Week 5

Question 1

What causes wind?

Answer 1

Wind originates when the sun heats the earth. The majority of this heat is received at the equator causing higher temperature than at the north
and south poles. The temperature differences cause convection currents
with the air ascending over the equator and descending at the poles.

Question 2

Closer to ground level, what can wind be affected by?

Answer 2

• Roughness of the surface of the earth.
• Obstacles (e.g. hills) that affect the wind in a non-linear way.
• Monsoons, sea breezes (local temperature change over land and sea).
• Mountain winds (caused by local temperature changes in valleys)

Question 3

Given general terrains on the earths surface rank their roughness from least to most rough.

Answer 3

Oceans and seas
Deserts
Grasslands
Rainforests
Mountains

Question 4

With consideration to an aerofoil section, In what direction does the lift force act with reference to the air stream?

Answer 4

It acts perpendicular to the air stream caused by the Bernoulli effect.

Question 5

With consideration to an aerofoil section, In what direction does the drag force act with reference to the air stream?

Answer 5

In line with the direction of the air stream

Question 6

How is the thrust force calculated?

Answer 6

The thrust force is the resultant of the lift and drag forces.

Question 7

Define pitch

Answer 7

Pitch refers to turning the angle of attack of the
blades of a turbine into or out of the wind to control the production or absorption of power. Wind turbines use this to adjust the rotation
speed and the generated power.

Question 8

Define chord in terms of an aerofoil

Answer 8

Chord refers to the imaginary
straight line joining the leading and trailing edges of an aerofoil. The chord length is the distance between the
trailing edge and the point on the leading edge where the chord intersects the leading edge

Question 9

Discuss the concept of ‘stall.’ Indicate how wind turbine designers can utilise this.

Answer 9

By tilting the body of the profile back, the lift increases. However in excess as it is tilted backwards the air flow on the upper surface stops sticking to the surface of the wing and flow separation occurs. Lift from the low pressure of the upper surface disapears and stall occurs.
Stall can be taken advantage to stop the wind rotor from rotation during gusty conditions.

Question 10

What is Betz’s Law?

Answer 10

Betz’s law indicates the maximum power that can be extracted from the wind,
independent of the design of a wind turbine in open flow.
According to Betz’s law, no turbine can capture more than 16/27 (59.3%) of the kinetic energy in wind.

The factor 16/27 (0.593) is known as Betz’s coefficient. Practical utility-scale wind
turbines achieve at peak 75% to 80% of the Betz limit.

Question 11

In the main, why is the three blade turbine preferred to a 2-blade turbine?

Answer 11

Due to its appearance and better ‘nonsymmetrical’
distribution of structural forces which gives a smaller dynamic response in realistic non – uniform wind conditions
2-bladed turbines demand a higher cost and complexity in the main

Question 12

In what situations would a two blade turbine most likely be utilised and why?

Answer 12

For offshore applications; because the rotor is

easier to transport and lift.

Question 13

What are the advantages and disadvantages of Upwind Machine wind turbines?

Answer 13

(Upwind machines have the rotor facing the wind)
Advantage: avoids the wind shade behind the tower.
Disadvantage: the rotor needs to be made rather inflexible, and placed at
some distance from the tower. In addition an upwind machine needs a yaw mechanism to keep the
rotor facing the wind.

Question 14

What are the advantages and disadvantages of Downwind Machine wind turbines?

Answer 14

Advantage: they may be built without an active yaw mechanism, if the rotor and
nacelle have a suitable design that makes the nacelle follow the wind passively. They also can have a more flexible rotor meaning the blade will bend at high wind speeds taking load off the tower. It can therefore be built lighter than an Upwind machine
Disadvantage: the fluctuation in the wind power due to the rotor passing through the wind shade of the tower.
This may give more fatigue loads on the turbine than with an upwind design

Question 15

What is the yaw mechanism used for?

Answer 15

To ensure that the rotor is

normal to the wind.

Question 16

What is an active Yaw system in a wind turbine?

Answer 16

The active yaw
systems are equipped with some sort of torque producing device able to
rotate the nacelle of the wind turbine against the stationary tower based on automatic signals from wind direction sensors or manual actuation
(control system override)

Question 17

What is an passive Yaw system in a wind turbine?

Answer 17

The passive yaw
systems utilise the wind force in order to adjust the orientation of the wind turbine rotor into the wind.
Therefore, the power of
the wind is responsible for the rotor rotation and the nacelle orientation

Question 18

What type of yaw systems will different turbines use?

Answer 18

Upwind wind turbines are equipped with an active yaw system
Upwind wind turbine equipped with a passive yaw system
Downwind wind turbine equipped with a passive yaw system.

Question 19

Why are rotor blades for large wind turbines always twisted?

Answer 19

To achieve an optimal flow ‘angle of attack’ throughout the length of the blade

Question 20

What are the advantages of HAWT?

Answer 20

• Minimal actual ground
space taken up by the
turbine bases.
• available "off the shelf"
• Relatively quick
construction and
installation.
• 25 year lifetime
• Simple decommissioning
• Well established
technology (20,000 grid
connected turbines
worldwide).
• Government backing

Question 21

What are the advantages of large turbines?

Answer 21

• Economies of scale,
cheaper electricity.
• Very well suited for
offshore wind power.
• Large turbines with a tall
towers use existing wind
resource more efficiently.

Question 22

What are the advantages of small turbines?

Answer 22

• Useful for remote areas.
• Low installation costs.
• Less Imposing on Landscape.
• Less fluctuation in the
electricity output from a
wind farm with a number
of smaller machines.

Question 23

Why can offshore wind turbines generate more power than similar land-based ones?

Answer 23

the surface roughness is low and so wind speeds are higher

Question 24

What are the different types of offshore turbine foundations? Give a brief description of each

Answer 24

Gravity – Reinforced concrete foundation floated to site then weighed down with sand and gravel

Mono Pile – Steel pile driven 10-20m into seabed – no seabedpreparations needed but not suitable for rocky locations

Steel Gravity foundation – Similar to concrete design but sits on a steel box on the sea floor.

Tripod (steel jacket) – Frame with three piles driven into sea bed – for larger depths.

Question 25

What are some of the challenges presented with the deployment of deep offshore technologies?

Answer 25

Technical and Infrastructure: Resource assessment, grid connection and wind farm
operation

Economic: Installation, operations and maintenance

Question 26

Describe a spar buoy deep offshore foundation

Answer 26

a very large cylindrical buoy stabilises the wind turbine using ballast. The centre of gravity is much lower in the water than the centre of buoyancy. Whereas the lower parts of the structure are heavy, the upper parts are usually empty elements near the surface, raising the centre of buoyancy.

Question 27

Describe a tension leg platform (TLP) deep offshore foundation

Answer 27

a very buoyant structure is semi submerged. Tensioned mooring lines are attached to it and anchored on the seabed to add buoyancy and stability

Question 28

Describe a semi-submersible deep offshore foundation

Answer 28

combining the main principles of the two previous designs, a semi submerged structure is added to reach the necessary stability. Wind- Float uses this technology