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.Other types of eolian features are playa lake, sediment plumes and wind scours.More information on the use of eolian features for locating windy areas is avail-able in [12].Another way to identify a windy site is to observe the biological indicators.Trees and bushes get deformed due to strong winds.The intensity and nature ofthis deformation depends on the strength of wind. 54 3 Analysis of wind regimes0 I II IIIIV V VI VII0 No deformation due to wind IV Strong flaggingI Brushing V Flagging and clippingII Light flagging VI Throwing and flaggingIII Moderate flagging VII Extreme flaggingFig.3.7.Biological rating scales for the wind speedThis method is specifically suitable to judge the wind in valleys, coasts and moun-tain terrains.Deformations of trees due to wind effect are classified by Putnam[17].There are five types of deformations under Putnam s classification.They arebrushing, flagging, wind throwing, clipping and carpeting.Wind brushing refers to the leeward bending of branches and twigs of the trees.Brushing may clearly be observed when the trees are off their leaves.This is anindication for light wind, which is not useful for wind energy conversion.In flag-ging, the branches are stretched out to leeward, with possible stripping of upwindbranches.The range of wind speeds corresponding to the flagging effect is of in-terest for energy conversion.In wind throwing, the main trunk and branches of thetree lean away from the coming wind.This indicates the presence of strongerwind.Under clipping, the lead branches of the tree are suppressed from growing toits normal height due to strong wind.With extreme winds, the trees are clippedeven at a very low height.This is termed as wind carpeting. 3.2 Measurement of wind 55Based on these deformations, the intensity of wind is rated on a seven pointscale put forth by Hewson and Wade [8].These are illustrated in Fig.3.7, showingthe top and front views of the tree trunk.However, it should be noted that the de-gree of this deformation may vary from one tree species to the other.For this rea-son, this method is to be calibrated with long term wind data for a given tree vari-ety.Once such calibration is available, the wind speed range can be directlyestimated on the basis of these biological indicators.3.2.2 AnemometersThe indicators discussed above, along with available wind data from meteorologi-cal stations, can give us an idea on the suitability of a given site for wind energyextraction.However, the final selection of the site should be made on the basis ofshort term field measurements.Anemometers fitted on tall masts are used for suchwind measurements.Height of the mast may be the hub height of the turbine toavoid further correction in wind speed due surface shear.As the power is sensitiveto the wind speed, good quality anemometers which are sensitive, reliable andproperly calibrated should be used for wind measurements.There are different types of anemometers.Based on the working principle, theycan be classified as:1.Rotational anemometers (cup anemometers and propeller anemometers)2.Pressure type anemometers (pressure tube anemometers, pressure plate ane-mometers and sphere anemometers)3.Thermoelectric anemometers (hot wire anemometers and hot plate anemome-ters)4.Phase shift anemometers (ultra sonic anemometers and laser doppler ane-mometers)Cup anemometerThe anemometer, most commonly used in wind energy measurements is the cupanemometer.It consists of three (or four) equally spaced cups attached to a cen-trally rotating vertical axis through spokes (Fig.3.8).The cups are hemisphericalor conical in shape and made with light weight material.This is basically a dragdevice.When kept in the flow, the wind exerts drag force on the cups.The dragforce is given by(3.5)12FD = CD A Áa V2where CD is the drag coefficient, A is the area of cup exposed to the wind, a is theair density and V is the wind velocity.As the drag coefficient of concave surface is more than on the convex surface,the cup with its concave side facing the wind experiences more drag force.Thiscauses the cups to rotate on its central axis.The intensity of rotation is directlyproportional to the velocity of incoming wind. 56 3 Analysis of wind regimesFig.3.8.Cup anemometer (Courtesy of THALES instruments GmbH, Werftweg 15,26135 Oldenburg, Germany, www.thales-instruments.de)This is further calibrated in terms of wind velocity, which can be directly sensedand recorded.Although these anemometers can sustain a variety of harsh environments, theyhave some limitations.It accelerates quickly with the wind but retards slowly aswind ceases.Due to this slow response, cup anemometers do not give reliablemeasurement in wind gusts.As the drag force is proportional to the density, anychanges in the air density will affect the accuracy of metered velocity [ Pobierz caÅ‚ość w formacie PDF ]
