# Wind Turbine Design

Primary objective in wind turbine design is to maximize the aerodynamic efficiency, or power extracted from the wind. But this objective should be met by well satisfying mechanical strength criteria and economical aspects. In this video we will see impact of number of blades, blade shape, blade length and tower height on wind turbine design.

Check following article to know more on wind turbine design aspects.

## Effect of Number of Blades

As the number of blades in the wind turbine increases aerodynamic efficiency increases, but in a diminishing manner. When we move from 2 blades to 3 blades design efficiency gain is about 3%. But as we move from 3 blades to 4 blades design, efficiency gain is marginal.

 Fig.1 Efficiency gain as number of blades in wind turbine is increased
As we increase number of blades, cost of the system increases drastically. Along with that mechanical design of blades also becomes a difficult affair. With more number of blades, blades should be thinner to be aerodynamically efficient. But blades with thinner portion at the root may not withstand bending stress induced due to axial wind load. So generally wind turbines with 3 blades which can accommodate a thicker root cross-section are used.
 Fig.2 Wind turbine blades have got thicker root to withstanad huge bending moment induced

The next big factor which is affecting performance of wind turbine is shape and orientation of blade cross section. A moving machine experiences fluid flow at a different velocity than the actual velocity. It is called as relative or apparent velocity. Apparent velocity of flow is difference between actual flow and blade velocity. Absolute velocity of the flow is shown in first figure, while apparent velocity in the second figure. It is clear that apparent velocity of flow is vectorial difference between actual and blade velocity. The vector difference is shown in the first figure at a particular cross section. A rotating blade will experience an apparent velocity of flow.

 Fig.3 Absolute & apparent velocity of wind
A close look at wind turbine blade will reveal that, it is having airfoil cross sections from root to tip. The driving force of wind turbine is, lift force generated, when wind flows over such airfoils. Lift force will be perpendicular to apparent velocity. Generally lift force increases with angle of attack. Along with that undesirable drag force also increases. While tangential component of lift force supports blade rotation, drag force opposes it. So a wind turbine can give maximum performance, when lift to drag ratio is maximum. This is called as, optimum angle of attack. Airfoil cross sections are aligned in a way to operate at this optimum angle of attack.
 Fig.4 Lift and drag force induced over a wind turbine blade
Even though flow velocity is uniform along the length of the blade, blade velocity increases linearly as we move to the tip. So angle and magnitude of relative velocity (apparent velocity) of wind varies along the length of the blade. Apparent velocity becomes more aligned to chord direction as we move to the tip.
 Fig.5 Change in apparent velocity along length of the blade
So there should be a continuous twist in the blade, so that at every airfoil cross section angle of attack is optimum.

Wind condition can change at any time. So it is also possible to rotate wind turbine blades in its own axis, in order to achieve optimum angle of attack with varying wind condition. This is known as pitching of blades. A clever algorithm which uses wind condition and characteristics of wind turbine as input, governs the pitch angle for the maximum power production.

 Fig.6 Schematic of algorithm which governs blade pitching