Wind Energy

Question 1

What is the cut in and cut out wind speeds? (definitions & graph)

Answer 1

Cut in wind speed: This is the wind speed at which the wind turbine will start generating power— typical cut-in wind speeds are 3 to 5 m/s.

Cut-out wind speed: This is the highest wind speed which the turbine will operate at. Above this speed, the turbine is stopped to prevent damage to the blades.

Question 2

What is the nominal wind speed?

Answer 2

Nominal wind speed: This is the lowest speed at which the wind turbine reaches its nominal power output. Above this speed, higher power outputs are possible, but the rotor is controlled to maintain a constant power to limit loads and stresses on the blades.

Question 3

What is the rated wind speed of a turbine?

Answer 3

This limit to the generator output is called the rated power output and the wind speed at which it is reached is called the rated output wind speed. At higher wind speeds, the design of the turbine is arranged to limit the power to this maximum level and there is no further rise in the output power. How this is done varies from design to design but typically with large turbines, it is done by adjusting the blade angles so as to to keep the power at the constant lev

Rated velocity is the velocity at which the turbine just reaches maximum outpu

Question 4

Give two reasons why Bertz limit isn’t achieved in practice

Answer 4

At low rotational speeds, the mill is imparting lost of rotational spin into the a ir. This represents an energy loss. At very high rotational speeds, the relative speed between the blades and the air is very significant and dissipation begins to become important.

Question 5

State the positives of wind energy

Answer 5

Renewable, green, no carbon/ low carbon foot print

Running costs low

Question 6

What are some of the negatives of wind energy? (5)

Answer 6

• Capital cost
• permission to operate presents difficulties.
• Intermittent power
• Connections to grid will be expensive
• Will require standby power to make up for intermittency

Question 7

Why is Betz limit less than unity?

Answer 7

From continuity, the air must go through the windmill. Hence it must leave the mill with finite speed. This means that all the available kinetic energy has Not been taken from the air, and so the power coefficient must be less than 1.0

Question 8

What is the equation for the power generated by a turbine?

Answer 8

P = 0.5 . rho . A . U^3 . Cp

Question 9

Does hot or cold air result in more power generation from a turbine?

Answer 9

Cold air because the density increases

Question 10

What is the buffeting effect?

Answer 10

Boundary layer - wind speed is lower by the surface therefore tall towers are good

Question 11

What could we use the electricity generated when demand is low for?

Answer 11

• charge batteries (Use EVs?)
• heat water or store in the ground (large thermal inertia)
• build industries which could use it (aluminium in Norway)

Question 12

What’s the equation for thrust force acting on the device?

Answer 12

The trust force is proportional to the change in momentum of the fluid before and after the turbine. For our system, Thrust = mass flow through the turbine x(velocity before –velocity after) = ρAtUtx (Uf–d), which for the Bertz optimised system (with Ut= 2/3Uf; Ud=1/3Uf) gives F =(2/3)^2ρAUf2=4/9 ρAU

Question 13

What’s the relationship between uf , ut and ud?

Answer 13

ut = 2/3uf & ud = 1/3uf

Question 14

what is the typical availability factor of a turbine?

Answer 14

1/3

Question 15

will the power extracted by the machine be proportional to the mean wind speed cubed or the mean of the cubed speed?

Answer 15

mean of the cubed speed

Question 16

Positives of vertical wind turbines? (3)

Answer 16

• takes the wind from any direction
• keep electronics on the group
• smoother loading

Question 17

Negatives of vertical wind turbines? (3)

Answer 17

• complex structure
• limited to relatively small devices
• not as developed

Question 18

Suggest reasons why the optimisation of λ is at the mean wind speed rather than the rated wind speed

Answer 18

Although a lot more power is generated at rated speed, the turbine spend more of it operational time at the means velocity, hence more sensible to design the unit to harvest the maximum possible at this velocity.

Question 19

how would you work out the power extracted by two turbine behind each other?

Answer 19

1) state the velocity at the first turbine and approaching the second which is v=(1-a)u
2) work out power at each p1 = normal sub u & (1-a)^2x4a for turbine 2, same equation but (1-a)^3 16/27
3) add together to get total power
4) differentiate and set to 0 look for a
5) sub optimal a into total power
6) workout Cp

Question 20

state three things that push up the cost of wind

Answer 20

• capital costs
• interest payment
• maintenance
• intermittent and unpredictable power supply
• typical power supply 1/3 of time

Question 21

what are the technical and physical limitations of making wind turbines much bigger?

Answer 21

• loads on structure become very big
• on land getting them into position is very hard (modular desing maybe to over come this?)
• become much more visible
• power outages can have a bigger impact
• increased noise

Question 22

what are typical lift coefficients?

Answer 22

1 - 1.2

Question 23

roughly, what is the ratio of Cd to CL?

Answer 23

Cd = 1/20 to 1/40 of lift coefficent

Question 24

how do we make sure that the voltage produced by the turbine is suitable for the grid?

Answer 24

Voltage, frequency and phase angle are all now dealt with by efficient power electronics. The turbine will be run (as far as is possible) at optimum speed, blade angle for the prevailing wind velocity. Electrical power from the alternator will be processed by a power inverter to ensure that the power being delivered to the grid id at the correct voltage and synchronised to the correct phase angle(s

Question 25

how can we control the rotational speed or te turbine? why is it important?

Answer 25

• altering the imposed torque (i.e. altering the amount of power taken from the machine) or by
• altering the attack angle of the blades.

ensure optimal tip speed ratio

Question 26

how is the thrust on the turbine calculated?

Answer 26

change in pressure drop using bernouli’s equation * AREA

Question 27

why does the blade twist?

Answer 27

• The tip of the blade travels faster than the root
• but the wind speed is independent of the radial location along the blade.
• i f we are to maintain the same effective ‘cutting angle’ between the velocity of the blade and the wind, then the angle of twist must reduce as the radial position increases.

Question 28

why is the blade thicker at the bottom?

Answer 28

• The maximum energy harvesting occurs when the Bertz criterion is met
• This means that the maximum (per unit swept area) is limited to this criterion everywhere.
• Hence there is a need to harvest the same thrust everywhere (per unit swept area)
• . As the twist is greater at the root, the width has to be greater so that the projected blade in this area is commensurate with maximum energy harvesting.

Question 29

what is the equation for the optimal blade twist?

Answer 29

angle of twist = phi - alpha

= tan-1(2R/3rlamda) - alpha