be less expensive than diesels, grid extension, or photovoltaics for virtually any wind resource above 4 m/s. This wind resource is available in most of the developing world. For larger daily load requirements the economics of wind power get progressively better. For a 10 kW wind turbine a wind resource of only 3-3.2 m/s will usually make wind the least cost option. There are not many areas of the world that have average wind speeds below 3 m/s .

In Asia, for example, 50 000 wind generators are currently in operation in Inner Mongolia. The success story in Mongolia was made possible by favorable climatic conditions, on the one hand, and a consistent development and marketing policy, on the other. A minimum monthly velocity above 5 m/s throughout the year in many parts of the vast grasslands provides for a continuous supply of electricity to the semi-nomads living in the region. Operating electric lights, a radio and a TV is one of the few modern technical conveniences available to the people living in these remote areas. On the other hand, several private companies competing with one another have developed cheap and affordable designs. The wind generators are sold locally. The local government subsidizes the price of the equipment with up to 50 % of the production costs.

Small wind turbines can be an attractive alternative, or addition, to those people needing more than 100-200 watts of power for their home, business, or remote facility. Unlike PV’s, which stay at basically the same cost per watt independent of array size, wind turbines get less expensive with increasing system size. At the 50 watt size level, for example, a small wind turbine would cost about USD 8/W compared to approximately USD 5/ for a PV module. This is why, all things being equal, PV is less expensive for very small loads. As the system size gets larger, however, this “rule-of-thumb” reverses itself. At 300 watts the wind turbine costs are down to USD 2,5/W, while the PV costs are still at USD 5/W. For a 1500 W wind system the cost is down to USD 2/W and at 10 000 watts the cost of a wind generator (excluding electronics) is down to USD 1,50/W. The cost of regulators and controls is essentially the same for PV and wind. Somewhat surprisingly, the cost of towers for the wind turbines is about the same as the cost of equivalent PV racks and trackers. The cost of wiring is usually higher for PV systems.

The wind systems for remote or rural application is essentially the same as used with a PV system. Most wind turbines are designed for battery charging and they come with a regulator to prevent overcharge. The regulator is specifically designed to work with that particular turbine. PV regulators are generally not suitable for use with a small wind turbine because they are not designed to handle the voltage and current variations found with turbines.
Small wind turbines usually consists of : blades, alternator, regulation and control electronics.

Blades are usually made of carbon fiber reinforced composite that twists as the turbine reaches its rated output. This twisting effect changes the shape of the blade, causing it to go into stall mode. This limits the revolving of the alternator, preventing damage in high winds.

Some small turbines do not have brakes and during period of strong winds they can change their orientation.

Alternator is optimized to match as close as possible the energy available in the wind. It is constructed with permanent magnets and is usually brushless for best performance and maintenance-free operation.
Regulation and control electronics performs several functions to assure maximum output and safety for the user. The control electronics maintains a load on the alternator at all times to make sure that the turbine never over speeds, regardless of the condition of the battery. In case of battery charging, the sophisticated regulator periodically checks the line, correcting for voltage loss and monitoring charge rate. Once the battery has reached its optimum charge level the regulator shuts the current off, preventing the battery from being overcharged while maintaining a load on the alternator at all times to prevent over speeding.


When considering renewable energy sources and their use in some remote areas wind energy is today once again a possible alternative to the diesel engine as an economical means of converting energy. The principal ways in which wind energy can be exploited in rural areas are as follows:
for pumping water and producing compressed air;
for generating electricity (and thus also for developing technologies which depend on electricity);
for powering mechanical devices.