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.Other types of eolian features are playa lake, sediment plumes and wind scours.More information on the use of eolian features for locating windy areas is avail-able in [12].Another way to identify a windy site is to observe the biological indicators.Trees and bushes get deformed due to strong winds.The intensity and nature ofthis deformation depends on the strength of wind. 54 3 Analysis of wind regimes0 I II IIIIV V VI VII0 No deformation due to wind IV Strong flaggingI Brushing V Flagging and clippingII Light flagging VI Throwing and flaggingIII Moderate flagging VII Extreme flaggingFig.3.7.Biological rating scales for the wind speedThis method is specifically suitable to judge the wind in valleys, coasts and moun-tain terrains.Deformations of trees due to wind effect are classified by Putnam[17].There are five types of deformations under Putnam s classification.They arebrushing, flagging, wind throwing, clipping and carpeting.Wind brushing refers to the leeward bending of branches and twigs of the trees.Brushing may clearly be observed when the trees are off their leaves.This is anindication for light wind, which is not useful for wind energy conversion.In flag-ging, the branches are stretched out to leeward, with possible stripping of upwindbranches.The range of wind speeds corresponding to the flagging effect is of in-terest for energy conversion.In wind throwing, the main trunk and branches of thetree lean away from the coming wind.This indicates the presence of strongerwind.Under clipping, the lead branches of the tree are suppressed from growing toits normal height due to strong wind.With extreme winds, the trees are clippedeven at a very low height.This is termed as wind carpeting. 3.2 Measurement of wind 55Based on these deformations, the intensity of wind is rated on a seven pointscale put forth by Hewson and Wade [8].These are illustrated in Fig.3.7, showingthe top and front views of the tree trunk.However, it should be noted that the de-gree of this deformation may vary from one tree species to the other.For this rea-son, this method is to be calibrated with long term wind data for a given tree vari-ety.Once such calibration is available, the wind speed range can be directlyestimated on the basis of these biological indicators.3.2.2 AnemometersThe indicators discussed above, along with available wind data from meteorologi-cal stations, can give us an idea on the suitability of a given site for wind energyextraction.However, the final selection of the site should be made on the basis ofshort term field measurements.Anemometers fitted on tall masts are used for suchwind measurements.Height of the mast may be the hub height of the turbine toavoid further correction in wind speed due surface shear.As the power is sensitiveto the wind speed, good quality anemometers which are sensitive, reliable andproperly calibrated should be used for wind measurements.There are different types of anemometers.Based on the working principle, theycan be classified as:1.Rotational anemometers (cup anemometers and propeller anemometers)2.Pressure type anemometers (pressure tube anemometers, pressure plate ane-mometers and sphere anemometers)3.Thermoelectric anemometers (hot wire anemometers and hot plate anemome-ters)4.Phase shift anemometers (ultra sonic anemometers and laser doppler ane-mometers)Cup anemometerThe anemometer, most commonly used in wind energy measurements is the cupanemometer.It consists of three (or four) equally spaced cups attached to a cen-trally rotating vertical axis through spokes (Fig.3.8).The cups are hemisphericalor conical in shape and made with light weight material.This is basically a dragdevice.When kept in the flow, the wind exerts drag force on the cups.The dragforce is given by(3.5)12FD = CD A Áa V2where CD is the drag coefficient, A is the area of cup exposed to the wind, a is theair density and V is the wind velocity.As the drag coefficient of concave surface is more than on the convex surface,the cup with its concave side facing the wind experiences more drag force.Thiscauses the cups to rotate on its central axis.The intensity of rotation is directlyproportional to the velocity of incoming wind. 56 3 Analysis of wind regimesFig.3.8.Cup anemometer (Courtesy of THALES instruments GmbH, Werftweg 15,26135 Oldenburg, Germany, www.thales-instruments.de)This is further calibrated in terms of wind velocity, which can be directly sensedand recorded.Although these anemometers can sustain a variety of harsh environments, theyhave some limitations.It accelerates quickly with the wind but retards slowly aswind ceases.Due to this slow response, cup anemometers do not give reliablemeasurement in wind gusts.As the drag force is proportional to the density, anychanges in the air density will affect the accuracy of metered velocity [ Pobierz caÅ‚ość w formacie PDF ]