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Aerofólio S809 - 3 - s2 0 - b9781856177931000109 - main

(Parte 1 de 6)

Wind Turbines 10

Take care your worship, those things over there are not giants but windmills. M. Cervantes, Don Quixote, Part 1, Chapter 8


Wind power comes from the conversion of wind energy into electricity using wind turbines to drive the electrical generators. Over the past three decades there has been a remarkable growth in global installed generating capacity. The data given in Figure 10.1 obtained from statistics published by the Global Wind Energy Council (GWEC), the European Wind Energy Association (EWEA), the American Wind Energy Association (AWEA), and others showing the regional and worldwide growth of installed wind power capacity. It is interesting to note that the global wind power capacity is now doubling every three years. The biggest regional contributors to this growth are Europe (particularly Germany and Spain) and the United States. Up to May 2009, 80 countries around the world contributed to the generation of wind power on a commercial scale. Predicting the growth of wind power generation is far from reliable. At the end of 2008 the total worldwide wind power capacity had reached 121 GW. The prediction by the German Wind Energy Association in March 2004 that the global market for wind power could reach 150 GW by 2012 turns out to be too pessimistic. This can be shown as follows, based upon the exponential law of growth:

where P0 is the initial installed power¼121 GW and P is the predicted installed power¼150 GW. We can determine the amount of time t needed to reach the predicted installed power if we know the value of

Wind Energy Availability

The Earth receives more energy from the Sun at the equator than at the poles. Dry land heats up (and cools down) more quickly than the oceans. This differential heating and cooling, which is greatest near the equator, drives an atmospheric convection system extending from sea level to the upper

© 2010 S. L. Dixon and C. A. Hall. Published by Elsevier Inc. All rights reserved.

DOI: 10.1016/B978-1-85617-793-1.00010-9 357 atmosphere. The warm air rises, circulates in the atmosphere, and gradually sinks back to the surface in the cooler regions. At the upper levels of the atmosphere, continuous wind speeds are known to exceed 150 km/h. The large scale motion of the air at altitude causes a circulation pattern with well-known features at sea level such as the trade winds.

The most striking characteristic of wind energy is its variability both spatially and temporally. This variability depends on many factors: climatic region, topography, season of the year, altitude, type of local vegetation, etc. Topography and altitude have a major influence on wind strength. The strength of wind on the high ground and mountain tops is greater than in the sheltered valleys. Coastal regions are often more windy than further inland because of the difference in heating between land and sea. On the other hand the presence of vegetation and its density is a factor that usually lessens wind strength.

At any given location temporal variability can mean that the amount of wind strength can change from one year to the next. The cause of these changes are not well understood but may be generated by large scale variations in weather systems and ocean currents.

The proper design and size of a wind turbine will depend crucially upon the site under consideration having a favourable wind. Briefly, to be favourable, the wind would need to be of sufficient strength and duration at an acceptable height. For the locations being considered as possible sites extended anemometric surveys (lasting over at least a year) are needed to determine the nature of the wind speed distribution with respect to time and height above the ground. These surveys are generally carried out at a fairly standard height of 30 m above the ground and, when required, some sort of extrapolation is made for estimating wind speeds at other heights. To assess the frequency of the

End of year results 2004

60 Installed power, GW

Europe Total worldwide

FIGURE 10.1 Operational Wind Power Capacity

358 CHAPTER 10 Wind Turbines occurrence of wind speeds at a site, a probability distribution function is often employed to fit the observed data. Several types of these distribution functions are commonly used:

(i) the simple, but less accurate, single-parameter Rayleigh distribution; (i) the complicated but more accurate, two-parameter Weibull distribution.

Some further details of these distributions and their application are given by Burton et al. (2001).

From these data, estimates of power output for a range of turbine designs and sizes can be made.

Wind turbine rotors have been known to suffer damage or even destruction from excessive wind speeds and obviously this aspect requires very careful consideration of the worst case wind conditions so the problem may be avoided.

An important issue concerning the installation of wind power plants involves their environmental impact. Walker and Jenkins (1997) have outlined the most significant benefits for installing wind turbines as well as the reasons put forward to counter their installation. It is clear that the benefits include the reduction in the use of fossil fuels, leading to a reduction in the emission of pollutants (the most important of these being the oxides of carbon, sulphur, and nitrogen). Any emissions caused by the manufacture of the wind turbine plant itself are offset after a few months of emission-free operation. Similarly, the energy expended in the manufacture of a wind turbine, according to the World Energy Council (1994), is paid back after about a year’s normal productive operation.

