Notes Topic 6 - 1 2 3 4 5 6 1 2 3 4 5 6 INTRODUCTION WIND RESOURCES WIND TURBINE COMPONENTS AND CONCEPTS WIND TURBINE AERODYNAMICS WIND TURBINE

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 1 2 3 4 5 6 1 2 3 4 5 6 INTRODUCTION WIND RESOURCES WIND TURBINE COMPONENTS AND CONCEPTS WIND TURBINE AERODYNAMICS WIND TURBINE BLADE DESIGN AND BLADE MANUFACTURE WIND TURBINE MECHANICAL DESIGN Introduction.................................................................................................................................................. 1 Wind Resources ........................................................................................................................................... 1 Wind Turbine Components and Concepts ................................................................................................... 1 Wind Turbine Aerodynamics....................................................................................................................... 1 Wind Turbine Blade Design and Blade Manufacture .................................................................................. 1 WIND TURBINE MECHANICAL DESIGN ............................................................................................. 1 6.1 Introduction to wind turbine design ...................................................................................................... 2 6.2 Wind turbine design loads..................................................................................................................... 3 6.2.1 Dynamic Loads............................................................................................................................... 6 6.2.2 Fatigue ............................................................................................................................................ 7 6.3 Mechanical design of wind turbine components................................................................................... 8 6.3.1 Rotor design.................................................................................................................................... 8 6.3.1.1 Rotor Position.............................................................................................................................. 8 6.3.1.2 Blade design ................................................................................................................................ 9 6.3.1.3 Hub Design.................................................................................................................................. 9 6.3.2 Transmission design ..................................................................................................................... 12 6.3.2.1 Gearbox concept........................................................................................................................ 12 6.3.2.2 Gearless direct drive concept .................................................................................................... 15 6.3.3 Tower design ................................................................................................................................ 16 6.3.3.1 Free standing tubular towers. .................................................................................................... 16 6.3.3.2 Lattice Towers........................................................................................................................... 18 6.3.3.3 Guyed towers............................................................................................................................. 18 6.3.4 Foundation Design........................................................................................................................ 18 6.3.4.1 Types of wind turbine foundations............................................................................................ 19 6.3.4.2 Geotechnical Investigations and foundation design.................................................................. 23 6.3.4.3 Foundation construction ............................................................................................................ 25 6.4 Design Economics, factors that influence design................................................................................ 26 6.4.1 Design criteria for stand alone turbines ........................................................................................ 26 6.4.1.1 Capacity of RAPS turbines........................................................................................................ 26 6.4.1.2 Weight concerns for transport and installation.......................................................................... 26 6.4.1.3 Tower designs for remote installation. ...................................................................................... 27 6.4.1.4 Designing for minimal maintenance. ........................................................................................ 28 Figure 6-1 Summarised engineering loadings on a wind turbine. ...................................................................... 4 Figure 6-2 Gravitational loading - illustration of the range of forces that a wind turbine blade experiences every revolution ........................................................................................................................................... 6 Figure 6-3 A Wind Turbine Company machine, showing deflection of the rotor under load. .......................... 9 Figure 6-4 Finite element analysis of a rigid three bladed hub ........................................................................ 10 Figure 6-5 Raw casting of a 1MW 3 bladed hub (casting weight 4500kg) ...................................................... 11 Figure 6-6 The Advanced Wind Turbine, AWT-26, incorporating a teetering hub......................................... 12 Figure 6-7 Cross-section of a spherical roller-bearing ..................................................................................... 13 Figure 6-8 The main shaft of a wind turbine (the large diameter flange bolts to the hub)............................... 13 6-1 Figure 6-9 Technicians connecting the shaft to the rotor bearing on a Nordex 1.3MW turbine...................... 14 Figure 6-10 Example of a parallel shaft helical gearbox made by Flender, manufacturer of gearboxes for Nordex wind turbines (amongst others)..................................................................................................... 14 Figure 6-11 A motorised slewing planetary gearbox made by Brevini, a leading manufacturer of gearboxes for wind turbines ........................................................................................................................................ 15 Figure 6-12 Wind loading on a free-standing tubular tower ............................................................................ 17 Figure 6-13 . Foundations of the 1.8MW Enercon turbines installed in Albany, West Australia.................... 17 Figure 6-14 Foundation being prepared for the Codrington wind farm, Victoria ............................................ 20 Figure 6-15 Foundations being prepared for the wind turbines at the Tararua wind farm, New Zealand, where a steel lattice tower was used ..................................................................................................................... 20 Figure 6-16 Typical variants of wind turbine foundation types ....................................................................... 21 Figure 6-17 Ground anchors being drilled for the Albany wind farm.............................................................. 22 Figure 6-18 A single 600kW Vestas turbine installed at Kooragang Island, Newcastle, NSW, which required a pile type foundation design ..................................................................................................................... 23 Figure 6-19 Typical geotechnical investigation underway at a proposed wind turbine site ............................ 24 Figure 6-20 Foundation close-up at the Albany wind farm showing arrangement of cabling conduits .......... 