Smaller wind turbines are used for applications such as battery charging and remote devices such as traffic warning signs. Larger turbines can contribute to a domestic power supply while selling unused power back to the utility supplier via the electrical grid.[3]
Wind turbines are manufactured in a wide range of sizes, with either horizontal or vertical axes, though horizontal is most common.[4]
The windwheel of Hero of Alexandria (10–70 CE) marks one of the first recorded instances of wind powering a machine.[5] However, the first known practical wind power plants were built in Sistan, an Eastern province of Persia (now Iran), from the 7th century. These "Panemone" were vertical axle windmills, which had long vertical drive shafts with rectangular blades.[6] Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind grain or draw up water, and were used in the gristmilling and sugarcane industries.[7]
Wind power first appeared in Europe during the Middle Ages. The first historical records of their use in England date to the 11th and 12th centuries; there are reports of German crusaders taking their windmill-making skills to Syria around 1190.[8] By the 14th century, Dutch windmills were in use to drain areas of the Rhine delta. Advanced wind turbines were described by Croatian inventor Fausto Veranzio in his book Machinae Novae (1595). He described vertical axis wind turbines with curved or V-shaped blades.
In Denmark by 1900, there were about 2500 windmills for mechanical loads such as pumps and mills, producing an estimated combined peak power of about 30 megawatts (MW). The largest machines were on 24-metre (79 ft) towers with four-bladed 23-metre (75 ft) diameter rotors. By 1908, there were 72 wind-driven electric generators operating in the United States from 5 kilowatts (kW) to 25 kW. Around the time of World War I, American windmill makers were producing 100,000 farm windmills each year, mostly for water-pumping.[15]
By the 1930s, use of wind turbines in rural areas was declining as the distribution system extended to those areas.[16]
A forerunner of modern horizontal-axis wind generators was in service at Yalta, USSR, in 1931. This was a 100 kW generator on a 30-meter (98 ft) tower, connected to the local 6.3 kV distribution system. It was reported to have an annual capacity factor of 32 percent, not much different from current wind machines.[citation needed]
In the autumn of 1941, the first megawatt-class wind turbine was synchronized to a utility grid in Vermont. The Smith–Putnam wind turbine only ran for about five years before one of the blades snapped off.[17] The unit was not repaired, because of a shortage of materials during the war.[18]
The first utility grid-connected wind turbine to operate in the UK was built by John Brown & Company in 1951 in the Orkney Islands.[13][19]
In the early 1970s, however, anti-nuclear protests in Denmark spurred artisan mechanics to develop microturbines of 22 kW despite declines in the industry.[20] Organizing owners into associations and co-operatives led to the lobbying of the government and utilities and provided incentives for larger turbines throughout the 1980s and later. Local activists in Germany, nascent turbine manufacturers in Spain, and large investors in the United States in the early 1990s then lobbied for policies that stimulated the industry in those countries.[21][22][23]
It has been argued that expanding the use of wind power will lead to increasing geopolitical competition over critical materials for wind turbines, such as rare earth elements neodymium, praseodymium, and dysprosium. However, this perspective has been critically dismissed for failing to relay how most wind turbines do not use permanent magnets and for underestimating the power of economic incentives for the expanded production of these minerals.[24]
Wind Power Density (WPD) is a quantitative measure of wind energy available at any location. It is the mean annual power available per square meter of swept area of a turbine, and is calculated for different heights above ground. Calculation of wind power density includes the effect of wind velocity and air density.[25]
Wind turbines are classified by the wind speed they are designed for, from class I to class III, with A to C referring to the turbulence intensity of the wind.[26]
The maximum theoretical power output of a wind machine is thus 16⁄27 times the rate at which kinetic energy of the air arrives at the effective disk area of the machine. If the effective area of the disk is A, and the wind velocity v, the maximum theoretical power output P is:
where ρ is the air density.
