Wind Turbines Blades [Materials, Trends and Applications]
Submitted to: Miss Saadia Riaz Submitted by: NS Umair Khalid NS Mudassir Hussain NS Hamza Ahmed 32 Mechanical(B) Date: May 31st, 2011
Wind Turbines A w ind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a w ind generator or w ind charger.
History For
thousands
of
years
people
have
used
windmills to pump water or grind grain. Even into the twentieth century tall, slender, multivaned wind turbines made entirely of metal were used in American homes and ranches to pump water into the house's plumbing system or into the cattle's watering trough. After World War I, work was begun to develop wind turbines that could
produce
electricity.
Marcellus
Jacobs
invented a prototype in 1927 that could provide power for a radio and a few lamps but little else. When demand for electricity increased later, Jacobs's small, inadequate wind turbines fell out of use. The first large-scale wind turbine built in
the United States was conceived by Palmer Cosslett Putnam in 1934; he completed it in 1941. The machine was huge. The tower was 36.6 yards (33.5 meters) high, and its two stainless steel blades had diameters of 58 yards (53 meters). Putnam's wind turbine could produce 1,250 kilowatts of electricity, or enough to meet the needs of a small town. It was, however, abandoned in 1945 because of mechanical failure. With the 1970s oil embargo, the United States began once more to consider the feasibility of producing cheap electricity from wind turbines. In 1975 the prototype Mod-O was in operation. This was a 100 kilowatt turbine with two 21-yard (19-meter) blades. More prototypes followed (Mod-OA, Mod-1, Mod-2, etc.), each larger and more powerful than the one before. Currently, the United States Department of Energy is aiming to go beyond 3,200 kilowatts per machine. Many different models of wind turbines exist, the most striking being the vertical-axis Darrieus, which is shaped like an egg beater. The model most supported by commercial manufacturers, however, is a horizontal-axis turbine, with a capacity of around 100 kilowatts and three blades not more than 33 yards (30 meters) in length. Wind turbines with three blades spin more smoothly and are easier to balance than those with two blades. Also, while larger wind turbines produce more energy, the smaller models are less likely to undergo major mechanical failure, and thus are more economical to maintain. Wind farms have sprung up all over the United States, most notably in California. Wind farms are huge arrays of wind turbines set in areas of favourable wind production. The great number of interconnected wind turbines is necessary in order to produce enough electricity to meet the needs of a sizable population. Currently, 17,000 wind turbines on wind farms owned by several wind energy companies produce 3.7 billion kilowatt-hours of electricity annually, enough to meet the energy needs of 500,000 homes.
Wind Turbine Components B lades: Most wind turbines have three blades, though there are some with two blades. Blades are generally 30 to 50 meters (100 to 165 feet) long, with the most common sizes around 40 meters (130 feet). Longer blades are being designed and tested. Blade weights vary, depending on the design and materials—a 40 meter LM Glass fiber blade for a 1.5 MW turbine weighs 5,780 kg (6.4 tons) and one for a 2.0 MW turbine weighs 6,290 kg (6.9 tons).
C ontroller: There is a controller in the nacelle and one at the base of the turbine. The controller monitors the condition of the turbine and controls the turbine movement. G earbox: Many wind turbines have a gearbox that increases the rotational speed of the shaft. A low-speed shaft feeds into the gearbox and a high-speed shaft feeds from the gearbox into the generator. Some turbines use direct drive generators that are capable of producing electricity at a lower rotational speed. These turbines do not require a gearbox. G enerators: Wind turbines typically have a single AC generator that converts the mechanical
energy
from
the
wind
turbine’s rotation into electrical energy. Clipper Windpower uses a different design that features four DC generators. N acelles: The nacelle houses the main components of the wind turbine, such as the controller, gearbox, generator, and shafts.
R otor: The rotor includes both the blades and the hub (the component to which
the
blades
are
attached).
Tow ers: Towers are usually tubular steel towers 60 to 80 meters (about 195 Vanes
Nacelle
to 260 feet) high that consist of three sections of varying heights.
Rotor
Vanes
Nacelle Pitch motors motors
Generators
Tower
Functionality As the wind starts to blow, yaw motors turn a turbine’s nacelle so that the rotor and blades face directly into wind. The blades are shaped with an aerofoil cross section (similar to an aircraft wing) and this causes air to move more quickly over one side than the other. This difference in speed causes a difference in pressure which in turn causes the blade to move, the rotor to turn and a rotational force (or torque) to be generated. The rotor is connected to a gearbox (on most turbines) and in turn to a generator housed in the nacelle that converts the torque into electricity. The electricity is then fed into a transformer located either inside or just outside the turbine which steps up the voltage to reduce losses in transportation. From there the electricity travels through underground cables to a small sub-station, usually on the wind farm site, where the voltage is stepped up through further transformers and exported to the local grid. Typically turbines start to generate electricity in wind speeds of 3-4 m/s (7-9 mph) which in the UK are generally experienced 75%-85% of the time. The amount of torque (and so electricity) generated increases with wind speed up to around 15 m/s (34 mph) where the maximum (or rated) capacity of the turbine is reached. Output is then maintained at this level until a turbine is shut down when the wind reaches high speeds of around 25m/s (57 mph) to protect it from excessive loads - though the turbines are in fact designed and certified to withstand wind speeds up to 70 m/s (157 mph).
