[ Pobierz całość w formacie PDF ]
.It was in-vented by Leon Battista Alberti as early as in 1450.This is further refined by3.2 Measurement of wind 57dxFig.3.9.Principle of pressure plate anemometerFig.3.9.Principle of pressure plate anemometerRobert Hooke (1664) and Rojer Pickering (1744).It basically consists of a swing-Robert Hooke (1664) and Rojer Pickering (1744).It basically consists of a swing-ing plate held at the end of a horizontal arm.This is attached to a vertical shafting plate held at the end of a horizontal arm.This is attached to a vertical shaftaround which the arm can rotate freely (Fig.3.9).A wind vane directs the platearound which the arm can rotate freely (Fig.3.9).A wind vane directs the platealways perpendicular to the wind flow.As the drag coefficient of a flat plate canalways perpendicular to the wind flow.As the drag coefficient of a flat plate canbe taken as unity, referring Eq.(3.5), the pressure exerted on the plate P by thebe taken as unity, referring Eq.(3.5), the pressure exerted on the plate P by thewind is given bywind is given by(3.6)12P = Áa V2This pressure makes the plate to swing inward.As the distance through which theplate swings depends on the wind strength, it can be directly calibrated in terms ofthe wind velocity.Pressure plate anemometers are suited for measuring gustywinds.Pressure tube anemometersAnother type of anemometer which uses the wind pressure to measure the velocityis the pressure tube anemometer.This works on the principle that, the wind flowP2P1Fig.3.10.Principle of pressure tube anemometer58 3 Analysis of wind regimespassing through the tube creates pressure where as the flow across a tube results insuction.Consider two tubes as shown in the Fig.3.10.The pressure in the tubeparallel to the wind is the sum of atmospheric pressure and the wind pressure.Thus(3.7)12P1 = PA + C1 Áa V2Similarly in the tube perpendicular to the wind, the pressure is(3.8)12P2 = PA C2 Áa V2where PA is the atmospheric pressure and C1 and C2 are coefficients.SubtractingP2 from P1 and solving for V, we get0.5(3.9)îøùø2 P1 P2( )V =ïøÁa C1 + C2 úø( )úøïøûøðøThus, by measuring the difference in pressure inside the two tubes, the wind ve-locity can be estimated.Values of C1 and C2 are available with the instrument.Thepressure is measured using standard manometers or pressure transducers.The ma-jor advantage of pressure tube anemometer is that it does not have any movingparts.This anemometer has limited application in the open field measurements asthe presence of dust, moisture and insects can affect its accuracy.Sonic anemometerFig.3.11.Sonic anemometer (Courtesy of Vaisala Oyj, Vanha Nurmijärventie 21,FIN  01670, Vantaa, Finland, www.vaisala.com)3.2 Measurement of wind 59Sonic anemometers measure the wind velocity by sensing the changes in thespeed of sound in air.It has three arms, mounted perpendicular to each other, asshown in Fig.3.11.Transducers fitted at the tips of each arm emit acoustic signalswhich travel up and down through the air.Speed of sound in moving air is differ-ent from that through still air.Let VS be the velocity of sound in still air and V isthe wind velocity.If both the sound and wind are moving in the same direction,then the resultant speed of sound waves (V1) isV1 = Vs + V (3.10)Similarly, if the propagation of the sound waves is opposite to the wind direction,then the resultant velocity of sound (V2) isV2 = Vs V (3.11)From Eq.(3.10) and Eq.(3.11) we get(3.12)V1 V2V =2Thus, by measuring the speed of sound waves between the transducer tips duringits upward and downward travel, the wind velocity can be estimated.Sonic ane-mometers also do not have any moving parts.They are reliable and accurate formeasuring wind velocity in the range of 0 to 65 m/s.However, they are costlierthan the other types of anemometers.Some other anemometers are bridled anemometers and hot wire anemometers.But they are not common in wind energy measurements.The bridled anemometersare similar to the cup anemometers, but have more number of cups (commonly32).The rotation of the central axis to which the cups are attached is checked by aspring.As the cups tend to rotate due to wind, tension builds up in the spring.In-tensity of this tension is translated into the wind speed.Hot wire anemometersutilize the cooling effect of wind for measuring its velocity.The rate at which ahot wire, maintained at a temperature, is cooled depends on the velocity of airmoving over it.Apart from the sensors discussed above, the complete anemometer assemblyincludes transducers, data loggers and processing units.For example, the cup andpropeller anemometers generally drive a mini generator to convert the rotationalspeed in to electrical signals.Some designs are fitted with dc generators whereassome recent models employ permanent magnet ac generators.The output voltageof the generator is directly proportional to the wind speed.With an analog to digi-tal (A/D) converter, the analog voltage can be converted to digital form which canfurther be processed and stored by a suitable data processing system.A typical ar-rangement powered by a solar panel is shown in Fig.3.12.The data processing unit in modern wind measurement systems includes anelectronic chip on a computer which receives the instantaneous data and averagesit over a specific time period.The standard time interval is 10 minutes.The instan-taneous turbulences, which are not of interest in wind energy conversion,60 3 Analysis of wind regimesFig.3.12.The data acquisition unit (Courtesy of THALES instruments GmbH, Werftweg15, 26135 Oldenburg, Germany, www.thales-instruments.de)smoothen out within this 10 minutes interval [ Pobierz caÅ‚ość w formacie PDF ]