View Section, Part I. View Section, 2. Introduction to QFT. View Section, 3. View Section, 4. Discrete Quantitative Feedback Technique. View Section, 5. View Section, 6. View Section, 7. View Section, 8.
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- Linear and Nonlinear Schemes Applied to Pitch Control of Wind Turbines.
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Nonlinear Switching Control Techniques. View Section, Part II. Wind Turbine Control. View Section, 9. Introduction to Wind Energy Systems. View Section, Standards and Certification for Wind Turbines. Wind Turbine Control Objectives and Strategies. Aerodynamics and Mechanical Modeling of Wind Turbines. Electrical Modeling of Wind Turbines. Advanced Pitch Control System Design. The varying output frequency and voltage can be matched to the fixed values of the grid using multiple technologies such as doubly fed induction generators or full-effect converters where the variable frequency current produced is converted to DC and then back to AC.
Although such alternatives require costly equipment and cause power loss, the turbine can capture a significantly larger fraction of the wind energy. In some cases, especially when turbines are sited offshore, the DC energy will be transmitted from the turbine to a central onshore inverter for connection to the grid. Gearless wind turbines also called direct drive get rid of the gearbox completely. Instead, the rotor shaft is attached directly to the generator, which spins at the same speed as the blades. Advantages of PMDD generators over gear-based generators include increased efficiency, reduced noise, longer lifetime, high torque at low rpm, faster and precise positioning, and drive stiffness.
PMDD generators "eliminate the gear-speed increaser, which is susceptible to significant accumulated fatigue torque loading, related reliability issues, and maintenance costs. To make up for a direct drive generator's slower spinning rate, the diameter of the generator's rotor is increased so that it can contain more magnets to create the required frequency and power.
Gearless wind turbines are often heavier than gear-based wind turbines. A study by the EU called "Reliawind"  based on the largest sample size of turbines has shown that the reliability of gearboxes is not the main problem in wind turbines. The reliability of direct drive turbines offshore is still not known, since the sample size is so small. In December , the US Department of Energy published a report stating critical shortage of rare-earth elements such as neodymium used in large quantities for permanent magnets in gearless wind turbines.
Hybrid drivetrains intermediate between direct drive and traditional geared use significantly less rare-earth materials. The ratio between the speed of the blade tips and the speed of the wind is called tip speed ratio. Modern wind turbines are designed to spin at varying speeds a consequence of their generator design, see above.
Wind Energy Systems - 1st Edition
Use of aluminum and composite materials in their blades has contributed to low rotational inertia , which means that newer wind turbines can accelerate quickly if the winds pick up, keeping the tip speed ratio more nearly constant. Operating closer to their optimal tip speed ratio during energetic gusts of wind allows wind turbines to improve energy capture from sudden gusts that are typical in urban settings.
In contrast, older style wind turbines were designed with heavier steel blades, which have higher inertia, and rotated at speeds governed by the AC frequency of the power lines. The high inertia buffered the changes in rotation speed and thus made power output more stable. It is generally understood that noise increases with higher blade tip speeds.
To increase tip speed without increasing noise would allow reduction the torque into the gearbox and generator and reduce overall structural loads, thereby reducing cost. The inability to predict stall restricts the development of aggressive aerodynamic concepts.
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A blade can have a lift-to-drag ratio of ,  compared to 70 for a sailplane and 15 for an airliner. In simple designs, the blades are directly bolted to the hub and are unable to pitch, which leads to aerodynamic stall above certain windspeeds. In other more sophisticated designs, they are bolted to the pitch bearing , which adjusts their angle of attack with the help of a pitch system according to the wind speed to control their rotational speed. The hub is fixed to the rotor shaft which drives the generator directly or through a gearbox. The number of blades is selected for aerodynamic efficiency, component costs, and system reliability.
Noise emissions are affected by the location of the blades upwind or downwind of the tower and the speed of the rotor. Given that the noise emissions from the blades' trailing edges and tips vary by the 5th power of blade speed, a small increase in tip speed can make a large difference. Wind turbines developed over the last 50 years have almost universally used either two or three blades.
Linear and Nonlinear Schemes Applied to Pitch Control of Wind Turbines
However, there are patents that present designs with additional blades, such as Chan Shin's Multi-unit rotor blade system integrated wind turbine. Increasing the number of blades from one to two yields a six percent increase in aerodynamic efficiency, whereas increasing the blade count from two to three yields only an additional three percent in efficiency. Theoretically, an infinite number of blades of zero width is the most efficient, operating at a high value of the tip speed ratio.
