This industry has come a long way from where it was in the 1980’s with 50kW to 100kW (approx.) output per generating station to 4MW generating capacity from land based generating stations and more than 5MW from offshore generating stations.

NEC and IEEE have enforcement in place for interconnection between generating stations and the utility for large scale transmission. However, this should not impede people interested in harnessing wind to produce energy to run small devices. Independence from the grid and using renewable resources for producing energy plays a big role in being energy efficient and conscious of the environment.

Wind energy production in the US is estimated to be at 36,000MW in 2010 as opposed to Europe where production is estimated at 75,000MW. In 2010, the estimated production of energy using wind is 160,000MW.[1]

Major advantages of harnessing wind are:
Commercial viability, short construction and risk involved, zero sustained fuel cost, general public acceptance, potential for localized job growth, tax benefits and minimal environmental impact. Of all these advantages listed for commercial producers of wind energy, as a hobbyist, the most attractive features would be zero sustained fuel cost, minimal environmental impact and low cost.

Some disadvantages are:
Intermittent nature of wind, opposition to altered landscape and maintenance.

Windmills were used in Persia (present-day Iran) as early as 200 B.C. The wind-wheel of Heron of Alexandria marks one of the first known instances of wind powering a machine in history. However, the first known practical windmills were built in Sistan, a region between Afghanistan and Iran, from the 7th century. These "Panemone" were vertical axle windmills, which had long vertical drive shafts with rectangular blades. Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind corn or draw up water, and were used in the milling and sugarcane industries. The concept of using wind energy for grinding grain spread rapidly through the Middle East and was widespread long before the first windmill appeared in Europe.[2]

Windmills first appeared in Europe during the middle ages. The first historical records for their use in England date to the 11th or 12th centuries. There are reports of German crusaders taking their windmill-making skills to Syria around 1190. By the 14th century, Dutch windmills were in use to drain areas of the Rhine delta.

The first electricity generating wind turbine, was a battery charging machine installed in July 1887 by Scottish academic, James Blyth to light his holiday home in Marykirk, Scotland. Some months later, American inventor Charles F Brush built the first automatically operated wind turbine for electricity production in Cleveland, Ohio.

The first utility grid-connected wind turbine to operate in the U.K. was built by John Brown & Company in 1951 in the Orkney Islands.[3]

How is wind produced?

Wind power all starts with the sun. When the sun heats up a certain area of land, the air around that land mass absorbs some of that heat. At a certain temperature, that hot air begins to rise very quickly because a given volume of hot air is lighter than an equal volume of cooler air. Faster-moving hot air particles exert more pressure than the slow moving particles, so it takes fewer of them to maintain the normal air pressure at a given elevation. When that lighter hot air suddenly rises, cooler air flows quickly in to fill the gap the hot air leaves behind. That air rushing in to fill the gap is wind.

How can energy be produced with wind?

If an object like a rotor blade is place in the path of wind, the wind will push on it, transferring some of its own energy of motion to the blade. This is how a wind turbine captures energy from the wind. This concept was adopted many thousands of years ago in sailboats. The large sails would capture wind and the energy in wind would propel the boat in that direction.

The simplest possible wind-energy turbine consists of three crucial parts as seen below in the image:

Basic Parts of a Wind Energy Turbine

1. Rotor blades - The blades act as barriers to the wind. When the wind forces the blades to move, it has transferred some of its energy to the rotor. Modern blade designs go beyond the barrier method and we will explore this in detail later.
    
2. Shaft - The wind-turbine shaft is connected to the center of the rotor. When the rotor spins, the shaft spins as well. In this way, the rotor transfers its mechanical, rotational energy to the shaft, which enters an electrical generator on the other end. Modern shafts also incorporate gear boxes to translate the actual wind speed to a higher speed.
    
