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Environmental Technology > Ch 7: Energy From The Wind > Flashcards

Ch 7: Energy From The Wind Flashcards

1
Q

What are the attributes of a HAWT?

A
  1. Requires a relatively constant wind direction.
  2. High speed required.
  3. High RPM.
  4. High vibration.
  5. High noise levels.
  6. Horizontal plane rotor axis.
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2
Q

What are the attributes of a VAWT?

A
  1. Any wind direction.
  2. Low speeds required.
  3. Low RPM.
  4. Low vibration.
  5. Low noise levels.
  6. Vertical plane rotor axis.
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3
Q

What are the main components of a HAWT?

A

Blades
Hub
Low speed shaft
High speed shaft
Wind vane
Anemometer
Gearbox
Generator
Tower
Rotor
Nacelle

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4
Q

What is the equation for kinetic energy?

A

Ke = 1/2 mv2

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5
Q

What does betz limit mean?

A

It’s an expression for the maximum proportion of energy that can be extracted from the wind in a wind turbine.

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6
Q

What is the the underlying principles of the betz limit?

A

The air coming into the swept area has a high velocity and takes up a small area. When the wind causes the blades and hub to turn, it looses energy and so has less energy downstream, so the velocity of the air downstream must be lower than the velocity up stream.

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7
Q

What is the equation for betz limit?

A

Velocity upstream + Swept area upstream = Velocity downstream + Swept area downstream

Vu + Au = Vd + Ad

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8
Q

What happens when a turbine has only 1 blade?

A

Most of the air would pass straight through the swept area, as it does not collide with the blade. The downstream velocity and swept area are not very different from upstream so this turbine would not be effective.

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9
Q

What happens if a turbine has an infinite number of blades?

A

These blades will collectively act as a solid disk. The velocity of air downstream is now 0. This acts like a blockage preventing any further air from flowing through the system from upstream so there would be no energy extracted from the wind.

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10
Q

What is the upper limit of the quantity of energy that can be extracted from wind?

A

59.3% when theoretically can never be reached.

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11
Q

What is the typical efficiency of a turbine?

A

Between 25% and 45%
As there are always sound and thermal losses from the wind energy.

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12
Q

What is the definition of rotor collected energy?

A

The exact amount of energy extracted from the wind.

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13
Q

What is the 1st reason for the upper betz limit not being reached?

A

Mechanical: Friction
Any moving parts rub against each other, causing some of the kinetic energy to change to thermal energy which increases the temperature of the material.

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14
Q

What is the 2nd reason for the betz limit not being reached?

A

Electrical: Resistance
Any current moving through a cable will encounter resistance. This causes the wire to heat up and so some electrical energy is converted to thermal energy.

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15
Q

What variables have direct proportionality to power out?

A

P out and A
P out and v cubed
P out and d squared
P out and p

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16
Q

What is meant by the cut in speed?

A

The minimum wind speed required to produce power.

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17
Q

What is meant by the rated power?

A

The max power that a wind turbine can produce.

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18
Q

What is the relationship between high temperature and air density?

A

A high temperature means that the molecules of the gases in air have more kinetic energy and so move with a greater speed between collisions. This high temperature gives a low air density as there is less mass per unit volume. So high temperature = low P out.

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19
Q

What is the relationship between low temperature and air density?

A

A low temperature means the molecules of the gases in air have less kinetic energy and so move with smaller speeds between collisions. This low temperature gives a higher air density as there is a higher mass per unit volume. So low temperature = high P out

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20
Q

What is meant by the term altitude?

A

How high a specific point is above the surface of the Earth.

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21
Q

What is the relationship between high altitude and air density?

A

At high altitude the air is thinner and so contains lower amounts of oxygen per cubic metre of air. So higher altitude means lower air density.

22
Q

What is the relationship between lower altitude and air density?

A

At lower altitude the air density is high and there has been no separation of molecules.

23
Q

What is a CBA (Cost Benefit Analysis)?

A

It enables a system analysis of a project to establish if the project is economically feasible. The aim of the CBA is to establish if the project will produce a satisfactory return for the investor.

24
Q

What are the parameters that affect the CBA?

A
  • Wind speed
  • Air density
  • COP
  • Swept area
  • Obstructions
  • Hub height
  • Temperature
  • Altitude
25
Q

How can you calculate the CBA of a project?

A

Repayment period in years = projects cost / revenue produced per year

26
Q

What is the hub height of a wind turbine?

A

The distance from the ground to the centre of the hub.