But other considerations lead to a compromise of only a few blades. Component costs that are affected by blade count are primarily for materials and manufacturing of the turbine rotor and drive train. Generally, the lower the number of blades, the lower the material and manufacturing costs will be. In addition, the lower the number of blades, the higher the rotational speed can be. This is because blade stiffness requirements to avoid interference with the tower limit how thin the blades can be manufactured, but only for upwind machines; deflection of blades in a downwind machine results in increased tower clearance.
Fewer blades with higher rotational speeds reduce peak torques in the drive train, resulting in lower gearbox and generator costs. System reliability is affected by blade count primarily through the dynamic loading of the rotor into the drive train and tower systems. While aligning the wind turbine to changes in wind direction yawing , each blade experiences a cyclic load at its root end depending on blade position. This is true of one, two, three blades or more. However, these cyclic loads when combined together at the drive train shaft are symmetrically balanced for three blades, yielding smoother operation during turbine yaw.
Turbines with one or two blades can use a pivoting teetered hub to also nearly eliminate the cyclic loads into the drive shaft and system during yawing.
Wind Turbine Control Systems
A Chinese 3. Finally, aesthetics can be considered a factor in that some people find that the three-bladed rotor is more pleasing to look at than a one- or two-bladed rotor. This narrows down the list of acceptable materials. Metals would be undesirable because of their vulnerability to fatigue. Ceramics have low fracture toughness, which could result in early blade failure. Traditional polymers are not stiff enough to be useful, and wood has problems with repeatability, especially considering the length of the blade.
That leaves fiber-reinforced composites, which have high strength and stiffness and low density, as a very attractive class of materials for the design of wind turbines. Wood and canvas sails were used on early windmills due to their low price, availability, and ease of manufacture. Smaller blades can be made from light metals such as aluminium. These materials, however, require frequent maintenance. Wood and canvas construction limits the airfoil shape to a flat plate, which has a relatively high ratio of drag to force captured low aerodynamic efficiency compared to solid airfoils.
Construction of solid airfoil designs requires inflexible materials such as metals or composites. Some blades also have incorporated lightning conductors. New wind turbine designs push power generation from the single megawatt range to upwards of 10 megawatts using larger and larger blades. A larger area effectively increases the tip-speed ratio of a turbine at a given wind speed, thus increasing its energy extraction.
As of the rotor diameters of onshore wind turbine blades are as large as meters,  while the diameter of offshore turbines reach meters. An important goal of larger blade systems is to control blade weight. Since blade mass scales as the cube of the turbine radius, loading due to gravity constrains systems with larger blades. Wind is another source of rotor blade loading. Lift causes bending in the flapwise direction out of rotor plane while air flow around the blade cause edgewise bending in the rotor plane.
Flapwise bending involves tension on the pressure upwind side and compression on the suction downwind side. Edgewise bending involves tension on the leading edge and compression on the trailing edge. Wind loads are cyclical because of natural variability in wind speed and wind shear higher speeds at top of rotation. Failure in ultimate loading of wind-turbine rotor blades exposed to wind and gravity loading is a failure mode that needs to be considered when the rotor blades are designed. The wind speed that causes bending of the rotor blades exhibits a natural variability, and so does the stress response in the rotor blades.
Also, the resistance of the rotor blades, in terms of their tensile strengths, exhibits a natural variability. In light of these failure modes and increasingly larger blade systems, there has been continuous effort toward developing cost-effective materials with higher strength-to-mass ratios. In order to extend the current 20 year lifetime of blades and enable larger area blades to be cost-effective, the design and materials need to be optimized for stiffness, strength, and fatigue resistance.
The majority of current commercialized wind turbine blades are made from fiber-reinforced polymers FRPs , which are composites consisting of a polymer matrix and fibers. The long fibers provide longitudinal stiffness and strength, and the matrix provides fracture toughness, delamination strength, out-of-plane strength, and stiffness. Manufacturing blades in the 40 to 50 metre range involves proven fibreglass composite fabrication techniques. Other manufacturers use variations on this technique, some including carbon and wood with fibreglass in an epoxy matrix.
Other options include preimpregnated "prepreg" fibreglass and vacuum-assisted resin transfer molding. Each of these options use a glass-fibre reinforced polymer composite constructed with differing complexity. Perhaps the largest issue with more simplistic, open-mould, wet systems are the emissions associated with the volatile organics released. Preimpregnated materials and resin infusion techniques avoid the release of volatiles by containing all VOCs.
