wind generator

A wind turbine is a device that converts the kinetic energy of wind into electrical energy. Hundreds of thousands of large turbines, in installations known as wind farms, now generate over 650 gigawatts of power, with 60 GW added each year. Wind turbines are an increasingly important source of intermittent renewable energy, and are used in many countries to lower energy costs and reduce reliance on fossil fuels. One study claimed that, wind had the "lowest relative greenhouse gas emissions, the least water consumption demands and the most favorable social impacts" compared to photovoltaic, hydro, geothermal, coal and gas energy sources.

Smaller wind turbines are used for applications such as battery charging for auxiliary power for boats or caravans, and to power 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.

Wind turbines are manufactured in a wide range of sizes, with either horizontal or vertical axes.

History

The windwheel of Hero of Alexandria (10 AD – 70 CE) marks one of the first recorded instances of wind powering a machine. 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. 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.

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. 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 Veranzios with curved or V-shaped blades.

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 was able to build the first automatically operated wind turbine after consulting local University professors and his colleagues Jacob S. Gibbs and Brinsley Coleberd and successfully getting the blueprints peer-reviewed for electricity production. Although Blyth's turbine was considered uneconomical in the United Kingdom, electricity generation by wind turbines was more cost effective in countries with widely scattered populations.

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 towers with four-bladed 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.

By the 1930s, wind generators for electricity were common on farms, mostly in the United States where distribution systems had not yet been installed.

A forerunner of modern horizontal-axis wind generators was in service at Yalta, USSR in 1931. This was a 100 kW generator on a 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.

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 1,100 hours before suffering a critical failure. The unit was not repaired, because of a shortage of materials during the war.

The first utility grid-connected wind turbine to operate in the UK was built by John Brown & Company in 1951 in the Orkney Islands.

Despite these diverse developments, developments in fossil fuel systems almost entirely eliminated any wind turbine systems larger than supermicro size. In the early 1970s, however, anti-nuclear protests in Denmark spurred artisan mechanics to develop microturbines of 22 kW. 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.

It has been argued that expanding 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 criticised for failing to recognise that most wind turbines do not use permanent magnets and for underestimating the power of economic incentives for expanded production of these minerals.

Resources

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.

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.

Efficiency

Conservation of mass requires that the amount of air entering and exiting a turbine must be equal. Accordingly, Betz's law gives the maximal achievable extraction of wind power by a wind turbine as (59.3%) of the rate at which the kinetic energy of the air arrives at the turbine.

The maximum theoretical power output of a wind machine is thus 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:

:$P=\frac\frac\rho v^3 A = \frac\rho v^3 A$,

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.
Further inefficiencies, such as gearbox losses, 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 at 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.

Efficiency can decrease slightly over time, one of the main reasons being dust and insect carcasses on the blades which alters the aerodynamic profile and essentially reduces 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.

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.

Different materials have been found to have varying effects on the efficiency of wind turbines. In an Ege University experiment, three wind turbines (Each with three blades with diameters 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.

The air velocity is the major contributor for 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 put it on mountain ridges. The higher the wind turbine will be, the wind velocity will be higher in average. Windbreak can also increase the wind velocity near the turbine.

Types

Wind turbines can rotate about either a horizontal or a vertical axis, the former being both older and more common. They can also include blades or be bladeless. Household-size vertical designs produce less power and are less common.

Horizontal axis

Large three-bladed horizontal-axis wind turbines (HAWT) with the blades upwind of the tower produce the overwhelming majority of wind power in the world today. These turbines 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 yaw system. 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. Some turbines use a different type of generator suited to slower rotational speed input. These don't need a gearbox and are called direct-drive, meaning they couple the rotor directly to the generator with no gearbox in between. While permanent magnet direct-drive generators can be more costly due to the rare earth materials required, these gearless turbines are sometimes preferred over gearbox generators because they "eliminate the gear-speed increaser, which is susceptible to significant accumulated fatigue torque loading, related reliability issues, and maintenance costs." There is also the pseudo direct drive mechanism, which has some advantages over the permanent magnet direct drive mechanism.