Historical Viewpoint

It may be of interest to mention a little about how the modern wind turbine evolved. Of course, the extraction of mechanical power from the wind is an ancient practice dating back at least 3000 years. Beginning with sailing ships the technical insight gained from them was extended to the early windmills for the grinding of corn etc. Windmills are believed to have originated in Persia in the seventh century and their use had spread across Europe by the twelfth century. The design was gradually improved, especially in England during the eighteenth century where millwrights developed remarkably effective self-acting control mechanisms. A brick built tower windmill, Figure 10.2,a classic version of this type, still exists on Bidston Hill1 near Liverpool, United Kingdom, and was used to grind corn into flour for 75 years up until 1875. It has now become a popular historical attraction.

The wind pump was first developed in Holland for drainage purposes while, in the United States the deep-well pump was evolved for raising water for stock watering. Most windmills employ a rotor with a near horizontal axis, the sails were originally of canvas, a type still in use today in Crete. The English windmill employed wooden sails with pivoted slats for control. The U.S. wind-pump made use of a large number of sheet-metal sails (Lynette and Gipe, 1998). The remarkable revival of interest in modern wind powered machines appears to have started in the 1970s because of the so-called fuel crisis. A most interesting brief history of wind turbine design is given by Eggleston and Stoddard (1987). Their focus of attention was the use of wind power for generating electrical energy rather than mechanical energy. A rather more detailed history of the engineering development of windmills from the earliest times leading to the introduction of the first wind turbines is given by Shepherd (1998).

1It is situated within 1 km of the Liverpool author’s home.


Wind turbines fall into two main categories, those that depend upon aerodynamic drag to drive them (i.e., the old style windmills) and those that depend upon aerodynamic lift. Drag machines such as those developed in ancient times by the Persians were of very low efficiency compared with modern turbines (employing lift forces) and so are not considered any further in this chapter.

The design of the modern wind turbine is based upon aerodynamic principles, which are elaborated later in this chapter. The rotor blades are designed to interact with the oncoming airflow so that an aerodynamic lift force is developed. A drag force is also developed but, in the normal range of pre-stall

FIGURE 10.2 Tower Windmill, Bidston, Wirral, UK. Circa 1875

360 CHAPTER 10 Wind Turbines

operation, this will amount to only about 1 or 2% of the lift force. The lift force, and the consequent positive torque produced, drives the turbine thereby developing output power.

In this chapter, the focus of attention is necessarily restricted to the aerodynamic analysis of the horizontal axis wind turbine (HAWT) although some mention is given of the vertical axis wind turbine (VAWT). The VAWT, also referred to as the Darrieus turbine after its French inventor in the 1920s, uses vertical and often slightly curved symmetrical aerofoils. Figure 10.3(a) shows a general view of the very large 4.2 MW vertical axis Darrieus wind turbine called the Eolé VAWT installed at Cap-Chat, Quebec, Canada, having an effective diameter of 64 m and a blade height of 96 m.

Figure 10.3(b), from Richards (1987), is a sketch of the major components of this aptly named eggbeater wind turbine. Guy cables (not shown) are required to maintain the turbine erect. This type of machine has one distinct advantage: it can operate consistently without regard to wind direction. However, it does have a number of major disadvantages:

(i) wind speeds are low close to the ground so that the lower part of the rotor is rather less productive than the upper part; (i) high fluctuations in torque occur with every revolution; (i) negligible self-start capability; (iv) limited capacity for speed regulation in winds of high speed.

(a) (b)

Brake discs Flexible coupling Building enclosure

Generator8.5 m

96 m 64 m


(a) The 4 MW Eolè VAWT Installed at Cap-Chat, Quebec; (b) Sketch of VAWT Eolé Showing the Major Components, Including the Direct-Drive Generator (Courtesy AWEA)

10.2 Types of Wind Turbine 361

Darrieus turbines usually require mechanical power input to start them but have been known to self-start. (Several VAWTs have been destroyed by such self-starts.) For assisted starting the method used is to run the generator as a motor up to a speed when aerodynamic wind forces can take over. Stopping a VAWT in high winds is difficult as aerodynamic braking has not been successful and friction braking is needed.

