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.
Read on for more information about wind power, or go to our database for films and case studies of Ashden Award winners who use wind power.
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.
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, wind monitoring.
Ashden Award winner Ecotricity installs large wind turbines (1 to 2 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 Cwmni Gwynt Teg is a small group of Welsh farmers who jointly built a wind farm, allowing them to generate income from the sale of electricity while retaining use of the land for farming purposes.
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 costs for installing wind turbines on land range from US$ 1,200 to 2,600 per kW. 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 4% of the turnkey costs over the lifetime of the turbine (note 1).
A 2 MW wind turbine operating at 30% capacity factor produces about 5 GWh of electricity in an average year, which would earn US$ 500,000/year if sold at 10c/kWh.
The global installed capacity of wind turbines has risen from 93.8 GW in 2007 to 120.6 GW in 2008, and 157.9 GW in 2009; an increase of 58% over two years (note 2). Several countries with high growth in installed capacity are shown below. Of particular interest is China, which more than doubled its capacity during 2008, and doubled it again in 2009.
|Country||Capacity at end of 2007||New capacity in 2008||New capacity in 2009|
GW of installed wind power in selected countries
In 2009, the global energy generated by wind turbines was 340 TWh, which is 2% of global electricity generation, and 0.26% of global primary energy use (note 3). The total global wind resource is estimated at 278,000 TWh/year (note 4), and at least a third of this is economically exploitable within the next 20 years, so a much larger proportion of global electricity could be generated by wind power. The UK has the largest wind resource share of any country in Europe, with the potential for economic generation of 4,400 TWh/year in 2030. In 2008 the UK generated 7.1 TWh from wind power (note 5).
1. Europe's onshore and offshore wind energy potential, EEA Technical report, 2009
2. Global installed wind power capacity (mw) – regional distribution, Global Wind Energy Council, 2009 Source 1 | Source 2
3. World Wind Energy Report 2009, World Wind Energy Association, 2010
4. World in Transition – Towards Sustainable Energy Systems, German Advisory Council on Global Change, 2003, p63
5. Digest of United Kingdom Energy Statistics 2009, DECC, 2009
Lead author: Dr Mike Pepler