3 WIND

Question 1

*Advantages of wind energy generation

Answer 1

• Each wind turbine can produce up to 12MW
• Wind farms can be constructed quickly (once planning permission is obtained)
• Low cost in high-wind-speed onshore sites (4-5 p/kWh)

Question 2

*Disadvantages of wind energy

Answer 2

• Noise and visual pollution
• Environmental impact
• Intermittent energy source
• Offshore- impacts fish/marine life (some research says they increase fish population as they act as artificial reefs)
• Onshore - impact on birds/bats
• Mining the copper ore emits CO2

Question 3

*How to calculate power from wind turbine

Answer 3

Pwt = Cp.Pair = Cp(1/2)𝜌AU^3

Question 4

*How to estimate energy output of wind turbine/farm

Question 5

*What is the momentum theory of wind turbine rotors?

Answer 5

Uses Bernoulli’s principle to illustrate conservation of momentum.

Question 6

Assumptions of momentum theory.

Answer 6

• rotor is a solid, infinitely thin disc
• disc is frictionless, without any rotational wake component (ie ideal)
• there is constant velocity induced along the axis of rotation on the disc
• the disk creates a flow around the rotor

Question 7

Benefits of moving wind farms offshore

Answer 7

• minimal visual impact
• higher mean wind speeds (larger blades)
• lower turbulence (usually caused by obstructions on land)

Question 8

Disadvantages of moving wind farms onshore

Answer 8

Higher capital and operating costs

Question 9

Why is the power of wind turbine shut off at the Rated power?

Answer 9

As not economically desirable to have power higher than this.
Higher powers are generated by high wind speeds which only occur a few hours in the year.
Also need to ensure turbine can withstand all the forces.

Question 10

How is wind turbine rotor shut off at rated power?

Answer 10

Power is cut off by rejecting the power in the wind before it gets into the turbine’s drivetrain for then we’d need to resist it with expensive equipment.

Question 11

What is capacity factor?

Answer 11

The ratio of (how much energy is being made) / (how much energy would be made if wind farm was operating 24/7 at max output)

Question 12

What is the potential difference between wind speeds in summer and winter?

Answer 12

Can be about double in winter as in summer

Question 13

How to read hourly mean wind speeds chart using method of bins (/Weibull distribution)

Answer 13

y-axis: Hours per year
x-axixs: Wind speed

The most common wind speed will be the point on the x-axis with the highest corresponding y-value.

Question 14

How to read binned power curve

Answer 14

y-aixs: power
x-axis: wind speed

Rated power occurs at the first x value (wind speed) for which the flat correlation occurs.

Question 15

How to find useful energy out from binned charts

Answer 15

Useful Energy OUT = each bin from energy output chart (power for that wind speed) * number of hours

Question 16

what is a streamtube?

Answer 16

A tube of constant mass flow rate

Question 17

What is the general working of a wind turbine?

Answer 17

1. the wind hits the blades
2. the blades rotate with the rotor
3. the rotor shaft rotates which is connected to the gearbox
4. the gearbox increases the speed of the shaft
5. the shaft goes into the generator, where the mech energy is converted into elec energy

Question 18

Force-Momentum theory

Answer 18

the rate of change of momentum is a force caused by the pressure difference across the rotor disk

Question 19

Limitation of momentum theory

Answer 19

When a > 0.5: momentum theory breaks down. As Wake velocity equation would become negative. An empirical modification is ∴ needed for high a values.

Question 20

What is the cut off speed of wind turbines?

Answer 20

~25m/s wind speed

Question 21

Which sides of the aerofoil have positive/negative pressure and why?

Answer 21

-ve pressure over upper surface as wind needs to accelerate to go a further way around than for the lower surface. (higher velocity -> higher Ek (Bernoullis))
+ve pressure over lower surface as wind is decelerating.

Question 22

How to resolve resultant force from aerofoil section?

Answer 22

Resolve into lift (useful) and drag forces (need to resist)

Fl = perpendicular to direction of wind
Fd = parallel to direction of wind

Question 23

How do drag and lift coefficients change with angle of attack?

Answer 23

Cl = Begins after 0 as the 𝛼 is what’s causing the lift. Rises rapidly until stall point where acceleration is too much and flow detaches from upper surface.

Cd = initially very low then suddenly rises at stall point (where flow detaches). Then increases

Question 24

How to test aerofoil for lift and drag coefs?

Answer 24

Test in wind tunnel by passing apparent wind over it. This gives the lift and drag forces which can be normalised to find Cl and Cd

Question 25

At which angle of attack does stall occur?

Answer 25

~12-13º

Question 26

What is the angle of attack?

Question 27

How does a blade stall?