25 Figure 6-21 Lattice type wind turbine towers................................................................................................... 27 Figure 6-22 Tow-up installation of a hinged type tower .................................................................................. 28 Table 6-1 Description of the loads experienced by a wind turbine and what causes them. ............................... 5 6.1 Introduction to wind turbine design The wind turbine classification presented in Topic 3 can be broadened so that machines fall into either of two families - small and large wind turbines. Small turbines are primarily used as de-centralised electricity generators in areas outside of the utility network, commonly called off-grid applications. Large turbines are utilised as major energy providers on established electricity grids, to replace or supplement conventional sources of electricity production. Both families of turbines are designed with the same objective – to generate electricity at the lowest possible cost over the systems lifetime. In simple terms, the cost of a unit of wind generated electricity, is the cost of the turbine including installation plus the lifetime (15 – 20 years) maintenance costs divided by the total power produced. It becomes apparent that not only is conversion efficiency important but the installation and ongoing maintenance expenses are also crucial to the economic performance of a turbine (see Topic 15 for more about the economics of wind energy). Thus different design philosophies are used when designing turbines for different applications, such as remote independent power supplies or large grid connected turbines installed near urban centres. The location of a turbine in relation to potential service centres will vary the cost of installation and the ongoing maintenance expenses. For example, a wind turbine installed in a remote location is fated to have considerably greater installation and maintenance costs than one installed near an urban centre. Hence design philosophies are tailored to the particular application for which the turbine will be marketed. It is the task of the wind project planner to understand these differences and select the turbine whose design will best suit the particular application and in turn produce electricity at the lowest possible life cycle cost. There are few machines that are as challenging to design as a wind turbine. A contrast may be drawn between the design of a motor car and a wind turbine. The engine of a small car may have a life of about 200 000 kilometres, which equates to approximately 5000 actual running hours. Within these 5000 hours, the engine will receive periodic maintenance (possibly every 200 hours) and breakdowns or malfunctions are almost a matter of course. In contrast to this, a wind turbine will be operating unattended for up to approximately 7000 hours each year, with a design life of 15 to 20 years. The wind turbine also must endure 6-2 the ferocities of weather without damage and rely on periodic maintenance sessions that may occur biannually only. Figure 6-1 Summarised engineering loadings on a wind turbine. Wind turbine design calls on knowledge in the disciplines of aerodynamic, mechanical and electrical engineering. Together these use a combination of principals in material science, structural dynamics, finite element analysis, power electronics and aerodynamics in order to optimise the cost to performance ratio of the turbine design. 6.2 Wind turbine design loads The determination of the design loads is an important first step in the design of a wind turbine. The local site conditions must be considered and the appropriate design standards and codes must be adhered to in the design process in order to qualify the design for international certification. The international standard for wind turbine design, for both small and large-scale wind turbines, is IEC 61400. For further information on this standard see www.nrel.gov/wind/certification/Certification/standards/iec_stds.html#WG1,2,3 Today, much research is being undertaken on the prediction of design loads, whether it be a once in fifty year extreme load, caused by extreme winds, or regular operating forces. By developing methods to better predict these forces, more efficient designs are possible. This results in more cost effective designs and also increased safety and reliability. Due to uncertainties in design loads, particularly in extreme conditions, large safety factors are often used producing conservative designs. A safety factor may be defined as the ratio of the design load to the actual load. The magnitude of the safety factor is usually proportional to the degree of uncertainty. By better understanding the actual loads, conservative safety factors may be reduced which results in the minimisation of material used in construction and hence a lowered manufactured cost. There are many research organizations worldwide developing computer simulation tools to predict the forces acting on wind turbines. Comparisons are then made between the predicted data and that of actual field measured loads to validate the simulation tools. By using these models, engineers are able to arrive at more reliable designs faster while minimising the cost of expensive physical simulation and field measurements on prototype turbines. The engineering loading on a wind turbine is a complex matter involving the interaction between numerous static and dynamic systems. Consider for example the rotor which operates in a frame of reference twice removed from that of the earth, and which is subject to cyclic gravitational and aerodynamic loadings, which vary in magnitude and frequency depending on the mode of operation and wind characteristics. Loadings on a wind turbine are caused by a mixture of forces, summarised in Figure 6-1 for a generic machine. Table 6-1 lists these loadings and provides a short indication of their source. 6-3 6-4 Table 6-1 Description of the loads experienced by a wind turbine and what causes them. Loading type Static load Steady load Cyclic load Stochastic load Transient load Resonance Figure 6-2 Gravitational loading - illustration of the range of forces that a wind turbine blade experiences every revolution Description Due to the weight of the wind turbine. The load is constant with time, which results in a constant deflection of the structure and is proportional to stiffness Primarily thrust load on the rotor due to the mean wind speed. Also loads that vary slowly enough so that the dynamics of the structure do not induce any effects in the loading case, hence the deflection of the structure is proportional to the load. Due to the rotation of the rotor. This includes gravitational loads on the blades, variations in aerodynamic load due to wind shear and yaw loading when the wind and rotor axis are misaligned. Loading due to wind turbulence, i.e. random and unpredictable. This is experienced as short term loading fluctuations due to changes in the aerodynamic loadings on the blades. Infrequent loads such as wind gusts, emergency stops or even controlled starting and stopping of the turbine. Aerodynamically derived loads also contribute significant forces into the wind turbine structure and are potentially complex to calculate due to the random nature of the wind. The predominant aerodynamically induced load is the thrust load acting on the rotor. The thrust load varies as a square of the wind velocity hence is relatively low under normal operating conditions but it can become devastating in high wind speeds. To overcome this, manufacturers have developed various designs to ‘unload’ the rotor in extreme conditions (which occur infrequently), in the effort of producing lighter more cost effective solutions. The foundation design calls on the understanding of the rotor aerodynamics and principles of civil engineering to determine the most cost effective solution to securely anchor the tower to the ground. Due to components of the wind turbine experiencing structural