1.7 Wind Turbines Flashcards
Describe two differences, other than the axis orientation, between Vertical Axis Wind Turbines (VAWT) and Horizontal Axis Wind Turbines (HAWT).
1.
2.
VAWT can operate with wind in any directionwhereas HAWT must yaw to face into the wind.
VAWT can operate at low wind speeds whereas HAWT requires higher wind speeds.
VAWT rotates at a low RPM whereas HAWT rotates at higher RPM.
VAWT is less noisy than HAWT and has less vibration than HAWT.
Describe two differences between Vertical Axis Wind
Turbines (VAWT) and Horizontal Axis Wind Turbines (HAWT).
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.
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.
*The maximum amount of the winds kinetic energy that a HAWT can convert to mechanical energy turning a rotor.
*Betz calculated this at 59.3% of the kinetic energy from the wind. Most *modern turbines however can only concert 35 – 45% of the winds energy
to electricity.
*Because of the energy losses in gear boxes etc.
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.
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).
• In addition, there will be further energy losses within the gearing and electrical components of the turbine.
Define what is meant by the term Betz Limit when applied to a wind turbine
Betz Limit: 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.
Explain how the Betz Limit is related to power efficiencies achievable by wind turbines in the real world
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
Give two reasons to explain why the rated energy output of a wind turbine is lower than the rotor collected energy
1.
2.
(i)The rotor collected energy refers to the energy in the wind utilised by the turbine blades.
(ii): The energy can be lost through inefficiencies such as energy loss between components in the turbine.
The rated energy output of a turbine can be limited by the size of the generator.
Explain the relationship between Power output and swept area for a HAWT.
The power output is directly proportional to the swept area.
If the length of the rotor blades in a HAWT is doubled, explain by what factor the shaded area will increase.
A = πr 2 therefore if r is doubled the swept area will be quadrupled
Explain the relationship between Power output and wind speed for a HAWT.
The power output increases with wind speed as Pout is directly proportional to v3.
Describe how the power output of a wind turbine is affected by the following factors;
Air density
When air density is lower the power output is less or when air density is higher the power output increases.
Describe how the power output of a wind turbine is affected by the following factors;
Temperature
When temperature is lower the turbine power output is greater or when temperature is higher the turbine power output reduces.
Explain one factor that is critical in determining the hub height of a wind turbine.
• Wind resource assessment on the site which considers average windspeed, potential obstructions to wind flow and local environmental conditions
• Topography of the site or terrain which will be analysed during a preliminary site assessment.
• Size of the turbine/blade length as the higher the hub height the greater the swept area that is possible
• Visual impact of the turbine which is dependent on size of turbine/ tower and topography of the surroundings
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.
1:
2:
Wind speed is higher as height increases from ground level. Wind velocity has a crucial impact on the power output of the wind turbine.
A larger hub height will allow for a larger blade diameter and therefore a larger swept area giving a greater power output.
A higher hub height prevents the air flow from becoming disturbed by any obstructions such as trees or buildings.
Outline two critical factors that must be taken into account when determining the hub height for a wind turbine installation.
Wind resource assessment of the site.
• Topography of the site.
• Size of the turbine / blade length.
• Visual impact of the turbine.
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;
1 – Blade length;
2 – Strength of materials;
3 – Siting requirements;
Blade length;
• 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;
• 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;
• Exposed locations provide stronger, more consistent wind.
• Obstacles (buildings / trees) can reduce performance.
• Hills facing towards prevailing winds can improve performance.
Explain how blade length and strength of materials affect turbine performance.
Blade length:
Strength of materials
Blade length:
Longer turbine blades have a greater swept area so can harness more wind power, increasing turbine performance. They may require higher cut-in wind speeds unless they are composed of lightweight/composite materials.
Strength of materials:
Strong materials are required to withstand the forces acting on the turbine blades. Stronger blades may be heavier and may need higher cut-in speeds to generate power which will reduce the efficiency/performance
of the turbine.
Define the term ‘wind survival speed’.
The maximum wind speed that a turbine is designed to withstand before sustaining damage.
State what is meant by the wind survival speed of a wind turbine
The maximum wind speed that a turbine is designed to withstand before it will sustain damage.
Wind turbines are designed with a range of power control systems. Name one power control system used in wind turbines.
Yawing
Describe the purpose of the Yaw mechanism.
Yaw mechanism ensures;
• Rotor faces the wind at all times.
• Maximum energy extraction.
Explain the function of yawing in the context of a wind turbine.
Where the turbine is turned to face into the wind in order to extract maximum energy from it.
