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|
|Fig.2 Wind turbine blades have got thicker root to withstanad huge bending moment induced|
Wind Turbine Blade Design
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|
|Fig.4 Lift and drag force induced over a wind turbine blade|
|Fig.5 Change in apparent velocity along length of the blade|
Pitching of Blades
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|
Next big factor affecting performance of wind turbine is length of the blade. As we discussed in first video lecture, power extracted by the wind turbine varies according to this equation. So it is clear that, a longer blade will favor the power extraction. But, with increase in blade length, deflection of blade tip due to axial wind force also increases. So blind increase in length of the blade may lead to dangerous situation of collision of blade and tower.
|Fig.7 Blade bending due to wind load acting on it|
Determination of Tower Height
Most critical factor of wind turbine design is determination of proper tower height. Power input available for wind turbine varies as cube of wind speed. So a small change in wind speed will have huge effect on power production. A typical wind speed increase from ground level is shown in figure. So from power extraction point of view, it is desired to have tower height as high as possible. But difficulty in road transportation and structural design problems put a limit on maximum tower height possible.
|Fig.8 Wind velocity increases with altitude resulting in more power extraction|