Notes Topic 5 - 1 2 3 4 5 1 2 3 4 5 The blades must also be...

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Unformatted text preview: 1 2 3 4 5 1 2 3 4 5 The blades must also be able to withstand repeated bending due to fluctuating winds. Fluctuating forces causes fatigue stresses on the blade and, if poorly designed, the blades may crack and ultimately break. The 3MW Growian machine built in Germany in 1982 illustrates this point. It was a huge machine with both rotor diameter and tower height equal to 100m but after three weeks of operation was removed from service due to blade fatigue. INTRODUCTION WIND RESOURCES WIND TURBINE COMPONENTS AND CONCEPTS WIND TURBINE AERODYNAMICS WIND TURBINE BLADE DESIGN AND BLADE MANUFACTURE Introduction.................................................................................................................................................. 1 Wind Resources ........................................................................................................................................... 1 WIND TURBINE COMPONENTS AND CONCEPTS ............................................................................. 1 WIND TURBINE AERODYNAMICS ....................................................................................................... 1 WIND TURBINE BLADE DESIGN AND BLADE MANUFACTURE ................................................... 1 5.1 Design Considerations........................................................................................................................... 1 5.2 Design Parameters................................................................................................................................. 2 5.2.1 Aerodynamic Design Parameters ................................................................................................... 2 5.2.2 Structural Design Parameters ......................................................................................................... 3 5.2.3 Operating Design Parameters ......................................................................................................... 4 5.3 Design Methods..................................................................................................................................... 5 5.4 Materials and methods........................................................................................................................... 7 5.4.1 Wood or Wood Laminate Blades ................................................................................................... 7 5.4.2 Steel or Aluminium Blades............................................................................................................. 8 5.4.3 Fibreglass or Carbon-fibre.............................................................................................................. 8 5.4.4 Small Wind Turbine Blades.......................................................................................................... 10 5.5 Protecting Wind Turbine Blades ......................................................................................................... 10 5.6 Testing Wind Turbine Blades.............................................................................................................. 13 5.6.1 Fatigue Testing ............................................................................................................................. 13 5.6.2 Modal Testing............................................................................................................................... 13 5.6.3 Static-strength testing ................................................................................................................... 13 Figure 5-1Cross-section of a wind turbine blade showing structural composition ............................................ 3 Figure 5-2 3-D computer simulation of buckling in wind turbine blades ......................................................... 4 Figure 5-3 Influence of drag/lift ratio and blade number on turbine performance............................................. 5 Figure 5-4 Finite Element Analysis of a wind turbine blade revealing the stresses on the blade ..................... 6 Figure 5-5 Illustration of the vacuum-moulding process ................................................................................... 7 Figure 5-6 Schematic diagram showing the structure of a fibreglass blade section........................................... 9 Figure 5-7 Illustration of the filament winding process ..................................................................................... 9 Figure 5-8 Illustration of the process of pultrusion .......................................................................................... 10 Figure 5-9 Using black blades on wind turbines to absorb heat and melt ice .................................................. 11 Figure 5-10 Lightning protection system in an LM blade ................................................................................ 12 Figure 5-11 Modal testing of a wind turbine blade .......................................................................................... 13 Figure 5-12 Static-strength testing of a wind turbine blade ............................................................................ 