Most horizontal axis turbines have their rotors upwind of the supporting tower. Downwind machines have been built, because they don't need an additional mechanism for keeping them in line with the wind. In high winds, the blades can also be allowed to bend, which reduces their swept area and thus their wind resistance. Despite these advantages, upwind designs are preferred, because the change in loading from the wind as each blade passes behind the supporting tower can cause damage to the turbine.

Turbines used in wind farms for commercial production of electric power are usually three-bladed. These have low torque ripple, which contributes to good reliability. The blades are usually colored white for daytime visibility by aircraft and range in length from . The size and height of turbines increase year by year. Offshore wind turbines are built up to 8 MW today and have a blade length up to . Designs with 10 to 12 MW were in preparation in 2018, and a "15 MW+" prototype with three blades is planned to be constructed in 2022. Usual multi megawatt turbines have tubular steel towers with a height of 70m to 120m and in extremes up to 160m.

Vertical axis

Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically. One advantage of this arrangement is that the turbine does not need to be pointed into the wind to be effective, which is an advantage on a site where the wind direction is highly variable. It is also an advantage when the turbine is integrated into a building because it is inherently less steerable. Also, the generator and gearbox can be placed near the ground, using a direct drive from the rotor assembly to the ground-based gearbox, improving accessibility for maintenance. However, these designs produce much less energy averaged over time, which is a major drawback.

Vertical turbine designs have much lower efficiency than standard horizontal designs. 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.

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, noise may be a concern and an existing structure may not adequately resist the additional stress.

Subtypes of the vertical axis design include:

Darrieus wind turbine

"Eggbeater" turbines, or Darrieus turbines, were named after the French inventor, Georges Darrieus. 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 blade area divided by the rotor area. Newer Darrieus type turbines are not held up by guy-wires but have an external superstructure connected to the top bearing.

Giromill

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. 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.

Savonius wind turbine

These are drag-type devices with two (or more) scoops that are used in anemometers, ''Flettner'' vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops.

Twisted Savonius is a modified savonius, with long helical scoops to provide smooth torque. This is often used as a rooftop wind turbine and has even been adapted for ships.

Parallel

The parallel turbine is similar to the crossflow fan or centrifugal fan. It uses the ground effect. Vertical axis turbines of this type have been tried for many years: a unit producing 10 kW was built by Israeli wind pioneer Bruce Brill in the 1980s.

Unconventional types

Design and construction

Wind turbine design is a careful balance of cost, energy output, and fatigue life.

Components

Wind turbines convert wind energy to electrical energy for distribution. Conventional horizontal axis turbines can be divided into three components:

* The rotor, which is approximately 20% of the wind turbine cost, includes the blades for converting wind energy to low speed rotational energy.
* The generator, which is approximately 34% of the wind turbine cost, includes the electrical generator, the control electronics, and most likely a gearbox (e.g., planetary gear box), adjustable-speed drive, or continuously variable transmission component for converting the low-speed incoming rotation to high-speed rotation suitable for generating electricity.
* The surrounding structure, which is approximately 15% of the wind turbine cost, includes the tower and rotor yaw mechanism.

A 1.5 ( MW) wind turbine of a type frequently seen in the United States has a tower high. The rotor assembly (blades and hub) weighs . The nacelle, which contains the generator, weighs . The concrete base for the tower is constructed using reinforcing steel and contains of concrete. The base is in diameter and thick near the center.

Turbine monitoring and diagnostics

Due to data transmission problems, structural health monitoring of wind turbines is usually performed using several accelerometers and strain gages attached to the nacelle to monitor the gearbox and equipment. Currently, digital image correlation and stereophotogrammetry are used to measure dynamics of wind turbine blades. These methods usually measure displacement and strain to identify location of defects. Dynamic characteristics of non-rotating wind turbines have been measured using digital image correlation and photogrammetry. Three dimensional point tracking has also been used to measure rotating dynamics of wind turbines.

