Wind turbine design and wind power systems design have made substantial progress in the last 20 years. Reliable and efficient turbines are being made that have capacities in the megawatt range. Modern utility sized wind turbines mostly utilize the three bladed rotor design pioneered by Dutch turbine manufacturers in the 1980’s. While a wind turbine is a fairly simple concept, modern wind power systems have cutting edge design and complex control systems to maximize power output, minimize the cost of electricity and ensure safety in operation. In the following, it will be shown how the kinetic energy of the wind is harvested by the wind turbine and changed into electrical energy.
The optimal design of a wind turbine depends heavily on the wind conditions of its siting. Designers of wind turbines optimize their machines to produce electricity at the lowest possible cost. The design that produces the lowest-cost energy is not necessarily the technically most efficient turbine. However, since the source of energy, the wind, is free, designers are not overly concerned with efficiency, low energy costs are more important. 
The design of wind turbines begins with evaluating the wind potential of a site. The variations in wind speed typically follow a Weibull probability distribution which is unique to each site. Knowledge of this distribution is necessary for optimizing the turbines performance. It is easy then to show that the power that is contained in the wind is proportional to the cube of the wind’s velocity. The power of a moving fluid is given by P=1/2 mV^2 where m is the mass flowrate of the wind. Knowing that the mass flowrate of a fluid is equal to ρAV where ρ is the air density and A is the area swept by the rotor, a quick substitution gives the following: P = 1/2ρAV^2.  A wind turbine cannot extract 100% of this energy however. Using the principles of fluid mechanics and thermodynamics, it can be shown that maximum percentage of the power of the wind that can be extracted is 59%. This is known as Betz’s law. .
Once the wind speed distribution of a site is known, a wind power system can be designed to produce the lowest cost electricity. The main components of a turbine include the rotor blades which are attached to the hub of the turbine. The hub is attached to the nacelle which sits on top of the supporting tower. Within the nacelle are usually a gearbox, the generator, and components of the control systems. Along with these components are the power electronics that make the electricity produced by the generator suitable for the electrical grid. Each component requires careful design to optimize the function of the wind power system.
The design of the rotor blades is similar to the design of aircraft wings and helicopter rotors. The rotor blades are shaped so that air moves faster over one surface of the blade, creating a pressure difference, and subsequently, lift. The blades are often twisted to optimize the angle-of-attack from the wind. Some blades are similar to the standard airfoil, others have more complicated profiles. 
The turbine blades are usually designed to have a maximum output at some moderated wind speed. Control systems are thus necessary to prevent damage to the turbine in higher wind speeds. One common method is to have pitch controlled turbine blades. With this method the pitch of the blades is adjusted with changes in wind speed to increase output in lower wind speeds and reduce speed, thus preventing damage, in higher wind speeds. This kind of system is computer controlled and hydraulically activated. A less common method is to have stall-controlled turbine blades. These blades are set at a fixed angle, but have profiles that are designed to create stall in higher wind speeds, thus preventing damage to the turbine. 
Another component that maximizes power output and reduces fatigue loading on the blades is the yaw mechanism. The yaw mechanism turns the blades so they are fully facing the wind, which helps to make the loading on the blades more even.  Other parts of the control system include braking mechanisms and vibration sensors.
Other considerations are taken into account when designing the rotor blades. They must be able to withstand extreme loadings that are relatively rare, but also to withstand the fatigue loading from normal winds. Metals are usually not used to make the blades because they are more prone to fatigue failure. Turbine blades are usually made from composite materials because of their strength and durability.
To increase the stability of the turbine blades, they are placed upwind of the tower and facing into the wind to avoid the wind shadow behind the tower. Also to increase stability, an odd number of blades, usually three, are used. If an even number of blades were used, when one blade was in the wind shadow in front of the tower, another blade is under its greatest load above the tower. This is avoided if three blades are used and results in a smaller fatigue load. 
While it is obvious that power output increases with the swept rotor area, bigger is not necessarily better. The size of rotor that produces the cheapest electricity depends on the wind speed distribution, as well as on the capabilities of the local electric grid.
The wind flowing through the rotor blades cause the rotor shaft to spin at a fairly slow rate. The generator in the nacelle requires a faster input, so a gearbox is necessary to step up the speed. A typical gear ratio is around 1:50.  The gearbox must be designed to withstand constant changes in torque from the changes in wind speed. It must also be properly lubricated to prevent wear from constant use.
After a proper step up in speed, the power is transferred from the gearbox to the generator. The most commonly used generator is 3-phase asynchronous generator with 4 to 6 poles, though synchronous generators are sometimes used. The advantage of an asynchronous generator is that it allows for variable input torques and input speeds. This results in less stress and wear on the gearbox.  However, if an asynchronous generator is used, it will have to be indirectly connected to the electrical grid. The asynchronous motor will produce an output voltage that varies in amplitude and frequency with variations in the input speed. This output voltage must then be put through a series of power equipment components which change the output to a DC voltage, then back to an AC voltage at the grid frequency, usually 60Hz. Also there will be control system components used to control power quality and to ensure the output is compatible with the electrical grid. This extra power equipment is expensive and causes some loss of power. Whether this extra cost is balanced by the increased power from being able to operate at variable speeds depends on the wind speed distribution of the site. 
The final component of the wind power system to be considered is the tower on which the nacelle sits. The height of the tower is also determined by the wind speed distribution of the site. Wind speed generally increases with altitude. The ideal height of the tower depends on the cost of building the tower and how much the wind varies with altitude. It also directly depends on the swept rotor area.
In summary, the principle of operation of a wind power system involves the kinetic energy of the wind being transferred to the rotor, then from the rotor to the gearbox. The energy is then transferred from the gearbox to the generator, where it is converted into electrical energy. The electricity produced goes through a series of components before being absorbed by the grid where it is available for consumer use. In addition to these basic components, there is a complex, computerized control system. This system includes pitch and yaw control, cooling of the generator, maintaining output power quality, and the collection and analysis of data from sensors that detect temperature, wind speed and component vibration. The modern wind turbine is a very sophisticated and meticulously designed machine that provides a significant challenge for today’s engineers.
 “Wind Turbine Design Optimization.” 4 April 2009.
 Munteanu, Iulian, Antoneta Iuliana Bratcu, Nicolaos-Antonio Cutululis, Emil Ceanga. “Optimal Control of Wind Energy Systems.” London, UK. Springer. 2008. pg 15-16
 “Rotor Blades.” 4 April 2009.
 Munteanu, Iulian, Antoneta Iuliana Bratcu, Nicolaos-Antonio Cutululis, Emil Ceanga. “Optimal Control of Wind Energy Systems.” London, UK. Springer. 2008. pg 73-74
 “The Wind Turbine Yaw Mechanism.” 4 April 2009.
”Wind Turbines: How Many Blades?” 4 April 2009.
 “Gearboxes for Wind Turbines.” 4 April 2009.
 “Asynchonous Generators.” 4 April 2009.
 “Indirect Grid Connection of Wind Turbines.” 4. April 2009.