14 5.1 Design Considerations In addition to fluctuating and extreme winds, the turbine may be sited in a region where the environmental conditions are harsh e.g. where there is high humidity that leads to corrosion of the blades or where there are high levels of ice or snow accumulation affecting the performance and the loading on the blades. Finally the impact on the blades of dirt and insects (particularly on the leading edge), and by birds and hail has to be taken in to account. 5.2 Design Parameters There are many factors that affect the performance of HAWT blades including aerodynamic parameters such as blade shape and blade solidity, structural parameters such as blade stiffness and operating parameters such as the tip speed ratio. 5.2.1 Aerodynamic Design Parameters A wind turbine has to cope with a wide range of wind speeds and wind directions, so that even the most sophisticated aerofoil is somewhat limited in its effect on wind turbine performance. In fact it has been said that said that the best conceivable wind turbine blade aerodynamically would only be 10% more efficient than a plank of wood! The improvement in performance is significant enough to identify a number of characteristics that must be taken into account when choosing the aerofoil design for the blades. 5.2.1.1 Aerodynamic Efficiency To maximize efficiency an aerofoil requires high lift to drag ratios (CL/CD) over a wide range of angles of attack corresponding to a range of wind speeds. The ideal shape of a blade is one that both tapers in chord and twist along its length in order to maintain a constant angle of attack corresponding to the optimum lift/drag ratio. 5.2.1.2 Reliability The aerodynamic performance of an aerofoil is affected by the roughness of the suction surface of the aerofoil. The roughness of the surface (particularly around the leading edge) will be influenced by the impacts of air-born dirt particles and insects. A robust aerofoil will have a degree of insensitivity of its aerodynamic characteristics to roughness. In addition, the general behaviour of aerofoils in stall is still not well understood and to avoid uncertainty in performance prediction, a designer will choose an aerofoil whose stall characteristics are relatively well known. 5.2.1.3 Low Noise Production In general, the cost of the rotor of a medium-large HAWT is roughly 30% of the cost of the entire turbine so much emphasis is placed on producing reliable, efficient blades. HAWT blades have to be designed to survive the bending forces placed on them due to extreme winds. Extreme winds are rare, occurring for e.g. a period of 10 minutes every 50 years, but potentially destructive if the HAWT blades are poorly designed. The wind has the ability to set surfaces vibrating, leading to an emission of sound e.g. sails on boats, roofs on buildings etc. If the wind is incident upon a sharp edge of a surface, it may produce a pure tone such as that produced by a musical instrument. For a wind turbine, the major source of aerodynamic noise is from the wind striking the sharp trailing edge of the blades and from the leakage of airflow around the tip of the blades 5-1 5-2 known as the tip vortices. Thus, the geometry of the trailing edge and the shape of the blade tip are particularly important in the design process. For instance, the strength of the tip vortex increases as the chord length of the aerofoil at the tip increases and thus a shaped (curved) tip is known to produce less aerodynamic noise than a blunt tip. The wind industry has conducted much research on minimizing blade noise but does not view noise as a major problem and the benefits of the research have been directed more towards increasing the energy output of the machine (since lower aerodynamic noise equates to lower aerodynamic losses). The subject of noise and wind turbines will be investigated in more detail in Topic 14. To conclude this section on aerodynamic design, the final blade shape is influenced by the economics of the project and a compromise is often reached between blade performance and manufacturing cost. There are large costs involved in developing a new blade design (and transferring the design to the machining equipment) and thus designers are inclined to stick to airfoils that have already demonstrated that they perform well e.g. the NACA 632 and 634 series of airfoils. In addition, a manufacturer may opt for e.g. an untwisted blade (fixed pitch angle along the length of the blade) because of ease of manufacture. This type of blade does not optimise the lift-to-drag ratios along the length of the blade and hence does not perform as well as a twisted blade but by producing slightly longer untwisted blades, the manufacturer can find a tradeoff between performance and cost. 5.2.2 Structural Design Parameters Figure 5-1shows a typical cross-section of a blade from a medium-sized HAWT. The blade’s principal structural support comes from a central section called a spar. The spar is placed to make the blade stronger and stiffer in the flapwise direction i.e. in the direction of the thrust on the wind turbine. Between the structural spar and the trailing edge is a triangular section referred to as the afterbody. Figure 5-2 3-D computer simulation of buckling in wind turbine blades (Source: http://www.ecn.nl/unit_de/wind/project/buck.html) The upcoming section on design methods explores in more detail the methods used in structural analysis of HAWT blades. 