excitation at its resonance frequency. The loads imposed onto a wind turbine may be either aerodynamically or mechanically derived. Mechanically induced loads are usually the product of the mass or momentum of components. Possibly the greatest mechanical loads are caused by gravity acting on components, such as on the blade roots, or bending of the main shaft of the turbine as a result of the rotor mass. This is particularly true of large turbines where components are becoming increasingly massive. Many of the loads acting on wind turbine components also undergo cyclic stress reversals, introducing classic fatigue conditions in components. This is particularly true of rotating blades. Figure 6-2 shows that gravitational loading causes compression, tension, positive bending or negative bending depending upon rotational position of a blade. The yawing of the turbines rotor, with change in wind direction, may also impose significant mechanical loads onto the blades. As the rotor yaws, gyroscopic forces are introduced which can impart further considerable loads onto the blades and main shaft. For this reason free or passive yaw designs, which use a tail vane to keep the rotor facing the wind, are only used in small turbines and controlled yaw using wind direction sensors and motors is used in larger turbines with more massive rotors. The analysis of engineering loads is important if wind turbines are to be safe, reliable and robust machines and hence this been the subject of numerous studies over the past twenty years to the point where the prime issues are well understood. For example, Refs. [1], [2] and [3] have sections related to the engineering loadings experienced by wind turbines and the International Electrotechnical Commission publishes standards which include many aspects of wind turbine loading. The effects of wind shear may also introduce significant aerodynamic induced loads, particularly in areas of relatively high surface roughness and on turbines with large diameter rotors. If the shear effect is coincident with severe wind gusting then the loadings on the rotor may become serious for the structural integrity of the rotor. Exercise: Calculate the difference in wind speed experienced by the rotor (top and bottom) of a turbine with a 60m rotor diameter installed on a 70m tall tower. Perform this calculation for a smooth, flat field installation and for a turbine installed in a metropolitan area. 6.2.1 Dynamic Loads The response of the wind turbine structure to variable forces is termed the dynamic response and it is important to understand these interactions to avoid potentially catastrophic failures caused structural resonances or fatigue 6-5 6-6 Vibrations, imposed by dynamic loads, can lead to failure either through wear, fretting, fatigue or in the case of resonance, direct material failure. The most common vibration dilemma encountered in structural engineering is resonance. This occurs where there is some regular forcing at a rate that coincides with the structure’s natural vibration frequency. If this occurs, a condition exists whereby small amounts of energy may be continuously added to the system resulting in progressively growing amplitude of vibration – like pushing a child on a swing. When applied to structures however, the result can be catastrophic due to substantial amplitudes of vibration resulting in excessive mechanical strains and consequently structural failure. With wind turbines, such resonances are prone to occur in the tower and blades as simple cantilevers, although problems may also occur as torsional resonances in shafts or other components. Therefore when designing a wind turbine, engineers calculate the dynamic response of critical components and also how the complete system behaves together. Common excitation forces that may cause resonances in are: • Aerodynamic impulses created as the blades pass the tower. • Blade out-of-balances (aerodynamic or mass) • Wind gusts • Rotating component out-of-balances. • Tower vortex shedding. analyse and optimise designs before they are production is started. An aeroelastic analysis of the complete wind turbine system is often used to predict the operating behaviour of individual components. On a more micro level the designer will focus on: • Eliminating stress rising shapes of components, such as sudden geometric transitions that may concentrate stresses. • Correct material selection. Certain materials have a low fatigue resistance and are prone to fatigue failure such as aluminium. Mild steel, however, is relatively fatigue resistant providing the component is subjected to a cyclic stress that is below its fatigue threshold (approximately half its ultimate tensile strength). • Designing for corrosion resistance. As a component corrodes and material is ‘lost’ the initial strength of the material may be significantly weakened resulting in increased operating stress levels that may induce fatigue. 6.3 Mechanical design of wind turbine components If an individual component or complete structure is found to be operating close to its natural frequency, standard techniques may be used to de-tune the resonance. This may involve, changing the stiffness and/or mass of the structure and by introducing damping. Damping is a term describing how fast the structure can return to equilibrium after the excitation force is removed. Exercise. A fixed speed turbine, with dual speeds, operates at 10 and 25 RPM at high and low speed respectively. Calculate the resulting forcing frequency cased by the blades passing the tower for a two and three blade turbine. 6.2.2 Fatigue Fatigue is a complex design issue that is prone to occur when a component is subjected to a relatively low, repeated stress with a high number of cycles. Fatigue can be defined, in engineering terms, as a mode of failure that involves the nucleation and growth of a crack in a structural component that is subjected to loads that vary with time and whose maximum amplitudes induce stresses that are equal or lower than the expected yield strength of the material used. As with many rotating machines, fatigue failure of components is of concern and this is particular true in wind turbine design. Therefore components that are subjected to continual cyclic loads are prone to fatigue failure. Cyclic loads may be induced as a result of rotating mass out-of-balances, rotating members under gravity loads (blades and heavy shafts) and wind gusts, to name a few. Wind turbines are subjected to roughly 100 million cycles in their lifetime, which produces much greater fatigue stresses than other structures such as bridges, aeroplanes ore even components in car engines. Wind turbine history is dotted with reminders of how important correct design and analysis is to avoid fatigue failures. Since structural fatigue of wind turbine components may be catastrophic, careful attention is focused on designs and predicting the service life of components. Various computer simulation methods are used to 6-7 The mechanical design of a wind turbine may be sub divided into key component areas. • Rotor including the hub • Transmission (rotor shaft(s), bearings, gearbox y/n, brakes) • Generator (further discussion Topic 7) • Nacelle including yaw mechanism • Tower & foundations 6.3.1 Rotor design In short, the rotor comprises the components of the wind turbine that may be seen to rotate as seen by a casual observer. The rotor comprises the blades and the blade mounting assembly (the hub), which may include a blade pitch changing mechanism and possibly a nose cone. Most large scale, modern wind turbines have evolved to fall within the class termed the Horizontal Axis Wind Turbine (HAWT). This broad class is noted by the rotor being on a horizontal axis located up-wind of the tower and comprises two or three blades. There are however numerous variations on this general design, many emerging particularly in the fast growing years of wind turbine development in the 1980’s. 6.3.1.1 Rotor Position The downwind or lee-position of the rotor (downwind of the tower or yaw axis) is inherently the most