Blade Structural design of a blade is optimised by adopting a shell structure with a long stiff central spar. The spar caps provide stiffness and strength in bending and extension, while the spar webs provide shear stiffness.
Why Materials Knowledge Is Critical? From an engineering perspective, the early structural failures and continuing risks had their genesis in an early lack of understanding of the wind forces acting on these large structures. This included not only the effects of the steady-state component of the incident wind flow field. Uncertainty about the properties of materials causes the wind turbine designer either to add more weight (and cost) than is required or to misjudge and inadequately size a component so that failure occurs (usually more costly). To further improve the economics of wind turbine systems and increase their range of use, improved materials properties are required. This is particularly true with regard to the long-term fatigue properties of composite materials. As used thus far in wind turbines, composite materials are combinations of glass, other synthetic fibers, or wood in a resin matrix.
With the anticipated increasing use of these and other composite materials, improved knowledge about both their static strength and their fatigue properties becomes critical in order to assure both short-term performance and the long-term life required of these power systems. This knowledge base is particularly critical for composites because of the wide variation in their geometries, constituents, and manufacture.
Materials choice as a prerequisite to Functionality and Design Viability The exact shape of the internal structure will determine the stiffness and strength of the blade under each loading mode for any given materials. In general terms however we need a material that is as light as possible for a given stiffness in order to satisfy the blade design criteria and to minimize the weight induced fatigue loads. Reducing the weight of the blades also will directly reduce the loads on the tower and foundations. Maximum stiffness required for optimum power generation.
.
Material must endure the continuous gyroscopic forces.
Wind turbines are really big fatigue testing machine-testing the blades! Fatigue life is a key design requirement Fiber composites are good in fatigue Blade Materials and Construction Wind turbine blades must be strong enough to withstand the applied loads without fracturing; thus, the ultimate strength must be sufficient to withstand the extreme loads, and the fatigue strength must be sufficient to withstand the time-varying loads throughout the intended life of the blade. The blades must also be stiff enough to prevent collision with the tower under extreme conditions. Material properties depend strongly on the fiber lay-up, the fiber content, and the chosen processing route. In much of the blade cross section, the stresses are predominantly longitudinal, because of flapwise and edgewise bending loads. In these parts of the blade, unidirectional laminates dominate as performance requirements of the materials are high stiffness and strength, both in tension and in compression. In the internal webs, the main requirement is to carry shear loads. Here, we find predominantly biaxial lay-ups with the fibers at +/− 45°. These laminates are often built as sandwich structures to reduce the tendency for buckling. Unsupported parts of the shell giving the aerodynamic shape are also often sandwich structures with multidirectional (usually triaxial) face laminates and a light core material, such as balsa wood or a polymer foam—often poly(vinyl chloride) (PVC). The critical properties of the sandwich structure are the shear strength and stiffness of the core and compressive strength and stiffness of the faces.
The blade parts are generally assembled using adhesive bonding. The strength and durability of the adhesive bonds are major design considerations and can become the main limiting performance factor together with the performance of the laminates themselves.
Materials used to date Wind turbine blades have developed a great deal since 1900. In the past wood was widely used in making European windmills, now other materials such as metal and fiberglass have been utilized by wind turbine makers. However, the material used in designing blades depends on the size and purpose of the wind technology. There are factors that contribute to the optimal efficiency of the blades. Some of these are the kind of material used, the shape and the number.
Wood Wood is often the material used for small turbines. Wooden blades are made from wood planks or wood laminates formed into the desired shape and finished with a weatherresistant coating. Its leading edge is covered by polyurethane tape, a tape similar to the one used on helicopter blades, for protection from erosion or hail damage. Wood planks are good for machines with a diameter of 5 meters (16 ft) or less while laminated wood is more preferred for bigger turbines. The latter has less tendencies of shrinking and warping and gives more control over the strength and stiffness of the blades. The laminated wood proves stronger than a single plank because it’s composed of slabs of wood joined together by a resin then shaped into your desired shape. The basic advantages of wood are: Its Strong, light weight, cheap, abundant, and flexible Metals
Steel Wooden blades were replaced by different metals in the late 19th century. The first one is galvanized steel, which is considered strong so it is used in bigger wind energy projects.
