Iec 61024 free download

The Evaluation Package is not intended to replace the standard, so a purchased copy should also be used. NOTE — This document is still under development, and only pages are being provided at this time. It will be updated here as it is completed. IEC , Edition 3. Medical Alarm Standards Cross-Reference meca-alarm-standards-cross-reference Note, IEC authorization has been received to publish wording from these standards.

Get a Quote. Thanks for your inquiry! We will be in touch with you shortly. The earliest designs, Persian windmills, utilised drag by means of sails made from wood and cloth.

These Persian windmills were principally similar to their modern counterpart the Savonius rotor No. Similar in principle is the cup type differential drag rotor No. The American farm windmill No. The Dutch windmill No. Table 2. Modern and historical rotor designs. Energies , 5 Ref No. HAWT are very sensitive to changes in blade profile and design. This section briefly discusses the major parameters that influence the performance of HAWT blades.

Energies , 5 5. The efficiency of a turbine can be increased with higher tip speeds [4], although the increase is not significant when considering some penalties such as increased noise, aerodynamic and centrifugal stress Table 3. Table 3. Tip speed ratio design considerations. This can lead to reduced material usage and lower production costs. Although an increase in centrifugal and aerodynamic forces is associated with higher tip speeds.

The increased forces signify that difficulties exist with maintaining structural integrity and preventing blade failure. As the tip speed increases the aerodynamics of the blade design become increasingly critical. A blade which is designed for high relative wind speeds develops minimal torque at lower speeds.

This results in a higher cut in speed [10] and difficulty self-starting. A noise increase is also associated with increasing tip speeds as noise increases approximately proportionately to the sixth power [4,11]. This has been found to produce efficient conversion of the winds kinetic energy into electrical power [1,6]. Several theories exist for calculating the optimum chord length which range in complexity [1,4,10,12], with the simplest theory based on the Betz optimisation [Equation 3 ] [1].

In instances of low tip speeds, high drag aerofoil sections and blade sections around the hub, this method could be considered inaccurate. In such cases, wake and drag losses should be accounted for [4,12]. The Betz method gives the basic shape of the modern wind turbine blade Figure 2. However, in practice more advanced methods of optimization are often used [12—14]. A typical blade plan and region classification. Assuming that a reasonable lift coefficient is maintained, utilising a blade optimisation method produces blade plans principally dependant on design tip speed ratio and number of blades Figure 3.

Low tip speed ratios produce a rotor with a high ratio of solidity, which is the ratio of blade area to the area of the swept rotor. It is useful to reduce the area of solidity as it leads to a decrease in material usage and therefore production costs.

However, problems are associated with high tip speeds Section 5. Figure 3. Optimal blade plan shape for alternate design tip speed ratios and number of blades [1]. Energies , 5 Generally, in practice the chord length is simplified to facilitate manufacture and which involves some linearization of the increasing chord length Figure 4. The associated losses signify that simplification can be justified by a significant production cost saving.

Figure 4. Efficiency losses as a result of simplification to ideal chord length [15]. A four bladed design offers marginal efficiency increases which do not justify the manufacturing cost of an extra blade. Tower loading must also be considered when choosing the appropriate blade quantity [6].

Four, three, two and one bladed designs lead to increased dynamic loads, respectively [16]. The imposing size and location of wind turbines signify that the visual impact must be considered. Three bladed designs are said to appear smoother in rotation therefore more aesthetically pleasing. Energies , 5 Faster one and two bladed designs have an apparent jerky motion [1].

Three bladed rotors are also thought to appear more orderly when in the stationary position [17]. Configuration A favourable reduction in rotor nacelle weight and manufacturing costs occur with the use of fewer blades [16]. However, dynamic structural and balancing difficulties of the polar asymmetrical rotor are apparent [16].

Increased wear, inferior aesthetic qualities and bird conservation problems are also associated with one and two bladed rotors [17,18]. Modern commercially available wind turbines include complex control and safety systems, remote monitoring and maintenance with provision for the survival of lightning strike Table 5.

Figure 5. Typical configuration of a modern large scale wind turbine www. Table 4. A selection of turbine size and weight configurations. Pitch or Rotor Dia No. A Typical modern 2MW wind turbine specification. Energies , 5 2. This prevents the gearbox from receiving additional loads. Reducing and facilitating its service. Brake Full feathering aerodynamic braking with a secondary hydraulic disc brake for emergency use. Conductors direct lightening from both sides of the blade tip down to the root joint and from there across the nacelle and tower structure to the grounding system located in the foundations.

