With continuing developments in technology and falling installation costs, wind is a fast-growing energy source, providing renewable electricity while reducing air pollution and climate-warming carbon emissions. The variability of wind is managed by linking turbines to an electricity grid, or by combining wind power with other forms of renewable energy.
- Wind turbines come in a huge range of sizes - from 50W to MW capacity.
- Global wind capacity more than tripled to over 318 GW between 2007.
- Wind turbines generated 4% of global electricity in 2013.
How wind power works
Wind turbines use blades shaped like aircraft wings, mounted on a central 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 8 MW turbines with 80 m long blades, for use in off-shore wind farms.
How wind power is used
Most grid-connected wind turbines are installed in groups or ‘wind farms’. Installing a large group of turbines reduces the average installation and operating cost, but can make them very visible in the landscape. As a result an assessment of the environmental impact of a wind farm is usually required for planning permission to be given. This is one area that Ashden Award winner Dulas specialises in, in addition to other services such as feasibility studies and wind monitoring.
Ashden Award winner Ecotricity installs large wind turbines (1 to 3 MW) individually and in small groups, rather than in large wind farms. By siting turbines sensitively and also using a design that does not need a gearbox, and therefore produces less noise, Ecotricity has been able to get local support for using sites which are closer to villages and towns.
What are the benefits of using wind power?
Wind power is one of the most mature renewable energy technologies. It provides renewable electricity, and avoids carbon emissions. It can also provide off-grid electrical and mechanical power, bringing useful services to remote areas.
The rapid development of wind technology over the past ten years has largely been driven by concerns about climate change and carbon emissions. How much carbon is saved by wind electricity depends on how electricity would otherwise have been generated. When electricity generated by wind power displaces electricity from the UK national grid, an average of 0.54 kg CO2 is saved per kWh supplied. If coal-fired generation is displaced, the saving is about 1.0 kg CO2 per kWh.
Wind power can also bring economic benefits when a community invests in a local wind farm. Ashden Award winner AliEnergy helped the Gigha Community Trust to raise finance for three 220 kW wind turbines that now provide 75% of the electricity needed on the island. All revenue from sales goes back to the Trust, to pay back loan finance and fund community development. Another winner, Energy4All, has enabled communities to invest in cooperatives that then build wind farms, or own a share of a larger wind farm, with the returns going back to investors and also to the community.
A number of Ashden Award-winning UK schools have installed smaller, grid-connected wind turbines in their school grounds, between 5 and 10 kW in size. These are valuable for teaching as well as reducing the schools’ energy bills and carbon emissions. How schools fund the installation of a turbine varies; Cassop Primary School, in Durham, had their turbine installed by the county council, while Sandhills Primary School, in Oxford, raised funds from local businesses, pupils and parents.
The cost of installing wind turbines in a wind farm depends on where they are sited, and how any are installed. Typical 2012 costs for installing wind turbines on land ranged from US$ 1,100 to 2,600 per kW, depending on the country, the type of turbine and the site in question . It is more difficult to install and maintain wind turbines offshore, and installation costs are higher. Smaller turbines cost more – under 100 kW capacity the price per kW ranges from US$ 3,000 to 5,000. The operational and maintenance costs are typically 15-25% of the turnkey costs over the lifetime of the turbine. In terms of the cost of energy, projects completed in 2012 were delivering power in the $30-50 per MWh range in the USA.
The global installed capacity of wind turbines more than tripled over the past six years from 94 GW in 2007 to 318 GW in 2013 . The five countries leading countries are shown in the table. Of particular interest is China, which has increased its capacity by seven-fold in three, and leapfrogged India, Spain, Germany and the USA to gain its current leadership position. (From WW India).
|Country||Capacity at end of 2007||New capacity in 2008||New capacity in 2009|
GW of installed wind power in selected countries
In 2013, the global electricity generated by wind turbines was 640 TWh. This is 4% of global generation, and is 80% more than the total electricity use in the UK . The UK supplied 27.4 TWh is estimated at 278,000 TWh/year , and over 90,000 TWh/year of this is assessed as economically exploitable by 2030. The UK has the largest wind resource in Europe, with an economic potential of 4,400 TWh/year by 2030.