Aluminum Aluminum is lighter and stronger. Possible of being extruded, aluminum poses two drawbacks:
It
costs
too
much
and
experiences metal fatigue.
PVC blades PVC blades have the light weight and easy to install but it is only applied in small wind turbines. Other significant factors include light weight and flexibility.
Composites Wind turbine blades are subjected to static and dynamic lift, drag and inertial loads over a wide range of temperatures and other severe environmental conditions (e.g., UV, rain, hail, bird strikes) during a typical 20‐year service life. Blades must possess Low weight and rotational inertia, High rigidity and Resistance to fatigue and wear. Because of the unique requirements for large‐scale wind turbine blades, advanced composites are the materials of choice. Composite fibers used as reinforcements in blade construction include: Traditional E‐glass fiber (70‐75% by weight) bonded with epoxy or unsaturated polyester resin (most common) Carbon fiber bonded with same resins (less common, although provides high stiffness and less weight for longer turbine blades).
Composite resins used in blade construction: Epoxy was preferred matrix as blades grew longer since it offers better mechanical performance – particularly tensile and flexural strength Polyester is easier to process (needs no post‐curing) and is less expensive Vinyl ester (limited use, but growing)
Fiber Glass Fiberglass is the dominant material used
in
wind
turbine
blade
construction. A wind turbine using a fiberglass blade (also called glass reinforced
polyester),
fiberglass
is
strong, sold at a relatively reasonable price, and possesses good fatigue characteristics. One thing that makes it in demand is that it can be made through different
processes. For
example,
aside
from
being
extruded, it can also be pultruded where instead of pushing the material to a die, fiberglass cloth is pulled through a vat of resin then through a die.
Future trends and latest industrial materials There are new component developments underway now that will significantly change the materials usage patterns. Generally there are trends toward lighter weight materials, as long as the life-cycle cost is low. Specific development trends in turbine components are discussed below:
Rotors:
Most rotor blades in use today are built from glassfiber-reinforced-plastic
(GRP). Other materials that have been tried include steel, various composites and carbon filament-reinforced-plastic (CFRP). As the rotor size increases on larger machines, the trend will be toward high strength, fatigue resistant materials. As the turbine designs continually evolve, composites involving steel, GRP, CFRP and possibly other materials will likely come into use.
Gearboxes:
The step-up gearbox used on large turbines today is expected to be
replaced in many future machines. Most small turbine designed for battery charging use a variable speed, permanent magnet, variable frequency generator connected to a rectifier. As high power solid state electronics are improved, larger and larger machines are likely to use AC-DC-AC cyclo converters. This is the case on turbines being developed by Northern Power Systems (100 kW), the ABB (3 MW), and in some commercial machines. This trend will increase the use of magnetic materials in future turbines. Large epicyclic gear boxes used in large ships, may continue to be the drive system for some large turbines.
Nacelles:
The nacelle contains an array of complex machinery including, yaw drives,
blade pitch change mechanisms, drive brakes, shafts, bearings, oil pumps and coolers, controllers and more. These are areas where simplification and innovation can pay off.
Towers:
Low cost materials are especially important in towers, since towers can
represent as much as 65% of the weight of the turbine. Prestressed concrete is a material that is starting to be used in greater amounts in European turbines, especially in off-shore or near-shore applications. Concrete in towers has the potential to lower cost, but may involve nearly as much steel in the reinforcing bars as a conventional steel tower.
The following observations are based on the results of the material usage analysis: Turbine material usage is and will continue to be dominated by steel, but opportunities exist for introducing aluminum or other light weight composites, provided strength and fatigue requirements can be met.
Small turbine production
volume
is increasing rapidly
which
can
be
accommodated by manufacturing mechanization and innovation that will lower costs.
Elimination of the gearbox by using variable speed generators will increase through use of permanent magnetic generators on larger turbines increasing the need for magnetic materials.
New high power electronics will help reduce the need for gearboxes and also decrease losses occurred during transmission of wind power to distant load centers. Simplification of the nacelle machinery may not only reduce costs, but also increase reliability.
Blades are primarily made of GRP, which is expected to continue. While use of CFRP may help to reduce weight and cost some, low cost and reliability are the primary drivers.
Increasing the use of offshore applications may partially offset this trend in favor of the use of composites. Prestressed concrete towers are likely to be used more, but will need a substantial amount of steel for reinforcement. Wood epoxy, used in early blade production, is not expected to be a material of choice despie excellent fatigue properties.
Fig: New Design Proposals for Wind Turbine Blades
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