3. Generator – A generator uses the properties of electromagnetic induction to produce electrical voltage. Voltage is the force that gives rise to electrical current. It is electrical current that transports energy from one point to another, using electrons. So generating voltage is, in effect, generating current. A simple generator consists of magnets and a conductor. The conductor is typically a coiled wire. Inside the generator, the shaft connects to an assembly of permanent magnets that surrounds the coil of wire. In electromagnetic induction, if you have a conductor surrounded by magnets, and one of those parts is rotating relative to the other, it induces voltage in the conductor. When the rotor spins the shaft, the shaft spins the assembly of magnets, generating voltage in the coil of wire. That voltage drives electrical current (typically alternating (AC) current) out through power lines for distribution.

Modern development of wind-energy technology and applications was well underway by the 1930s, when an estimated 600,000 windmills supplied rural areas with electricity and water-pumping services. Once broad-scale electricity distribution spread to farms and country towns, use of wind energy in the United States started to subside, but it picked up again after the U.S. oil shortage in the early 1970s. By the mid-1980s, wind turbines had a typical maximum power rating of 150 kW.

Modern wind turbines have two primary designs:

1. Vertical-axis wind turbines (VAWTs) are pretty rare. The only one currently in commercial production is the Darrieus turbine.
    
2. Horizontal-axis wind turbines (HAWTs)

Vertical Axis Wind Turbines (VAWT)

The Darrieus type is theoretically just as efficient as the propeller type if wind speed is constant, but in practice this efficiency is rarely realized due to the physical stresses and limitations imposed by a practical design and wind speed variation. There are also major difficulties in protecting the Darrieus turbine from extreme wind conditions and in making it self-starting. This design of wind turbine was patented by Georges Jean Marie Darrieus, a French aeronautical engineer in 1931.

In the original versions of the Darrieus design (VAWT), the aerofoils are arranged so that they are symmetrical and have zero rigging angle, that is, the angle that the aerofoils are set relative to the structure on which they are mounted.

When the Darrieus rotor is spinning, the aerofoils are moving forward through the air in a circular path. Relative to the blade, this oncoming airflow is added vectorially to the wind, so that the resultant airflow creates a varying small positive angle of attack (AoA) to the blade. This generates a net force pointing obliquely forwards along a certain 'line-of-action'. This force can be projected inwards past the turbine axis at a certain distance, giving a positive torque to the shaft, thus helping it to rotate in the direction it is already traveling in. The aerodynamic principles which rotate the rotor are equivalent to that in autogiros, and normal helicopters in auto-rotation.

As the aerofoil moves around the back of the apparatus, the angle of attack changes to the opposite sign, but the generated force is still obliquely in the direction of rotation, because the wings are symmetrical and the rigging angle is zero. The rotor spins at a rate unrelated to the wind speed, and usually many times faster. The energy arising from the torque and speed may be extracted and converted into useful power by using an electrical generator.

In this mechanism, it is the tangential force pulling the blade around, and the radial force acting against the bearings that is important.

Advantages of a Darrieus turbine (VAWT) over conventional turbines (HAWT) –

1. Because of the arrangement of the aerofoils, this turbine is equally effective no matter which direction the wind is blowing as opposed to the conventional type (HAWT), which must be rotated to face into the wind.
    
2. In this configuration, the Darrieus design is theoretically less expensive than a conventional type, as most of the stress is in the blades which torque against the generator located at the bottom of the turbine. The only forces that need to be balanced out vertically are the compression load due to the blades flexing outward (thus attempting to "squeeze" the tower), and the wind force trying to blow the whole turbine over, half of which is transmitted to the bottom and the other half of which can easily be offset with guy wires. By contrast, a conventional design has all of the force of the wind attempting to push the tower over at the top, where the main bearing is located. Additionally, one cannot easily use guy wires to offset this load, because the propeller spins both above and below the top of the tower. Thus the conventional design requires a strong tower that grows dramatically with the size of the propeller. Modern designs can compensate most tower loads of that variable speed and variable pitch.

Disadvantages of a Darrieus turbine (VAWT) over conventional turbines (HAWT) –

1. When the rotor is stationary, no net rotational force arises, even if the wind speed rises quite high—the rotor must already be spinning to generate torque. Thus the design is not normally self-starting. Under rare conditions, Darrieus rotors can self-start, so some form of brake is required to hold it when stopped.
    
