Characteristics

Question 1

How is the characteristic curve of cP for a fast running turbine?

Answer 1

• starts with a value of zero (rotor is not rototating)
• increase uo to a peak (~0.5, Betz Limit can not be reached because losses are included)
• peak is located at a tip speed ratio sligthly smaller than design speed ratio (drag loss)
• decreases down to zero (tip speed ratio sligthly smaller than 2*design speed ratio)

Question 2

What is the major loss type for a fast running wind turbine?

Answer 2

• drag loss

Question 3

What is the idling state?

Answer 3

• point where power coefficient reverts to zero

Question 4

How is cq definied?

Answer 4

cq = cp/lambda

Question 5

How is the characteristic curve of cP for a fast running turbine?

Answer 5

• similar to power coefficient curve
• but not zero at zero tip speed
• the peak of the torque coefficient can be determined by drawing the tangent through zero in the power coeffiecient curve

Question 6

How is the characteristic curve of cT for a fast running turbine?

Answer 6

• close to zero at zero tip speed ratio (low number of blades in fast running windturbines , low solidity , results in low thrust)
• continuisly increases with tip speed ratio (1.with increasing tip speed ratio, inflow angle phi decreases approaches zero, cosin of phi approaches 1, lift coefficient approaches the thrust coefficient, lift force approaches thrust force
  2. lift force scales with the square of relativ inflow velocity, tip speed ratio increases and inflow velocity increases because the rotational component of inflow increases)

Question 7

How is the characteristic curve of cP for a slow running turbine?

Answer 7

• for zero tip speed ratio is zero
• up to a peak, peak is located a bit higher than design tip speed ratio (wake rotation loss makes the rotor a bit faster running)
• decreases and reaches zero in idling state, approx. 2* tip speed ratio at peak)

Question 8

Why is the peak in the cp curve much smaller for slow running turbines?

Answer 8

• due to the high impact of the wake rotation loss forslow running windturbines

Question 9

What is the major loss type for a s low running wind turbine?

Answer 9

wake rotation loss

Question 10

Why is the torque coefficient for slow running turbines that high for a tip speed ratio of zero?

Answer 10

• due to the high twist angles of the blades

results in high lift forces for small tip speed ratio

Question 11

Why is the peak of the torque coefficient for slow running turbines that much higher than for fast running turbines?

Answer 11

• the power of the rotor is equal to the torque times the rotational veocity of the rotor (omega)
• to obtain the same amount of power, the torque has to increase because the rotation speed is much smaller

Question 12

Why is the thrust coefficient for slow running turbines that high for a tip speed ratio of zero?

Answer 12

• high solidity of the rotor (15 blades)

- much higher than low running turbines

Question 13

How is the characteristic curve of cT for a slow running turbine?

Answer 13

• starting at a high value for zero tip speed ratio

- increasing to a tip and then decreasing

Question 14

Why does cT for slow running turbines decrease?

Answer 14

• in the inboard region of the blade you obtain negative angles of attack at high tip speed ratio (due to high twist angles), negative lift coefficient, results in reduction of thrust coefficent

Question 15

What are the characterstic curves?

Answer 15

• Power coefficient
• Thrust coefficient
• Torque coefficient

Question 16

How can physical quantities can be non-dimensioned?

Answer 16

• use of a Reference Force Fref (resultant areodynamic pressure of the wind)