Part 2: Aerodynamics, Power generation and Conceptual design Flashcards
Explain why the blades must be twisted.
The angle of attack has to be optimal in each section of the blade length. Since the velocity is higher at the tip compared to the base due to rotation the blade must be twisted to keep the optimal lift-to-drag-ratio.
What are the advantages/ disadvantages of having two or three blades?
Advantages: High efficiency compared to more blades Less noise than two bladed turbines Lower cost (less material) Visual acceptance. Disadvantages: Smaller starting torque than turbines with more blades, More material needed than two bladed turbines
Explain why angle of attack is dependent on rotational speed and pitch angle.
Rita figur. Angle of attack is defined as the angle between the chord and the relative wind speed. The relative wind speed is made of two components; normal wind faced by the turbine and wind due to the rotation. If the rotational speed of the blade changes the magnitude and direction of relative wind will change and hence the AoA will also change. Pitch angle = angle of attack + twist angle. Twist angle can not be changed after manufacturing. If the if the pitch angle is reduced than AoA is also reduced.
Explain why larger turbines often have lower rotational speed in rpm.
For each type of wind turbine we have the highest efficiency at the optimum tip speed ratio. Lambda_opt=(rotational speed*Rotor radius)/wind speed. Rewriting this equation is can be seen that there is an inverse relationship between rotational speed and rotor radius (size)
What is the direct driven concept? What are its main characteristics?
Similar to type 4 concept. Generator shaft directly connected to to rotor blades. Characteristics: It can operate over a wide range of wind speed from zero to rated speed. This means that power can be retrieved from all the wind that passes the rotor shaft.
What is the advantage of a variable speed turbine?
The rotor speed can be controlled. This means that the tip speed can be kept at it optimum speed even if the wind speed changes which improves the efficiency. There will be smaller fluctuations in the power output. Lastly they produce less noise.
How much (approximate numerics) can the speed be varied for each type of turbine concept?
- Danish concept - About 1 %
- Wounded induction generator with variable resistance - +10%
- Double-fed induction generator - up to +-30%
- Fully converted synchronous generator - +-100%
Explain why speed is varied in different amounts and which component is limiting the speed variation.
- The squirrel cage induction generator is the limiting factor as it cannot change its torque.
- The heat losses from the resistance would become too great.
- The limiting factor is the capacity of the back-to-back converter connected to the rotor.
- No limitation.
In large turbines it is possible to apply power regulation. Mention 4 ways and explain. Which methods are most common?
Pitch control: Uses a motor to increase the AoA of the blade to optimize the power output. When the rated power is reached the AoA is reduced to avoid overloading.
Variable speed control: Regulate the power by changing the generators torque.
Stall control: Does not need any special equipment which meant that it is robust. The blades are designed so that higher wind speed it offers high AoA to the relative wind. Adds complexity to design and manufacturing.
Active stall control: Like pitch control to speed below rated speed. Than it increases the AoA instead of reducing it. This results in drag and reduces the lift.
The most commonly used are pitch and variable speed control.
What functions do the different turbines have?
Squirrel Vari. slip DFIG Multiple sync.
Gearbox yes yes yes no
frequency converter no no yes yes
active pc no yes yes yes
reactive pc no no yes yes
variable speed no yes yes yes
Draw Type A
Kolla slides
Draw Type B
Kolla slides
Draw type C
Kolla slides
Draw Type D
kolla slides
What is the power coefficient?
Cp=P_rotor/P_wind<16/27
