Wind

Question 1

Describe two differences between VAWT and HAWT. {2}

Answer 1

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.

Question 2

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 2

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 3

What is the KE equation?

Answer 3

KE = 1/2 mv2

Question 4

For a rotor diameter of 5.6m 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 5.2Kw. {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

Explain the relationship between Power output and swept area for a HAWT. {1}

Answer 5

The power output is directly proportional to the swept area. {1}

Question 6

What is the equation for swept area?

Answer 6

A = pi x r2

Question 7

Explain the relationship between Power output and wind speed for a HAWT. {1}

Answer 7

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

Question 8

What is the cut in speed?

Answer 8

The wind speed at which the turbine begins to turn and generate electricity.

Question 9

What is the cut out speed?

Answer 9

The wind speed at which the turbine stops rotating in order to protect itself from damage.

Question 10

Describe how the power output of a wind turbine is affected by Air density. {1}

Answer 10

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

Question 11

Describe how the power output of a wind turbine is affected by Temperature. {1}

Answer 11

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

Question 12

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

Answer 12

Wind resource assessment of the site. {1}

Topography of the site. {1}

Size of the turbine / blade length. {1}

Question 13

Describe two ways in which the performance of a turbine could be influenced by Blade length.

Answer 13

Longer blades may need stronger wind speeds to generate power.

Longer blades can increase the stresses within the turbine.

Question 14

Describe two ways in which the performance of a turbine could be influenced by the Strength of the Materials.

Answer 14

Lightweight blade materials may be too weak and may break.

Stronger, heavier blades need stronger wind speeds to generate power.

Question 15

Describe two ways in which the performance of a turbine could be influenced by the Siting requirements.

Answer 15

Exposed locations provide stronger, more consistent wind.

Obstacles (buildings / trees) can reduce performance.

Question 16

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

Answer 16

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

Question 17

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

Answer 17

Yawing. {1}

Question 18

Describe the purpose of the Yaw mechanism. {2}

Answer 18

Rotor faces the wind at all times. {1}

Maximum energy extraction. {1}