2. The angle of attack changes as the turbine spins, so each blade generates its maximum torque at two points on its cycle (front and back of the turbine). This leads to a sinusoidal (pulsing) power cycle that complicates design. In particular, almost all Darrieus turbines have resonant modes where, at a particular rotational speed, the pulsing is at a natural frequency of the blades that can cause them to (eventually) break. For this reason, most Darrieus turbines have mechanical brakes or other speed control devices to keep the turbine from spinning at these speeds for any lengthy period of time.
    
3. Another problem arises because the majority of the mass of the rotating mechanism is at the periphery rather than at the hub, as it is with a propeller. This leads to very high centrifugal stresses on the mechanism, which must be stronger and heavier than otherwise to withstand them. One common approach to minimize this is to curve the wings into an "egg-beater" shape.

In overall comparison, while there are some advantages in Darrieus design there are many more disadvantages, especially with bigger machines in MW class. The Darrieus design uses much more expensive material in blades while most of the blade is too near of ground to give any real power. Traditional designs assume that wing tip is at least 40m from ground at lowest point to maximize energy production and lifetime.

Parts of a simple Horizontal Axis Wind Turbine

Horizontal Axis Wind Turbine (HAWT)

HAWTs have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

Major HAWT components:

1. Rotor blades – They capture wind's energy and converts it to the rotational energy of shaft
    
2. Shaft – It is the medium through which wind’s energy is captured and transferred to the generator.
    
3. Tower - supports the rotor blades and nacelle and lifts the entire setup to a higher elevation where the blades can safely clear the ground during rotation
    
4. Electrical equipment - carries electricity from the generator down through the tower. Controls required for safety of the turbine are also a part of the electrical equipment.
   
   
   

   

   Parts internal to the nacelle of a Horizontal Axis Wind Turbine
5. Nacelle – This is the casing that holds:
   
   • Gearbox - increases speed of shaft between the rotor hub and the generator
   
   • Generator - uses rotational energy of the shaft to generate electricity using principles of electromagnetism
   
   • Electronic Control Unit (not shown) - monitors the system, shuts down turbine in case of malfunction and controls yaw mechanism
   
   • Yaw Controller (not shown) – moves the rotor blades to align with direction of wind. By minimizing the yaw angle (the misalignment between wind and turbine pointing direction), the power output is maximized and non-symmetrical loads minimized. However, since the wind direction varies quickly the turbine will not strictly follow the direction and will have a small yaw angle on average.
   
   • Brakes - stops rotation of the shaft in case of a power overload or system failure
   
   
    

Loads on a Rotor Blade

Lift and Drag:

Turbine blades are shaped a lot like airplane wings - they use an aerofoil design. In an aerofoil, one surface of the blade is somewhat rounded, while the other is relatively flat. Lift is a pretty complex phenomenon. One simplified explanation of lift, when wind travels over the rounded, downwind face of the blade, it has to move faster to reach the end of the blade in time to meet the wind traveling over the flat, upwind face of the blade (facing the direction from which the wind is blowing). Since faster moving air tends to rise in the atmosphere, the downwind, curved surface ends up with a low-pressure pocket just above it. The low-pressure area sucks the blade in the downwind direction, an effect known as "lift." On the upwind side of the blade, the wind is moving slower and creating an area of higher pressure that pushes on the blade, trying to slow it down. Like in the design of an airplane wing, a high lift-to-drag ratio is essential in designing an efficient turbine blade. Turbine blades are twisted so they can always present an angle that takes advantage of the ideal lift-to-drag force ratio.

Aerodynamics is not the only design consideration at play in creating an effective wind turbine. Other factors that play a significant role are:

1. Size matters - the longer the turbine blades (and therefore the greater the diameter of the rotor), the more energy a turbine can capture from the wind and the greater the electricity-generating capacity. Generally speaking, doubling the rotor diameter produces a four-fold increase in energy output. In some cases, however, in a lower-wind-speed area, a smaller-diameter rotor can end up producing more energy than a larger rotor because with a smaller setup, it takes less wind power to spin the smaller generator, so the turbine can be running at full capacity almost all the time.
    
