Aerofólio S809 - 3 - s2 0 - b9781856177931000109 - main

Aerofólio S809 - 3 - s2 0 - b9781856177931000109 - main

(Parte 5 de 6)

therefore,

10.9 Rotor Configurations 395

Some values of λ are shown in Table 10.8. Equation (10.62) enables j to be calculated directly from . These equations also allow the optimum blade layout in terms of the product of the chord l and the lift coefficient CL (for CD¼ 0) to be determined. By ascribing a value of CL at a given radius the corresponding value of l can be determined.

Example 10.10 A three-bladed HAWT, with a 30 m tip diameter, is to be designed for optimum conditions with a constant lift coefficient CL of unity along the span and with a tip–speed ratio J ¼5.0. Determine a suitable chord distribution along the blade, from a radius of 3 m to the blade tip, satisfying these conditions.

Solution It is obviously easier to input values of to determine the values of the other parameters than attempting the reverse process. To illustra te the procedure, choose ¼10°, ands ow ed etermine jλ ¼0.0567, using eqn. (10.63). From eqn. (10.59) we determine λ¼0.0152 and then find j¼3.733. Now

j ¼ Ωr cx1

jλ ¼ J rR

ZlCL

and Table 10.9 shows the optimum blade chord and radius values.

Figure 10.20 shows the calculated variation of blade chord with radius. The fact that the chord increases rapidly as the radius is reduced would suggest that the blade designer would ignore optimum conditions at some point and accept a slightly reduced performance. A typical blade planform (for the Micon 65/13 HAWT; Tangler et al. 1990) is also included in this figure for comparison.

Table 10.9 Values of Blade Chord and Radius (Optimum Conditions) (deg) j 4jλ r (m) l (m)

396 CHAPTER 10 Wind Turbines

10.10 THE POWER OUTPUT AT OPTIMUM CONDITIONS

Equation (10.17) expresses the power output under general conditions, i.e., when the rotational interference factor a0 is retained in the analysis. From this equation the power coefficient can be written as

This equation converts to optimum conditions by substituting eqn. (10.56) into it, i.e.,

where the limits of the integral are changed to jh and J¼ΩR/cx1. Glauert (1935) derived values for CP for the limit range j¼0t o J (from 0.5 to 10) by numerical integration and the relative maximum power coefficient ζ. These values are shown in Table 10.10. So, to obtain a large fraction of the possible power it is apparent that the tip–speed ratio J should not be too low.

r /R (a)

(b)

FIGURE 10.20

Examples of Variation of Chord Length with Radius: (a) Optimal Variation of Chord Length with Radius, According to Glauert Theory, for CL¼1.0; (b) a Typical Blade Planform (Used for the Micon 65/13 HAWT)

Table 10.10 Power Coefficients at Optimum Conditions

10.10 The Power Output at Optimum Conditions 397

10.1 HAWT BLADE SECTION CRITERIA

The essential requirements of turbine blades clearly relate to aerodynamic performance, structural strength and stiffness, ease of manufacture, and ease of maintenance in that order. It was assumed, in the early days of turbine development, that blades with high lift and low drag were the ideal choice with the result that standard aerofoils, e.g., NACA 4X, NACA 230X, (where the X denotes thickness to chord ratio, as a percentage), suitable for aircraft were selected for wind turbines. The aerodynamic characteristics and shapes of these aerofoils are summarised by Abbott and von Doenhoff (1959).

The primary factor influencing the lift–drag ratio of a given aerofoil section is the Reynolds number. The analysis developed earlier showed that optimal performance of a turbine blade depends on the product of blade chord and lift coefficient, lCL. When other turbine parameters such as the tip–speed ratio J and radius R are kept constant, the operation of the turbine at a high value of CL thus allows the use of narrower blades. Using narrower blades does not necessarily result in lower viscous losses, instead the lower Reynolds number often produces higher values of CD. Another important factor to consider is the effect on the blade structural stiffness, which decreases sharply as thickness decreases.

The standard aerofoils just mentioned also suffered from a serious fault; namely, a gradual performance degradation from roughness effects consequent on leading-edge contamination. Tangler commented that “the annual energy losses due to leading-edge roughness are greatest for stall-regulated3 rotors.” Figure 10.21,a daptedf rom Tangler et al. (1990) illustrates the surprising loss in power output of a stall-regulated, three-bladed rotor on a medium scale (65 kW) turbine. The loss in performance is proportional to the reduction in maximum lift coefficient along the blade. The roughness also degrades the aerofoil’s lift-curve slope and increases profile drag, further contributing to losses. Small scale wind turbines are even more severely affected because their lower elevation allows the accretion of more

Clean bladesFouled blades

Generator power, kW

FIGURE 10.21 Power Curves from Field Tests for NACA 4415-4424 Blades (Adapted from Tangler, 1990, Courtesy of NREL)

3Refer to Section 10.13, Control Methods.

