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Sustainable Energy Systems > L3 - Wind energy > Flashcards

L3 - Wind energy Flashcards

1
Q

Wind speed based on the beaufort scale

A
  • 0: Calm
  • 3: Light
    Start up by few turbines
  • 6: Strong
    Rated capacity reached
  • 9: gale
    Machines shut down
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2
Q

Characteristics of wind

A
  • Vary with time
  • Vary with height: frictional retardation, certain height above ground a maximum wind speed is reached
  • Affected by terrain: Zg = height at which wind speed is equal to free air (geotrophic wind speed). In urban areas, this height would be higher due to tall buildings
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3
Q

Wind speed profile calculation

A

(Wind speed at higher/Wind speed at lower) = (height at higher/height at lower)^p

  • P = wind profile exponent which is related to atmospheric stability and if in rural or urban environment
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4
Q

Selecting site for wind turbines

A

Want a site with consistent reasonable wind speeds rather than a site with occassional high wind speeds

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5
Q

Resource capture/conversion technologies

A
  • From windmills to wind energy conversion systems (WECS)
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6
Q

Wind turbines

A
  • Another term for wind energy conversion systems (WECS)
  • Classified by:
  • Horizontal axis
  • Vertical axis
  • Concentrators
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7
Q

Horizontal configuration wind turbines

A
  • Most commonly used for power generation and smaller water pump applications
  • Can have 1 blade, 2, 3 and multi
  • 3 blade most commonly used
  • Sensors on top of nacelle detect wind speed and direction and then electric motor rotates nicell to face wind
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8
Q

Vertical configuration wind turbines

A
  • Cup anenometer
  • Savonius rotor
  • Darrieus ‘egg beater’

Not very common

  • However can start at lower wind speeds and fit in narrower spaces
  • Can be less efficient and uneven distribution = bad reliability
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9
Q

Concentrator wind turbines

A
  • Not currently deployed commercially

Adv:
- Higher efficiency

Disadv:

  • More materials = higher cost
  • Greatly decreased swept area cancels out gains
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10
Q

Kinetic energy of the wind

A

ke = 1/2 mv^2

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11
Q

Mass of air per second

A

m = density * area * velocity

m = air density * volume of air flowing per second

volume of air flowing per second = area * length of cylinder of air flowing per second

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12
Q

Kinetic energy per second

A

ke/s-1 = 0.5 * density * area * velocity^3

  • in J/s
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13
Q

Power in the wind and equation analysis

A

P = 1/2 * density * area * velocity^3 * Cp

  • Cp values with wind speed for individual machines. Maximum theoretical Cp = 59%. Betz limit. eg a max Cp or 0.4 to relative to betz limit = 0.4/0.59 = 68%
  • By doubling area = double the power
  • Doubling velocity (air speed) = x8 the power potential
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14
Q

What does air density depend on

A
  • Height

- Meteorological condition

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15
Q

What does air speed (velocity) depend on

A
  • Height
  • Local topography
  • Time
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16
Q

Extraction efficiency

A

Will decrease from an optimum if:
- Blades are so close together or rotating so rapibly that a following blade moves into the turbulent air created by preceding blade

or

  • Blades so far apart or rotating so slowly that much of the air passes through the cross section of the rotor without interfering with the blade
17
Q

Economic assessment of WECS factors

A
  • Annual energy production
  • Capital costs
  • Annual capital charge rate
  • Length of the contract with the purchaser or electricity produced
  • Number of years over which the investment should be covered (often same as length of contract)
  • Operation and maintenance costs, insurance and land leasing
18
Q

Offshore vs onshore

A

Offshore:

  • Capital costs greater
  • Winds higher than land
  • More likely to utilise very large scale wind turbines

Currently, offshore more expensive but still competitive

19
Q

Environmental impacts of wind

A
  • Noise: mechanical and aerodynamic
  • Electromagnetic interferance - TV services etc
  • Visual impact
  • Birds killed (small amount)
  • Resource depletion: large turbine contains >400 t steel as well as copper and aluminium. However they are highly recyclable, less so for offshore bc of foundations
20
Q

Summary

A
  • Wind speeds are variable and depend on location and height above ground
  • Wind turbine types: Horizontal, vertical and concentrator
  • Power efficiency dependent on type of turbine
  • Costs are now economic in relation to conventional power regeneration
  • Very large scale wind farms are in existence

Sustainable Energy Systems (12 decks)

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