Wind is caused by known physical processes occurring within the Earth’s atmosphere. The Sun heats the Earth’s surface unevenly, and the air above hot land rises and is replaced by air from cooler regions. These bulk movements of air result in winds. Man has sought to harness the power of wind for thousands of years, first by utilizing sails to power ships, and then from about the 7th century, by dedicated machinery — windmills. Now, as we prepare to leave the first decade of the 21st century, with diminishing fossil fuel reserves and increasing pressure worldwide for greater use of cleaner, renewable energy production, man once more has turned his attention to harnessing the power of wind by using wind turbines.
Wind turbines are modern windmills. They convert the kinetic energy (movement energy) of the wind into energy which can be applied to do other forms of work (for instance, to drive electric motors). The force of the wind acts on the turbine’s blades which turn its shaft. As the shaft spins, it turns a gear coupling which move electrical wires between a series of magnets. This action generates a current in the wires which is then stored for later use, or fed directly into circuitry to power machines.
There are two principal wind turbine designs: horizontal-axis wind turbines (HAWTs), in which the blades turn about a shaft which is parallel to the ground, and vertical-axis wind turbines (VAWTs) where the blades turn about an upright shaft.
HAWTs are by far the most common type of turbine in use today. Rotors comprising one, two or three blades are mounted on a horizontal shaft connected to a nacelle. The nacelle houses gear mechanisms, electrical generators and control systems and is mounted atop a tower. Towers are usually made from cylindrical steel and are 25m to 75m tall. Wiring connects the nacelle to external battery storage, or regulators which normalize the generated current before feeding it into an electrical grid or appliance.
Blades are usually made from reinforced polyester or wood-epoxy and can be 35m in length, giving overall rotor diameters of up to ~75m. Ordinarily, blades face upwind (into the wind) and rotate between 10 and 30 times per minute at constant speed, although, increasingly, newer turbines operate at variable speed. As wind speed increases, the rotor of a wind turbine rotates faster. If unchecked, the rotor would continue to accelerate until internal forces and resonances cause instability — a phenomenon known as overspeeding. To avoid overspeeding, modern wind turbines have mechanisms for controlling their rotor speeds.
To spin slower, the turbine blades must catch less wind. To achieve this, the angle the blades make with the wind direction can be altered by turning the blade. Increasing the blade angle means a smaller component of the incident wind’s force goes into pushing the rotor. This is known as pitch control and is adjusted by the turbine’s internal control system.
A second means of regulating the rotor speed (and therefore the turbine’s power output) is stall control. Stall control utilizes the inherent properties of the rotor blades to self-regulate rotation speeds. By designing blades with a certain degree of twist and lateral thickness, wind turbulence acting against the natural movement of the rotor can be introduced at higher rotation speeds. This turbulence acts as a natural damping mechanism. In very high winds turbines it can be stopped by a separate internal braking system.
In order to achieve maximum efficiency, it is necessary to ensure the rotor always faces into the wind direction. Sensors are used to monitor the wind direction and a small motor is used to auto-adjust the turbine head to ensure that is always faces upwind. This is known as a yaw adjustment mechanism. Downwind designs have also been implemented with the advantage of not requiring yaw adjustment mechanisms. However, turbulence generated by wind passing the tower and hub prior to reaching the rotor decrease the turbine efficiency.
Recent vertical wind turbines are based on a machine patented in 1930s by a French engineer G.J.M. Darrieus. Its two blades consist of twisted metal strips affixed to the shaft at the top and bottom and bowed out in the middle similar to the blades on a food mixer. VAWTs have several advantages over their horizontal counterparts. They allow for ground mounted nacelles, making installation and maintenance easier. Also, they do not require additional equipment to ensure they always face the upwind – this makes them particularly well suited to areas where wind direction changes frequently.
However, they do require an initial impetus from an external motor to start rotating, are often prone to blade fatigue due to cyclical motion against the wind, and under constant conditions are less efficient than HAWTs.
Wind turbines are unable to extract the entire amount of kinetic energy available from the wind, as the wind leaves the blades with a finite velocity. The maximum efficiency (assumed to be energy extracted divided by energy available in the captured wind area) obtainable is about 59%, although practical wind turbines extract only a portion of this theoretical quantity. At present, the maximum efficiency obtainable with a propeller-type wind turbine is about 45%. This efficiency is reached when the propeller-tip speed is between five and six times the incident wind velocity.
For any given rotor speed, the efficiency decreases rapidly as the wind velocity decreases. The extractable power varies as the square of the rotor diameter and the cube of the wind velocity. Therefore, the theoretical maximum energy obtainable from a rotor with a diameter of 30 metres in a wind with a speed of 14 metres per second would be about 690 kW. If the wind speed decreases by 50% to 7 metres per second, the theoretical maximum drops to about 86 kW. At this lower wind speed, it would require upward of 17,000 wind turbines (with rotors of 30 metres across) operating at an efficiency of 40% to match the output of a single Giga Watt central power station.
When these limitations are coupled to the need for suitable sites with steady winds, it becomes apparent that major challenges remain if wind turbines are to play a major role in meeting the power demands of a 21st century industrialized nation.