stable position, as the wind constantly pushes the downwind rotor away from the tower, hence a natural tracking of the wind is achieved. Some small-scale turbines take advantage of this intrinsic feature in order to simplify their designs. However, experience has shown that the operating life of downwind rotor blades can be dramatically shortened due to the ‘wind shadow’ that exists on the lee side of the tower. As a blade passes through the aerodynamic wake that exists in the lee side of the tower, the lift and drag forces suddenly collapse and then regain as the blade passes the affected region. This cyclic loading, as each blade passes the tower, can produce significant fatigue stresses within the blade and hub structure that has been shown to severely reduce service life of the rotor. To reduce the inherent cyclic loadings of downwind turbines, novel flexible hub designs are often used. 6-8 Downwind turbines nearly always produce more aerodynamic noise than upwind designs. As the downwind blades pass the tower, into a region of lower wind speed and higher turbulence, the sudden change of aerodynamic conditions often produce a low frequency, audible beat. This cyclic beating tends to make downwind turbines noisier than up-wind models. Figure 6-4 Finite element analysis of a rigid three bladed hub Another advantage of locating the rotor downwind is that the blades may be made lighter and more flexible than that of an upwind design. This is because the downwind rotor blades deflect safely away from the tower when under load. Therefore the blades require relatively low structural stiffness, hence may be made lighter. A flexible rotor has the additional bonus that the blades are able to deflect under load, particularly in gusty conditions. This greatly reduces the loads imparted onto the hub, drive-train and nacelle of the turbine. Consequently this allows for the construction of a lighter, less-massive and thus lower cost, nacelle. Nearly all turbines currently manufactured, are of the up-wind design. Although some research is still being performed on downwind machines, particularly in the U.S. (see Figure 6-3). Figure 6-3 A Wind Turbine Company machine, showing deflection of the rotor under load. (Source: http://www.power-technology.com/contractors/renewable/wind/) 6.3.1.2 Blade design Blade design is covered in Topic 5. 6.3.1.3 Hub Design The rotor blades are usually directly connected to the hub that transmits the torque produced by the blades to the turbines main shaft. The hub is a structurally important component of the rotor as it transmits all blade loadings to the nacelle and in turn to the ground via the tower. Because of the high cyclical loads experienced by the hub, considerable effort is devoted to optimising the design for high strength (and fatigue resistance) as well as minimising mass. Finite element analysis is commonly used in the design of such components where complex shapes and complex load cases exist. This method of examination uses computer numerical analysis to determine stresses and deflections within the component. This enables the designers to arrive at optimal designs more quickly and with a higher degree of accuracy than with conventional methods. 6-9 (Source: To be confirmed) The turbine hub for a fixed blade pitch machine may be a relatively simple component compared to that of a variable pitch machine. The hub of a variable pitch turbine is quite complex as it houses the blade pitch bearings and the actuating mechanism. This mechanism must be supplied with control signals, electricity and sometimes hydraulic oil from the nacelle. This necessitates the need for slip rings and rotating unions to transmit these supplies from the stationary nacelle to the rotating hub. Some small turbines use hubs that have been fabricated from steel, but due to the high fatigue conditions experienced castings are commonly used. The major loads acting on the blade hub are an axial load produced by the centrifugal force of the rotating blade and the bending load caused by the wind thrust. The hubs of most large machines, which may weigh up to 10,000kg (10 metric tonnes), are made from an iron alloy castings. The alloy chosen for this application is usually a ductile type of cast iron that has a similar strength to that of steel. Ordinary cast iron has a low tensile strength, which makes it unsuitable for this application due to its high carbon content and inherent crystalline structure. 6-10 Figure 6-5 Raw casting of a 1MW 3 bladed hub (casting weight 4500kg) Figure 6-6 The Advanced Wind Turbine, AWT-26, incorporating a teetering hub (Source: http://www.gamorris.co.uk/) The hub must withstand the sizeable bending moment introduced by the blades due to their loading. In order to counter this effect, the blades are sometimes angled leeward slightly, at what is termed the blade-coning angle. This results in moving the blade centre of gravity of rearward. As the blades rotate, the centrifugal force tends to move the blades windward to rotate in a flat disk. This counter load in-effect reduces the blade bending moment and hence the necessary strength of the blade root and hub. Hubs may further be subdivided into two types, rigid and teetering. A rigid hub is as the name implies, rigid in that it holds the blades fixed at a particular angle to that of the main shaft. A teetering hub allows the blade-to-main shaft angle to vary by the inclusion of a hinge in the blade hub. Teetering hubs are used in many two blades designs. The obvious advantage of a two-blade rotor over the traditional three-blade design is in reduced rotor cost and weight. However a two bladed rotor generates considerably higher dynamic loads than a three bladed rotor. The dynamic imbalance is pronounced as a two bladed rotor yaws with wind direction changes. As the two bladed rotor is in the vertical position it creates only a small resistance to yaw due to aerodynamic and inertial resistance. As the rotor moves through to the horizontal position, the resistance increases to a maximum, which produces a cyclic fluctuation of loads that occurs twice per revolution. This increased dynamic loading necessitates a more complex hub design with a hinged (teetering hub) rotor as shown in Figure 6-6. The teetered hub allows the blades to move backward and forward and consequently reduce the bending stresses on the blade root and the hub. To achieve this the rotor is allowed to pivot on an axis which is perpendicular to the main shaft, and which rotates along with the main shaft. In order to reduce the cyclic loadings on downwind turbines teetering hubs are often used, but are relatively costly to manufacture due to their inherent mechanical complexity. (Source: http://www.nrel.gov/wind/awt-26a.html) 6.3.2 Transmission design The transmission comprises the drive train components that transmit power from the rotor to the generator. In the traditional Danish concept, the transmission includes the main shaft and its support bearings, the gearbox and brake. The complexity of the transmission system has been greatly reduced with the advent of the modern direct drive turbines where the rotor is coupled directly to a low speed generator. 6.3.2.1 Gearbox concept Many turbine designs utilise a gearbox to increase the rotational speed of the rotor, so that a conventional high-speed asynchronous (induction) motor may be used as a generator. Motors of this type are mass produced and hence relatively cost effective, but have the disadvantage that they must be run at high speed, typically 1000-1500RPM, necessitating the need for a speed-increasing gearbox. With this design, the rotor hub is connected to the main shaft. The main shaft not only transmits the torque of the rotor to the gearbox but also supports the rotor and transfers all the loads to the nacelle. Commonly, a pair of high capacity bearings supports the main shaft and also takes the shaft axial loads, resulting from the wind pressure acting on the rotor. The Main Shaft Bearings Spherical roller bearings are used as the as main shaft bearings in most large turbines. Spherical roller bearings have two rows of barrel-shaped rollers with a common outer race. This type of bearing is selfaligning, which accommodates any small axial misalignments of the shaft and the bearing housing due to shaft deflections / bending or slight misalignments during assembly. The bearings are contained within bearing housings that are bolted to the nacelle structure. The bearings may be lubricated by manual greasing or by an automatic dispensing system that ensures a supply of clean grease to the rollers, within the bearings. Normally labyrinth type seals are used to prevent ingress of any water or dirt particles that would result in premature bearing failure. 