A wind turbine is a large rotating device that converts the energy of the wind into mechanical energy. This mechanical energy can then be used for simple mechanical tasks, such as turning large wheels to grind grain, in which case the wind turbine is referred to as a windmill. It can also be used as a source of electrical energy, either for a nearby application, or after being transferred via high-voltage lines to a central switching station, in which case it is referred to as a wind generator.
The wind turbine has been in use for centuries in windmill form. Many parts of the world still use turbines extensively to grind grain, and pump water. Windmills allow people without access to electrical pumps to get water from deep below the groundwater level, helping to ensure access to clean water, and to protect against water-borne illnesses.
In recent years, the wind turbine as electrical generator has begun to be used as a significant source of power generation throughout the world. In the global move away from fossil-fuel energy to more sustainable forms of energy, wind power has become an important player. It is relatively affordable, can be used in many different regions of the world, and can be quite efficient. Although there are some environmental concerns over wind generators, they are generally minor, and more than 120 gigawatts (GW) of electricity is produced worldwide through the use of wind energy.
Turbines themselves can be separated into two distinct groups: horizontal-axis wind turbines (HAWT), and vertical-axis wind turbines (VAWT). Most turbines people are used to seeing are HAWTs, based on the familiar windmill design. Modern HAWTs are generally three-bladed, at the top of enormous towers to bring them to an altitude where wind shear is much higher.
Most HAWTs are controlled by computers to automatically turn into the wind, to make the most of whatever wind is present. HAWTs don’t require any sort of backtracking, since their blades are always moving perpendicular to the wind. As a result, the entire time they rotate they are generating electricity, in contrast to VAWTs. However, HAWTs are incredibly expensive to transport, since they are enormous, and it is common for 20% of total cost to be spent in transportation alone. Additionally, their height can make them aesthetically displeasing, and installation is a difficult, costly, and time-consuming process.
A vertical-axis wind turbine (VAWT) on the other hand has the rotator shaft sticking straight up into the air, with foils that catch the wind and turn the entire shaft. The most common type seen is the Darrieus wind turbine, which looks roughly like a large eggbeater. VAWTs require much less height, and are much less costly to transport and install, and are able to produce electricity with much slower wind speeds than HAWTs. Their overall efficiency is roughly 50% that of HAWTs, however, and because they are lower to the ground they tend to experience much lower wind speeds.
In addition to the two major types of wind turbine, which make up the vast majority of turbines in the world, there are a number of non-standard designs as well. These tend to be variations of HAWTs or VAWTs, such as the aerogenerator, or ducted rotor turbine. Even stranger variants, such as the magnetic levitation wind turbine, and airborne wind turbine, have also been proposed and in some cases experimented with. Most alternative designs, however, have not yet seen widespread commercial acceptance.