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Berkeley ENE,RES C200 - WIND POWER SYSTEMS

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~fl/lcuJ:CVuJJL7}IbiJvT(I'{.t42~1}l{bLg-bAEfAc4hd&1£(.f:y.~&wS'l-skrns..CHAPTER 6WIND POWER SYSTEMS6.1 HISTORICAL DEVELOPMENT OF WIND POWERWind has been utilized as a source of power for thousands of years for suchtasks as propelling sailing ships, grinding grain, pumping water, and poweringfactory machinery. The world's first wind turbine used to generate electric-ity was built by a Dane, Poul la Cour, in 1891.Itis especially interestingto note that La Cour used the electricity generated by his turbines to elec-trolyze water, producing hydrogen for gas lights in the local schoolhouse. Inthat regard we could say that he was 100 years ahead of his time since thevision that many have for the twenty-first century includes photovoltaic andwind power systems making hydrogen by electrolysis to generate electric powerin fuel cells.In the United States the first wind-electric systems were built in the late1890s; by the 1930s and 1940s, hundreds of thousands of small-capacity, wind-electric systems were in use in rural areas not yet served by the electricitygrid. In 1941 one of the largest wind-powered systems ever built went intooperation at Grandpa's Knob in Vermont. Designed to produce 1250 kW froma 175-ft-diameter, two-bladed prop, the unit had withstood winds as high as115 miles per hour before it catastrophically failed in 1945 in a modest 25-mph wind (one of its 8-ton blades broke loose and was hurled 750 feet away).Renewable and Efficient Electric Power Systems. By Gilbert M. MastersISBN 0-471-28060-7 © 2004 John Wiley & Sons, Inc.307309Germany12,001.199902002TYPESOFWINDTURBINESDenmarkt==-2,8800mcmUl m"00m;;:en'cx0UlCg~-<D:cccUlro~mU5:500Figure6.2 Total installed capacity in 2002, by country. AWEA data.Figure6.3 Installed wind capacity in the United States in1999and2002.2000I I.c'0mg-1000oD~roUic31500~Most early wind turbines were used to grind grain into flour, hence the name"windmill." Strictly speaking, therefore, calling a machine that pumps water orgenerates electricity a windmill is somewhat of a misnomer. Instead, people are6.2 TYPES OF WIND TURBINESIII Net AdditionsIII Installed CapacityWINDPOWERSYSTEMS»40,00035,00030,000~25,000~z-20,000'0Cll0.Cll15,000o10,000500003081995 1996 1997 1998 199920002001 2002 2003Figure6.1 Worldwide installed wind-power capacity and net annual additions to capac-ity have grown by over 25% per year since the mid-1990s. Data from AWEA.Subsequent interest in wind systems declined as the utility grid expanded andbecame more reliable and electricity prices declined. The oil shocks of the 1970s,which heightened awareness of our energy problems, coupled with substantialfinancial and regulatory incentives for alternative energy systems, stimulated arenewal of interest in windpower. Within a decade or so, dozens of manufac-turers installed thousands of new wind turbines (mostly in California). Whilemany of those machines performed below expectations, the tax credits and otherincentives deserve credit for shortening the time required to sort out the besttechnologies. The wind boom in California was short-lived, and when the taxcredits were terminated in the mid-1980s, installation of new machines in theUnited States stopped almost completely for a decade. Since most of the world'swind-power sales, up until about 1985, were in the United States, this sud-den drop in the market practically wiped out the industry worldwide until theearly 1990s.Meanwhiley, wind turbine technology developmentcontinued-especiallyinDenmark, Germany, andSpain-andthose countries were ready when salesbegan to boom in the mid-1990s. As shown in Fig. 6.1, the global installedcapacity of wind turbines has been growing at over 25% per year.Globally, the countries with the most installed wind capacity are shown inFig. 6.2. As of 2003, the world leader is Germany, followed by Spain, the UnitedStates, Denmark, and India. In the United States, California continues to have themost installed capacity, but as shown in Fig. 6.3, Texas is rapidly closing the gap.Large numbers of turbines have been installed along the Columbia River Gorge inthe Pacific Northwest, and the windy Great Plains states are experiencing majorgrowth as well.311TYPES OF WIND TURBINESand inexpensive since they don't have to handle the constant flexing associatedwith blades on horizontal axis machines.There are several disadvantages of vertical axis turbines, the principal onebeing that the blades are relatively close to the ground where windspeeds arelower. As we will see later, power in the wind increases as the cube of velocityso there is considerable incentive to get the blades up into the faster windspeedsthat exist higher up. Winds near the surfaceofthe earth are not only slower butalso more turbulent, which increases stresses onVAWTs.Finally, in low-speedwinds, Darrieus rotors have very little starting torque; in higher winds, whenoutput power must be controlled to protect the generator, theycan'tbe made tospill the wind as easily as pitch-controlled blades on a HAWT.While almost all wind turbines are of the horizontal axis type, there is stillsome controversy over whether an upwind machine or a downwind machine isbest. A downwind machine has the advantage of letting the wind itself control theyaw (theleft-rightmotion) so it naturally orients itself correctly with respect towind direction. They do have a problem, however, with wind shadowing effectsof the tower. Every time a blade swings behind the tower, it encounters a briefperiodofreduced wind, which causes the blade to flex. This flexing not only hasthe potential to lead to blade failure due to fatigue, but also increases blade noiseand reduces power output.Upwind turbines, on the other hand, require somewhat complex yaw controlsystems to keep the blades facing into the wind. In exchange for that addedcomplexity, however, upwind machines operate more smoothly and deliver morepower. Most modem wind turbines are of the upwind type.Another fundamental design decision for wind turbines relates to the numberof rotating blades. Perhaps the most familiar wind turbine for most people is themultibladed, water-pumping windmill so often seen on farms. These machines areradically different from those designed to generate electricity. For water pumping,the windmill must provide high starting torque to overcome the weight andfrictionofthe pumping rod that moves up and down in the well. They must alsooperate in low windspeeds in order to provide nearly continuous water


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