1. Wind Energy

3. The Wind

6. Energy And Power In The Wind The energy contained in the wind is its kinetic energy, and as we saw in Chapter 1 the kinetic energy of any particular mass of moving air is equal to half the mass, m, of the air times the square of its velocity, V: kinetic energy = half mass × velocity squared i.e kinetic energy = 0.5 mV 2 where m is in kilograms and V is in metres per second (ms -1 )

7. In other words: mass (m) of air per second = air density × volume of air flowing per second = air density × area × length of cylinder of air flowing per second = air density × area × velocity i.e. m =ρAV Substituting for m in (1) above: kinetic energy per second = 0.5ρAV 3 (joules per second) where ρis in kilograms per cubic metre (kg m -3 ), A is in square metres (m 2 ) and V is in metres per second (m s -1).

9. Wind Turbines

12. Wind Turbine Types

14. Effect Of The Number Of Blades

15. A wind turbine of a particular design can operate over a range of tip speed rations, but will usually operate with its best efficiency at a particular tip speed ratio, i.e., when the velocity of its blade tips is a particular multiple of the wind velocity. The optimum tip speed ratio for a given wind turbine rotor will depend upon both the number of blades and the width of each blade. As we have seen, the term solidity describes the fraction of the swept area that is solid. Wind turbines with large numbers of blades have highly solid swept areas and are referred to as high-solidity wind turbines ； wind turbines with small numbers of narrow blades are referred to as low-solidity wind turbines. Multi-blade wind pumps have high-solidity rotors and modern electricity-generation wind turbines (with one, two or three blades) have low-solidity rotors.

18. Aerodynamics Of Wind Turbines

22. HAWT Rotor Blades Wind Forces And Velocities Because the blade is in motion, the direction from which the blade sees the relative wind velocity, W, is the resultant of the tangential velocity, u, of the blade at that position and the wind velocity, V1, at the rotor. The tangential velocity, u, (in metres per second) at a point along the blade is the product of the angular velocity,Ω (in radians per second) of the rotor and the local radius, r, (in metres), at that point, that is : u=Ωr The wind velocity at the rotor, V1, is the undisturbed wind velocity upstream of the rotor, V0, reduced by a factor that takes account of the wind being slowed down as result of power extraction. This factor is often referred to as the axial interference factor, and is represented by a.

25. Vertical Axis Wind Turbines