Water pumping
Wind energy has always been used extensively for pumping water, since there are no major problems involved in storing sufficient quantities of water without loss. Current estimates calculate that 100 000 wind pumps are installed around the world. Most of them are located in rural, non-electrified areas. They are used primarily by farmers for drinking water supply and livestock-watering. Wind pump technology is still of major interest for applications in the developing countries because of the importance of water supplies in rural areas, and the relative simplicity and transparency of the technology.
In view of the varying amount of wind energy available and the fact that, for economic reasons, the amount of storage capacity is limited, it can only be assumed in extremely rare cases that a single wind pump installation will be capable of ensuring a 100 percent reliable power supply. Hence, as a rule, these renewable energy sources can only be used as part of a combination of different systems appropriate to the case in question.
This means that for pumping water, be it for a drinking water supply, irrigation, or drainage, a suitable combination of different pumping systems with an optimized storage capacity should be installed. For small pump capacities up to approx. 10 m3/day, systems such as hand and foot pumps, capstans and, with certain limitations, solar pumps may be considered in addition to wind pumps where the water requirement is greater, motor pumps (diesel or electric) become competitive.
The question as to which combination of possible systems is the right one, i.e., the one which is most economical and best adapted to local conditions, depends on a variety of physical, socio-economic and sociocultural conditions which can differ considerably from one region to another. All of these conditions, which are not dealt with in more detail here for reasons of space, are of vital importance in the planning of rural water supply systems. Failures of projects for the introduction of wind pumps can, without exception, be traced back to the non-observance of one or more of these conditions or prerequisites.
Thus, for example, a combination of wind and hand pumps can be the right solution for providing a drinking water supply for a settlement, always provided that there is a sufficient amount of wind available. In the case of a small-scale irrigation system with wind pumps, a small, transportable diesel pump which can be used by several farmers is more suitable as a back-up system.
Other factors which have proved to be essential for dissemination on a larger scale are the existence and financial and technical capability of potential operators as well as the availability of marketing and service facilities in the area.
Today there are several water-pumping windmills on the market. They are designed to pump water in wind speeds as low as 2 m/s to 4 m/s from depths reaching 1000 meters. Typical water pumping windmill with a 3-m rotor can draw up to 2000 liters per hour from a depth of 10 meters at a wind speed of 3 m/s. Windmill with a 7-m rotor, can draw up to 8000 liters per hour under the same conditions. These systems can be used for irrigation, land reclamation or drinking water in remote areas. Windmills are designed for easy installation and require minimal maintenance.

The use of wind pumps for irrigation purposes seems to be problematic, since the water requirement and the availability of wind energy were generally subject to wide variations over the year. A good and above all constant wind regime is required to make them a viable option. Generally speaking, an annual average wind speed of four meters per second is a prerequisite for economic operation.
Typical project involving wind pump for irrigation was realized in Eastern Indonesia. This area has a short rainy season and traditional practice is for farmers to raise one rice crop per year. Two thirds of the time, during the dry season, the rice paddies are used only for grazing cattle. But many areas have substantial ground water resources which can be used for irrigation. In one project they dig wells, installed pumps, and trained the local farmers to use irrigation to raise higher value crops year-round. In most cases small 5 horsepower kerosene pumps are used for irrigation. These pumps are inexpensive and the fuel costs are partially subsidized by the government. But they also only last a few years and they operate at poor efficiency, so their life-cycle costs are quite high. Small wind systems cost more initially, but they have lower life-cycle costs. Project in Oesao, where the water table is only 2-5 meters below ground level, was based on use of the wind turbine which drives a surface mounted centrifugal pump. Pump is operated at variable voltage and frequency and its speed varies with the rotor speed of the wind turbine. The peak flow rate is ~3 liters/second. The system requires no fuel and no regular maintenance. A kerosene pump is, however, used for back-up. The Oesao system was installed in 1992 as a pilot project to show that wind power could be effective for water pumping in Eastern Indonesia. Since that time fifteen additional systems have been installed and more systems are planned.

Wind power is an excellent source of power for telecommunications sites because the height and exposure that make for a good antenna site also make for a good wind energy site. But wind turbines for this application must be particularly rugged because of the harsh conditions often encountered on mountains.

