What is wind energy?

Wind energy is an indirect form of solar energy. Wind is a stream of air caused to move by the uneven heating of the earth's surface. On a global scale, warm air rises from the areas near the equator to be replaced by cool air from the poles. About 1 - 2 per cent of the total solar radiation reaching the earth is converted into wind energy.

This air movement is complicated by a number of global scale factors such as the Earth's rotation, continents, oceans and mountain ranges and on a local scale by hills, vegetation and lakes. Air flow is rarely smooth, with most places experiencing fairly rapid changes in wind speed and direction. The wind speed also increases with the height above the ground, due to the frictional drag of the ground, vegetation and buildings. It is clear that any plans to harness the wind must take into account these variables.

The best known machine is the windmill which gives its name to any wind machine whether it mills (grinds) or not. Wind machines convert the kinetic energy of the wind to mechanical power. The impact of the air stream turns a set of sails or blades attached to a shaft. The rotational force can be applied to do work in a mill or turn the rotor in a generator. The general name for the types of machines which generate electricity is a turbine.

How do wind turbines work?

A wind turbine consists of blades attached to a rotor which drives an electric generator through some type of transmission. In the more common type of turbine, the transmission, generator, and various controls are housed inside a nacelle which is mounted on a tower to position the turbine at an appropriate height.

The multi-bladed turbine is the most common type of wind turbine. The number of blades can vary and determines such properties as the wind speed required to start the turbine and the speed of rotation for a given wind speed. Generally, most machines have three blades as they are more stable and have fewer vibration problems. Peak efficiency also occurs at a lower tip speed with three blades rather than two. 

Various materials have been used for turbine blades including wood, aluminium, fibreglass and steel. Fibreglass and composites tend to have improved resistance to fatigue and lighter materials place less stress on the hubs and bearing of larger machines.

Horizontal axis wind turbines must be mounted on towers in order to raise the blades above the ground. For a small turbine this is relatively simple, but for larger turbines the tower is a very large structure and forms a significant portion of the total cost of the installation.

Horizontal-axis turbines must face into the wind or be down wind for successful operation. The whole turbine must therefore be able to rotate and follow changes in wind direction. For a small wind machine this is easily achieved by using a tail vane. Large machines require more complex equipment.

Vertical-axis turbines are available but are less common than the horizontal-axis variety due to there lower aerodynamic efficiency.

What is a wind farm?

A wind farm is the name used for any group of wind turbines in one location that are connected electrically. Each wind turbine acts independently and the electricity generated flows through common cabling out into the grid. The turbines are usually arranged to maximise use of the wind available and placed sufficiently far apart to avoid interference with one another.

Links

Australian Wind Energy Association: www.auswind.org .