Technology

Generally, efficiency increases along with turbine blade lengths. The blades must be stiff, strong, durable, light and resistant to fatigue. Materials with these properties include composites such as polyester and epoxy, while glass fiber and carbon fiber have been used for the reinforcing. Construction may involve manual layup or injection molding. Retrofitting existing turbines with larger blades reduces the task and risks of redesign.
As of 2021, the longest blade was , producing 15 MW with a maximum noise level of 118 dB(A). Blades need to function over a 100 million load cycles over a period of 20–25 years.

Blade materials

Materials commonly used in wind turbine blades are described below.

Glass and carbon fibers

The stiffness of composites is determined by the stiffness of fibers and their volume content. Typically, E-glass fibers are used as main reinforcement in the composites. Typically, the glass/epoxy composites for wind turbine blades contain up to 75% glass by weight. This increases the stiffness, tensile and compression strength. A promising composite material is glass fiber with modified compositions like S-glass, R-glass etc. Other glass fibers developed by Owens Corning are ECRGLAS, Advantex and WindStrand.

Carbon fiber has more tensile strength, higher stiffness and lower density than glass fiber. An ideal candidate for these properties is the spar cap, a structural element of a blade which experiences high tensile loading. A glass fiber blade could weigh up to , while using carbon fiber in the spar saves 20% to 30% weight, about . However, because carbon fiber is ten times more expensive, glass fiber is still dominant.

Hybrid reinforcements

Instead of making wind turbine blade reinforcements from pure glass or pure carbon, hybrid designs trade weight for cost. For example, for an blade, a full replacement by carbon fiber would save 80% of weight but increase costs by 150%, while a 30% replacement would save 50% of weight and increase costs by 90%. Hybrid reinforcement materials include E-glass/carbon, E-glass/aramid. The current longest blade by LM Wind Power is made of carbon/glass hybrid composites. More research is needed about the optimal composition of materials

Nano-engineered polymers and composites

Additions of small amount (0.5 weight %) of nanoreinforcement (carbon nanotubes or nanoclay) in the polymer matrix of composites, fiber sizing or interlaminar layers can improve fatigue resistance, shear or compressive strength, and fracture toughness of the composites by 30% to 80%. Research has also shown that incorporating small amounts of carbon nanotubes (CNT) can increase the lifetime up to 1500%.

Costs

, the capital cost of a wind turbine was around $1 million per megawatt of nameplate capacity, though this figure varies by location; from a half million in South America to $1.7 million in Asia.

For the wind turbine blades, while the material cost is much higher for hybrid glass/carbon fiber blades than all-glass fiber blades, labor costs can be lower. Using carbon fiber allows simpler designs that use less raw material. The chief manufacturing process in blade fabrication is the layering of plies. Thinner blades allow reducing the number of layers and so the labor, and in some cases, equate to the cost of labor for glass fiber blades.

Offshore has significantly higher installation costs.

Non-blade materials

Wind turbine parts other than the rotor blades (including the rotor hub, gearbox, frame, and tower) are largely made of steel. Smaller turbines (as well as megawatt-scale Enercon turbines) have begun using aluminum alloys for these components to make turbines lighter and more efficient. This trend may grow if fatigue and strength properties can be improved.
Pre-stressed concrete has been increasingly used for the material of the tower, but still requires much reinforcing steel to meet the strength requirement of the turbine. Additionally, step-up gearboxes are being increasingly replaced with variable speed generators, which requires magnetic materials. In particular, this would require a greater supply of the rare earth metal neodymium.

Modern turbines use a couple of tons of copper for generators, cables and such. , global production of wind turbines use of copper per year.

Material supply

A study of the material consumption trends and requirements for wind energy in Europe found that bigger turbines have a higher consumption of precious metals but lower material input per kW generated. The current material consumption and stock was compared to input materials for various onshore system sizes. In all EU countries the estimates for 2020 doubled the values consumed in 2009. These countries would need to expand their resources to meet the estimated demand for 2020. For example, currently the EU has 3% of world supply of fluorspar and it requires 14% by 2020. Globally, the main exporting countries are South Africa, Mexico and China. This is similar with other critical and valuable materials required for energy systems such as magnesium, silver and indium. The levels of recycling of these materials are very low and focusing on that could alleviate supply. Because most of these valuable materials are also used in other emerging technologies, like light emitting diodes (LEDs), photo voltaics

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