Answer 27

An increase in U (free wind speed) causes 𝛼 and 𝜑 to change. If 𝛼 increases to speeds of ~12-13, the blade will stall, whereby the lift forces will suddenly drop causing Fr to also decrease.

Question 28

How to control the lift force acting upon a blade?

Answer 28

Can either reduce 𝛼 to 0, or increase 𝛼 to the stall point; both of which would cause Cl to drop, hence Fl would reduce. Rotate the blades about their axis to vary 𝛼.

Question 29

How to control thrust and rotational forces acting on a blade?

Answer 29

By either - controlling Fl and Fd (with 𝛼), or - operating rotor at a constant 𝜔. If 𝜔R is constant, then 𝜑 will automatically increase as U increases. (↑𝜑 => ↑𝛼 => stall at some point, causing Fr to drop)

Question 30

What is 𝜑?

Question 31

What does an actuator do?

Answer 31

Rotates the blades if it reads too much power in the Power transducer and controller compared to Psetpoint

Question 32

Why does the pitch regulated rotor have a cleaner and crisper power curve than for stall regulated?

Answer 32

As less power is lost as blade approaches stall.

Question 33

What is pitch regulation?

Answer 33

..... All modern turbines are variable speed and use pitch regulation.

Question 34

What are the two types of wind turbines?

Answer 34

1. Fixed Speed. Induction generator locked onto the 50Hz mains. Generates power from the torque, which is then injected into the mains. 2. Variable Speed. Uses power electronics to apply a controlled/restraining torque to the rotor shaft which controls the rotational speed.

Question 35

What does the drivetrain of a fixed speed turbine consist of?

Answer 35

Rotor with blades fixed rigidly to the hub, gearbox, simple induction generator

Question 36

Why do modern turbines operated at varying rotational speed?

Answer 36

- to operate at max power - to allow the speed to vary a little in response to gusts

Question 37

How is energy generation affected if rotor operates too slowly?

Answer 37

It would allow some wind to pass through the rotor disk without interacting with the blades. So no power would be generated from that wind

Question 38

How is energy generation affected if rotor operates too quickly?

Answer 38

The blades will chop up the wind causing turbulence which is wasted energy.

Question 39

How is energy generation affected if turbine operates at an ideal speed?

Answer 39

The blades interact with all the wind that passes through the rotor and can extract all energy possible from the wind; without any unnecessary turbulence.

Question 40

What is power coefficient in relation to turbines?

Answer 40

The fraction of wind able to extract from turbine rotor

Question 41

What is Betz limit?

Answer 41

The max value of Cp possible = 0.59 It's impossible for a system to be more efficient than this. = the best wind turbine design that could ever be obtained.

Question 42

Why would you want to control torque applied to turbine rotor?

Answer 42

Need to break the link between mains' 50Hz and speed of generator.

Question 43

How to control torque applied to turbine rotor

Answer 43

Using power electronics to vary 𝜔. - by passing all power into DC then inverting back to AC, using two power converters. (Convert 50Hx mains' AC to DC. Invert the DC back to AC where the generator is and hence the rotor.)

Question 44

How is speed controlled in variable speed rotor?

Answer 44

Indirectly control speed to control torque.

Question 45

How is pitch angle control used to shut down a variable speed rotor?

Answer 45

A Pitch Controller twists the blades to reject some of the power before it reaches the generator and needs to be resisted. Resisting the power would be costly and inefficient.

Question 46

What does the generator side converter of a variable speed rotor do?

Answer 46

Controls the rotational speed of the (synchronous) generator

Question 47

What does the Grid side converter of a variable speed rotor do?

Answer 47

Moves power from capacitor to mains at 50Hz.

Question 48

What frequency is mains power fixed at by power stations?

Answer 48

50Hz

Question 49

What is measured on the shaft of the generator in variable speed rotor?

Answer 49

Rotational speed on the shaft which provides speed of rotation of synchronous generator. This is used to control the amount of torque applied to the generator to ensure we stay on the locus; to maintain optimum operation. Wind speed can't be measured.

Question 50

What does the locus of variable speed operation show?

Answer 50

What torque to provide the rotor with for the generator speed to remain on the locus of the Power/Generator rotational speed graph. (it gives operation at peak Cp over range of wind speeds)

Question 51

What is pitch regulation?

Answer 51

Adjusting the angle of the blade to adjust 𝛼. To keep the power captured by the rotor approximately constant.

Question 52

How to find apparent wind on aerofoil?

Answer 52

Found from free wind speed (U) and rotational speed (𝜔).

Question 53

At what windspeed does the Turbine power curve start at?

Answer 53

~4m/s Usually no electricity generated below this.

Question 54

Why is a rotor shut down?

Answer 54

To avoid mechanical damage.