6-11 6-12 Figure 6-9 Technicians connecting the shaft to the rotor bearing on a Nordex 1.3MW turbine Figure 6-7 Cross-section of a spherical roller-bearing (Source: http://products.skf.com/) The Main Shaft The main shaft of a wind turbine is usually made from high quality forged steel. Because of the low speed, high torque transmitted by this shaft, the axel is usually relatively large in diameter and in many cases hollow. The rotational speed for this shaft is typically 20 to 30 revolutions per minute for a turbine of approximately 750kW in capacity. The shaft may contain pipes for the hydraulic system and electrical cables to provide power for blade variable pitch motors in the hub. (Source: http://www.electricalline.com/images/mag_archive/13.pdf) The Gearbox In the traditional approach, the main shaft is connected to the gearbox usually with a rigid type coupling. A gearbox is used to increase the rotational speed of the rotor, from approximately 15 – 50 RPM (for a three bladed turbine) to 1500 RPM suitable for an asynchronous (induction) generator. The gearbox is one of the major capital components of a turbine and the correct design of a gearbox is essential for long, trouble-free operation. Wind turbine gearboxes, over the past 12 to 15 years, have evolved from relatively standard industrial components to specialised products specifically designed for wind turbine applications. History has shown the difficulties in designing a gearbox to cope with the dynamic conditions that are inherent within the wind turbine transmission, with widely reported gearbox problems occurring with numerous manufactures. Gearbox reliability has, however, been improving over recent years—mainly due to a better understanding of the dynamic forces they are subjected to throughout their life cycle. Figure 6-8 The main shaft of a wind turbine (the large diameter flange bolts to the hub) Wind turbine gearboxes are commonly of the helical or epicyclic (planetary) type or may be a combination of both. Helical gearboxes are larger and heavier than comparable capacity epicyclic units but have the advantage that they are quieter. Figure 6-10 Example of a parallel shaft helical gearbox made by Flender, manufacturer of gearboxes for Nordex wind turbines (amongst others). (Source: http://www.cmasa.com/piezas.html) (Source: http://www.flender-power.co.uk/index2.htm) 6-13 6-14 The concept of direct drive generators has been used in the conventional power generation industry for many years. However using direct drive generators for wind turbines has specific problems related to their relatively low rotational speed. The main concern is the large physical size required for the low speed, high torque design of such generators, often resulting in increased mass. Large capacity synchronous generators, suitable for wind turbines, are not commonly available and must be specially designed and manufactured for the particular application. Enercon is perhaps the most well known manufacturer of large-scale turbines using the direct-drive synchronous generator topology today. (Further information may be found in Topic 7). Figure 6-11 A motorised slewing planetary gearbox made by Brevini, a leading manufacturer of gearboxes for wind turbines Many RAPS style wind turbines, designed for battery charging applications, use direct drive synchronous generators. The advantage of using this design in such applications is similar to the philosophy of their larger cousins –lower maintenance and fewer components hence better reliability. More detailed information on synchronous and asynchronous generators is presented in Topic 7. (Source: http://www.breviniuk.com) Gearboxes can increase the speed of the input shaft up to a maximum of approximately 6:1 per stage. So the relatively faster turning rotor of a smaller turbine may only require a two-stage gearbox, but a larger turbine may require a three-stage gearbox. For example, a 150kW turbine with a rotor speed of 45 RPM and a generator speed of 1500RPM, the gearbox must have a total ratio of 45/1500 or 33.3:1, which would be capable with a two stages. The number of stages is directly reflected by the cost of the gearbox and also its efficiency. Typically the loss of 1 to 2% of input power, results from each stage in the gearbox, hence minimising the number of stages in the gearbox not only reduces the cost of the transmission but also reduces energy loss. A 750kW turbine, operating at rated capacity, would loose about 33kW in the gearbox alone. This energy, which is rejected as heat, must be dissipated to reduce the chance of premature gearbox failure from overheating. Air-cooled heat exchangers are commonly used on large turbines to keep gear oil temperature within limits. A practical disadvantage of using gearboxes in wind turbines is the fact that the gearbox may contain up to 350L of oil, stored some 50 metres or more above the ground. This makes oil changes extremely difficult, time consuming and expensive. In order to reduce the number of oil changes needed during the operating life of a turbine, advanced lubricants are often specified and designers take careful steps to ensure high oil quality through oil temperature monitoring and filtration. Gear noise from meshing gear teeth is an inherent characteristic of all gearboxes. In order to reduce the noise transmission of the gearbox through the nacelle and the tower, the gearbox is often directly coupled to the main shaft and the torque reaction is taken by rubber pads or springs. This type of design also builds a form of ‘torque compliance’ into the drive train that absorbs fluctuations in torque produced by gusts in the wind. 6.3.2.2 Gearless direct drive concept Some manufactures have moved away from the traditional gear driven high-speed generator and towards direct drive, gearless, designs using low speed synchronous generators. Low speed generators are significantly more expensive than a comparable output, conventional generator, but the additional cost may be offset by the exclusion of the gearbox. This design philosophy also has additional advantages due to decreased maintenance (associated with the gearbox) hence minimising operating costs, reduced mechanical noise and increased gear train efficiency. 6-15 6.3.3 Tower design Many manufacturers offer a range of different tower heights for each model wind turbine. This allows the customer to analyse the wind speed data from the prospective site and to perform predictions of the cost of power generated at various hub heights (considering the towers increasing cost versus the increase in power produced with increasing height). The trend today is of increasing tower heights with some manufacturers offering towers over 100m tall. Over the years numerous tower designs have been used to elevate the rotor of the turbine into less turbulent and higher winds experienced at higher distances from the ground. Many early turbines were installed on squat lattice style towers, while during the mid-eighties many US built turbines were installed on relatively slender lattice towers. The majority of utility scale turbines are installed on tubular freestanding towers. This has become the industry norm due to this design being generally considered to be the least aesthetically intrusive. The tower must be capable of resiting the thrust load of the rotor as well as the wind loading acting on the tower. The appropriate wind loading standards (AS1100) are used to determine the maximum design wind speed that must be used when designing the tower. In Australia the basic design wind speeds vary from approximately 45m/s up to 65m/s in some regions affected by tropical cyclones. Any local topographical effects, such as hills, must also be factored in which may increase local wind speeds. 