2. Tower height - this is a major factor in production capacity. The higher the turbine, the more energy it can capture because wind speeds increase with elevation increase. Ground friction and ground-level objects interrupt the flow of the wind. Scientists estimate a 12% increase in wind speed with each doubling of elevation.

The ratio between the speed of the blade tips and the speed of the wind is called tip-speed ratio. High efficiency 3-blade-turbines have tip speed/wind speed ratios of 6 to 7.
Most modern rotor blades on large wind turbines are made of glass fiber reinforced plastics, (GRP), i.e. glass fiber reinforced polyester or epoxy. Using carbon fiber or aramid (Kevlar) as reinforcing material is another possibility, but usually such blades are uneconomic for large turbines. Wood, wood-epoxy, or wood-fiber-epoxy composites have not penetrated the market for rotor blades, although there is still development going on in this area. Steel and aluminum alloys have problems of weight and metal fatigue respectively. They are currently only used for very small wind turbines.

The speed and torque at which a wind turbine rotates must be controlled for several reasons:

1. To optimize the aerodynamic efficiency of the rotor in light winds.
2. To keep the generator within its speed and torque limits.
3. To keep the rotor and hub within their centripetal force limits. The centripetal force from the spinning rotors increases as the square of the rotation speed, which makes this structure sensitive to overspeed.
4. To keep the rotor and tower within their strength limits. Because the power of the wind increases as the cube of the wind speed, turbines have to be built to survive much higher wind loads (such as gusts of wind) than those from which they can practically generate power. Since the blades generate more downwind force (and thus put far greater stress on the tower) when they are producing torque, most wind turbines have ways of reducing torque in high winds.
5. To enable maintenance; because it is dangerous to have people working on a wind turbine while it is active, it is sometimes necessary to bring a turbine to a full stop.
6. To reduce noise. As a rule of thumb, the noise from a wind turbine increases with the fifth power of the relative wind speed (as seen from the moving tip of the blades). In noise-sensitive environments, the tip speed can be limited to approximately 60 m/s (200 ft/s).

To sum it up, the desirable features of rotor blades are:

1. Aerodynamic shape and lightweight
2. Twist in the rotor blade
3. Tapered rotor blade
4. Length of the rotor blade

Some interesting articles on rotor design can be found at: http://infogreenglobal.com/extreme-testing-for-rotor-blades/#more-2932
 

Why are rotor blades tapered?[4]

The mechanical stress increases moving towards the center, but the wind speed increases moving towards the radial tip. Aiming for an optimal wind speed, all factors are calculated and then wind tunnel tested to verify and further optimize so that all radial sections of the blade contribute the most to rotating the rotor under the chosen wind speed. The results are the shapes we see out there achieving optimal performance from both mechanical and aerodynamic standpoints. On a wind turbine rotor, designing the blade so that it is thinner/ narrower near the blade tip can provide the same energy as the thicker root because it will have more air flowing past it, and at a different vector. This can also be achieved with a bit of twist in the blade.

Why are rotor blades twisted?

Rotor blades for large wind turbines are always twisted. Seen from the rotor blade, the wind will be coming from a much steeper angle, as you move towards the root of the blade, and the center of the rotor. The rotor blade will stop giving lift and stall, if the wind hits the blade at an angle too steep. If the rotor blade is twisted, an optimal angle of attack is achieved throughout the length of the blade. This is more important in the case of stall-controlled wind turbines as having a twist in the blade will stall gradually from the blade root and outwards at high wind speeds.
 

What is the importance of rotor Tip-Speed Ratio (TSR)?

The Tip Speed Ratio (TSR) is of vital importance in the design of wind turbine generators. If the rotor of the wind turbine turns too slowly, most of the wind will pass undisturbed through the gap between the rotor blades. Alternatively if the rotor turns too quickly, the blurring blades will appear like a solid wall to the wind. Therefore, wind turbines are designed with optimal tip speed ratios to extract as much power out of the wind as possible.