Utilization of small wind turbines for lighting, TV or refrigeration is quite simple through battery charging. Storing wind produced electricity in battery gives a homeowner a possibility to use this power whenever it is needed. Many small wind turbines directly produce 14 or 28 V . Some smaller wind turbines and other larger types produce higher voltages. 12 V o 24 V output from the battery can be used directly for DC appliances or inverted to 240 VAV current. For standard domestic appliances. It is usually best to directly charge the battery from the wind as this will not load the wind turbine at low speed causing stalling of the rotor.

If there is a need for hot water it is better to use direct wind generated electricity via an immersion heater to standard hot water tank and store the hot water. Battery storage is always more expensive than heat storage. The simplest system for water heating uses a thermostat to protect the water from boiling. The immersion heater should match the wind turbine rating. If a 1 kW turbine is used the immersion heater should also be rated at 1 kW (most domestic immersion heaters are 3 kW).

Wind - Solar Hybrid Systems
Solar and wind energy are complementing each other well under average seasonal conditions. In winter, when there is much wind, room heating is needed while in summer with much sun domestic hot water is needed. The combination of solar-wind is very interesting in the so-called off-grid electricity systems. These are self-supplying plants which are not coupled to the public electricity grid. A photovoltaic plant has a relatively high production in summer and a relatively small production in winter. This means that an off-grid system will either result in a heavy over-production in summer or should be equipped with a seasonal storage. Both solutions will be very expensive. A wind power supply can have serious problems in summer when periods with no wind may occur. The combination of solar-wind is therefore evident.
The important question, what the proportion between the solar and wind plant should be, have to be answered by the planner of the facility. It is obvious that the answer depends on energy needs during the year and a site conditions.

In many part of the world, there is such a dearth of electricity generation that the public welcomes wind turbines with open arms. Where there are alternative choices, however, environmental impact is of major significance for development. Note that impacts may be judged as either beneficial or harmful. The impacts of wind turbines and the factors influencing these are:

Noise is mostly generated from blade tips (high frequencies), from blades passing towers and perturbing the wind (low frequencies) and from machinery, especially gearboxes. Since noise is essentially a sign of inefficiency and because of complaints, manufacturers have reduced noise-generation intensities greatly over the last five years. The critical noise intensity is usually considered to be 40 dBA, or less, as judged necessary for sleeping. This level of acceptance is usually attained at distances of about 250 m or less. However, attitudes to noise are strongly psychological; the owner of a machine probably welcomes the noise as a sign of prosperity; whilst neighbors may be irritated by intrusion into “their space”.

Turbines should be separated by at least five to ten tower heights; this allows the wind strength to reform and the air turbulence created by one rotor not to harm another turbine downwind. Consequently, only about 1 % of land area is taken out of use by the towers and the access tracks. The taller and larger the turbines, the greater the separation. Megawatt machines should be spaced between half and one kilometer apart. Neither buildings nor commercial forestry can be established between, so the land is thereafter safeguarded against such development and can be used for agriculture, leisure or natural ecology.

Wind turbines are always visible from places in clear line of sight. The larger the machines, the greater the distance between them. The need for a long fetch of undisturbed wind, and the economic bias to large machines, means that machines will potentially be visible from distances of tens of kilometers. However, at such distances, the majority of the public will have their view obscured by hills, trees, buildings etc. The most likely people to notice the machines on land are walkers and pilots. For the former, beauty is in the eye of the beholder, and for the latter there is danger for exceptionally low flying. For offshore machines, visual impact is largely, as yet, unassessed.

