Power has been extracted from wind for thousands of years, and modern wind turbines are one of the most mature renewable energy technologies available. The cost of electricity produced by wind turbines is competitive with fossil fuel generation, and the number being constructed across the world has been increasing rapidly for several years now. Although the strength of the wind at a particular location varies over time, the effect of this can be minimised by linking turbines to a national or international electricity grid, and by combining them with other forms of renewable energy generation.
How wind power works
Wind turbines use blades shaped like aircraft wings which are mounted on a shaft. The force of the wind turns the blades, converting the energy of the wind into mechanical energy of the rotating shaft. This shaft is then used to turn a generator to produce electricity, or to operate a mechanical pump or grinding mill. Most modern wind turbines are used for electricity generation.
The amount of energy available in the wind is proportional to the cube of the wind speed — so when the wind speed doubles, the energy available increases eightfold. To operate effectively a wind turbine must be therefore be situated in an area with high average wind speeds and minimal turbulence. The speed of the wind increases with the height above the ground, so turbines are mounted on tall towers. Wind speeds can vary significantly with location, even between sites a few kilometres apart, so a potential site for a large wind scheme is monitored for up to a year to find if the wind speeds are high enough. Concerns over affects on radar, wildlife and aesthetics sometimes limit the number of potential onshore turbine locations, which has led to the construction of offshore wind farms. The wind is often stronger and less variable over the sea, so a growing number of wind farms have been built offshore, but the increased difficulty of construction and maintenance means an offshore wind farm costs more to build and operate than an onshore one.
In most wind turbines the rotating shaft is horizontal. A mechanism is therefore needed to automatically turn the blades to keep facing the wind, whatever direction the wind comes from. In small wind turbines, the vane down-wind of the blades keeps the blades aligned, but larger machines use electronic control. Some turbines use a vertical axis of rotation, so don’t need to be turned to face the wind, but they are less efficient than a turbine with a horizontal axis.
Most wind turbines supply electricity to a national grid, so it is essential that they produce electricity at the right voltage and frequency. Until recently, most used a gearbox to couple the shaft to the faster-rotating generator. More advanced turbine designs use variable speed generators and electronics to feed power to the grid at the correct frequency, allowing the turbine to rotate at the ideal speed for the wind conditions. Small wind turbines are also used in off-grid systems, usually with rechargeable batteries so that the variable wind supply can be matched to the demand for the electricity.
The rated output of an electricity-generating wind turbine is the electrical power produced (in watts, W) at a standard wind speed, usually between 8 and 15 m/s. The wind does not blow steadily at this speed and the average power generated is typically 30% of the rated power: this is termed the ‘capacity factor’. The range of wind turbines on the market is enormous — from 50 W battery chargers with blades 0.25 m long, up to 6 MW turbines with 60 m long blades, for use in off-shore wind farms.
Implementation in Bangladesh
Small wind turbines may be installed in the coastal region and off-shore islands of the country, but there is the consideration of building the infrastructure to carry power to the major cities/rural areas.