5.2.3 Operating Design Parameters 5.2.3.1 Number of Blades and Tip Speed Ratio Figure 5-1Cross-section of a wind turbine blade showing structural composition Figure 5-3 shows that, in accordance with momentum theory, the ideal HAWT would have a very large number of very thin blades operating at high tip speed ratio. In practice, the manufacture of a HAWT with a small number of slender blades is both easier and economical in order to reduce both cost and weight of the HAWT. The figure shows that a HAWT with a small number of blades can have still have a high efficiency by operating at moderate tip speed ratios. Typically a HAWT will have a three-bladed rotor and will operate in a tip speed ratio range of λ = 4 – 8. Although a two-bladed rotor is only 2-3% less efficient, three-bladed machines are usually chosen over 2-bladed machines due to the issues of aesthetics, noise and cyclic loading as discussed in Topic 3. (Source: Gipe, P. Wind Power for Home and Business, p120) For structural analysis, the spar acts like a main beam upon which has been glued a thin skin which defines the aerofoil shape. The choice of aerofoil should allow enough room for a structural spar that will take the load that the aerofoil develops. The afterbody of an aerofoil is typically constructed from lightweight material and can be badly distorted if the aerofoil is unsupported. Finally, internal air pressure and centrifugal forces causes loads on the shell structure. The structural design process requires calculation of these pressureinduced stresses and a means to combat them by the use of compartment walls and venting. The weight of the blades of a large wind turbine is of concern to manufacturers in terms of the material cost and the load due to gravity on the blade. However if the walls of the skin of the blade are too thin and are made from a material with a high stiffness then the rotor blades may be prone to ‘buckling’ under loading (See Figure 5-2). 5-3 5-4 5.3.1 Aerodynamic Design In Topic 4, various aerodynamic analysis tools were presented that predict the performance of the wind turbine rotor. These prediction tools rely on assuming the geometry of the blade prior to undertaking the analysis and feature no design algorithm as such. There have been various design methods developed in recent years most notably inverse design methods and optimisation methods. Figure 5-3 Influence of drag/lift ratio and blade number on turbine performance Inverse Methods specify, under fixed flow conditions, the desired pressure distributions across the surfaces of the aerofoils comprising the wind turbine rotor. The method then determines the shape of the aerofoils that correspond to the specified surface pressure distributions. The Direct Method of optimisation combines aerodynamic analysis with numerical optimisation techniques and minimises a particular objective (e.g. the aerofoil drag) subject to aerodynamic constraints (e.g. lift at certain angle of attack) plus side constraints (e.g. thickness of the aerofoil). Genetic Algorithms (GA’s) are among are range of emerging optimisation techniques that offers a more advanced approach in terms of trying to mimic the natural evolution process to develop a wind turbine blade that “adapts” to its environment e.g. a blade that maximizes the annual energy production of the turbine for a given wind distribution. Each blade consists of a “population” of aerofoils; each individual aerofoil can be likened to a chromosome where the properties of the aerofoil are encoded in the “genes” of the structure. The algorithm simulates crossover, mutation and selection of genes as the blade evolves and improves its aerodynamic performance. Structural Analysis Finite Element Analysis (FEA) is widely used to predict the structural dynamics of wind turbine blades. The basic principle involves modelling the wind turbine blade as a series of small cells or elements (see Figure 5-4). Using a number of assumptions to simplify the analysis of each cell, the structural properties of the whole blade can be calculated by solving a series of differential equations. In particular the model is used to predict the result when a stress is placed at a particular point on the blade. (Source: Lysen, E.H., Introduction to Wind Energy, p64) 5.2.3.2 Choice of Power Control Although stall-regulated control has the advantage of being less expensive and mechanically simpler than pitch regulation, the behaviour of aerofoils in stall can be unpredictable. In particular, for an aerofoilexperiencing stall, fluctuating winds lead to fluctuating angles of attack and stall hysterisis can arise (where the lift behaviour of the aerofoil lags behind the changes in angle of attack). The phenomenon is known as dynamic stall and the HAWT designer is faced with the problem of predicting and controlling the periodic and transient loads arising from stall hysterisis. Based on the current designs of large HAWT's, the choice of power control remains evenly split between stall-regulation and pitch-regulation. Figure 5-4 Finite Element Analysis of a wind turbine blade revealing the stresses on the blade 5.3 Design Methods To design a blade and analyse its behaviour, the wind turbine manufacturer uses computer models for aerodynamic design and structural analysis. Aerodynamic design methods involve calculating the geometry of the blade that, within the constraints of cost, will optimise energy capture. Structural analyses involve calculating the deflections, stresses, and strains due to the forces on the blade and takes into account both static and dynamic loads as well as the problem of buckling (see Section - Structural Design Parameters). The structural analysis often includes a modal analysis that looks at the natural frequencies of vibration of the blade. The wind turbine manufacturer must ensure that other components of the turbine do not have the same natural frequencies as the rotor blade to avoid resonance vibrations that may destroy the whole turbine. 