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.Statistical models are being successfully used fordefining the distribution of wind velocity in a regime, over a given period of time.Once the wind velocity and its distribution at a prospective site are available,we can proceed further with the assessment of the energy potential.One of the ini-tial questions to be addressed is How much energy is available per unit area ofthe rotor? Percentage of time for which the wind is within a useful velocity range,the most frequent wind velocity, and the velocity contributing maximum energy tothe regime are some other factors of interest.Similarly, for the safe structural de-sign, possibilities of extremely high wind at the site should be identified.In this chapter, let us discuss the basic nature of wind along with the methods ofmeasuring its strength.Statistical models commonly used for wind resource analy-sis are described, indicating their application in wind energy conversion.The dis-cussions are further extended to the derivation of methods and indices for assess-ing the energy potential of a given wind regime. 46 3 Analysis of wind regimesHigh pressure regionCoriolis forceResulting pathLow pressure regionFig.3.1.Wind direction affected by the Coriolis force3.1 The windThe earth receives around 1.7Ø1014 kW of power from the sun in the form of solarradiation.This radiation heats up the atmospheric air.The intensity of this heatingwill be more at the equator (0o latitude) as the sun is directly overhead.Air aroundthe poles gets less warm, as the angle at which the radiation reaches the surface ismore acute.The density of air decreases with increase in temperature.Thus,lighter air from the equator rises up into the atmosphere to a certain altitude andthen spreads around.This causes a pressure drop around this region, which attractsthe cooler air from the poles to the equator.This movement of air causes thewind.Thus, the wind is generated due to the pressure gradient resulting from the un-even heating of earth s surface by the sun.As the very driving force causing thismovement is derived from the sun, wind energy is basically an indirect form of so-lar energy.One to two per cent of the total solar radiation reaching the earth s sur-face is converted to wind energy in this way.The wind described above, which is driven by the temperature difference, iscalled the geostrophic wind, or more commonly the global wind.Global winds,which are not affected by the earth surface, are found at higher altitudes.The rota-tion of earth leads to another phenomenon near its surface called the Coriolis ef-pressure gradientForce due to 3.1 The wind 47fect, named after the famous mathematician Gustave Gaspard Coriolis.Due to theCoriolis effect, the straight movement of air mass from the high pressure region tothe low pressure region is diverted as shown in Fig.3.1.Under the influence ofCoriolis forces, the air move almost parallel to the isobars.Thus, in the northernhemisphere, wind tends to rotate clockwise where as in the southern hemispherethe motion is in the anti-clockwise direction.3.1.1 Local effectsChanges in velocity and direction of wind near the surface, say up to 100 m abovethe ground, is more important as far as energy conversion is concerned.In this re-gion, the wind pattern is further influenced by several local factors.Land and sea breezes are examples for the local wind effects.During the daytime, land gets heated faster than the sea surface.As a result, the air near the landrises, forming a low pressure region.This attracts cool air to the land from the sea.This is called the sea breeze.During night time, the process gets reversed as cool-ing is faster on land.Thus wind blows from the land to the sea, which is called theland breeze.In mountain valleys, the air above the surface gets heated and rises up along theslopes during the day time.This is replaced by the cool air, resulting in the valleywinds.During the night, the flow is from the mountain to the valley which isknown as the mountain wind.Quite often, this phenomenon may create verystrong air currents, developing powerful wind.Wind shear, turbulence and accel-eration over the ridges are some other examples for local wind effects.3.1.2 Wind shearThe flow of air above the ground is retarded by frictional resistance offered by theearth surface (boundary layer effect).This resistance may be caused by the rough-ness of the ground itself or due to vegetations, buildings and other structures pre-sent over the ground.For example, a typical vertical wind profile at a site is shownin Fig.3.2.Theoretically, the velocity of wind right over the ground surfaceshould be zero.Velocity increases with height upto a certain elevation.In theabove example, the velocity increases noticeably upto 20 m, above which the sur-face influence is rather feeble
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