A wind turbine is the main element of a wind farm, and is used to convert the kinetic energy of the wind to mechanical energy and then to electricity. In the context of electricity production the wind turbine is also known as wind generator. A wind turbine consists of the rotor, the blades that are attached to the rotor, a generator and the tower structure.
The rotor is the element of the wind turbine that collects energy from the wind. The blades of the wind turbine are attached to the hub of the rotor. The blades spin by the flow of the wind with the use of complex aerodynamic designs, rotating the rotor. The rate at which the blades spin depends on the speed of the wind and the shape of the blades.
In order to generate electricity a generator is also required, in order to convert the kinetic energy into electricity. In commercial wind turbines a gearbox is placed between the rotor and the generator, in order to convert the low speed rotation of the blades into high speed rotation required for electricity generation. The rotation speed of a wind turbine is usually between 40-400 rpm (rotations per minute) while to produce electricity we need 1200-1800 rpm.
Wind turbines are mounted on top of high tower structures (usually above 80m) in order to operate at the required altitude. Wind turbines exploit wind flow at higher altitudes in order to take advantage of the higher and more constant wind speeds usually found at higher altitudes (due to the reduced influence of drag).
Electricity is produced when the blades of the wind turbine spin by the flow of the wind allowing the rotor to rotate. The rotor transfers the power towards the generator (via the gearbox) which in turn transmits the converted power towards a transformer and eventually to the grid network. A typical commercial wind turbine can produce between 1,5-7 MW of electricity, depending on its size, its design, and the wind flow at the specific location.
There are two major designs of wind turbines, the horizontal axis wind turbines and the vertical axis wind turbines. A horizontal axis wind turbine rotates around the horizontal axis of the wind turbine. Modern wind turbine blades are controlled by computer controlled motors and are optimized so that they always face the wind irrelevant of its direction, thus sustaining high performance for longer periods of time. Horizontal axis wind turbines are the older type of wind turbines and the most common currently used in commercial wind farms. On the other hand, a vertical axis wind turbine rotates around the vertical axis (they are omni-directional) which means that it doesn’t need to be pointed towards the wind direction in order to spin. Vertical axis wind turbines do not require high and constant wind speeds in order to operate like the horizontal ones and so they can be positioned in lower heights. This advantage together with their omni-directional ability makes them more favorable in urban areas and on top of rooftops.
Wind turbines can be positioned either on land (known as onshore wind turbines) or in the sea (offshore wind turbines). Onshore wind turbines are usually cheaper as they are easier to install. Offshore wind turbines are more expensive, but they take advantage of the stronger and more constant winds found at the sea, allowing for larger capacities.
For large scale production of electricity wind turbines are organized in wind farms. Large wind farms extend over areas of several square miles and consist of hundreds of wind turbines. Wind farms located on land are called onshore wind turbines and wind turbines positioned in the sea are called offshore wind farms. A good location of a wind turbine is one that has constant, non-turbulent wind speeds of at least 10m/h (16km/h), and is located near a transmission system. Before selecting a wind farm site, the winds at the location are monitored and measured for at least a year. Measurements are taken at different spots and heights. The collected data will determine the design, heights, specific locations of the wind turbines within the wind farm and the distances between wind turbines. A substation is also required at the local site, where all generated electricity from the individual wind turbines (medium voltage) is collected and transmitted in the local transmission system (transformed to high voltage). In onshore installations, the land between the wind turbines can be used for agricultural purposes.