Wind turbines

N.B. The information herein provided is a synthesis and integration of the ELREN Training Manual content as prepared by the Tipperary Institute, Ireland on behalf of CARLOW LEADER core partner.

Wind Turbine Types & Scale

Wind turbines can generally be classified into two main configurations:
a) horizontal axis, and
b) vertical axis.

Horizontal Axis

With Horizontal Axis Wind Turbine (HAWT) the wind direction is parallel to the turbines's rotation axis.
They consist of a tower on top of which is located a rotor connected to blades. Generally, the position of the blades can be rotated so that they may face into the wind, therefore capturing the maximum amount of energy from it.
The large scale HAWT have predominantly two or three blades. These types of machines dominate the wind turbines industry for electricity production. They have evolved from the traditional wind mills with streamlined appearances as a result of improved knowledge in the area of design, aerodynamics and availability of new materials and technologies.
Smaller scale HAWT typically have multiple blades, to improve the ability to deal with high torque requirements. Such wind turbines are referred to as "high solidity turbines", with modern electricity generating wind turbines being referred to as "low solidity turbines", as they have a lower number of blades.

HAWTs in a farming field at Middenmeer, The Netherlands
Source: ELREN field visit, 15.05.2006

Vertical Axis

While not as dominant as the HAWT, Vertical Axis Wind Turbines (VAWT) present particular benefits in relation to efficiency and reduced stress loads on components. Vertical axis machines can capture the wind from any direction, therefore are not required to rotate to face into it.
The main VAWT type machine evolved from the 'egg beater' design developed in France by Darrieus and consists of a vertical shaft to which the two curved blades are attached at the top and bottom.
A variety of VAWT types have been developed afterwards, including the H-Type VAWT and V-Type VAWT.

Darrieus windmill
Source: Wikipedia (link)

Wind Turbine Size

The size of HAWT has grown considerably in the past 20 years. Due to significant developments and technological advancements the maximum size of turbine has increased to 5MW in 2006. The dramatic increase in the physical size and electricity capacity of wind turbines from 1990 to 2005 is shown in the following figure.

Source: IEA (link)

Wind Turbine Components

Source: Nordex GmbH (link)

Wind turbines can essentially be divided into a number of key components.

1. Blades. Blades are responsible for capturing the energy available in the wind.
Much of the developments in the area of blade design have evolved from the aeronautical industry.
The number of blades on a modern HAWT is defined by: a) the need to capture as much of the wind passing through the turbine, b) the size of the turbine and c) costs.
The blades on a HAWT machines with a small number of blades will rotate faster when compared to a machine with a larger number of blades. While increasing the number of blades will result in a gain in power and energy the effect is limited (a few percent increase) and does not justify the additional capital cost.
While in the past blades were constructed from wood, steel, aluminium etc., today blades are typically manufactured from composite materials which provide maximum strength with reduced weight.

2. Hub. The blades are connected to a hub. The hub provides the connection point between the blades and the rotor.

3.   Rotor & Shaft. The rotor is the term used for the blades and hub combined. This is connected to a shaft to transfer the rotational force through the gearbox and generator.

4.  Gearbox. Typically generators operate at high rotational speeds and therefore it is necessary to have a gearbox to increase the rotational speed from the rotor to match the requirements of the generator.

5.  Generator. The generator is the turbine's component which produces electricity. In large scale HAWT the generators are 3-phase AC generators, as are used in conventional power plants.
The majority of new turbines are including variable speed drives/generators in the design. Traditional generators operated efficiently at a set speed. New generator and turbine designs now allow the turbine to produce electricity at maximum efficiency at variable speeds - which suits the nature of the variable wind resource.

6.  Nacelle. The rotor, gearbox and generator are all housed in the nacelle.

7.  Tower. Wind speed varies with height and therefore to capture the maximum amount of wind the blades and nacelle are hoisted on a tower. The tower typically has a slightly conical shape and are hollow internally. Normal turbine tower heights are >40m.

8.  Controls. Wind is a variable resource and therefore the controls on the turbine are vital to maximise the amount of energy that can be captured from the wind. The controls required on a typical turbine include:
• Ability to start-up when wind speed above a certain level
• Ability to stop when wind spend exceeds safe limits
• Ability to maximise output as wind speed varies
• Ability to rotate to face into the wind direction
• Ability to monitor and control quality of electricity output to the grid.
For medium/large scale turbines two main control methods are used to control energy output:
• Pitch Control: Blades regulate power delivered by pitching the blades to reduce the lifting force of aerofoil sections
• Stall: Aerofoils sections are designed to stall as wind speed and relative flow angle increases. Power is regulated by progressive loss of rotor efficiency as stall extends.
As the size of wind turbines increases there is an increased use of pitch control as the mechanism for control.
Turbines must also rotate the nacelle so that the blades are facing into the wind direction. This is controlled by the Yaw mechanism which allows the turbines to rotate the entire nacelle based on data it receives from wind direction measurements.
Turbines will also have a mechanical brake which will be used to stop the turbine as required. This generally will work in combination with the stall/pitch controls.