Wind is not homogeneous along the space. Its distribution depends on:
The kinetic energy has to be converted into useful energy. To achieve that, wind reaches the wind turbines that convert the kinetic energy into mechanical power thanks to the blade motion. This mechanical power can be converted into electricity thanks to a generator for power supply.
There are places in the world that are more suitable than others for wind power harvesting. This suitability depends mainly on the wind speed. Here is an interactive map where we can observe the speed of the wind the world.
Turbines with a horizontal axis have the main rotor and electrical generator at the top of a tower. Small turbines have a simple wind blade, while large turbines use a wind sensor with a servo motor. Most have a gearbox, which turns the rotation into a quicker rotation to achieve a higher efficiency. Turbines that are used in wind farm for large-scale electrical production are normally three-bladed and have computer-controlled motors
The main advantages of offshore wind turbines are:
However, they are more expensive since their integration to the electrical grid is more complex. Maintenance is also more complex and costly than in onshore wind turbines.
According to the design of the blades, horizontal axis wind turbines can be classified as:
UP WIND TURBINES
These turbines operate with the blades upwind to the tower. Small-scale wind turbines use a tail vane to keep the blades facing into the wind
SHROUDED WIND TURBINES
These turbines have an augmentor that increases the amount of wind passing through the blades.
DARRIEUS WIND TURBINE
They have good efficiency but poor reliability. They also generally require some external power source, or an additional Savonius rotor to start turning. The torque ripple is reduced by using three or more blades which results in greater solidity of the rotor. Solidity is measured by blade area divided by the rotor area.
Giromill wind turbine is a subtype of Darrieus turbine with straight vertical blade sections attached to the central tower with horizontal supports. Vertical blades change their orientation whilst the rotation of the rotor occurs for a greater use of the force of the wind.
SAVONIUS WIND TURBINE
The Savonius turbine is a drag-type device, consisting of two or three scoops. The scoops experience less drag when moving against the wind than when moving with the wind.The differential drag causes the wind turbine to spin. Savonius turbines are used whenever cost or reliability is much more important than efficiency. For this reason, anemometers are Savonius wind turbines since the efficiency is not as important as the reliability. Much larger Savonius turbines have been used to generate electric power in such places where small amounts of power and a low cost of maintenance are needed.
Twisted Savonius turbine is similar to Savonius wind turbines. The difference is that they have longer helical scoops to provide smooth torque. They are normaly used as a rooftop windturbine and have been adapted for ships.