The sun heats the earth unevenly, creating thermal air currents. In order to achieve equal temperatures around the earth these air pockets move about the earth as global wind. The energy that travels in the wind can be captured and converted to provide electricity.
Wind energy provides a clean, sustainable solution to our energy problems. It can be used as an alternative to fossil fuels in generating electricity, without the direct emission of greenhouse gases. And there will always be wind; it is inexhaustible and renewable.
It is envisaged that wind power will make the most significant contribution to the achievement of national and international targets for green electricity, due to its environmental benefits, technological maturity and competitiveness.
Wind Farms in Ireland
Since the first wind farm project was realised in 1992 at Bellacorrick, Co. Mayo, 1,379 MW of wind capacity has been installed at the end of June, 2010. In order to achieve our national targets for renewable electricity by 2020 (40%) an estimated 5,500-6,000 MW of wind generation is required.
Wind energy's contribution to Ireland's electricity supply continues to rise with additional capacity. By June 2010 a total of 110 wind farms were metered, bringing the total installed capacity for wind to 1,379. In 2009, wind power displaced approximately 1.28 million metric tonnes of CO2 emissions (estimated with reference to the grid average CO2) and primary energy imports of 215,000 metric tonnes of oil equivalent to a nation which is 89% dependent on imported energy supplies.
Guidelines for wind farm planning, development and construction have been produced and are widely available, particularly through National and International Industry Associations such as the IWEA and EWEA .
Meitheal na Gaoithe provide good information on how to begin the process in their FAQ section.
IWEA also give good guidelines. The Irish Wind Energy Association "Best Practice Guidelines for the Irish Wind Energy Industry" published in September 2007 and part-funded by SEAI can be downloaded at no charge from the publications section of the IWEA website.
Where does wind come from?
It is possible to compare the earth's atmosphere to a thermal engine where air masses move due to their different thermal conditions. This large scale motion can be observed in the air streams which are the result of the transformation of thermal energy into kinetic energy. The source of this energy is the sun.
The motion of air in the atmosphere is evident as a global circulation with seasonal cycles or as a regional phenomenon. The movement can also be determined locally by orographic or surface conditions.
As the earth has a spherical form, the incident radiation at the equator is greater than that occurring at the polar regions. This energy imbalance created, with excess in the atmosphere around the equator and a deficit around the southern and northern latitudes, causes the heat to move by air masses into the cooler regions. These global movements are called geostrophic winds.
There may also be local imbalances in radiation, however these are mainly due to differences in the earth's surface. Local winds with site specific characteristics result. The most important local winds in an Irish context are the sea-land breeze and the mountain-valley wind.
These winds are mainly influenced by temperature differentials as well as the surface structure. For example, in the case of sea-land breezes, when the solar radiation heats the land during the day the temperature over the sea is lower resulting in an exchange of air masses as the land air rises. The wind may be noticed up to 40km inland and reach speeds of 10m/s.
In analysing potential sites and yield from wind energy the geostrophic winds are less important than the surface winds. When a fluid, in this case air, flows over a surface it interacts with that surface creating a boundary layer. The visualisation of this boundary layer depicts a velocity of zero at the surface rising to a maximum at some point in the atmosphere. Wind turbines always operate in this boundary layer, as it varies between tens and hundreds of meters.
In addition to the interaction of the air stream with a plain surface of the earth, in reality there are changes in orography, landscapes, buildings and natural obstacles which locally effect the wind speed profile by causing turbulence, degraded velocity and backflows.
For these reasons the only way to get a realistic assessment of the wind resource and energy yield from a prospective turbine site is to erect instruments such as anemometers and vanes over long periods, typically not less than a year to record full seasonal variations, and to use this data with the nearest met station's historical data.