Past Paper Questions Wind

Question 1

Describe two differences between Vertical Axis Wind Turbines (VAWT) and Horizontal Axis Wind Turbines (HAWT).

Answer 1

Any two from;
• A VAWT can operate with wind in any direction whereas HAWT must yaw to face the wind.
• VAWT can operate at low wind speeds whereas HAWT requires higher wind speeds.
• VAWT rotates at low RPM whereas HAWT rotates at higher RPM.
• VAWT has rotor in vertical direction; HAWT has rotor in horizontal direction.
• VAWT is less noisy than HAWT.
• VAWT has less vibration than HAWT.

Question 2

Calculate the wind speed required to produce 17,280 joules of energy from 540 kg of air.

Answer 2

K.E.= 0.5 mv²
17280 = 0.5 x 540 x v²
v² =17280/ 0.5 x 540
=64 
= 8m/s

Question 3

Calculate the wind speed required to produce a maximum theoretical energy of 15,680 joules from 640kg of air. Show your working out in the space below.

Answer 3

K.E. = 0.5 mv²
15680 = 0.5 x 640 x v²
v²=15680/ 0.5 x 640
v²= 49
7m/s

Question 4

Define what is meant by the term Betz Limit when applied to a wind turbine and explain how it is related to power efficiencies achievable by wind turbines in the real world.

Answer 4

The maximum amount of the winds kinetic energy that a HAWT can convert to mechanical energy turning a rotor. {1}
Betz calculated this at 59.3% of the kinetic energy from the wind. {1}
Most modern turbines however can only concert 35 – 45% of the winds energy to electricity. {1}
Because of the energy losses in gear boxes etc. {1}

Question 5

For a rotor diameter of this size and with a wind speed of 11m/s the maximum available rated energy in the wind is 16.4 kW. Identify two reasons which explain why there is an energy shortfall between the maximum energy available in the wind and the actual rated energy output of the turbine.

Answer 5

Because a significant portion of the available wind energy has to pass through the blades and is unavailable for energy conversion (i.e. the Betz limit). {1}
• In addition there will be further energy losses within the gearing and electrical components of the turbine. {1}

Question 6

Explain the relationship between Power output and swept area for a HAWT.

Answer 6

The power output is directly proportional to the swept area

Question 7

If a turbine has a rotor diameter of 6.0m calculate the rotor swept area for the turbine. Show your working out in the space below.

Answer 7

Radius = 1/2 x diameter = 3m
Swept area = pie r²
A= pie x 3² = 28.27m²

Question 8

Describe how the power output of a wind turbine is affected by the following factors;
Question (I); Air density.

Answer 8

When air density is lower the power output is less or when air density is higher the power output increases.

Question 9

Describe how the power output of a wind turbine is affected by the following factors;
Question (I); Temperature

Answer 9

When temperature is lower the turbine power output is greater or when temperature is higher the turbine power output reduces.

Question 10

Outline two critical factors that must be taken into account when determining the hub height for a wind turbine installation.

Answer 10

Any two from;
• Wind resource assessment of the site. {1}
• Topography of the site. {1}
• Size of the turbine / blade length. {1}
• Visual impact of the turbine. {1}

Question 11

A turbine has a mass of 10 tonnes.

In the space below, show that is the blade length is doubled the new mass of the turbine will be 80 tonnes.

Answer 11

1: Existing mass (10 tonnes) is proportional to r³
2: If we double blade length to 2R then
New mass (x) is proportional to (2R)³
3: Which is 8R³
4: If R corresponds to a mass of 80 tonnes

Question 12

John is considering a wind turbine to power his home. Describe two ways in which the performance of his turbine could be influenced by each of the following factors; {6}
1 – Blade length;

2 – Strength of materials;

3 – Siting requirements;

Answer 12

Blade length; {2} Any 2 from; • Longer blades could generate more power than short blades due to larger swept area. • Longer blades may need stronger wind speeds to generate power. • Longer blades can increase the stresses within the turbine.

Strength of materials; {2} Any 2 from; • Lightweight balde materials may be too weak and may break. • Stronger, heavier blades need stronger wind speeds to generate power.
• Composite materials can provide a good mix of strength and weight. • Turbine blades need to resist corrosion / rust.

Siting requirements; {2} Any 2 from; • Exposed locations provide stronger, more consistent wind. • Obstacles (buildings / trees) can reduce performance. • Hills facing towards prevailing winds can improve performance.

Question 13

Wind turbines are designed with a range of power control systems. Name one power control system used in wind turbines.

Answer 13

Yawing.

Question 14

Describe the purpose of the Yaw mechanism.

Answer 14

Rotor faces the wind at all times. {1}

• Maximum energy extraction.