5-5 (Source: http://www.eng.newcastle.edu.au/me/wind/feblade.gif) 5-6 5.4 Materials and methods The choice of blade materials is an engineering decision involving considerations of size, strength, stiffness and weight. Additional factors in the decision-making process are design and manufacturing experience, maintenance and cost. Traditionally, HAWT blades have been constructed from either • • • Wood or Wood Laminate Steel or Aluminium Fibreglass or Carbon-fibre The advantages and disadvantages of using these materials for making wind turbine blades, and the manufacturing methods associated with the materials are explored in this section. 5.4.1 Wood or Wood Laminate Blades Wood was used for the earliest historical wind turbines and many small machines still use wooden blades. The advantage of wood is that it is low weight, low cost and excellent fatigue strength. Additionally, it is readily available and easy to work with. The disadvantages of working with wood are its sensitivity to moisture and the machining costs. The manufacturer takes solid wood planks and machines the wood into the desired blade shape. The blade is coated with a tough weather-resistant finish and the leading edge is covered with fibreglass tape to protect the blade from wind erosion and hail damage. Blades fashioned from solid wood planks perform well for small machines of up to 5m in rotor diameter but for larger machines, designers prefer to use laminated wood because they can better control the blades strength and stiffness. In the laminating process, a resin is used to bond slabs of wood together and the resulting block is then carved into a blade. By varying the resin used, the types of wood and the direction of their grains, a block can be produced which is stronger than a single plank of the same size. To overcome the problems of moisture sensitivity and cost associated with machining blades from planks or blocks, a technique called ‘cold-moulding’ is often employed. The technique has been adapted from the manufacture of sailboat hulls (see Figure 5-5) and uses wafer-thin slices of wood called veneers. Layer upon layer of the wood veneers are sandwiched together and, under vacuum pressure, are pressed into a mould of the aerofoil shape. At the same time a resin is infused into the mould to saturate the fibres until the blade is fully cured. Blades up to 43m in diameter have been fabricated by this method and are referred to as woodcomposite blades. Figure 5-5 Illustration of the vacuum-moulding process 5.4.2 Steel or Aluminium Blades Years ago steel was considered as potential HAWT blade material and welded steel rotors were chosen for some large wind turbines in the USA e.g. the 3.2MW Mod-5B machine. The advantage of steel is its strength and the properties of steel are well understood from e.g. bridge construction. The disadvantage of steel is that it has relatively low fatigue strength. In addition steel is heavy so that the hub, drive train and tower of a wind turbine with steel blades must be made big enough to withstand the self-weight induced stresses. Compared to steel, aluminium is lighter and stronger for its weight. Aluminium blades can be fabricated using the same techniques used in the aircraft industry; stretching an aluminium skin over a rib or, on smaller machines, folding the sheet metal over the spars and riveting in place. Based on techniques used in the building industry, a process of extrusion can also be used to make aluminium blades, whereby melted aluminium is forced through a shaping steel block called a die. Unfortunately, using aluminium blades on commercial wind turbines hasn’t been successful and aluminium blades have only been used for experimental purposes. The weakness of aluminium lies in the fact that it is expensive and its fatigue strength is worse than that of steel. Another drawback to metal blades, whether steel or aluminium is electromagnetic interference with telecommunications like radio and TV. Metal reflects television signals and this can cause double or ‘ghost’ images on the TV screen. Electromagnetic interference will be explored further in Topic 14. No major manufacturer builds wind turbines today with metal blades. 5.4.3 Fibreglass or Carbon fibre Most wind turbine blades in the world are made from fibreglass (also known as glass-fibre reinforced polyester/plastic or GRP) and hence the manufacturing process is described below in more detail than for the alternative blade materials. Fibreglass has many of the properties of wood - it is strong, has good fatigue characteristics and is relatively inexpensive. In addition, a variety of designs and manufacturing processes can be applied to fibreglass. Most medium-large wind turbine blades are manufactured using a lay-up process similar to the wood-composite methods and adapted from techniques used in the construction of fibreglass boats, car kits garden equipment, play equipment etc. The blade is made in two half-blade moulds - one mould for the suction side of the blade and one mould for the pressure side of the blade. The first step is to lay a gel coat in the base of the moulds in order to provide a smooth white finish to the blades (thus it is not necessary to paint the blades after they come out of the moulds). Layer after layer of fibreglass cloth are placed into the half- blade moulds. As they add each additional, the fibreglass cloth is coated with polyester epoxy resin to bind the cloths into a hard matrix. The