However, these contained processes have their own challenges, namely the production of thick laminates necessary for structural components becomes more difficult. As the preform resin permeability dictates the maximum laminate thickness, bleeding is required to eliminate voids and ensure proper resin distribution. During evacuation, the dry fabric provides a path for airflow and, once heat and pressure are applied, resin may flow into the dry region resulting in a thoroughly impregnated laminate structure.
Epoxy-based composites have environmental, production, and cost advantages over other resin systems. Epoxies also allow shorter cure cycles, increased durability, and improved surface finish. Prepreg operations further reduce processing time over wet lay-up systems. As turbine blades pass 60 metres, infusion techniques become more prevalent; the traditional resin transfer moulding injection time is too long as compared to the resin set-up time, limiting laminate thickness. Injection forces resin through a thicker ply stack, thus depositing the resin where in the laminate structure before gelation occurs.
Specialized epoxy resins have been developed to customize lifetimes and viscosity. Carbon fibre-reinforced load-bearing spars can reduce weight and increase stiffness. Carbon fibres have the added benefit of reducing the thickness of fiberglass laminate sections, further addressing the problems associated with resin wetting of thick lay-up sections.
Wind turbines may also benefit from the general trend of increasing use and decreasing cost of carbon fibre materials. Recent developments include interest in using carbon nanotubes CNTs to reinforce polymer-based nanocomposites. CNTs can be grown or deposited on the fibers, or added into polymer resins as a matrix for FRP structures. They have very low density, and improve the elastic modulus, strength, and fracture toughness of the polymer matrix.
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The addition of CNTs to the matrix also reduces the propagation of interlaminar cracks which can be a problem in traditional FRPs. Further improvement is possible through the use of carbon nanofibers CNFs in the blade coatings. A major problem in desert environments is erosion of the leading edges of blades by wind carrying sand, which increases roughness and decreases aerodynamic performance. The particle erosion resistance of fiber-reinforced polymers is poor when compared to metallic materials and elastomers, and needs to be improved.
It has been shown that the replacement of glass fiber with CNF on the composite surface greatly improves erosion resistance. CNFs have also been shown to provide good electrical conductivity important for lightning strikes , high damping ratio, and good impact-friction resistance.
These properties make CNF-based nanopaper a prospective coating for wind turbine blades. Another important source of degradation for turbine blades is lightning damage, which over the course of a normal year lifetime is expected to experience a number of lightning strikes throughout its service. Based on a study carried out by the European Wind Energy Association, in the year alone, between and kilotons of composites were consumed by the wind turbine industry for manufacturing blades.
GFRPs hinder incineration and are not combustible. Currently, depending on whether individual fibres can be recovered, there exists a few general methods for recycling GFRPs in wind turbine blades:. Wind velocities increase at higher altitudes due to surface aerodynamic drag by land or water surfaces and the viscosity of the air. The variation in velocity with altitude, called wind shear , is most dramatic near the surface. Typically, the variation follows the wind profile power law , which predicts that wind speed rises proportionally to the seventh root of altitude. To avoid buckling , doubling the tower height generally requires doubling the diameter of the tower as well, increasing the amount of material by a factor of at least four.
At night time, or when the atmosphere becomes stable, wind speed close to the ground usually subsides whereas at turbine hub altitude it does not decrease that much or may even increase. A stable atmosphere is caused by radiative cooling of the surface and is common in a temperate climate: it usually occurs when there is a partly clear sky at night. A daytime atmosphere is either neutral no net radiation; usually with strong winds and heavy clouding or unstable rising air because of ground heating—by the sun.
Indiana had been rated as having a wind capacity of 30, MW, but by raising the expected turbine height from 50 m to 70 m, the wind capacity estimate was raised to 40, MW, and could be double that at m. For HAWTs , tower heights approximately two to three times the blade length have been found to balance material costs of the tower against better utilisation of the more expensive active components.
Road size restrictions makes transportation of towers with a diameter of more than 4.
Swedish analyses show that it is important to have the bottom wing tip at least 30 m above the tree tops, but a taller tower requires a larger tower diameter. Height is typically limited by the availability of cranes. This has led to a variety of proposals for "partially self-erecting wind turbines" that, for a given available crane, allow taller towers that put a turbine in stronger and steadier winds, and "self-erecting wind turbines" that can be installed without cranes. Currently, the majority of wind turbines are supported by conical tubular steel towers. The use of lighter materials in the tower could greatly reduce the overall transport and construction cost of wind turbines, however the stability must be maintained.
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