Wind-to-rotor efficiency (including rotor blade friction and drag) are among the factors affecting the final price of wind power.[30]Further inefficiencies, such as gearbox, generator, and converter losses, reduce the power delivered by a wind turbine. To protect components from undue wear, extracted power is held constant above the rated operating speed as theoretical power increases as the cube of wind speed, further reducing theoretical efficiency. In 2001, commercial utility-connected turbines delivered 75% to 80% of the Betz limit of power extractable from the wind, at rated operating speed.[31][32]
Efficiency can decrease slightly over time, one of the main reasons being dust and insect carcasses on the blades, which alter the aerodynamic profile and essentially reduce the lift to drag ratio of the airfoil. Analysis of 3128 wind turbines older than 10 years in Denmark showed that half of the turbines had no decrease, while the other half saw a production decrease of 1.2% per year.[33]
In general, more stable and constant weather conditions (most notably wind speed) result in an average of 15% greater efficiency than that of a wind turbine in unstable weather conditions, thus allowing up to a 7% increase in wind speed under stable conditions. This is due to a faster recovery wake and greater flow entrainment that occur in conditions of higher atmospheric stability. However, wind turbine wakes have been found to recover faster under unstable atmospheric conditions as opposed to a stable environment.[34]
Different materials have varying effects on the efficiency of wind turbines. In an Ege University experiment, three wind turbines, each with three blades with a diameter of one meter, were constructed with blades made of different materials: A glass and glass/carbon epoxy, glass/carbon, and glass/polyester. When tested, the results showed that the materials with higher overall masses had a greater friction moment and thus a lower power coefficient.[35]
The air velocity is the major contributor to the turbine efficiency. This is the reason for the importance of choosing the right location. The wind velocity will be high near the shore because of the temperature difference between the land and the ocean. Another option is to place turbines on mountain ridges. The higher the wind turbine will be, the higher the wind velocity on average. A windbreak can also increase the wind velocity near the turbine.[36]
Wind turbines can rotate about either a horizontal or a vertical axis, the former being both older and more common.[37] They can also include blades or be bladeless.[38] Household-size vertical designs produce less power and are less common.[39]
Most horizontal axis turbines have their rotors upwind of the supporting tower.[43] Downwind machines have been built, because they don''t need an additional mechanism for keeping them in line with the wind. In high winds, downwind blades can also be designed to bend more than upwind ones, which reduces their swept area and thus their wind resistance, mitigating risk during gales. Despite these advantages, upwind designs are preferred, because the pulsing change in loading from the wind as each blade passes behind the supporting tower can cause damage to the turbine.[44]
Vertical turbine designs have much lower efficiency than standard horizontal designs.[50] The key disadvantages include the relatively low rotational speed with the consequential higher torque and hence higher cost of the drive train, the inherently lower power coefficient, the 360-degree rotation of the aerofoil within the wind flow during each cycle and hence the highly dynamic loading on the blade, the pulsating torque generated by some rotor designs on the drive train, and the difficulty of modelling the wind flow accurately and hence the challenges of analysing and designing the rotor prior to fabricating a prototype.[51]
When a turbine is mounted on a rooftop the building generally redirects wind over the roof and this can double the wind speed at the turbine. If the height of a rooftop mounted turbine tower is approximately 50% of the building height it is near the optimum for maximum wind energy and minimum wind turbulence. While wind speeds within the built environment are generally much lower than at exposed rural sites,[52][53] noise may be a concern and an existing structure may not adequately resist the additional stress.
Subtypes of the vertical axis design include:
"Eggbeater" turbines, or Darrieus turbines, were named after the French inventor, Georges Darrieus.[54] They have good efficiency, but produce large torque ripple and cyclical stress on the tower, which contributes to poor reliability. They also generally require some external power source, or an additional Savonius rotor to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades, which results in greater solidity of the rotor. Solidity is measured by the blade area divided by the rotor area.
A subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety has variable pitch to reduce the torque pulsation and is self-starting.[55] The advantages of variable pitch are high starting torque; a wide, relatively flat torque curve; a higher coefficient of performance; more efficient operation in turbulent winds; and a lower blade speed ratio, which lowers blade bending stresses. Straight, V, or curved blades may be used.[56]
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