6.3.3.1 Free standing tubular towers. This type of design may be though of as a cantilever, where the tower foundation resists the overturning moment resulting from the rotor’s thrust and the wind loading acting on the tower as seen in Figure 6-12. Since the bending moment is determined in simple terms as the product of the load by the distance of where it is applied to the base of the tower. It can be seen that the bending moment is of greatest magnitude at the base of the tower and minimum at hub height. Thus the maximum strength of the tower is required at the base. In order to reduce costs, most free -standing towers are tapered to a maximum diameter at the tower base (where the most bending strength is required) hence minimising material. Tapering the tower with height not only reduces material but also minimises the wind loading at increasing distance from the tower base. Tubular towers are usually manufactured from rolled steel plate and welded along the longitudinal seam. For practical reasons tower section are made in lengths of up to twelve metres and are flange jointed on site with a circumferential ring of bolts on the inside of the pipe. 6-16 Free-standing towers inherently have a relatively large overturning moment, which is resited by the foundation at the base of the tower. Generally the foundation is made from steel reinforced concrete, which distributes the overturning moment of the turbine to a large area of soil. The foundation design is dependant on the local soil type and its capacity to resit compressive force generated by the overturning moment. Figure 6-13 shows the foundations used in the Albany wind farm. Foundation design is covered in some detail in Section 6.3.4. 6.3.3.2 Lattice Towers Space frame structures such as lattice towers offer exceptional strength to weight ratios. Thus substantial material savings may be made over other tower designs. If we examine a beam in bending, it is the extreme fibres from the centroid that offer the greatest resistance to bending. A lattice tower uses this theory by locating comparatively slender beams a relatively large distance from the centroid of the structure. These beams are then ‘tied’ together via a network of tie bars that prevent the slender structural beams from buckling. The drawbacks of lattice towers is that they require a large amount of work at site to erect, since all the components must be assembled and all bolts must be individually torqued. Since a turbine is a dynamic machine and will always produce vibrations, some engineers fear that the movement may cause loosening of bolted connections or fatigue failures. Lattice towers are generally larger in diameter and hence more visually intrusive than monopole type towers. Figure 6-12 Wind loading on a free-standing tubular tower 6.3.3.3 Guyed towers Currently no turbine manufacturer uses guyed towers for large-scale machines. This is because of the very large footprint that is required by this design for tall towers that make it impractical to install. It is also argued that a large scale guyed wind turbine tower would be more visually intrusive that a cantilevered design because of the heavy guy wires that would be required. Guyed towers for small wind turbines are covered in Section 6.4.1.3 and also in Topic 11. 6.3.4 Foundation Design In simple terms the foundation must securely anchor the tower to the ground. The principal force that must be resited by the foundation is the overturning moment that is a product of the rotor thrust and the height from which this acts above the ground (hub height). While a full discussion of the engineering aspects of the loads is far beyond this course it is worth considering the main load passed through to the foundations, shown in Figure 6-1, and which is known as the overturning moment. While this load will obviously vary with time and have aspects of all those loads in Table 6.1, experience has shown that the foundation design is largely driven by the magnitude of the overturning moment, which peaks under extreme wind conditions. Figure 6-13 . Foundations of the 1.8MW Enercon turbines installed in Albany, West Australia While very few wind turbines fail because of inadequate foundations there is a considerable amount of work required by wind farm designers in analysing the foundation design and often what type of foundation is required is different for every project. It is easy to get a sense of the magnitude of the overturning moment by calculating the thrust forces acting on a wind turbine at some wind speed. For a 2MW wind turbine, the thrust force (steady load in Figure 20.1) at rated power is equivalent to about 65 tonnes. This means that at this wind speed the foundation must be capable of supporting a load equivalent to the weight of approximately 65 small passenger vehicles acting horizontally from the top of the tower! Exercise 6-1 Calculation of overturning moment for a wind turbine at rated wind speed conditions (Source: http://www.wpcorp.com.au/html/about_us/environment/renewable_energy/renewable_wind.html) 6-17 Problem: You have hired a structural engineer to provide you with an independent verification that the wind turbine you propose to use for your project, a 2MW unit with a 80m tower height and 75m rotor diameter, meets all relevant Australian wind loading standards (AS1170). To do this, the manufacturer of the wind turbine has provided an estimation of the overturning moment at the extreme wind speed of 60m/s based on empirical evidence and calculations. As a matter of interest, and to do some crosschecking, the engineer has 6-18 requested that you provide him also with a rough estimate of the overturning moment at rated wind speed conditions. How do you calculate this? Solution: The manufacturer has already provided you with a power curve for the wind turbine in question, which includes a thrust coefficient (CT) curve. The wind turbine reaches its rated wind speed at 14.5m/s at which point its thrust coefficient is approximately 0.85 (interestingly, this is very close to the optimum thrust coefficient of 8/9 which comes about from momentum theory which leads to the Betz limit). Wind turbine towers are usually of steel or concrete construction but there are several variants on these, which influence the type of wind turbine foundation used. For example, Figure 6-14 and Figure 6-15 show the difference in foundation design for wind turbines with steel cylindrical and lattice type towers. Figure 6-14 Foundation being prepared for the Codrington wind farm, Victoria If we assume that the wind turbine operates under steady load conditions then the major loading causing the overturning moment MT is that due to the thrust force on the rotor. The thrust coefficient is defined as the thrust force over the dynamic force as follows; Thrust force Thrust force = CT = 1 ρU 2 A Dynamic force 2 where U is the wind speed (m/s), ρ is the air density (assumed to be 1.22 kg/m3) and A is the swept area of the rotor. Solving for the thrust force gives; 2 Thrust force = 1 ρU 2 AC T ×85 = 0.5 ×1.22 ×14.52 × π × 80 = 644667 N 2 2 () which is a force approximately equal to 65,000 kg or 65 tonne. This is approximately the weight of 65 small passenger vehicles which are trying to turn the turbine over! If we assume that the thrust force acts through the centre of the wind turbine rotor, then the overturning moment is simply the thrust force times the moment arm which, in this case, is the tower height. Solving this gives; M T ≈ Thrust force × tower height = 644667 × 80 = 56.7 MN.m As the wind turbine gets subjected to winds above its rated wind speed the blades pitch to keep the wind turbine power output constant. This has the effect of decreasing the thrust coefficient. However, at extreme wind speeds the wind turbine has stopped rotating to protect itself and the thrust force is entirely due to drag on the wind turbine structure - here CT is simply replaced in the equations above by the drag coefficient, CD, which lies around values of 0.5 to 0.9. At such wind speeds, the analyses is dominated by U2 term and the overturning moments reached are significantly higher than that calculated above. (Source: Photo courtesy of AN WindEnergy) Figure 6-15 Foundations being prepared for the wind turbines at the Tararua wind farm, New Zealand, where a steel lattice tower was used If you consider that such loadings are much higher under extreme wind conditions the ability to design a cost effective and reliable foundation can be seen to be very important. 