When a rotor blade passes through the air it leaves turbulence in its wake. If the next blade on the spinning rotor arrives at this point while the air is still turbulent, it will not be able to extract power efficiently from the wind. However if the rotor span a little more slowly the air hitting each turbine blade would no longer be turbulent. Therefore the tip speed ratio is chosen so that the blades do not pass through turbulent air.

Tip Speed of the blade is the rotational speed of the tip of the blade and wind speed is the actual wind velocity.
It has been empirically shown that the tip speed ration for the maximum power output will occur at:

The optimum tip speed ratio depends on the number of blades in the wind turbine rotor. The fewer the number of blades, the faster the wind turbine rotor needs to turn to extract maximum power from the wind. A two-bladed rotor has an optimum tip speed ratio of around 6, a three-bladed rotor around 5, and a four-bladed rotor around 3.

Highly efficient aerofoil rotor blade design can increase these optimum values by as much as 25-30% increasing the speed at which the rotor turns and therefore generating more power. A well designed typical three-bladed rotor would have a tip speed ratio of around 6 to 7.

The graph below shows efficiency at different tip speed ratios.
 

Betz Limit

If the tip speed ratio is too low - for example if poorly designed rotor blades are used - the wind turbine will tend to slow and/or stall. If the tip speed ratio is too high the turbine will spin very fast through turbulent air, power will not be optimally extracted from the wind, and the wind turbine will be highly stressed and at risk of structural failure.[5]

Wind turbines extract energy by slowing down the wind. For a wind turbine to be 100% efficient it would need to stop 100% of the wind - but then the rotor would have to be a solid disk and it would not turn and no kinetic energy would be converted. On the other extreme, if you had a wind turbine with just one rotor blade, most of the wind passing through the area swept by the turbine blade would miss the blade completely and so the kinetic energy would be kept by the wind.

The theoretical maximum power efficiency of any design of wind turbine is 0.59 (i.e. no more than 59% of the energy carried by the wind can be extracted by a wind turbine). Once you also factor in the engineering requirements of a wind turbine - strength and durability in particular - the real world limit is well below the Betz Limit with values of 0.35-0.45 common even in the best designed wind turbines. By the time you take into account other inefficacies in a complete wind turbine system - e.g. the generator, bearings, and power transmission and so on - only 10-30% of the power of the wind is ever actually converted into usable electricity.

Horizontal axis wind turbines (HAWT) theoretically have higher power efficiencies than vertical axis wind turbines (VAWT) however wind direction is not important for a VAWT and so no time (and power) is wasted chasing the wind. In turbulent conditions with rapid changes in wind direction more electricity will be generated by a VAWT despite its lower efficiency.

Pitching is turning of the blade angle relative to the wind direction. Pitching towards feather is to turn the blade angle so that the trailing edge moves away from the wind (just like the way a feather does under high wind). Pitching towards stall turns the blade oppositely to increase the angle of wind attack towards the right angle stall. Because the blades' shape was optimized for one wind speed, some turbine systems pitch in real time to get optimal power from the current wind speed. Other turbines pitch only to reduce rotational power to protect itself in high wind.

Yaw mechanism as seen from the nacelle

The wind turbine yaw mechanism is used to turn the wind turbine rotor against the wind.

Yaw Error - The wind turbine is said to have a yaw error, if the rotor is not perpendicular to the wind. A yaw error implies that a lower share of the energy in the wind will be running through the rotor area. The share will drop to the cosine of the yaw error.

If this was the only thing that happened, then yaw control would be an excellent way of controlling the power input to the wind turbine rotor. That part of the rotor which is closest to the source direction of the wind, however, will be subject to a larger force (bending torque) than the rest of the rotor. On the one hand, this means that the rotor will have a tendency to yaw against the wind automatically, regardless of whether we are dealing with an upwind or a downwind turbine. On the other hand, it means that the blades will be bending back and forth in a flapwise direction for each turn of the rotor. Wind turbines which are running with a yaw error are therefore subject to larger fatigue loads than wind turbines which are yawed in a perpendicular direction against the wind.

