Chapter 7 And 8 : Energy From Wind And Sun

Question 1

Describe two differences, other than the axis orientation, between Vertical Axis Wind Turbines (VAWT) and Horizontal Axis Wind Turbines (HAWT). (2 marks each)

Answer 1

VAWT can operate with wind in any direction [1] whereas HAWT must yaw to face into the wind [1].

VAWT rotates at a low RPM [1] whereas HAWT rotates at higher RPM [1].

Question 2

Describe two differences between Vertical Axis Wind
Turbines (VAWT) and Horizontal Axis Wind Turbines (HAWT). (2)

Answer 2

A VAWT can operate with wind in any direction whereas HAWT must yaw to face the wind. (1)

VAWT rotates at low RPM whereas HAWT rotates at higher RPM. (1)

Question 3

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. (4)

Answer 3

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. (1)

Question 4

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. {2}

Answer 4

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 5

Define what is meant by the term Betz Limit when applied to a wind turbine. {2}

Answer 5

The maximum amount of the wind’s kinetic energy that a HAWT can convert to mechanical energy turning a rotor

Betz calculated this at 59.3% of the kinetic energy of the wind.

Question 6

Explain how the Betz Limit is related to power efficiencies achievable by wind turbines in the real world. {2}

Answer 6

Most modern wind turbines can only convert 35–45% of the wind’s energy into electricity

This is because of energy losses in gearboxes, generators, etc.

Question 7

Define the term ‘Rotor Collected Energy’. {1}

Answer 7

The rotor collected energy refers to the energy in the wind utilised by the turbine blades. {1}

Question 8

Give two reasons to explain why the rated energy output of a wind turbine is lower than the rotor collected energy. (2 marks each)

Answer 8

The energy can be lost through inefficiencies such as energy loss between components in the turbine. {2}

The rated energy output of a turbine can be limited by the size of the generator. (2)

Question 9

Use the Betz limit to calculate the maximum theoretical limit of kinetic energy that can be converted by the turbine from 43000 J of wind energy. (2)

Answer 9

43,000 x 0.593 = 25,499J

Question 10

Explain the relationship between Power output and swept area for a HAWT. (1)

Answer 10

The power output is directly proportional to the swept area. (1)

Question 11

If the length of the rotor blades in a HAWT is doubled, explain by what factor the shaded area will increase. {2}

Answer 11

A = πr 2 therefore if r is doubled the swept area will be quadrupled (2)

Question 12

Explain the relationship between Power output and wind speed for a HAWT. (1)

Answer 12

The power output increases with wind speed as Pout is directly proportional to v3. {1}

Question 13

Describe how the power output of a wind turbine is affected by the following factors;

Air density (1)

Temperature (1)

Answer 13

When air density is lower the power output is less or when air density is higher the power output increases. {1}

When temperature is lower the turbine power output is greater or when temperature is higher the turbine power output reduces. {1}

Question 14

Explain one factor that is critical in determining the hub height of a wind turbine. {2}

Answer 14

Visual impact of the turbine [1] which is dependent on size of turbine/ tower and topography of the surroundings [1]

Question 15

A local quarry owner has applied for planning permission to install a Horizontal Axis Wind Turbine. The Planning Service has requested that the hub height of the turbine be lowered

Discuss two reasons why this will have a detrimental impact on the power output of the turbine. (2 marks each)

Answer 15

Wind speed is higher as height increases from ground level. Wind velocity has a crucial impact on the power output of the wind turbine.(2)

A larger hub height will allow for a larger blade diameter and therefore a larger swept area giving a greater power output. (2)

Question 16

Outline two critical factors that must be taken into account when determining the hub height for a wind turbine installation. {2}

Answer 16

1. Visual impact
2. Size of the turbine/blade length

Question 17

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 17

Blade length: (2)

• Longer blades could generate more power than short blades due to larger
swept area.
• Longer blades may need stronger wind speeds to generate power.

Strength of materials; (2)

Stronger, heavier blades need stronger wind speeds to generate power. (1)
Turbine blades need to resist corrosion / rust. (1)

Siting requirements

Exposed locations provide stronger, more consistent wind. (1)
Obstacles (buildings / trees) can reduce performance. (1)

Question 18

Explain how blade length and strength of materials affect turbine performance. (2 marks each)

Answer 18

Blade length:
Longer turbine blades have a greater swept area so can harness more wind power, increasing turbine performance. [1] They may require higher cut-in wind speeds unless they are composed of lightweight/composite materials. [1]

Strength of materials:
Strong materials are required to withstand the forces acting on the turbine blades. [1] Stronger blades may be heavier and may need higher cut-in speeds to generate power which will reduce the efficiency/performance
of the turbine. [1]

Question 19

Define the term ‘wind survival speed’. {1}

Answer 19

The maximum wind speed that a turbine is designed to withstand before sustaining damage. (1)

Question 20

Wind turbines are designed with a range of power control systems. Name one power control system used in wind turbines. {1}

Answer 20

Yawing (1)

Question 21

Describe the purpose of the Yaw mechanism. {2}

Answer 21

• Rotor faces the wind at all times. {1}
• Maximum energy extraction. {1}

Question 22

Explain the function of yawing in the context of a wind turbine. {2}

Answer 22

Where the turbine is turned to face into the wind in order to extract maximum energy from it. (2)

Question 23

Explain how an automated tracking system can maximise energy output from solar devices. (3)

Answer 23

• Daily variations of the position of the sun during day light hours (earth spinning on its own axis).

• Annual variations in the position of the sun in the sky depending on the season / time of the year (Earths elliptical orbit around the Sun).

• Tracking device must be able to change tilting angle and rotate on its own axis to achieve optimum tracking.

Question 24

Outline two ways in which automated solar tracking can maximise the energy output from solar collectors. {2}

Answer 24

• It can tilt and rotate on its own axis to achieve optimum tracking.

• It can adjust according to location in northern or southern hemispheres.

Question 25

State one advantage provided by an evacuated tube solar collector compared to a flat plate solar collector. {1}

Answer 25

It eliminates conduction losses (1)

Question 26

Explain the operation of a flat plate solar collector. {2}

Answer 26

The suns energy is captured by the absorber plate {1} and transferred to the water which heats up a tank in a house {1}.

Question 27

Name one other type of solar thermal collector. {1}

Answer 27

Evacuated tube solar collector. {1}

Question 28

Compare the operation of the solar thermal collector with an evacuated tube solar thermal collector (6)

Answer 28

Both solar collectors transfer solar energy to water in a storage tank [1] via a pipe network containing a fluid, often water. [1]

Evacuated tube solar collectors are more efficient than flat plate solar collectors. [1] A smaller area of evacuated tube solar collectors is required to deliver the same heat energy as flat plate solar collectors. [1]

In a flat plate collector, the absorber plate is in direct contact with the pipe network. [1] In an evacuated tube collector, the absorber plate is contained in a vacuum tube and is not in direct contact with the pipe network. [1]

Question 29

State three factors which should be considered when calculating the roof area required to install flat plate thermal solar panels on a house. {3}

Answer 29

• Solar radiation levels of site / roof. (1)

• Family size and hot water requirements. (1)

• Shading. (1)

Question 30

Identify two factors, other than cost, that should be taken into consideration by the occupants when deciding whether to install the solar collector. {2}

Answer 30

Solar radiation levels [1]

Shading [1]

Question 31

Explain the main benefit to households of installing a flat plate solar collector. {1}

Answer 31

Economic reasons. Cost more important in the application than efficiency. {1}

Question 32

State one benefit to households of installing a flat plate
collector. (1)

Answer 32

• Financial benefit – reducing energy costs. (1)