number of fibreglass cloths determines the thickness of the blade shell. Typically, the blade shell is thicker around the middle of the aerofoil and thins toward the leading and trailing edges. Before the two half-blades are glued together, structures called webs may be glued between the shells to make the blade stronger and stiffer in the flapwise direction. Additionally foam panels may be glued on to stiffen the trailing edge. Figure 5-6 shows that, once the upper and lower parts of the blade are assembled, the webs and the thick blade shell in the centre of the aerofoil form a boxlike structure that is the main spar of the aerofoil. (Source: http://www.mdacomposites.org/Manufacturing.htm) 5-7 5-8 it becomes much cheaper. Some fibreglass blades are lined with small amounts of carbon-fibre to bolster areas that are likely to experience the greatest structural stress. Figure 5-6 Schematic diagram showing the structure of a fibreglass blade section 5.4.4 Small Wind Turbine Blades Traditionally, the economics of repairing and maintaining small wind turbines has forced manufacturers to focus on reliable blades of simple design. In addition, the type of material used for small wind turbine blades is often driven by production efficiency rather than blade characteristics such as weight, stiffness etc. The blades for the Bergey Windpower range of small wind turbines (adapted for use on some of the Westwind machines) are made through a process of pultrusion (see Figure 5-8). Instead of pushing the material through a die (as in extrusion), fibreglass cloth is pulled through a vat of resin and then through a die. In this way many consumer products are made e.g. the side rails for fibreglass ladders. The pultruded blades on the Bergey machines can be easily identified by their symmetry and their constant width and thickness. The advantage of pultrusion is to give the blades a torsional flexibility not found in other manufacturing techniques. As the market for small wind turbines grows, there are indications that more sophisticated aerodynamic profiles are being considered for small wind turbines blades e.g. the University of Newcastle blades designed for use on the 20kW Westwind turbine. (Source: Hansen, M.O.L, Aerodynamics of Wind Turbines, p96) This simple hand laid procedure offers the lowest cost option for large fibreglass blades. The glass can be laid more accurately by applying it in pre-impregnated sheets, the half-blade moulds are often vacuum bagged and the mould is cured at a raised temperature. As stated above, however, fibreglass lends itself to a variety of manufacturing methods. In a process known as filament winding, fibreglass strands are pulled through a vat of resin and wound around a structure known as a mandrel (see Figure 5-7). The mandrel can be a simple shape like a tube, or a more complex shape like that of an aerofoil. Though some blades have been entirely filament wound, the process is often used only to produce the main structural spar for the blade. The blade is then assembled in a mould with a smooth fibreglass shell using the lay-up technique. Figure 5-8 Illustration of the process of pultrusion Figure 5-7 Illustration of the filament winding process (Source: http://www.mdacomposites.org/Manufacturing.htm) 5.5 Protecting Wind Turbine Blades 5.5.1 Leading-edge Tape Fibrecloth tape is used to cover the leading edge and protect it from erosion due to impact from dust, dirt and insects. The blades are inspected during maintenance check-ups (and the leading-edge tape may be replaced) in order to ensure that the blade is clean and free of any particles that may affect the performance of the blade. (Source: http://www.mdacomposites.org/Manufacturing.htm) Carbon-fibre blades (also known as carbon fibre reinforced polyester/plastic or CFRP) have been made successfully in prototype and limited production runs. It gives the highest stiffness and lowest weight of all the materials but unfortunately carbon-fibre is expensive and it will not have widespread use for blades until 5-9 5-10 5.5.2 Blade Heating Wind turbines in cold climates suffer increased loading from ice accumulation on the blades. The ice accretion affects the streamlined shape of the aerofoil, leading to reduced performance. In these conditions, wind turbine productivity can fall by as much 10-20%, particularly for large turbine on tall towers where the risk of icing increases. Wind farms with roads passing close to them may have an additional problem of ‘ice throw’ where the rotating blade can throw the ice that has built up. A variety of systems have been employed to prevent icing on wind turbine blades. Some turbines are fitted with black blades that absorb heat from the sun to melt the ice (see Figure 5-9), others have blades coated with Teflon so that snow and ice do not stick. Currently, there is a market for blade heating systems in locations such as Scandinavia, Alpine regions, North America, Russia and China. Different heating systems are marketed based on electrical, hot air or microwave heating. The heating mechanism within the blade structure is triggered by signals from an ice detector and thermistors at the first signs of icing. Research into blade icing problems continues as the wind turbine industry looks toward large offshore expansion. Most large turbine blades have inbuilt lightning protection systems usually of the form of a lightning receptor at the tip of the blade (see Figure 5-10) and a correctly sized cable or strap, that conducts the current from the receptor to the blade mounting