Almost all horizontal axis wind turbines use forced yawing, i.e. they use a mechanism which uses electric motors and gearboxes to keep the turbine yawed against the wind.
The image shows the yaw mechanism of a typical 750 kW machine seen from below, looking into the nacelle. We can see the yaw bearing around the outer edge, and the wheels from the yaw motors and the yaw brakes inside. Almost all manufacturers of upwind machines prefer to brake the yaw mechanism whenever it is unused. The yaw mechanism is activated by the electronic controller which several times per second checks the position of the wind vane on the turbine, whenever the turbine is running.

Cable Twist Counter

If the turbine changes direction based on a yaw mechanism, what happens to the cables carrying power from the generator output to the ground through the tower?

Cables carry the current from the wind turbine generator down through the tower. The cables, however, will become more and more twisted if the turbine by accident keeps yawing in the same direction for a long time. The wind turbine is therefore equipped with a cable twist counter which tells the controller that it is time to untwist the cables.

Occasionally you may therefore see a wind turbine which looks like it has gone berserk, yawing continuously in one direction for five revolutions.

Like other safety equipment in the turbine there is redundancy in the system. In this case the turbine is also equipped with a pull switch which is activated if the cables become too twisted.

Modern large-turbine designs use several different types of braking systems:

1. Pitch control - The turbine's electronic controller monitors the turbine's power output. At wind speeds over 45 mph, the power output will be too high, at which point the controller tells the blades to alter their pitch so that they become unaligned with the wind. This slows the blades' rotation. Pitch-controlled systems require the blades' mounting angle (on the rotor) to be adjustable.
    
2. Passive stall control - The blades are mounted to the rotor at a fixed angle but are designed so that the twists in the blades themselves will apply the brakes once the wind becomes too fast. The blades are angled so that winds above a certain speed will cause turbulence on the upwind side of the blade, inducing stall. Simply stated, aerodynamic stall occurs when the blade's angle facing the oncoming wind becomes so steep that it starts to eliminate the force of lift, decreasing the speed of the blades.
    
3. Active stall control - The blades in this type of power-control system are pitchable, like the blades in a pitch-controlled system. An active stall system reads the power output the way a pitch-controlled system does, but instead of pitching the blades out of alignment with the wind, it pitches them to produce stall.

The tower of the wind turbine carries the nacelle and the rotor. Towers for large wind turbines may be either tubular steel towers, lattice towers, or concrete towers. Guyed tubular towers are only used for small wind turbines (battery chargers etc.)

The different types of Towers are:

1. Tubular Steel Towers - Most large wind turbines are delivered with tubular steel towers, which are manufactured in sections of 20-30 meters with flanges at either end, and bolted together on the site. The towers are conical (i.e. with their diameter increasing towards the base) in order to increase their strength and to save materials at the same time.
    
2. Lattice Towers - Lattice towers are manufactured using welded steel profiles. The basic advantage of lattice towers is cost, since a lattice tower requires only half as much material as a freely standing tubular tower with a similar stiffness. The basic disadvantage of lattice towers is their visual appearance, (although that issue is clearly debatable). Be that as it may, for aesthetical reasons lattice towers have almost disappeared from use for large, modern wind turbines.
    
3. Guyed Pole Towers - Many small wind turbines are built with narrow pole towers supported by guy wires. The advantage is weight savings, and thus cost. The disadvantages are difficult access around the towers which make them less suitable in farm areas. Finally, this type of tower is more prone to vandalism, thus compromising overall safety.
    
4. Hybrid Tower Solutions - Some towers are made in different combinations of the techniques mentioned above. One example is the three-legged Bonus 95 kW tower which you see in the photograph, which may be said to be a hybrid between a lattice tower and a guyed tower.
    

How do you decide on the type of a Wind Turbine Tower?

Some considerations before deciding on the type of a Wind Turbine Tower are:

1. Cost considerations - The price of a tower for a wind turbine is generally around 20 per cent of the total price of the turbine. For a tower around 50 meters' height, the additional cost of another 10 meters of tower is about $15,000. It is therefore quite important for the final cost of energy to build towers as optimally as possible. Lattice towers are the cheapest to manufacture, since they typically require about half the amount of steel used for a tubular steel tower.
    