Birds often collide with high voltage overhead lines, masts, poles, and windows of buildings. They are also killed by cars in the traffic. Birds are seldom bothered by wind turbines. Radar studies from Tjaereborg in the western part of Denmark, where a 2 megawatt wind turbine with 60 meter rotor diameter is installed, show that birds - by day or night - tend to change their flight route some 100-200 meters before the turbine and pass above the turbine at a safe distance. In Denmark there are several examples of birds (falcons) nesting in cages mounted on wind turbine towers. The only known site with major bird collision problems is located in the Altamont Pass in California. A "wind wall" of turbines on lattice towers is literally closing off the pass. There, a few bird kills from collisions have been reported. A study from the Danish Ministry of the Environment says that power lines, including power lines leading to wind farms, are a much greater danger to birds than the wind turbines themselves. Some birds get accustomed to wind turbines very quickly, others take a somewhat longer time. The possibilities of erecting wind farms next to bird sanctuaries therefore depend on the species in question. Migratory routes of birds will usually be taken into account when siting wind farms. Offshore wind turbines have no significant effect on water birds. That is the overall conclusion of a three year offshore bird life study made at the Danish offshore wind farm Tunø Knob.

There have been many independent studies of birds killed by rotating blades. This undoubtedly happens, but perhaps to a similar or lower frequency than strikes by a car, against the windows of a building or : against grid transmission cables. Every death is regretted. The counter argument, again attested by experts, is that land around wind turbines may provide excellent breeding conditions. The exception to this argument is the possibility of strikes by large migratory birds flying in the dark and by raptors intent on their prey.

TV, FM and radar waves are perturbed in line of sight by electrically conducting materials. Therefore, the metallic parts of rotating blades can produce dynamic interference in signals. It is easy, but not necessarily cheap; to install TV and FM repeater stations to provide another direction of signal for receivers. Radar interference is, as yet, a largely undocumented effect, of most concern to the military. However, wind turbines are a fact of life that has to be accepted by the military on an international scale. There are many sites of wind turbines close to airfields, and no significant difficulties occur.

Wind turbines have to compete with many other energy sources. It is therefore important that they be cost effective. They need to meet any load requirements and produce energy at a minimum cost . When you have decided that it is time to consider buying and installing a wind turbine you have to examine first two things: how much energy you require, and what is the average wind speed at the height of the wind turbine. Sometimes, it sure seems windy in your area, at least part of the time any way. But how can you tell if a wind turbine generator will really be optimized in term of power output versus wind speed. The common response is that you must monitor the wind speed at your site for at least one year and compare the results with historical data that had been recorded for some years. Or, contract a professional who will do a ‘feasibility study’ to estimate the yearly average wind speed and the estimated annual energy that would be captured by the wind turbine. Usually, which way to choose depends on the amount of investment you are willing to pay for having the wind turbine. For small applications when the amount of investment is relatively small, it is unrealistic to pay more than the cost of the wind turbine for obtaining the yearly average wind speed.

Wind systems are at the mercy of their site survey. Without an extended site survey or real wind data for a specific location, it is really impossible to specify a wind turbine for the system. While PV and micro hydro systems are often effectively designed by their users, wind systems should seek help from someone who really knows wind power. Here are some guidelines for siting and sizing small wind turbines.

A common way of siting wind turbines is to place them on hills or ridges overlooking the surrounding landscape. In particular, it is always an advantage to have as wide a view as possible in the prevailing wind direction in the area. On hills, one may also experience that wind speeds are higher than in the surrounding area. You may notice that the wind can bend some time before it reaches the hill, because the high pressure area actually extends quite some distance out in front of the hill.  Also, you may notice that the wind becomes very irregular, once it passes through the wind turbine rotor.  As before, if the hill is steep or has an uneven surface, one may get significant amounts of turbulence, which may negate the advantage of higher wind speeds.

The distance between the obstacle and the turbine is very important for the shelter effect. In general, the shelter effect will decrease as you move away from the obstacle, just like a smoke plume becomes diluted as you move away from a smokestack. In terrain with very low roughness (e.g. water surfaces) the effect of obstacles (e.g. an island) may be measurable up to 20 km away from the obstacle. If the turbine is closer to the obstacle than five times the obstacle height, the results will be more uncertain, because they will depend on the exact geometry of the obstacle.

