Design Considerations in Blade Design

Design Considerations in Blade Design

The blades of a wind turbine are designed based on the forces caused by the rate of change of momentum theory, such blades are not efficient on account of boundary layer separation from blade surfaces. It causes the loss of pressure due to formation of eddies with distortion of flow.

It should be noted that the problem of boundary flow separation does not occur in radial blades since the increase in pressure is due to centrifugal head imparted to it.

The problem of boundary layer separation can be avoided by using aerofoil blading.

1. Nomenclature Related to Aerofoil Blades :

Figure A

The related nomenclature used in aerofoil blades are as follows (Refer Figure A).

1. Camber line : It represents the locus of all points midway between the upper and lower mean surfaces of an acrofoil section as measured perpendicular to the mean line.
2. Blade thickness, t : It represents the perpendicular distance of the outer or inner surface from camber line.
3. Camber angle, θ : The angle made by the tangents to camber line at the leading and trailing edge of the blade profile.
4. Chord line : It represents the line joining the ends of the mean camber line.
5. Camber : It is the maximum rise of camber line from the chord line.
6. Distance, a : It is the distance along the X-axis from leading edge upto the point of maximum camber.
7. Pitch, s : It represents the distance between the two consecutive blades.

2. Cross-section of Rotor Blade :

The cross-section of a rotor blade is identical to that of the propeller of an aeroplane or helicopter. Such a section is called an aerofoil section. The major points and dimensions of an acrofoil rotor blade is shown in Figure B.

Figure B

The wind velocity, C acting on the rotor blade at any point has two components, namely :

1. Real wind velocity, C
2. Wind of rotation, ω or

Where w is the speed of rotation and ‘r’ is the radius of the point under consideration. It is shown in Figure A and B.

Figure C

The ratio ω . r / cf is called λ

The angle of effective wind will be different at different radii r. The angle of attack should be around 10° for its best performance. To keep this constant the blade is twisted along its length.

3. Lift and Drag :

The effective wind velocity produces the total force acting on the blade section called aerodynamic force. It is proportional to the kinetic energy of stream and the projected area of blading.

The resultant force can be resolved into two components as follows :

1. Lift, F_L which is normal to the direction of approach velocity. It is responsible for an aeroplane to maintain its lift . It is caused due to unbalanced pressure distribution over aerofoil surface.
2. Drag, F_D which is parallel to the direction of approach velocity. It represents the friction forces.

Lift is the useful component which gives rotation to the turbine. These forces F_L and F_D are related by their coefficients called lift coefficient F_L and the drag coefficient F_D. These are defined as :

C_L = Lift, force, F_L / 1/2p . C²_m

C_D = Drag force, F_D / 1/2p . C²_m

Where C is the length of the chord and Cm is the average velocity.

C_L and C_D depends on the angle of attack and they are determined experimentally.

The variation of C_L and C_D are shown in Figure D against the angle of attack, α.

Figure D

4. Blade Sections :

Figure E

Figure E shows the cross section of the blade along the length. It could be observed that the angle of attack is different at different radii from the centre of the rotor. Due to the variation in the effective wind speeds and change in wind speed and the rotor speed, the rotor can give the best performance only at one wind speed.

The other aspects to be considered under aerodynamic considerations are :

1. Tower height
2. Number of blades.

There aspects are briefly discussed in the succeeding sections.

5. Tower Height :

It is a known fact that as we go higher, the wind blows faster. Variation of wind velocity with altitude is called wind shear.

The wind velocity varies proportional to the seventh root of altitude. Hence, if the height of tower is doubled, the wind velocity increases by 10% and power varies by about 34%. But when the height of the tower is doubled, the rotor diameter also has to be doubled. This increases the cost of material, size and weight.

6. Number of Blades :

Determination of number of blades involves the design considerations of acrodynamic efficiency, component cost, system reliability etc.

The universally accepted standard configuration now selects either two or three number of blades. When number of blades are increased from two to three, the acrodynamic efficiency goes up by 6%. But further increase to four blades will yield only 3% increase in efficiency.

Thereafter any further increase would yield only marginal increase in efficiency.

Increasing the number of blades will increase the material cost and the rotational speed. Fewer blades with high rotational speeds are preferable. This is because, it reduces peak torque and hence reduces the size of the gear box and generators. Thus, it leads to reduced cost.

Number of blades also affects the system reliability. This is due to two factors firstly due to the dynamic loading of rotor and the drive train. Secondly when the turbine is aligned in the direction of the wind, each blade experiences a cyclic load at its root. But for three bladed rotors, these cyclic loads get balanced.

Another factor which may also be considered is the top speed ratio. This is the ratio of the speed of the wind to the speed of the tip of the blade. Three blade turbines can have a tip speed ratio of 6 to 7.