What is wind energy?
Wind energy is a form of solar energy. About 1% to 2% of the energy coming from the sun is converted into wind energy. Generation of air currents is a direct effect of the combination of two phenomena: circulation of hot air and Earth rotation. Hot air is lighter than cold air, and will rise into the sky until it reaches approximately 10-km (6 miles) altitude and then spread to the North and the South. The equator represents a low-pressure area. At the Poles, there will be high pressure due to the cooling of the air.
- Since Earth is rotating, any movement on the Northern Hemisphere is diverted to the right, (In the Southern Hemisphere it is bent to the left). This apparent bending force is known as the Coriolis force. Riverbeds are dug deeper on one side than the other is. (Which side depends on which hemisphere we are in). The combination of these two factors gives rise to the more complex movement of air, known as wind.
- This so-called geostrophic wind, a stable air stream from West to East is, however, disturbed by the turbulent phenomenon which we call weather. Clouds formed by the cooling of moist air cast shadow on the ground, thus leading to differences of the surface temperature, which in turn give rise to air movements with fairly chaotic patterns. The roughness of the Earth’s surface increases the turbulence of these air movements and reduces the wind speed.
How does it work?
- Wind energy offers a completely clean form of energy production. No waste is produced and the very source of energy, air motion generated by solar radiation, is everlasting!
Wind blows and sets the blades of a turbine in motion, generating power that can be converted into electricity. Turbines can be located on land, preferably close to the shoreline, where higher wind speeds and more steady wind conditions are available or off shore, in shallow waters near the coastline, where wind conditions are even better.
How are wind turbines made?
- The structure of a wind turbine is rather simple: a steel or concrete tower is equipped with a kind of machine house (called nacelle) on top, which is turning around a vertical axis in a way that the rotor (usually equipped with two or three blades) always faces the wind. Many turbine manufacturers make special efforts for the design of their turbines to allow them to blend well into the landscape. Most wind turbines have three blades, a more aesthetically appealing set-up than two-blade turbines, which are, however, lighter and easier to assemble and might, therefore, be preferred for offshore installations.
What is the principle on which wind turbines operate?
When wind flows, the rotor turns and drives an electric generator. The generation of electric power depends on wind speed, area of the rotor and air density in the following way:
- cube of wind speed, i.e. double wind speed gives eight times more power
- square of rotor diameter: i.e. double rotor diameter gives four times more power
- density of the air: if e.g. the air is 10 C colder, density - and power production – increase by about 3 %, while moist air (containing high amounts of water vapour) is less dense and will thus decrease power production.
What are the current uses of wind technology?
- Wind turbines have become a mature state-of-the-art technology during the last decade, and wind farms are operating actively all across Europe.
- Since 1998 world-wide production of wind power has increased at a rate of up to 40% per year, creating jobs for over 70000 people and providing electricity for 35 million people.
- In 2001, 70% of wind installation were in Europe and at the moment European companies produce over 80% of the wind turbines.
Although wind is intrinsically intermittent, it is still possible to meet at least 10 to 15% of a country’s needs. More could be achieved, but that would require extra storage systems, which at the moment would not be a commercially advantageous step to take.
Overview of the technology
How does a wind turbine work?
The rotor of a wind turbine is similar to the rotor of a helicopter. Horizontal rotation of the helicopter’s rotor creates a downward air stream, which lifts the helicopter. In the case of the wind turbine a horizontal air stream (the wind) causes the rotor to turn around its horizontal axis.
Wind turbines for electricity generation come in all sizes, and they include:
- A rotor with blades which convert the wind’s energy into rotational movement
- A nacelle (machine house) containing the rotor shaft linked to a generator by a drive train, often including a gearbox
- A tower to support the nacelle and to put it at a sufficient height above ground to make use of the higher wind speeds usually available further above ground and to keep the rotor blades at a safe distance above the ground
- Electronic equipment for the general control of the turbine
The blades are mounted on a central hub connected to the generator. Typical rotor diameters range from 30 to 65 meters, and the tower height is between 25 and 80 meters. Rotational mechanical energy is converted into electricity through the generator. Usually wind turbines start to operate at wind speeds of 4 to 5 metres per second and reach their maximum power at around 15 meters/second. At even higher wind speeds the turbines are shut down to avoid damage to the structure by excessive mechanical load.
As the technology is by now quite mature, wind turbines can be expected to function for a period of 20-25 years. High reliability is especially important for offshore wind turbines, because maintenance and repair visits are more expensive and not always possible (e.g. in rough sea conditions).