L3 - Wind energy Flashcards
Wind speed based on the beaufort scale
- 0: Calm
- 3: Light
Start up by few turbines - 6: Strong
Rated capacity reached - 9: gale
Machines shut down
Characteristics of wind
- 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
Wind speed profile calculation
(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
Selecting site for wind turbines
Want a site with consistent reasonable wind speeds rather than a site with occassional high wind speeds
Resource capture/conversion technologies
- From windmills to wind energy conversion systems (WECS)
Wind turbines
- Another term for wind energy conversion systems (WECS)
- Classified by:
- Horizontal axis
- Vertical axis
- Concentrators
Horizontal configuration wind turbines
- 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
Vertical configuration wind turbines
- 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
Concentrator wind turbines
- Not currently deployed commercially
Adv:
- Higher efficiency
Disadv:
- More materials = higher cost
- Greatly decreased swept area cancels out gains
Kinetic energy of the wind
ke = 1/2 mv^2
Mass of air per second
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
Kinetic energy per second
ke/s-1 = 0.5 * density * area * velocity^3
- in J/s
Power in the wind and equation analysis
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
What does air density depend on
- Height
- Meteorological condition
What does air speed (velocity) depend on
- Height
- Local topography
- Time
Extraction efficiency
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
Economic assessment of WECS factors
- 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
Offshore vs onshore
Offshore:
- Capital costs greater
- Winds higher than land
- More likely to utilise very large scale wind turbines
Currently, offshore more expensive but still competitive
Environmental impacts of wind
- 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
Summary
- 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