flange and to the main shaft. The rollers in the main shaft bearings conduct the current from the main shaft to the nacelle and then down the tower to ground. Because of the relatively small contact area between the rollers and the bearing housing, arcing within the bearing often occurs as high currents are conducted, which may seriously damage the bearing. To prevent this some manufacturers use specially manufactured bearings with higher contact areas. Depending on the type of protection needed, a number of copper grounding rods are driven into the ground to provide an earth. Some systems also include a sensor to register the frequency of lightning strikes. Figure 5-10 Lightning protection system in an LM blade Figure 5-9 Using black blades on wind turbines to absorb heat and melt ice (Source: To be confirmed) 5.5.3 Lightning Strikes The height of a wind turbine makes it a natural target for lightning and almost all wind turbines are struck by lightning at some point. A bolt of lightning can have a highly destructive effect on unprotected turbine blades and damage due to lightning is the major component of total damage costs for wind turbines. Lightning damage has the potential to be extremely costly due to the expense of blade replacements (since blades are the single most expensive component of the turbine) and the high installation costs of refitting a damaged blade. It is crucial that wind turbine owners protect their blades against lightning strikes to prevent damage and long periods out of operation. Studies have shown that the blade tip has the greatest probability of being struck because it is the highest point of the structure and it is the low pressure side of the blade that typically gets struck. When a strike occurs, an electric arc carrying up to 200 000 Amps is created between the point of contact and the ground, via any conductive path. The creation of this high current arc, results in extreme temperatures that may result in a sudden violent expansion of air within the blade. This may result in cracking, delamination and an irreversible breakdown of glue joints within the blade structure. 5-11 (Source: LM Lightning Protection, LM Brochure, Internet http://www.lm.dk/) 5-12 Figure 5-12 Static-strength testing of a wind turbine blade 5.6 Testing Wind Turbine Blades 5.6.1 Fatigue Testing The aim of fatigue testing is to verify that when a blade is subjected to repeated stress, the fibres of the blade do not break and the layers of the rotor blade do not separate (i.e. delamination). A fatigue test consists of using a servo-hydraulic system to repeatedly bend the blade in both flapwise and edgewise (lead-lag) directions with a frequency cycle close to the natural frequency of the blade. A typical flapwise fatigue test uses millions of cycles and takes about 3 months to complete! The bending and stretching of the rotor blades are measured by strain gauges glued to the surface of the rotor blade. Data from the strain gauge are monitored to look for non-linear changes in the bending results that may indicate structural damage to the rotor blade. In addition, infrared cameras are used to scan the blade and look for areas of heat build-up throughout the blade. This may reveal an area of delamination or an area where the blade fibres have reached breaking point. (Source: To be confirmed) 5.6.2 Modal Testing As stated in Section 7.3, a wind turbine manufacturer must ensure that the vibration frequencies of wind turbine blades do not coincide with the natural frequencies of the wind turbine on which the blades are to be mounted. Otherwise the whole turbine may be subjected to resonance and the undampened vibrations may cause the turbine to shake itself to pieces. Modal testing of a blade is conducted by instrumenting the blade with excitation devices to induce the blade to vibrate at different frequencies and in different directions. In this way the different modal forms of vibration of each blade can be verified (see Figure 5-11). Figure 5-11 Modal testing of a wind turbine blade References: [1] Goldberg D. E. (1989) Genetic Algorithms in Search, Optimization and Machine Learning, Addison Wesley, USA. [2] Lysen, E.H. (1982) Introduction to Wind Energy. SWD Steering Committee Wind Energy Developing Countries, Amersfoort, The Netherlands, p 64. [3] Hansen, M. O. L. (2000), Aerodynamics of Wind Turbines, James and James Ltd., UK, p 95-96. [4] Gipe, P. (1993) Wind Power for Home and Business, Chelsea Green Publishing Company, USA, p 117-120. [5] Manwell, J.F., McGowan, J.G. and Rogers, A.L. (2002) Wind Energy Explained – Theory, Design and Application, John Wiley and Sons, UK, pp 284-292. Useful Websites: (Source: http://www.nrel.gov/wind) http://www.windpower.org/tour/manu/bladtest.htm 5.6.3 Static strength testing In static-strength testing, the ability of the blades to withstand extreme (i.e. hurricane) loads is tested by bending the blade once with a very large force. The static-strength test is an ultimate test and involves testing the blade until it breaks to reveal the method of failure under maximum strength loading. The test is carried out on blades that have been in operation for a substantial amount of time (see Figure 5-12). 5-13 http://www.lm.dk/UK/home/default.htm http://www.iesd.dmu.ac.uk/wind_energy/m31rotex.html 5-14 ...
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This note was uploaded on 06/09/2011 for the course PV 5053 taught by Professor Aasd during the Three '11 term at University of New South Wales.

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