6.3.4.1 Types of wind turbine foundations The type of foundation used for a wind turbine depends largely on; • • • The type of tower used The strength of the sub-strata under the ground, and The magnitude of the overturning moment that is largely dictated by the likely extreme wind speed. (Source: Photo courtesy of PB Power) 6-19 6-20 farm, steel cables (known as ground anchors) inserted into holes drilled into the sand sub-strata and grouted into place (see Figure 6-17) – this latter method simply relies on the weight of the cone of material around and above the cable for its strength. Figure 6-16 Typical variants of wind turbine foundation types Figure 6-17 Ground anchors being drilled for the Albany wind farm (Source: Photo courtesy of Western Power) Sometimes the material under the turbine is simply not strong enough for a normal slab foundation and under these circumstances a pile type foundation is required. Bored and cast in-situ piles are also normally used for lattice type wind turbine towers. Pile type foundations rely on depth rather than width for primary resistance to the overturning moment. In particularly weak ground piles can extend to great depths, up to 50m, before they reach ground solid enough to withstand the static weight of the turbines. An example of a turbine in Australia that required such foundations was that at Kooragang Island in Newcastle, NSW, which was built on land reclaimed from the adjacent Hunter River, see Figure 6-18. Piles can be driven into the ground or drilled and cast in situ. Typically wind turbine foundations fall into the category of slab or pile types and an example of the variants possible are shown in Figure 6-16. A slab foundation relies on the bearing pressure of the soil and the weight of the foundation and overburden to act against the overturning moment to prevent the turbine from toppling or rocking. This latter issue can lead to uneven compression and settlement of the ground under the edges of the foundation that gets increasingly worse and finally results in the foundation cracking and failing. Sometimes to prevent foundation rocking slab foundations are “tied” down using different types of anchors. These can be steel rods inserted and grouted into holes drilled into bedrock, or in the case of the Albany wind 6-21 6-22 Figure 6-18 A single 600kW Vestas turbine installed at Kooragang Island, Newcastle, NSW, which required a pile type foundation design Figure 6-19 Typical geotechnical investigation underway at a proposed wind turbine site (Source: Photo courtesy of Energy Australia) A final point in regard to foundations types is for offshore wind farms. In other countries offshore wind is particularly attractive primarily due to the paucity of land and the better wind resource that results across large bodies of water. However, positioning turbines in the ocean is more expensive that on land due to the increased construction and electrical connection costs. In Australia there does not appear to be the economic and social drivers to pursue off shore locations. However, for the interested reader the issues for foundation design for offshore wind farms are covered in Ref [1]. 6.3.4.2 Geotechnical Investigations and foundation design Foundation design is a matter for civil engineering experts who understand the complexities of soil types and the issues associated with protecting the wind turbine over long periods of time. Such issues are well beyond this course and it is normal practice for the wind farm designer and developer to bring in expertise for this. To examine the ground structure it is necessary to undertake a geotechnical investigation and a typical example is shown underway in Figure 6-19. In these investigations, a hollow drill hole is bored to between 5 and 25m depth and the material is removed as a core sample and sent off for analyses in a civil engineering laboratory. There the material’s strength and differential settlement – or its compressibility – are examined and this information is given to the foundation designer. Their job is to design a foundation based on the soil type that can support the structure given the estimated size of the overturning moment. 6-23 (Source: Photo courtesy of Western Power) Depending on the type of soil and the size of wind turbine, from one to about five drill holes can be required at each turbine location. Sometimes the results indicate that the turbine should be moved, for example if underground caverns are found which occasionally happens in sandy soil. Some developers try to limit the cost of geotechnical investigations by only undertaking such bores at selected locations. This is possible if the ground structure is uniform though the experience is that the cost saved can be lost easily by underestimating what is below ground at other locations. Foundation design also has to include other services that are required through the concrete. These can include earthing stakes or grids and the electrical cabling which for a large wind turbine can be a severe limitation on the placement of reinforcing. Figure 6-20 shows the cabling conduits being positioned within the foundation reinforcing at the Albany wind farm, Western Australia. Such conduits are required so that if a cable fails it can easily be replaced at a latter date. 6-24 Figure 6-20 Foundation close-up at the Albany wind farm showing arrangement of cabling conduits During pouring it is normal for concrete air operated “vibrators” to be used to ensure that the concrete fills all the foundation space, that it flows around the foundation reinforcing and cable conduits and that no voids are present. It is always necessary to have a spare vibrator present; as such pouring cannot stop if one breaks down. Concrete can take up to a month to set to its design strength and usually test samples are poured concurrently with the foundation and these are tested at regular intervals following the pouring to see how the strength is improving with time. Sometimes for expediency fast setting concrete is used but this is normally more expensive. Once the required strength has been reached the construction of the wind turbine can commence. In very cold climates (such as the wind turbines installed Mawson in Antarctica) it is sometimes necessary to heat the foundation using warm water passed through conduits immersed in the concrete during setting or by some other form of heating to ensure that the concrete does not freeze, which would prevent it from setting. 6.4 Design Economics, factors that influence design. It has already been noted that the primary design objective is to build a turbine and its associated system to provide electricity at the lowest possible cost, including the installation and lifetime maintenance costs. Therefore it becomes obvious that no single design will triumph for all applications. The challenge facing the design engineer is to produce a turbine that is economical for a particular application, but not so specific that is reduces its potential in the broad market. RAPS turbines particularly illustrate this point due to the relatively high cost of installation, maintenance and operation compared to urban grid-connected machines. 