The roughness of the terrain between the obstacle and the wind turbine has an important influence on how much the shelter effect is felt. Terrain with low roughness will allow the wind passing outside the obstacle to mix more easily in the wake behind the obstacle, so
that it makes the wind shade relatively less important. A good rule of thumb is that we deal with individual obstacles which are closer than about 1000 meters from the wind turbine in the prevailing wind directions. The rest we deal with as changes in roughness classes.

The taller the obstacle, the larger the wind shade. If the turbine is closer to the obstacle than five times the obstacle height, or if the obstacle is taller than half the hub height, the results will be more uncertain, because they will depend on the exact geometry of the obstacle. In that case the programme will put a warning in the text box below the results.

Since a wind turbine generates electricity from the energy in the wind, the wind leaving the turbine must have a lower energy content than the wind arriving in front of the turbine. This follows directly from the fact that energy can neither be created nor consumed.  A wind turbine will always cast a wind shade in the downwind direction. In fact, there will be a wake behind the turbine, i.e. a long trail of wind which is quite turbulent and slowed down, when compared to the wind arriving in front of the turbine. Wind turbines in parks are usually spaced at least three rotor diameters from one another in order to avoid too much turbulence around the turbines downstream. In the prevailing wind direction turbines are usually spaced even farther apart.

Turbulence decreases the possibility of using the energy in the wind effectively for a wind turbine. It also imposes more tear and wear on the wind turbine, as explained in the section on fatigue loads. Towers for wind turbines are usually made tall enough to avoid turbulence from the wind close to ground level.

To correctly site and size a wind turbine, it is helpful to have the information about average wind speed for the location. The annual average wind speed is used to describe the general windiness of a place. Shorter-term averages (monthly, hourly) are used in more precise analyses where the time relation between wind energy availability and energy demand is particularly important. The time variation of wind speed at a given site is described by the relative probability of the wind speed at any moment being greater or less than the average wind speed. A typical distribution of wind speed (called the Rayleigh Distribution, special case of Weibull Distribution) usually means that there is little probability of absolutely no wind; the most frequent wind speed is about 75% of the average wind speed; and wind speeds above twice the average wind speed do occur, but not often.

Wind Speed Measurement
Don't consider wind power without a thorough measurement of the wind speed at your specific location. In most cases, four months should be the minimum recording interval and one year is preferred. If you are going to spend a lot of money on a wind system, this extra eight months could mean the difference between a good investment and a bad one.
The measurement of wind speeds is usually done using a cup anemometer. The cup anemometer has a vertical axis and three cups which capture the wind. The number of revolutions per minute is registered electronically. Normally, the anemometer is fitted with a wind vane to detect the wind direction. Other anemometer types include ultrasonic or laser anemometers which detect the phase shifting of sound or coherent light reflected from the air molecules. Hot wire anemometers detect the wind speed through minute temperature differences between wires placed in the wind and in the wind shade (the lee side). The advantage of  the non-mechanical anemometers may be that they are less sensitive to icing. In practice, however, cup anemometers tend to be used everywhere, and special models with electrically heated shafts and cups may be used in arctic areas.

Determining the exact average annual wind speed is not an easy task and it is an expensive process. After all it might be unnecessary. For small wind turbines applications what we need to do is get some idea of the average annual wind speed for the area, and that can be available by observing few physical phenomena around the site. Start by your feeling, while they are hardly scientific, then try to check the airport and weather station data for your area. Use these data as a raw baseline, which you have to tune to make them represent your area.