27
Q

What is the rotor diameter of a wind turbine?

A

The diameter of the swept area of the wind turbine.

28
Q

What are the 4 factors that are critical when determining the hub height of a wind turbine?

A
  1. Wind resource assessment
  2. Terrain / topography
  3. Wind turbine size
  4. Visual impact
29
Q

How is wind resource assessment a factor when determining hub height?

A

It considers average wind speed, potential obstructions to wind flow and local environmental conditions.

30
Q

How is terrain/topography a factor when determining hub height?

A

It will be analysed during a preliminary site assessment.

31
Q

How is size of the turbine a factor when determining hub height?

A

As the higher the hub height the greater the swept area that is possible.

32
Q

How is visual impact a factor when determining hub height?

A

It is dependent on the size of the turbine/tower and topography of the surroundings.

33
Q

What is meant by topography?

A

This is the shape of the land so elevations, slopes and geological characteristics.

34
Q

How can a turbine’s performance be influenced by the blade length?

A

Longer blades could generate more power than shorter blades due to larger swept area. But longer blades may need stronger wind speeds to generate power. Longer blades can increase the stresses within the turbine.

35
Q

How can a turbine’s performance be influenced by the strength of the material?

A

Lightweight blade materials may be too weak and may break. Stronger, heavier blades need stronger wind speeds to generate power.

36
Q

How can a turbine’s performance be influenced by siting requirements?

A

Exposed locations provide stronger, more consistent wind. Obstacles can reduce performance. Hills facing towards prevailing winds can improve performance.

37
Q

What is wind survival speed?

A

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

38
Q

What is destructive testing?

A

Where the product will be tested under extreme conditions until it fails. Allows the maximum wind speed to be established in which the turbine will break.

39
Q

What is pitching?

A

A method of power control in which the blades of the turbine change orientation relative to the wind direction, to control the rate of power production.

40
Q

What is feathering?

A

The method used to shut the wind turbine down, this is done when the blades orientate themselves such that they are parallel to the incoming wind direction. The wind no langer causes the rotor to turn, hence no power produced.

41
Q

What is passive pitching?

A

Pitching method used for small scale wind turbines which keeps production costs to a minimum. They naturally ‘adapt’ at higher wind speeds so slightly twist the blades when under higher wind loads.

42
Q

What is active pitching?

A

Method of pitching used for bigger scale projects achieving maximum energy production for all higher wind speeds. Uses hydraulic systems or an electric motor. A signal from the anemometer at the back of the nacelle is sent to the pitching system which will precisely adapt the angle of pitch at higher wind speeds.

43
Q

What is yawing?

A

A technique that can be utilised to offer power control. This technique ensures the wind turbine faces into the wind so that maximum energy is extracted from the wind.

44
Q

What is the purpose of yaw mechanism?

A

It ensures the rotor faces the wind at all times and maximum energy extraction.

45
Q

What are the issues affecting energy output of a turbine?

A
  • Topography
  • Orientation of the site
  • Wind speeds
  • Air density and altitude
46
Q

What are the issues affecting the cost of a wind turbine?

A
  • Installation costs
  • Grid connection costs (depends how far the project is from the nearest transformer, £65,000 per km)
  • Servicing
  • End of life costs (disposal and transportation of the turbine when finished with)
  • Surveys (for noise, shadow flicker and wildlife)
47
Q

What are the environmental and social issues of a wind turbine?

A
  • Visual issues
  • Noise pollution (noise assessment to see how close it is to nearby houses)
  • Communication links (might affect the communication if near certain zones)
  • Environmental impact assessment (prevent habitat destruction)
  • Local objections
48
Q

What is meant by the term Betz limit when applied to a wind turbine and how is it related to power efficiencies achievable by wind turbines?

A
  • 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 convert 35-45% of the winds energy to electricity.
  • Because of the energy losses in the gear boxes.
49
Q

Why is the actual rated energy output of a wind turbine lower than the maximum energy available in the wind?

A
  • Because a significant portion of the available wind energy has to pass through the blades and is unavailable for energy conversion (the Betz limit).
  • In addition, there will be further energy losses within the gearing and electrical components of the turbine.
50
Q

Why is the rated energy output of a wind turbine lower than the rotor collected energy?

A
  • The energy can be lost through inefficiencies such as energy losses within between components in the turbine.
  • The rated energy output of a turbine can be limited by the size of the generator.
51
Q

What is the rated power of a wind turbine?

A

The power limit of the electrical generator (the max power on the curve).

Environmental Technology (7 decks)

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