6.4.1 Design criteria for stand alone turbines Stand-alone turbines may be defined as units that are used to provide electricity for decentralised (off grid) applications. Commonly, a stand-alone turbine may be installed to provide electricity for use in isolated or remote area applications such as in remote homesteads, communities or for un-manned applications such as telecommunication stations. (Source: Photo courtesy of Western Power) 6.3.4.3 Foundation construction Once the design is finalised the construction of the foundation can begin. For a slab type foundation this typically starts by the removal of overburden and the placement of what is known as a “blinding layer” of concrete at the base of the foundation excavation, which is there to site reinforcing material onto and prevent concrete pours from scouring into the ground surface. Depending on the ground material, formwork can be used to define the outside of the foundation and to restrict the movement of concrete during pouring. Reinforcing is then built up to the designer’s pattern. Sometimes the lower tower section of the turbine is cast into the concrete and this is tied into the reinforcing in place before the concrete is poured. For other machines this is not the case, with the lower tower section bolting onto threaded rods (sometimes called a pin-basket) which stand up out of the foundation and which are cast in place during pouring – with this latter type a grout is usually placed under the bottom of the tower after it has been placed and bolted onto the foundation. For a large wind turbine there can be up to 100 concrete truck movements required to fill a slab foundation and this can take many hours. The selection of the concrete type in terms of aggregate size, sand fines and cement/water ratio is typically specified by the foundation designer and tests are performed on site for each concrete truck load to ensure the correct concrete mix is being delivered. 6-25 Common design features of stand alone wind turbines used in RAPS. • Generally up to 30kW capacity. • Light weight for ease of transport and installation without specialised equipment. • Towers designed for erecting without cranes. • Designed for minimal maintenance /optimised for reliability. The primary design concerns for stand alone turbines focus on ease of installation and a rugged design for low maintenance. Together these two criteria may greatly reduce the lifetime cost of power generated. 6.4.1.1 Capacity of RAPS turbines Generally speaking the energy requirements of remote users is relatively small, often between 5 and 20kWhrs/day but possibly up to 100kWhr/day for small communities. Hence the installed capacity of the turbines is relatively small, usually up to 30kW. 6.4.1.2 Weight concerns for transport and installation. The cost of transport is usually a function of the capacity of the transport vehicle and the time taken for a return journey. Hence by reducing the mass of the equipment, it is possible to make considerable cost savings. Heavy machinery for unloading trucks and for manoeuvring components is not often available in 6-26 remote sites and the increased cost of providing this machinery, adds extra expense and inconvenience to the installation phase. It is the duty of the engineer to take this into account while designing the turbine. This calls for selection of appropriate materials and accurate structural analysis of components in order to optimise designs for not only strength but also mass. Figure 6-22 Tow-up installation of a hinged type tower 6.4.1.3 Tower designs for remote installation. In order to reduce the installation cost of a remotely installed turbine, novel tower designs are used to negate the need for cranes and other heavy machinery that would otherwise have to travel to the particular site, greatly adding to the cost of installation. Towers are usually constructed in sections that can be easily transported on conventional trucks and may be unloaded and moved easily by regular machinery like tractors or on smaller machines by hand. Lattice or tubular guy supported towers are commonly used. However a few manufactures offer free-standing lattice type (windmill style) towers. Very few small turbines use self-supporting monopole towers, such as those used by large-scale machines. Guy supported towers offer very efficient use of material and require the least foundation material of all tower designs. They also require the largest plan area or ‘footprint’ due to the guy span. Figure 6-21 Lattice type wind turbine towers. (Source: Courtesy of Westwind Turbines) Maintenance costs in remote areas have the potential to be quite significant due to the increased cost of transporting maintenance teams and replacement parts to site. Often a diagnosis team must first be sent to diagnose the problems and recommend an action plan to rectify the problem, before the maintenance crew arrive. Increased down time is common in such applications resulting in lower availability and hence increased electricity costs. Therefore it become obvious that for a turbine to be economical in remote applications it must be optimised for low maintenance operation and designed for low cost installation. 6.4.1.4 Designing for minimal maintenance. All turbines are designed for minimal maintenance, but this is especially true for turbines that are destined for remote areas. This is due to the cost of cost of mobilising maintenance teams to isolated places and the associated costs of transporting of any necessary replacement parts. (Source: U.S. made Jacobs wind turbines – see http://www.windturbine.net/) For ease of installation and future maintenance, small wind turbine towers are sometimes designed so that all assembly is performed at ground level. These types of towers incorporate a hinge mechanism at the base of the tower to allow the tower to be tilted into the vertical position with the aid of a gin-pole. (see Figure 6-22). A gin-pole is a separate pole that is fixed at approximately 90 degrees to the tower, near the tower hinge and provides the leverage to raise the tower. Such systems allow easy raising and lowering of the turbine without the need for cranes, which enables quick, simple assembly and future maintenance may be performed safely at ground level. 6-27 Most turbine manufactures specify a preventative maintenance schedule based on past field experience. This schedule advises on the frequency of required maintenance and the specific nature of what is necessary. Adherence to such a maintenance plan aims at minimising turbine downtime similar to the regular servicing of an automobile. The frequency of maintenance is also dependant on local site conditions. A turbine installed in a coastal location which experiences continuous high winds as well as abrasion from airborne particles and salt will require more maintenance than a similar turbine installed in a relatively low wind inland location. As a general rule a well-designed remote area turbine installed may only require a singular, annual inspection. In order to achieve such minimal maintenance demands, designers strive to minimise components –especially dynamic components. A well-designed remote area turbine, will have minimal moving parts and for this reason most are direct drive or gear-less. In this case the rotor is bolted directly to the generator, which eliminates the maintenance requirements of the gearbox, such as oil changes, bearing maintenance etc. The designer must also consider protective coatings for components that may be subject to corrosion, such as galvanising, painting or plating. 6-28 References: [1] Manwell, J.F., McGowan, J.G. & Rogers, A.L. (2002) “Wind Energy Explained – Theory, Design and Application”, John Wiley & Sons. [2] Burton, T., Sharpe, D., Jenkins, N. & Bossanyi, E. (2001) “Wind Energy Handbook”, John Wiley & Sons. [3] Hau, E. (2000) “Wind-turbines. Fundamentals, Technologies, Application, Economics”, Springer Verlag. Useful websites: www.nrel.gov/wind/certification/Certification/standards/iec_stds.html#WG1,2,3 6-29 ...
View Full Document

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.

Ask a homework question - tutors are online