2. Aerodynamic considerations - Generally, it is an advantage to have a tall tower in areas with high terrain roughness, since the wind speeds increases farther away from the ground, as we learned on the page about wind shear. Lattice towers and guyed pole towers have the advantage of giving less wind shade than a massive tower.
    
3. Structural Dynamic considerations - The rotor blades on turbines with relatively short towers will be subject to very different wind speeds (and thus different bending) when a rotor blade is in its top and in its bottom position, which will increase the fatigue loads on the turbine.

How do you choose between Low and Tall Towers?

You get more energy from a larger wind turbine than a small one, but if you take a look at the three wind turbines below, which are 225 kW, 600 kW, and 1,500 kW respectively, and with rotor diameters of 27, 43, and 60 meters, you will notice that the tower heights are different as well.

Clearly, we cannot sensibly fit a 60 meter rotor to a tower of less than 30 meters. But if we consider the cost of a large rotor and a large generator and gearbox, it would surely be a waste to put it on a small tower, because we get much higher wind speeds and thus more energy with a tall tower. (See the section on wind resources). Each meter of tower height costs money, so the optimum height of the tower is a function of:

1. Tower costs per meter (10 meter extra tower will presently cost you about $15,000)
    
2. How much the wind locally varies with the height above ground level, i.e. the average local terrain roughness (large roughness makes it more useful with a taller tower)
    
3. The price the turbine owner gets for an additional kilowatt hour of electricity

Manufacturers often deliver machines where the tower height is equal to the rotor diameter. Aesthetically, many people find that turbines are more pleasant to look at, if the tower height is roughly equal to the rotor diameter.

The amount of power generated by a windmill is calculated as follows:

Based on the above equation, the two most important factors for producing power from wind are:

1. Area swept by the rotor blades – doubling the area swept by the rotor blades will double the power output.
    
2. Wind velocity – doubling the wind velocity with increase power output by a factor of 8.

The area of the disc covered by the rotor, (and wind speeds, of course), determines how much energy we can harvest in a year.

The image above gives you an idea of the normal rotor sizes of wind turbines: A typical turbine with a 600 kW electrical generator will typically have a rotor diameter of some 44 metres (144 ft.). If you double the rotor diameter, you get an area which is four times larger (two squared). This means that you also get four times as much power output from the rotor.

Rotor diameters may vary somewhat from the figures given above, because many manufacturers optimize their machines to local wind conditions: A larger generator, of course, requires more power (i.e. strong winds) to turn at all. So if you install a wind turbine in a low wind area you will actually maximize annual output by using a fairly small generator for a given rotor size (or a larger rotor size for a given generator) For a 600 kW machine rotor diameters may vary from 39 to 48 m (128 to 157 ft.) The reason why you may get more output from a relatively smaller generator in a low wind area is that the turbine will be running more hours during the year.

To calculate the amount of power a turbine can actually generate from the wind at a particular location, you need to know the wind speed at the turbine site and the turbine power rating. Most large turbines produce their maximum power at wind speeds around 15 meters per second (33 mph). Considering steady wind speeds, it is the diameter of the rotor that determines how much energy a turbine can generate. As a rotor diameter increases, the height of the tower increases as well, which means more access to faster winds.
 

The table below shows the change in power output as the rotor diameter increases.

At 33 mph, most large turbines generate their rated power capacity, and at 45 mph (20 meters per second), most large turbines shut down.

A good read on how redesigning windmills lower the cost per Megawatt can be found at: http://infogreenglobal.com/better-redesigned-windmills-will-get-lower-cost-per-megawatt/#more-1692

There are a number of factors to consider before deciding to use a particular turbine.

Reasons for Choosing Large Turbines are:

1. There are economies of scale in wind turbines, i.e. larger machines are usually able to deliver electricity at a lower cost than smaller machines. The reason is that the cost of foundations, road building, electrical grid connection, plus a number of components in the turbine (the electronic control system etc.), are somewhat independent of the size of the machine.
    