398 CHAPTER 10 Wind Turbines insects and dust particles and the debris thickness is actually a larger fraction of the leading-edge radius. Some details of the effect of blade fouling on a small scale (10 m diameter) rotor are given by Lissaman (1998). Estimates of the typical annual energy loss (in the United States) caused by this increased roughness are 20–30%. The newer NREL turbine blades described in the next section are much less susceptible to the effects of fouling.

10.12 DEVELOPMENTS IN BLADE MANUFACTURE

Snel (1998) remarked, “in general, since blade design details are of a competitive nature, not much information is present in the open literature with regard to these items.” Fortunately, for progress, efficiency, and the future expansion of wind energy power plants, the progressive and enlightened policies of the U.S. Department of Energy, NASA, and the National Renewable Energy Laboratory allowed the release of much valuable knowledge to the world concerning wind turbines. Some important aspects gleaned from this absorbing literature follows.

Tangler and Somers (1995) outlined the development of special-purpose aerofoils for HAWTs, which began as a collaborative venture between the National Renewable Energy Laboratory (NREL) and Airfoils Incorporated. Seven families of blades comprising 23 aerofoils were planned for rotors of various sizes. These aerofoils were designed to have a maximum CL that was largely insensitive to roughness effects. This was achieved by ensuring that the boundary layer transition from laminar to turbulent flow on the suction surface of the aerofoil occurred very close to the leading edge, just before reaching the maximum value of CL. These new aerofoils also have low values of CD in the clean condition because of the extensive laminar flow over them. The tip–region aerofoils typi- cally have close to 50% laminar flow on the suction surface and over 60% laminar flow on the pressure surface.

The preferred choice of blade from the NREL collection of results rather depends on whether the turbine is to be regulated by stall, by variable blade pitch or by variable rotor speed. The different demands made of the aerofoil from the hub to the tip preclude the use of a single design type. The changing aerodynamic requirements along the span are answered by specifying different values of lift and drag coefficients (and, as a consequence, different aerofoil sections along the length). For stall-regulated turbines, a limited maximum value of CL in the blade tip region is of benefit to passively control peak rotor power. Figures 10.2 to 10.25 show families of aerofoils for rotors originally desig- nated as “small-, medium-, large-, and very large-sized” HAWTs,4 designed specifically for turbines having low values of maximum blade tip CL. A noticeable feature of these aerofoils is the substantial thickness–chord ratio of the blades, especially at the root section, needed to address the structural requirements of “flap stiffness” and the high root bending stresses.

According to Tangler (2000) the evolutionary process of HAWTs is not likely to deviate much from the now firmly established three-bladed, upwind rotors, which are rapidly maturing in design. Further refinements, however, can be expected of the various configurations and the convergence towards the best of the three options of stall-regulated, variable-pitch, and variable-speed blades. Blades on large,

4With the top end size of HAWTs growing ever larger with time, the size categories of “large” or “very large” used in the 1990s are rather misleading and, perhaps, better described by stating either the relevant diameter or power range.

10.12 Developments in Blade Manufacture 399

stall-regulated wind turbines with movable speed control tips may be replaced by variable-pitch blades for more refined peak power control and reliability.

With the very large HAWTs [i.e., 104 m diameter, refer to Figure 10.4(a)] being brought into use, new blade section designs and materials will be needed. Mason (2004) has described “lightweight” blades being made from a carbon/glass fibre composite for the 125 m diameter, 5 MW HAWT to be deployed in the North Sea as part of Germany’s first deepwater off-shore project.

10.13 CONTROL METHODS (STARTING, MODULATING, AND STOPPING)

Referring to Figure 10.9, the operation of a wind turbine involves starting the turbine from rest, regulating the power while the system is running, and stopping the turbine if and when the wind speed becomes excessive. Startup of most wind turbines usually means operating the generator as a motor to overcome initial resistive torque until sufficient power is generated at “cut-in” speed assuming, of course, that a source of power is available.

Blade Pitch Control

The angle of the rotor blades is actively adjusted by the machine control system. This, known as blade pitch control, has the advantage that the blades have built-in braking, which brings the blades to rest. Pitching the whole blade requires large actuators and bearings, increasing the weight and expense of

NREL S822 Tip–region airfoil, 90% radius

Root–region airfoil, 40% radius

Design Specifications

Airfoil S822 S823

CLmaxCD (min)

NREL S823

FIGURE 10.2 Thick Aerofoil Family for HAWTs of Diameter 2 to 1 m (P¼2 to 20 kW) (Courtesy NREL)

400 CHAPTER 10 Wind Turbines

the system. One solution to this problem is to use partial span blade pitch control where only the outer one third of the blade span is pitched.

Passive or Stall Control

The aerodynamic design of the blades (i.e., the distribution of the twist and thickness along the blade length) varies in such a way that blade stall occurs whenever the wind speed becomes too high. The turbulence generated under stall conditions causes less energy to be transferred to the blades minimising the output of power at high wind speeds.