As a result, the blade and sensitive electrical components are protected. Real time operation and remote control of turbines, meteorological mast and substation is facilitated via satellite-terrestrial network.

A predictive maintenance system is in place for the early detection of potential deterioration or malfunctions in the wind turbines main components. Aerodynamics Aerodynamic performance is fundamental for efficient rotor design [19]. Aerodynamic lift is the force responsible for the power yield generated by the turbine and it is therefore essential to maximise this force using appropriate design.

A resistant drag force which opposes the motion of the blade is also generated by friction which must be minimised. Traditionally aerofoils are tested experimentally with tables correlating lift and drag at given angles of attack and Reynolds numbers [24]. Historically wind turbine aerofoil designs have been borrowed from aircraft technologies with similar Reynolds numbers and section thicknesses suitable for conditions at the blade tip.

However, special considerations should be made for the design of wind turbine specific aerofoil profiles due to the differences in operating conditions and mechanical loads. The effects of soiling have not been considered by aircraft aerofoils as they generally fly at altitudes where insects and other particulates are negligible. Turbines operate for long periods at ground level where insect and dust particulate build up is problematic.

This build up known as fouling can have detrimental effects on the lift generated. Provision is therefore made for the reduced sensitivity to fouling of wind turbine specific aerofoil designs [25].

The structural requirements of turbine blades signify that aerofoils with a high thickness to chord ratio be used in the root region. Such aerofoils are rarely used in the aerospace industry. Thick aerofoil sections generally have a lower lift to drag ratio. Special consideration is therefore made for increasing the lift of thick aerofoil sections for use in wind turbine blade designs [25,26]. The angle of attack is the angle of the oncoming flow relative to the chord line, and all figures for CL and CD are quoted relative to this angle.

The use of a single aerofoil for the entire blade length would result in inefficient design [19]. Each section of the blade has a differing relative air velocity and structural requirement and therefore should have its aerofoil section tailored accordingly.

At the root, the blade sections have large minimum thickness which is essential for the intensive loads carried resulting in thick profiles. Approaching the tip blades blend into thinner sections with reduced load, higher linear velocity and increasingly critical aerodynamic performance. The differing aerofoil requirements relative to the blade region are apparent when considering airflow velocities and structural loads Table 6.

Table 6. The aerofoil requirements for blade regions [26]. Stall typically occurs at large angles of attack depending on the aerofoil design. The boundary layer separates at the tip rather than further down the aerofoil causing a wake to flow over the upper surface drastically reducing lift and increasing drag forces [6].

This condition is considered dangerous in aviation and is generally avoided. However, for wind turbines, it can be utilised to limit the maximum power output to prevent generator overload and excessive forces in the blades during extreme wind speeds and could also occur unintentionally during gusts.

It is therefore preferable that the onset of the stall condition is not instantaneous for wind turbine aerofoils as this would create excessive dynamic forces and vibrations [1]. The sensitivity of blades to soiling, off design conditions including stall and thick cross sections for structural purposes are the main driving forces for the development of wind turbine specific aerofoil profiles [1,26].

The use of modern materials with superior mechanical properties may allow for thinner structural sections with increased lift to drag ratios at root sections. Thinner sections also offer a chance to increase efficiency through reducing drag.

Higher lift coefficients of thinner aerofoil sections will in turn lead to reduced chord lengths reducing material usage [Equation 3 ]. Angle of Twist The lift generated by an aerofoil section is a function of the angle of attack to the inflowing air stream Section 5.

The inflow angle of the air stream is dependent on the rotational speed and wind speed velocity at a specified radius. The angle of twist required is dependent upon tip speed ratio and desired aerofoil angle of attack. Generally the aerofoil section at the hub is angled into the wind due to the high ratio of wind speed to blade radial velocity.

In contrast the blade tip is likely to be almost normal to the wind. The total angle of twist in a blade maybe reduced simplifying the blade shape to cut manufacturing costs. However, this may force aerofoils to operate at less than optimum angles of attack where lift to drag ratio is reduced.

Such simplifications must be well justified considering the overall loss in turbine performance. For larger modern turbines this is no longer applicable and it is suggested that the gearbox maybe obsolete in future turbines [27].