2. Larger machines are particularly well suited for offshore wind power. The cost of foundations does not rise in proportion to the size of the machine, and maintenance costs are largely independent of the size of the machine.
    
3. In areas where it is difficult to find sites for more than a single turbine, a large turbine with a tall tower uses the existing wind resource more efficiently.

Reasons for Choosing Smaller Turbines are:

1. The local electrical grid may be too weak to handle the electricity output from a large machine. This may be the case in remote parts of the electrical grid with low population density and little electricity consumption in the area.
    
2. There is less fluctuation in the electricity output from a wind park consisting of a number of smaller machines, since wind fluctuations occur randomly, and therefore tend to cancel out. Again, smaller machines may be an advantage in a weak electrical grid.
    
3. The cost of using large cranes, and building a road strong enough to carry the turbine components may make smaller machines more economic in some area
    
4. Several smaller machines spread the risk in case of temporary machine failure, e.g. due to lightning strikes.
    
5. Aesthetical landscape considerations may sometimes dictate the use of smaller machines. Large machines, however, will usually have a much lower rotational speed, which means that one large machine really does not attract as much attention as many small, fast moving rotors. (See the section on wind turbines in the landscape).

Vibration Sensor

1. Sensors - A remarkably simple vibration sensor is used in some turbines. This sensor basically consists of a metal ball attached to a chain, poised on a tiny pedestal. If the turbine starts vibrating above a certain threshold, the ball falls off the pedestal, pulling on the chain and triggering a shut down.
    
2. There are many other sensors in the nacelle, e.g. electronic thermometers which check the oil temperature in the gearbox and the temperature of the generator.
    
3. Rotor Blades - Safety regulations for wind turbines vary between countries. Denmark is the only country in which the law requires that all new rotor blades are tested both statically, i.e. applying weights to bend the blade, and dynamically, i.e. testing the blade's ability to withstand fatigue from repeated bending more than five million times.
    
4. Overspeed Protection - It is essential that wind turbines stop automatically in case of malfunction of a critical component. Eg. if the generator overheats or is disconnected from the electrical grid it will stop braking the rotation of the rotor, and the rotor will start accelerating rapidly within a matter of seconds. In such a case it is essential to have an overspeed protection system.
    
5. 

   

   Mechanical Brakes
   Automatic Braking System - The most commonly activated safety system in a turbine is the "braking" system, which is triggered by above-threshold wind speeds. These setups use a power-control system that essentially hits the brakes when wind speeds get too high and then "release the brakes" when the wind is back below 45 mph. These systems are usually spring operated, in order to work even in case of electrical power failure, and they are automatically activated if the hydraulic system in the turbine loses pressure. The hydraulic system in the turbine is used turn the blades or blade tips back in place once the dangerous situation is over.
    
6. Mechanical Braking System - The mechanical brake is used as a backup system for the aerodynamic braking system, and as a parking brake, once the turbine is stopped in the case of a stall controlled turbine. Pitch controlled turbines rarely need to activate the mechanical brake (except for maintenance work), as the rotor cannot move very much once the rotor blades are pitched 90 degrees.

You can refer to Wind Power Density (WPD) charts for your area. These charts show a quantitative measure of wind energy available at a particular location. It is a calculation of the mean annual power available per square meter of swept area of a turbine, and is tabulated for different heights above ground. Calculation of wind power density includes the effect of wind velocity and air density. Color-coded maps are prepared for a particular area.

Areas designated Class 3 or greater are suitable for most utility-scale wind turbine applications, whereas Class 2 areas are marginal for utility-scale applications but may be suitable for rural applications. Class 1 areas are generally not suitable, although a few locations (e.g., exposed hilltops not shown on the maps) with adequate wind resource for wind turbine applications may exist in some Class 1 areas.

The degree of certainty with which the wind power class can be specified depends on three factors:

1. The abundance and quality of wind data
2. The complexity of the terrain
3. The geographical variability of the resource

The wind power density limits for each wind power class are shown in the table below:

What does it mean to have a small wind turbine?

A small, 10-kW-capacity turbine can generate up to 16,000 kWh per year, and a typical U.S. household consumes about 10,000 kWh in a year.