Meteorologists already collect wind data for weather forecasts and aviation, and that information is often used to assess the general wind conditions for wind energy in an area. Precision measurement of wind speeds, and thus wind energy is not nearly as important for weather forecasting as it is for wind energy planning, however. Wind speeds are heavily influenced by the surface roughness of the surrounding area, of nearby obstacles (such as trees, lighthouses or other buildings), and by the contours of the local terrain. Unless you make calculations which compensate for the local conditions under which the meteorology measurements were made, it is difficult to estimate wind conditions at a nearby site. In most cases using meteorology data directly will underestimate the true wind energy potential in an area.
It is because weather stations monitor wind speeds at or slightly above street level, where people live. They don't monitor wind speeds at 20 - 30 meters, where the wind turbine is usually located. Similarly, airports data has limited value. Because airplanes traditionally had problems taking off and landing in windy locations, airports were sited in rather sheltered locations. Virtually all airports are sheltered.  After having the raw data from nearby airport or weather station, you need to extrapolate these numbers to your location using a concept know as shear ‘factor’. Based on these numbers and the topographical difference or similarity between your site and theirs (weather station and airport), you can theoretically estimate your average wind speed at any proposed height.

Very simple anemometer can be build by yourself. Here is the way how to construct it. Materials needed : five paper Dixie cups, two straight plastic soda straws, a pin scissors, paper punch, small stapler, sharp pencil with an eraser.
Procedure: Take four of the Dixie cups. Using the paper punch, punch one hole in each, about a half inch below the rim. Take the fifth cup. Punch four equally spaced holes about a quarter inch below the rim. Then punch a hole in the centre of the bottom of the cup. Take one of the four cups and push a soda straw through the hole. Fold the end of the straw, and staple it to the side of the cup across from the hole. Repeat this procedure for another one-hole cup and the second straw. Now slide one cup and straw assembly through two opposite holes in the cup with four holes. Push another one-hole cup onto the end of the straw just pushed through the four-hole cup. Bend the straw and staple it to the one-hole cup, making certain that the cup faces in the opposite direction from the first cup. Repeat this procedure using the other cup and straw assembly and the remaining one-hole cup. Align the four cups so that their open ends face in the same direction (clockwise or counter clockwise) around the centre cup. Push the straight pin through the two straws where they intersect. Push the eraser end of the pencil through the bottom hole in the centre cup. Push the pin into the end of the pencil eraser as far as it will go. Your anemometer is ready to use. Your anemometer is useful because it rotates at the same speed as the wind. This instrument is quite helpful in accurately determining wind speeds because it gives a direct measure of the speed of the wind. To find the wind speed, determine the number of revolutions per minute. Next calculate the circumference of the circle (in feet) made by the rotating paper cups. Multiply the revolutions per minute by the circumference of the circle (in feet per revolution), and you will have the velocity of the wind in feet per minute. The anemometer is an example of a vertical-axis wind collector. It need not be pointed into the wind to spin.

Another useful tool to help determine the potential of a wind site is to observe the area's vegetation. Trees, especially conifers or evergreens, are often influenced by winds. Strong winds can permanently deform the trees. This deformity in trees is known as flagging. Flagging is usually more pronounced for single, isolated trees with some height. On the upwind side of the tree, the branches are noticeably stunted. On the downwind side, they're long and horizontal. The flagging was caused by persistent winds from, more or less, one direction. Look around especially for single trees, or trees on the outskirts of a grove. Unless they have grown considerably above the common tree line, trees in a forest will not show flagging because the collective body of trees tends to reduce the wind speed over the area. While the presence of flagging positively indicates a wind resource, you should not conclude that the absence of flagging in your area precludes any suitable average wind speeds. Other factors that you are not aware of may be affecting the interaction of the wind with the trees.

For very rough estimate of the average wind speed Griggs-Putman Index of Deformity can be used. 

While average wind speed is meaningful, there are other wind parameters that are just as meaningful. Other wind parameters worth knowing are maximum wind speed, number of days (hours) between winds of greater than 5m/s. Number of consecutive days (hours) where the wind is in excess of 5 m/s, and the times of year where the either wind or not wind periods occur. The wind speed is always fluctuating, and thus the energy content of the wind is always changing. Exactly how large the variation is depends both on the weather and on local surface conditions and obstacles. Energy output from a wind turbine will vary as the wind varies, although the most rapid variations will to some extent be compensated for by the inertia of the wind turbine rotor.

