This page explains how wind turbines work, the type and scale of wind turbines that are available, and how they tend to be applied in projects.
How they work
Wind turbines use the energy in the wind to turn a rotor, which drives a generator. The rotor either has a horizontal axis, which is the most common type, or a vertical axis.
Horizontal axis turbines: These often have three blades, although this varies in some models. The blades are designed as aerofoils, which use the wind to create lift and turn the rotor. They work in a similar way to aeroplane wings. The turbine rotor is designed to face either directly into or away from the wind. They are designed with a yawing mechanism which aligns them according to the wind direction.
Vertical axis turbines: These rotate around a vertical axis, turning a shaft which is in line with the mast or tower it is mounted on. The rotors can harness wind from any direction. The generator can be located inside the top of the mast, or the shaft can pass down through the inside of the mast to drive a generator located at the base. These turbines place less stress on the tower and base, which means they can be better for roof mounting.
Scale
Wind turbines are available at a wind range of scales. Micro-wind turbines can be installed on rooftops or on poles in back gardens. Very large turbines are used for onshore or offshore wind farms. Horizontal axis wind turbines tend to be more common for all of these applications, so the examples here focus on this type.
Comparing capacity and output
The electricity generated by a wind turbine increases with the wind speed and the area of the rotor. The capacity of a wind turbine tends to be quoted in terms of the peak rate of electricity generation. The wind speed required to reach peak output varies for different types of turbines. Therefore, it is not necessarily easy to compare turbines just on the basis of their rated capacity. Most turbines will operate in wind speeds ranging from around 5m/s up to peak output at around 15m/s.
The capacity factor is a simple way of expressing the typical output from a wind turbine over the course of the year, in relation to its rated peak capacity. It describes the output during a year as a proportion of the theoretical maximum output if it was operating at its peak capacity all of the time. Common industry average capacity factors are:
- 30 per cent for large wind turbines in reasonable locations
- five to 15 per cent for smaller wind turbines installed in urban areas (as buildings and other tall structures affect wind speeds and directions).
This graph shows the sizes and typical annual electricity output for a range of different scales of turbine.
These are calculated for an average mean wind speed of 6m/s.
Typical electricity output is expressed in terms of the number of new semi-detached homes it could supply electricity to.
Wind speeds
Wind speeds vary with location and weather conditions, so the output from a wind turbine is also variable. Unless the location being considered is very windy, average wind speeds will often be less than the speed required to reach peak generating capacity.
Wind speeds also vary with height, so as well as having a bigger rotor area to capture the wind, the larger turbines tend to have access to higher wind speeds because they are taller. Average wind speeds by location and height are available from the NOABL wind speed database.
Some turbines will not operate at all until a minimum wind speed has been reached, and may have a small motor to get them to start turning before the wind takes over. Most turbines also have a braking mechanism that stops them from turning when wind speeds are too high to avoid damage.