Siting
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Siting
What do wind farm developers look for?
What about other land users?
How far away from houses are wind turbines built?
Why so much emphasis on wind speed?
Where are the windy sites?
What do wind farm owners avoid?
Why are wind turbines so tall?
Why are wind farms put on top of hills?
Why do we not build more wind farms inland?
Why do we not build offshore wind farms?
How far apart are the turbines?
What other issues impact wind farm layout?
What influences the wind farm size?
How much land is needed?
What do wind farm developers look for?
Developers favour sites with the following characteristics:
- Strong and consistent winds
- Proximity to a suitable electricity grid
- Land where wind farm development is appropriate, away from areas of high conservation value or areas with endangered flora or fauna species (e.g. national parks and wetlands)
- Identifiable and manageable cultural heritage issues
- Open land without current or future obstacles to the wind flow
- Broad community support and acceptance
- Good access for wind farm construction and maintenance
- Suitable geology for access tracks and foundations.
A compromise will usually be needed between these various factors.
What about other land users?
Wind farming is compatible with many land uses ranging from cropping and grazing properties, to industrial estates, port breakwaters and sometimes even forestry. In New Zealand wind farms have been built on or are currently proposed for, most of these types of land. Impact on local amenities, such as airports, must be considered when siting a wind farm. The long life of wind energy developments (20 to 25 years) means that it is also important to consider future users of adjacent land.
How far away from houses are wind turbines built?
Modern production wind turbines are not noisy in operation. They nonetheless need to comply with very strict noise standards and as a result it is normally noise criteria that determine the distance between turbines and houses (called 'setback'). Setback distances may range from 300 metres to as much as one kilometre according to a variety of factors. These include the noise standard prescribed, the local topography, prevailing wind conditions and the wind farm layout.
Why so much emphasis on wind speed?
The commercial success of a wind farm depends on two key factors:
- the amount of electricity that can be generated
- the price at which that electricity can be sold.
The amount of electricity that can be generated is obviously determined by the average wind speed at the site. It should therefore be no surprise that the key factor affecting the economics of a wind farm is the average speed of the wind in and around it. The relationship between the wind speed and the energy contained within it is not linear. In fact a doubling of the wind speed leads to an eight-fold increase in the wind energy passing the turbine blades. For this reason the average speed of the wind is a critical factor in siting both a wind farm and the individual turbines within it.
Where are the windy sites? New Zealand is well suited to wind energy developments lying as it does across the prevailing north-westerly winds in an area referred to by sailors as the 'Roaring Forties'. The country has a good wind resource because of its oceanic location and a fairly steady succession of troughs and depressions passing to the east. Each weather system induces a pressure pattern over the mountains that can lead to almost continual wind generation in some parts of the country. Nonetheless anti-cyclones can break this pattern from time to time. A May 2001 study identified 13 general areas of land around New Zealand that would be suitable for wind farming and of these most were on or near the coast. For a variety of reasons coastal winds are generally of a higher speed and consistency throughout the year and consequently coastal regions are often more suited for wind farms. The same report noted that the total electricity generation potential from wind was 100 Terawatt hours (TWh) annually or approximately three times more than our current electricity consumption.
What do wind farm owners avoid?
Wind farm operators will seek to avoid sites with features that might significantly slow down the wind or increase its turbulence. The impact of any obstacle will be determined by its height, its width and its porosity to the wind. Obstacles can be natural, ranging from dense forests to scattered trees, or man-made such as windbreaks or buildings. Such artificial and natural structures can not only slow the wind down but they can also induce turbulence which has a negative impact on the energy yield and the machine 'wear and tear'. 'Surface roughness' is another factor that must be considered. Rougher surfaces slow down the wind and introduce turbulence into the air flow. Even a mature wheat crop will slow down the wind more than a closely grazed pasture. Scattered trees will have more of an impact than a wheat field and big cities will have the largest affect. The ocean has one of the lowest 'surface roughness' ratings of all surfaces and this is why coastal sites generally have the best wind speeds.
Why are wind turbines so tall?
Wind speeds increase with distance above ground level. In addition to which power output dramatically increases with rotor diameter. For these two reasons it is easy to see that as the tower increases in height it is possible to expose the generator to higher winds and also to have larger rotors which can then tap into more of the wind's energy.
Why are wind farms put on top of hills?
Bernoulli's Law describes the behaviour of a fluid (i.e. the air) under varying conditions of flow and pressure. In essence: As a moving mass of air approaches an obstacle, such as a hill, it speeds up in order to pass over the obstacle. The result is that the wind speed is significantly higher at the top of a hill or ridgeline.
Why do we not build more wind farms inland?
There are already a number of inland wind farm sites such as Hau Nui, Tararua and Te Apiti in addition to the recently announced White Hill project in Southland. Clearly then there are a number of excellent inland sites available around the country. However these generally need to be in elevated terrain for the reasons already discussed. Nonetheless many coastal regions are also under investigation as they typically enjoy stronger winds because of their exposure and proximity to the ocean where sea breeze effects are greatest.
Why do we not build offshore wind farms?
Offshore wind farms are being actively investigated in various parts of the world and most of all in Europe, which has the most advanced offshore wind industry in the world. The rapid growth in the industry in Europe has led some to ask why we do not build turbines offshore here as well. The fundamental reason is that the cost of electricity from an offshore wind farm is 30 to 50 per cent more expensive than is the case from an onshore development. In Europe this is not so much of an issue due to higher electricity prices in that part of the world in addition to the much higher population densities there, which put onshore land at a significant premium. It is also significant that the sea floor around New Zealand shelves away rapidly as you move away from the coast. In other words it gets too deep, too quickly. All this is not to say that we will not have offshore wind farms in the future. It is simply that their development in the near term is unlikely to be a high priority.
How far apart are the turbines?
Wind turbines within a wind farm will generally be separated from each other by three to five rotor diameters if the line of the wind turbines is at 90 degrees to the prevailing wind direction. If the line of turbines is in the same direction as the prevailing wind direction the turbine spacing will be more like five to seven rotor diameters.
What other issues impact wind farm layout?
The various issues governing the layout of a wind farm are all interrelated and the final siting decision is very complex as a result. Major factors include local terrain, noise constraints, aesthetic appearance and avoidance of areas of important native vegetation and sites of cultural and archaeological significance. Sophisticated three dimensional computer models help developers to plot the many complex and often competing issues involved in designing a wind farm. The layout of any wind farm will usually go through many changes before the final design is reached.
What influences the wind farm size?
A large variety of factors have already been mentioned that affect the size of a wind farm. However, the principal factor affecting the size of wind farms in New Zealand is economics. Larger wind farms tend to enjoy greater economies of scale than smaller ones. For this reason and, given that there are no taxpayer subsidies for wind energy in New Zealand, wind farms here tend to be larger than those in Europe. There are some single turbines, such as Brooklyn and Gebbies Pass. However these were constructed as demonstration units. Operational wind farms typically vary from five to 50 or more turbines in number.
How much land is needed?
Turbines within a wind farm are spread out and, as a result, the total area of a single wind farm may be quite significant. However the land area that is physically taken up by the turbine towers, access roads, transformer stations etc is actually very small and is typically in the region of two to three per cent of the total area of the entire wind farm. The remaining 97 to 98 per cent of the land area can continue to be used for its original activities such as stock grazing etc. In terms of the overall land area required for a wind farm, a good rule of thumb is that a single square kilometre of land (1000 metres X 1000 metres) can support 10 to 15 MW of wind power.
Last updated: Tuesday, 24 October 2006
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