All important data is not available from garden variety recording anemometers. A recording anemometer that will take all the data mentioned above will cost much. Such anemometers are more computer than wind sensor and cost between USD 2,000 and USD 4,000.

This is a job for someone with experience with all types of wind turbines. Not only must the wind turbine be well made, but it also must fit the wind conditions at your particular site and must produce the power that the system requires. Modern turbines usually produce some specie of low voltage and only the very large units make 60 cycle, 120/240 VAC directly.
When choosing a turbine the rated power for a wind turbine is not a good basis for comparing one product to the next. This is because manufacturers are free to pick the wind speed at which they rate their turbines. If the rated wind speeds are not the same then comparing the two products is very misleading. Usually manufacturers will give information on the annual energy output at various annual average wind speeds. These figures allow you to compare products fairly, but they don't tell you just what your actual performance will be.

The power in the wind is a function of (among other things) the cube of the wind speed. Therefore, the easiest way to increase the power available to a wind generator is to increase the wind speed. We can increase wind speed by either installing a taller tower or by moving to a windier location. Note that as a percentage, wind speed increases much faster over terrain cluttered with trees and buildings than over flat open ground. With the exception of the middle of a lake or desert, wind speed increases significantly with height. For example, power available at 30 meters can be up to 100% higher than power available at 10 meters. Said another way, two wind generators on two 10 meters towers will produce as much power as one wind generator on a 30 meter tower. And the system with the 30 meters tower will be cheaper to install than the “twin” systems at 10 meters. The rule of thumb for siting is that the wind generator must be at least 10 meters above any obstacle within 100 meters. Consider 15 meters to be a realistic minimum and after that, go as high as you can. Smaller turbines typically go on shorter towers than larger turbines. A 250 watt turbine is often, for example, installed on a 15-20 meter tower, while a 10 kW turbine will usually need a tower of 20-30 meter. A wind turbine must have a solid tower to perform efficiently. Turbulence, which is highest close to the ground and diminishes with height, reduces the  performance of the turbine.
For small wind mills the least expensive tower type is the guyed-lattice tower, such as those commonly used for ham radio antennas. Smaller guyed towers are sometimes constructed with tubular sections or pipe. Self-supporting towers, either lattice or tubular in construction, take up less room and are more attractive but they are also more expensive. Telephone poles can be used for smaller wind turbines. Towers, particularly guyed towers, can be hinged at their base and suitably equipped to allow them to be tilted up or down using a winch or vehicle. This allows all work to be done at ground level. Some towers and turbines can be easily erected by the purchaser, while others are best left to trained professionals. Anti-fall devices, consisting of a wire with a latching runner, are available and are highly recommended for any tower that will be climbed. Aluminium towers should be avoided because they are prone to developing cracks. Towers are usually offered by wind turbine manufacturers and purchasing one from them is the best way to ensure proper compatibility. Be sure that the tower is strong and well installed. Sloppy tower installation can bring the whole system crashing down. Guyed towers are more secure and less expensive than unguided towers.

Choosing a wind controller
In almost every case, the manufacturer of the wind machine also makes a regulator for that specific model. So, the user doesn't have to select a regulator because it is bundled in with the wind machine. These controls are shunt types that divert the turbine's output to maintain control of the system's voltage. Diversion regulator schemes are really the only type used, because unloading the wind machine will cause over speeding and damage to the turbine.

Sizing the Wind system's battery
The size of a wind system battery storage is determined by the longest period of windless weather. This can be very difficult to determine in advance. For this reason wind systems usually have more days of battery storage than do PV systems. Shoot for a minimum of seven days of storage and extend this to fourteen days if you can afford it. Wind power comes in gusts and spurts, having a large battery makes more effective use of nature's least consistent power source.