According to Armstrong and Brown (1990) there is some competition between the advocates of the various systems used in commercial wind farms. The classical European machines are usually stall regulated, while most American designs are now either pitch regulated or, for large turbines, use some form of aileron control.

NREL S820 Tip–region airfoil, 95% radius

Root–region airfoil, 40% radius

Design Specifications

Airfoil S820 S819

CLmaxCD (min)

NREL S819

NREL S821 Primary outboard airfoil, 75% radius

FIGURE 10.23 Thick Aerofoil Family for HAWTs of Diameter 1 to 21 m (P¼20 to 100 kW) (Courtesy NREL)

10.13 Control Methods (Starting, Modulating, and Stopping) 401

Aileron Control

Aerodynamic control surfaces have been investigated by the U.S. DOE and NASA as an alternative to full blade-pitch control. The aileron control system has the potential to reduce cost and weight of the rotors of large HAWTs. The control surfaces consist of a moveable flap built into the outer part of the trailing edge of the blade, as shown in Figure 10.26(a). Although they appear similar to the flaps and ailerons used on aircraft wings, they operate differently. Control surfaces on an aircraft wing deflect downwards towards the high-pressure surface in order to increase lift during takeoff and landing, whereas on a wind turbine blade the flaps deflect towards the low-pressure surface (i.e., downwind side) to reduce lift and cause a braking effect. Figure 10.26(b) shows sketches of two typical control surface arrangements in the fully deflected position, included in a paper by Miller and Sirocky (1985). The configuration marked plain was found to have the best braking performance. The configuration

NREL S813 Tip–region airfoil, 95% radius

Root–region airfoil, 40% radius Primary outboard airfoil, 75% radius

Design Specifications

Airfoil S813 S812

CLmaxCD (min)

NREL S812

NREL S814

FIGURE 10.24

Thick Aerofoil Family for HAWTs of Diameter 21 to 35 m (P¼100 to 400 kW) (Note: Blade Profile for S815 Was Not Available) (Courtesy NREL)

402 CHAPTER 10 Wind Turbines

marked balanced has both a low pressure and a high pressure control surface, which helps to reduce the control torque.

Ailerons change the lift and drag characteristics of the basic blade aerofoil as a function of the deflection angle. Full-scale field tests were conducted on the Mod-O wind turbine5 with ailerons of 20% chord and 38% chord. Results from loss of load to shutdown showed that the 38% chord ailerons were the better aerodynamic braking device than the 20% chord ailerons. Also, the 38% chord ailerons effectively regulated the power output over the entire operating range of the Mod-O turbine. Figure 10.27 shows the variation of the lift and drag coefficients for the 38% chord ailerons set at 0°, 60°, and 90°.

Although wind tunnel tests normally present results in terms of lift and drag coefficients, Miller and Sirocky (1985) wisely chose to represent their aileron-controlled wind turbine results in terms of a

NREL S817 Tip–region airfoil, 95% radius

Root–region airfoil, 40% radius

Design Specifications

Airfoil S817 S816

CLmaxCD (min)

NREL S816

NREL S818 Primary outboard airfoil, 75% radius

FIGURE 10.25 Thick Aerofoil Family for HAWTs with D > 36 m (Blade Length 15 to 25 m, P¼400 to 1000 kW) (Courtesy NREL)

5Details of the Mod-O wind turbine are given in Divone (1998).

10.13 Control Methods (Starting, Modulating, and Stopping) 403

Direction of rotation

Aileron blade tip

Plain

Direction of rotation

Wind

Balanced

(b) ,

FIGURE 10.26

Aileron Control Surfaces: (a) Showing Position of Ailerons on Two-Bladed Rotor; (b) Two Types of Aileron in Fully Deflected Position (Adapted from Miller and Sirocky, 1985)

Angle of attack, deg (a) Lift coefficient

260 deg 0 deg

Angle of attack, deg (b) Drag coefficient

260 deg

C D 0 deg

FIGURE 10.27

Variation of (a) Lift and (b) Drag Coefficients for the 38% Chord Ailerons when Set at 0°, 60°, and at 90° (Adapted from Savino, Nyland, and Birchenough, 1985; Courtesy of NASA)

404 CHAPTER 10 Wind Turbines

chordwise force coefficient, C (also called a suction coefficient). C is a combination of both the lift and drag coefficients, as described next:

where α¼angle of attack.

The reason for using C to describe aileron-control braking effectiveness is that only the chordwise force produces torque (assuming a wind turbine blade with no pitch or twist). Because of this direct relationship between chordwise force and rotor torque, C serves as a convenient parameter for evaluating an aileron’s braking effectiveness. Thus, if C is negative it corresponds to a negative torque producing a rotor deceleration. Clearly, it is desirable to have a negative value of C available for all angles of attack. Figure 10.28 shows some experimen tal results, Snyder, Wentz, and Ahmed

(Parte 5 de 6)

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