Today the use of fixed speed turbines is limited to smaller designs therefore the associated off-design difficulties are omitted. The design wind speed is used for optimum dimensioning of the wind turbine blade which is dependent upon site wind measurements. However, the wind conditions are variable for any site and the turbine must operate at off-design conditions, which include wind velocities higher than rated. Hence a method of limiting the rotational speed must be implemented to prevent excessive loading of the blade, hub, gearbox and generator.

The turbine is also required to maintain a reasonably high efficiency at below rated wind speeds. As the oncoming wind velocity directly affects the angle of incidence of the resultant airflow onto the blade, the blade pitch angle must be altered accordingly.

This is known as pitching, which maintains the lift force of the aerofoil section. Generally the full length of the blade is twisted mechanically through the hub to alter the blade angle. This method is effective at increasing lift in lower than rated conditions and is also used to prevent over speed of the rotor which may lead to generator overload or catastrophic failure of the blade under excessive load [1].

Two methods of blade pitching are used to reduce the lift force and therefore the rotational velocity of the rotor during excessive wind speeds. Firstly decreasing the pitch angle reduces the angle of attack which therefore reduces the lift generated.

This method is known as feathering. The alternative method is to increase the pitch angle which increases the angle of attack to a critical limit inducing the stall condition and reducing lift. The feathering requires the maximum amount of mechanical movement in pitching the blade. However, it is still favoured as stalling can result in excessive dynamic loads.

These loads are a result of the unpredictable transition from attached to detached airflow around the blade which may lead to undesirable fluttering [1].

Utilising the stall condition a limiting speed can be designed into the rotor blade known as passive stall control [1]. Increased wind velocity and rotor speed produce an angle at which stall is initiated therefore automatically limiting the rotor speed. In practice accurately ensuring stall occurs is difficult and usually leads to a safety margin. The use of a safety margin indicates that normal operation occurs at below optimum performance, consequently this method is utilised only by smaller turbines [28].

Feathered pitching offers increased performance, flexibility and the capability of fully pitching the blades to a parked configuration. Manufacturers are reported as using collective pitch [29], in that all the blades are pitched at identical angles. However, further load reductions can be found by pitching blades individually [30]. This requires no extra mechanism in most designs and it is expected to be widely adopted [29,30].

Within this category of blade design are numerous approaches which are either aerodynamic control surfaces or smart actuator materials. An extensive review of this subject is given by Barlas [31]. The driver behind this research is to limit ultimate extreme loads and fatigue loads or to increase dynamic energy capture. Research is mainly initiated based on similar concepts from helicopter control and is being investigated by various wind energy research institutes. The proceedings show a variety of topics, methods and solutions, which reflects the on-going research [32,33].

The use of aerodynamic control surfaces includes aileron style flaps, camber control, active twist and boundary layer control. Figures 6 and 7 show a comparison graph of aerodynamic performance lift control capability of a variety of aerodynamic control surface based concepts Figure 6.

Schematics of smart structure concepts. Figure 7. Comparison of aerodynamic device concepts in terms of lift control capability [31].

Energies , 5 Smart actuator materials include conventional actuators, smart material actuators, piezoelectric and shape memory alloys. Traditional actuators probably do not meet minimum requirements for such concepts. Furthermore, proposed concepts of aerodynamic control surfaces distributed along the blade span require fast actuation without complex mechanical systems and large energy to weight ratios. Promising solution for this purpose is the use of smart material actuator systems.

By definition, smart materials are materials which possess the capability to sense and actuate in a controlled way in response to variable ambient stimuli. Generally known types of smart materials are ferroelectric materials piezoelectric, electrostrictive, magnetostrictive , variable rheology materials electrorheological, magnetorheological and shape memory alloys.

Piezoelectric materials and shape memory alloys are generally the most famous smart materials used in actuators in various applications. The development of their technology has reached a quite high level and commercial solutions are available and widely used [31].

Blade Shape Summary An efficient rotor blade consists of several aerofoil profiles blended at an angle of twist terminating at a circular flange Figure 8 [4,34]. It may also include tip geometries for reducing losses. All manufacturing simplifications are detrimental to rotor efficiency and should be well justified.

The introduction of new moulding techniques and materials has allowed the manufacture of increasingly complex blade shapes. However, the economics of production coupled with difficulty of design analysis still dictate final geometry. Leading wind turbine suppliers now include most optimisation features such as angle of twist, variable chord length and multiple aerofoil geometries.

