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Wind energy has the attractive attribute that the fuel is free and this will be the case for the project lifetime and beyond. The economics of a project are thus crucially dependent on the site wind resource. At the start of the project development process, the long-term mean wind speed at the site is unknown. To illustrate the importance of the long-term mean wind speed, Table I.2.2 shows the energy production of a 10 MW project for a range of long-term annual mean wind speeds.
Table I.2.2: Sensitivity of Wind Farm Energy Production to Annual Mean Wind Speed
Wind speed (m/s) Wind speed
normalised to 6 m/s (percent)
of 10 MW wind farm (MWh/annum)*
normalised to 6 m/s site (percent)
normalised to 6 m/s site(percent)
5 83 11,150 63 100 6 100 17,714 100 100 7 117 24,534 138 102 8 133 30,972 175 105 9 150 36,656 207 110 10 167 41,386 234 120
Source: Garrad Hassan
Note: * Assumes typical turbine performance, air density of 1.225 kg/m3, total losses of 12 percent and Rayleigh wind speed distribution.
It can be seen that when the long-term mean wind speed is increased from 6 m/s to 10 m/s, about 67 per cent, the energy production increases by 134 per cent. This range of speeds would be typical of Bavaria at the low end and hill-top locations in Scotland or Ireland at the high end. As the capital cost is not strongly dependent on wind speed, the sensitivity of the project economics to wind speed is clear. Table I.2.2 illustrates the importance of having as accurate a definition of the site wind resource as possible.
The sensitivity of energy yield to wind speed variation varies with the wind speed. For a low wind speed site, the sensitivity is greater than for a high wind speed site. For example, at a low wind speed site, a 1 per cent change in wind speed might result in a 2 per cent change in energy, whereas for a high wind speed site the value might be only 1.5 per cent. Table I.2.2 is in fact a simplification of the reality of the situation, where different specifications of turbine model would typically be selected for low and high wind speed sites, but it serves to illustrate the importance of wind speed to energy production.
The commercial value of a wind farm development is therefore crucially dependent on the energy yield, which in turn is highly sensitive to the wind speed. A change of wind speed of a few per cent thus makes an enormous difference in financial terms for both debt and equity.
In summary, the single most important characteristic of a wind farm site is the wind speed. Thus every effort should be made to maximise the length, quality and geographical coverage across the wind farm site of the data collected. However, measurement is undertaken at the very beginning of the project and some compromise is therefore inevitable.
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