Wind Energy

Wind is a vast potential source of renewable energy. Winds are generated by complex mechanisms involving the rotation of the Earth, the heat capacity of the Sun, the cooling effect of the oceans and polar ice caps, temperature gradients between land and sea, and the physical effects of mountains and other obstacles.

Wind energy is generated by converting wind currents into other forms of energy using wind turbines. Turbines extract energy from the passing air by converting kinetic energy from rotational movement via a rotor. The effectiveness of this conversion at any given site is commonly measured by its energy density or, alternatively, as a capacity factor. Wind energy is primarily used for electricity generation, both onsite and for transport to the grid. Wind energy is also used to pump bore water, particularly in rural areas.

The wind energy industry is the fastest growing renewable energy source in many countries and is expected to continue to grow rapidly over the period to 2030. Production of wind energy is largely concentrated in Europe and the United States. However, there has also been rapid growth in the wind energy industries in China and India.

World wind energy resources

The world's wind energy resource is estimated to be about one million GW for total land coverage. Assuming only 1 per cent of the area is utilised and allowance is made for the lower load factors of wind plant, the wind energy potential would correspond to around the world total electricity generation capacity (WEC 2007).

The windiest areas are typically coastal regions of continents at mid-to high latitudes and in mountainous regions. Locations with the highest wind energy potential include the westerly wind belts between latitudes 35? and 50?. This includes the coastal regions of western and southern Australia, New Zealand, southern South America, and South Africa in the southern hemisphere, and northern and western Europe, and the north eastern and western coasts of Canada and the United States. These regions are generally characterised by high, relatively constant wind conditions, with average wind speeds in excess of 6 metres per second (m/s) and, in places, more than 9m/s.

Regions with high wind energy potential are characterised by:

• high average wind speeds;
• winds that are either constant or coinciding with peak energy consumption periods (during the day or evening);
• proximity to a major energy consumption region (i.e. urban/industrial areas); and
• smooth landscape, which increases wind speeds, and reduces the mechanical stress on wind turbine components that results from variable and turbulent wind conditions associated with rough landscape.

Because of wind variability, the energy density at a potential site - commonly described as its capacity factor - is generally in the range of 20-40 per cent. While the majority of areas in locations convenient for electricity transfer to the grid are located onshore, offshore sites have also been identified as having significant potential for wind energy, both to take advantage of increased wind speeds and to increase the number of available sites. Offshore locations also help reduce turbulence and hence stress on machine components. There have been wind turbines deployed in shallow seas off northern Europe for more than a decade. Offshore sites are expected to make an increasingly significant contribution to electricity generation in some countries, notably in Europe, where there are increasing difficulties in gaining access to onshore sites.

Australia's wind energy resources

Australia has some of the best wind resources in the world. Australia's wind energy resources are located mainly in the southern parts of the continent (which lie in the path of the westerly wind flow known as the 'roaring 40s') and reach a maximum around Bass Strait . The largest wind resource is generated by the passage of low pressure and associated frontal systems whose northerly extent and influence depends on the size of the frontal system. Winds in northern Australia are predominantly generated by the monsoon and trade wind systems. Large-scale topography such as the Great Dividing Range in eastern Australia exert significant steering effects on the winds, channelling them through major valleys or deflecting or blocking them from other areas (Coppin et al. 2003).

Deflection of weaker fronts from frontal refraction around the ranges of the Divide in south eastern Australia creates winds with a southerly component ('southerly busters') along the east coast. In addition to the refractions by topography and heat lows over northern Australia, other major factors influencing wind resources are seasonal and diurnal variation in wind speed. Winds are strongest in winter and spring in western and southern Australia but the monthly behaviour differs from region to region. Variations in average monthly wind speed of up to 15-20 per cent over the long term annual average are not uncommon. There may be similar daily variations at individual locations, with increased wind speeds in the afternoon (Coppin et al. 2003).

Meso-scale maps show that Australia's greatest wind potential lies in the coastal regions of western, south-western, southern and south-eastern Australia. Coastal regions with high wind resources (wind speeds above 7.5m/s) include the west coast south of Shark Bay to Cape Leeuwin, along the Great Australian Bight and the Eyre Peninsula in South Australia, to western Victoria and the west coast of Tasmania. Good wind resources extend hundreds of kilometres inland and many of Australia's wind farms (current and planned) are located some distance from the coast. Inland regions of Western Australia, South Australia and western Victoria all have good wind resources. Areas with high wind potential also lie along the higher exposed parts of the Great Dividing Range in south-eastern Australia, such as the Southern Highlands and New England areas.

The New South Wales Wind Atlas (Sustainable Energy Development Authority, NSW 2002) shows that the areas with the highest wind energy potential lie along the higher exposed parts of the Great Dividing Range and very close to the coast except where there is significant local sheltering by the escarpment. The best sites result from a combination of elevation, local topography and orientation to the prevailing wind. Significantly, the map shows that some inland sites have average wind speeds comparable with those in coastal areas of southern Australia.

The Victorian Wind Atlas (Sustainable Energy Authority Victoria 2003), shows a modelled average wind speed of 6.5m/s across the state with the highest average wind speeds (> 7m/s) found in coastal, central and alpine regions of Victoria (figure 9.8). The atlas also presents modelled average wind speed data in relation to land title (national parks, other public land and freehold land), land use and proximity to the electricity network. Effective wind resources are defined as those located within a commercially viable distance from the electricity network. The atlas delineates corridors within 10 and 30km of the network. It presents wind resource maps for each of the local government areas in relation to the electricity network according to land title.

Local topography and other variability in the local terrain such as surface roughness exert a major influence on wind speed and wind variability. Wind speed varies with height and with the shape and roughness of the terrain. Wind speed decreases with an increasingly rough surface cover, but can be accelerated over steep hills, reaching a maximum at the crest and then separating into zones of turbulent air flow. There are also thermal effects and funnelling which need to be considered when assessing wind resources. All of these effects impact on capacity factors (Coppin et al. 2003; ESIPC 2005). Australia's high capacity factors reflect the large development potential.

Because of these factors meso-scale maps such as figure 9.8 do not account for fine-scale topographical accelerations of the flow. In particular, the effect of any topographical feature smaller than 3km is unlikely to be accounted for. In mountainous country, topographical accelerations (and decelerations) because of these finer scale features commonly exceed 20 per cent. As such, these maps are useful only for preliminary selection of sites: detailed assessment of wind energy resources for potential wind farm location sites requires integration of high quality monitoring measurements with a micro-scale model of wind flow incorporating the effects of topography and terrain roughness.