Figure 8. A typical modern HAWT blade with multiple aerofoil profiles, twist and linear chord length increase. Energies , 5 6. Blade Loads Multiple aerofoil sections and chord lengths, 22 specified stochastic load cases and an angle of twist with numerous blade pitching angles results in a complex engineering scenario.

Therefore, the use of computer analysis software such as fluid dynamics CFD and finite element FEA is now commonplace within the wind turbine industry [35]. To simplify calculations, it has been suggested that a worst case loading condition be identified for consideration, on which all other loads may be tolerated [4].

The worst case loading scenario is dependent on blade size and method of control. For small turbines without blade pitching, a 50 year storm condition would be considered the limiting case. In practice several load cases are considered with published methods detailing mathematical analysis for each of the IEC load cases [6]. For modern large scale turbine blades the analysis of a single governing load case is not sufficient for certification. Therefore multiple load cases are analysed.

The most important load cases are dependent on individual designs. Aerodynamic 2. Gravitational 3. Gyroscopic 5. Operational The load magnitude will depend on the operational scenario under analysis. If the optimum rotor shape is maintained, then aerodynamic loads are unavoidable and vital to the function of the turbine, considered in greater detail Section 6.

As turbines increase in size, the mass of the blade is said to increase proportionately at a cubic rate. The gravitational and centrifugal forces become critical due to blade mass and are also elaborated Section 6. Gyroscopic loads result from yawing during operation. They are system dependant and generally less intensive than gravitational loads. Operational loads are also system dependant, resulting from pitching, yawing, breaking and generator connection and can be intensive during emergency stop or grid loss scenarios.

Gyroscopic and operational loads can be reduced by adjusting system parameters. Blades which can withstand aerodynamic, gravitational and centrifugal loads are generally capable of withstanding these reduced loads.

Therefore, gyroscopic and operational loads are not considered within this work. The angle of attack is dependent on blade twist and pitch. The aerodynamic lift and drag produced Figure 9 are resolved into useful thrust T in the direction of rotation absorbed by the generator and reaction forces R. It can be seen that the reaction forces are substantial acting in the flatwise bending plane, and must be tolerated by the blade with limited deformation.

Figure 9. Aerodynamic forces generated at a blade element. For calculation of the blade aerodynamic forces the widely publicised blade element momentum BEM theory is applied [4,6,37].

Gravitational and Centrifugal Loads Gravitational centrifugal forces are mass dependant which is generally thought to increase cubically with increasing turbine diameter [38]. Therefore, turbines under ten meters diameter have negligible inertial loads, which are marginal for 20 meters upward, and critical for 70 meter rotors and above [4]. The gravitational force is defined simply as mass multiplied by the gravitational constant, although its direction remains constant acting towards the centre of the earth which causes an alternating cyclic load case.

The centrifugal force is a product of rotational velocity squared and mass and always acts radial outward, hence the increased load demands of higher tip speeds. Centrifugal and gravitational loads are superimposed to give a positively displaced alternating condition with a wavelength equal to one blade revolution. Structural Load Analysis Modern load analysis of a wind turbine blade would typically consist of a three dimensional CAD model analysed using the Finite Element Method [39].

Certification bodies support this method and conclude that there is a range of commercial software available with accurate results [40].

These standards also allow the blade stress condition to be modelled conservatively using classical stress analysis methods. Traditionally the blade would be modelled as a simple cantilever beam with equivalent point or uniformly distributed loads used to calculate the flap wise and edgewise bending moment.

The direct stresses for root sections and bolt inserts would also be calculated. The following simple analysis Sections 6. In practice a more detailed computational analysis would be completed including local analysis of individual features, bonds and material laminates. Flapwise Bending The flap wise bending moment is a result of the aerodynamic loads Figure 9 , which can be calculated using BEM theory Section 6.

Aerodynamic loads are suggested as a critical design load during 50 year storm and extreme operational conditions [6]. Once calculated, it is apparent that load case can be modelled as a cantilever beam with a uniformly distributed load Figure 10 [4]. This analysis shows how bending occurs about the chord axis creating compressive and tensile stresses in the blade cross section Figure To calculate these stresses the second moment of area of the load bearing material must be calculated [Equation 6 ].

Using classical beam bending analysis bending moments can be calculated at any section along the blade [41]. Local deflections and material stresses can then be calculated at any point along the beam using the fundamental beam bending equation [Equation 7 ]. Figure The blade modelled as a cantilever beam with uniformly distributed aerodynamic load.

Energies , 5 Figure Flapwise bending about the axis xx.