According to Ackermann and Söder (2002), VAWTs were developed and produced commercially in the 1970s until the 1980s. Since the end of the 1980s research and development on VAWTs has virtually ceased in most countries, apart from Canada (see Gasch, 2002; Walker and Jenkins, 1997; and Divone, 1998).

Large HAWTs

The HAWT type is currently dominant in all large scale applications for extracting power from the wind and seems likely to remain so. The large HAWT, Figure 10.4(a), operating at Barrax, Spain, is 104 m in


(a) First General Electric Baseline HAWT, 3.6 MW, 104 m Diameter, Operating at Barrax, Spain, Since 2002. (Courtesy U.S. Department of Energy). (b) The Bergey Excel-S, Three-Bladed, 7 m Diameter Wind Turbine, Rated at 10 kW at Wind Speed of 13 m/s (With Permission of Bergey Windpower Company)

362 CHAPTER 10 Wind Turbines diameter and can generate 3.6 MW. (This size of wind turbine has now, in 2010, become fairly commonplace, especially in the coastal waters around Great Britain.) Basically, a HAWT comprises a nacelle mounted on top of a high tower, containing a generator and, usually, a gearbox to which the rotor is attached. Increasing numbers of wind turbines do not have gearboxes but use a direct drive. A powered yaw system is used to turn the turbine so that it faces into the wind. Sensors monitor the wind direction and the nacelle is turned according to some integrated average wind direction. The number of rotor blades employed depends on the purpose of the wind turbine. As a rule, three-bladed rotors are used for the generation of electricity. Wind turbines with only two or three blades have a high ratio of blade tip speed to axial flow velocity (the tip–speed ratio), but only a low starting torque and may even require assistance at startup to bring it into the useful power producing range of operation. Commercial turbines range in capacity from a few hundred kilowatts to more than 3 MW. The crucial parameter is the diameter of the rotor blades, the longer the blades, the greater is the “swept” area and the greater the possible power output. Rotor diameters now range to over 100 m. The trend has been towards larger machines as they can produce electricity at a lower price. Most wind turbines of European origin are made to operate upwind of the tower, i.e., they face into the wind with the nacelle and tower downstream. However, there are also wind turbines of downwind design, where the wind passes the tower before reaching the rotor blades. Advantages of the upwind design are that there is little or no tower “shadow” effect and lower noise level than the downwind design.

Small HAWTs

Small wind turbines with a horizontal axis were developed in the nineteenth century for mechanical pumping of water, e.g., the American farm pump. The rotors had 20 or more blades, a low tip– speed ratio but a high starting torque. With increasing wind speed pumping would then start automatically. According to Baker (1985), the outgrowth of the utility grid caused the decline of the wind driven pump in the 1930s. However, there has been a worldwide revival of interest in small HAWTs of modern design for providing electricity in remote homes and isolated communities that are “off grid.” The power output of such a wind powered unit would range from about 1 to 50 kW. Figure 10.4(b) shows the Bergey Excel-S, which is a three-blade upwind turbine rated at 10 kW at a wind speed of 13 m/s. This is currently America’s most popular residential and small business wind turbine.

Tower Height

An important factor in the design of HAWTs is the tower height. The wind speed is higher the greater the heightabovetheground.Thisisthemeteorologicalphenomenonknownaswindshear.Thiscommoncharacteristicofwindcanbeusedtoadvantagebyemployingwindtowerswithincreasedhubheightstocapture more wind energy. A study by Livingston and Anderson (2004) investigated the wind velocities at heights up to 125 m on the Great Plains (United States) and provide a compelling case for operating wind turbines with hub heights of at least 80 m. Typically, in daytime the variation follows the wind profile one-seventh power law (i.e., wind speed increases proportionally to the seventh root of height above the surface):

cx=cx,ref ¼ð h=hrefÞn, where cx is the windspeed at heighth, cx,ref isthe known windspeed at a reference heighthref. The exponent n is an empirically derived coefficient. In a neutrally stable atmosphere and over open ground (the normal

10.2 Types of Wind Turbine 363 condition), n ≈ 1/7 or 0.143. Over open water a more appropriate coefficient is n ≈ 0.1. As an example it is required to estimate the wind speed at a height of 80 m above the ground using a reference velocity of 15 m/s measured at a height of 50 m:

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