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4.2 Wind

Wind turbines converts the air’s motion to mechanical energy Links to an external site.. Wind is caused by differences in atmospheric pressure. Wind speeds are varied based on geography, topography and season. Therefore, there are some locations better suited than others for wind energy Links to an external site. generation . In general, wind speeds are higher near offshore and the coast, since there are fewer objects like mountains, vegetation and buildings to slow them down.

A turbine is a structure with several spinning blades. The blades are connected to an electromagnetic generator that generates electricity when the blades spin. 
(Source : https://www.studentenergy.org/topics/wind-power Links to an external site.)

How does a wind turbine work?

(Source : https://www.youtube.com/channel/UCqZQJ4600a9wIfMPbYc60OQ Links to an external site.)

For the basic theories of generators please check Chapter 3.3 Electromagnetic Theory

 

Types of wind turbines

Wind turbines are classified into two general types: horizontal axis and vertical axis.

 

  • Horizontal axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

    HAWT advantages :
     - High efficiency. Variable blade pitch, which gives the turbine blades the optimum angle of attack, so the turbine collects the maximum amount of wind energy for the time of day and season.
     - The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.

    HAWT disadvantages :
     - The tall towers and blades up to 90 meters long are difficult to transport. Transportation can now cost 20% of equipment costs. Also difficult and expensive to install. 
     - Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.
     - Their height makes them obtrusively visible across large areas.
    - HAWTs require an additional yaw mechanism to turn the blades toward the wind.
  • Vertical axis wind turbines (VAWT) have the main rotor shaft arranged vertically. With a vertical axis, the generator and other primary components can be placed near the ground. VAWT has few different types including, Darrieus wind turbine, Giromill, Savonius wind turbine, Helical Wind Turbine, etc.

    VAWT advantages :
     - No yaw mechanisms is needed.
     - Can be located nearer the ground, making it easier to maintain the moving parts.
     - Lower wind startup speeds.
     - VAWTs situated close to the ground can take advantage of locations where rooftops, mesas, hilltops, ridgelines, and passes funnel the wind and increase wind velocity.

    VAWT disadvantages :
     - Most VAWTs have a average decreased efficiency, mainly because of the additional drag that they have as their blades rotate into the wind. Versions that reduce drag produce more energy, especially those that funnel wind into the collector area.
     - Having rotors located close to the ground where wind speeds are lower.
    (Source : http://centurionenergy.net/types-of-wind-turbines Links to an external site.)

 

 

Factors affecting wind turbine efficiency

  • Air temperature The power from a wind turbine will increase almost 16% as the temperature drops from + 20° C to - 20° C for any given wind speed. Colder air is denser and increases power output.
  • Altitude Wind speed increases by 12% as the distance between the turbine and ground is doubled. Because of various atmospheric factors. Higher places also have lesser obstructions from the surrounding hills, trees and buildings.
  • Wind power The faster the wind speed and the stronger the wind force, the greater the amount of power generated by a wind turbine.
  • Blade aerodynamics The amount of lift for a given airfoil depends heavily on the angle that it makes with the direction of the relative wind, known as the angle of attack. When the angle of attack gets too large, turbulence develops and drag increases significantly, while lift is lost.
  • Blade strength When the wind turns a turbine blade, the rest of the blades are forced to move with it since they are attached to the same hub (the rotating part at the centre of the turbine), however, the weight of the blades impedes that by holding them back (this is inertia itself). It is almost as if something is dragging back on the blades and bending them, they are not very flexible, so if they are stressed too much, they break.
  • Blade weight Inertia is the same force that you have to overcome when you first start running or when you are trying to accelerate your vehicle. Wind turbine blades also have to overcome drag, which is an air force exerted on them. This is why vehicles burn extra fuel to accelerate, and when braking. Both bringing an object to a stop and moving it from a stop is a change of velocity (speed).

    Weight adds to this inertia problem, which is why heavier objects are harder to stop and harder to get moving. Typical wind turbine blades are designed to withstand the stress that normal operation would cause, so they don’t break too often, however, it is very difficult to pull this off.
    (Source : http://www.slideshare.net/windturbinesnet/factors-affecting-wind-turbine-efficiency-7146602 Links to an external site.)

 


Recommended courses :

  • coursera - Wind Energy Links to an external site. 
    Wind energy MOOC-course from DTU (Dansk teknisk universitet). 
    "You will gain a rational understanding of wind energy engineering and, through hands-on exercises, you will learn to perform wind energy calculations based on simple models. Working with the different course disciplines will give you a taste of what wind energy engineering is all about. This allows you to identify the most interesting or relevant aspects of wind energy engineering to be pursued in your future studies or in your professional career."
    • Duration : 5 weeks, 7-8 hours/week
    • Free
    • Introductory course
    • The course must be followed during a given period.

 

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