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The importance of the wind resource

Wind energy has the attractive attribute - the fuel is free and this will be the case for the project lifetime and beyond.  The economics of a project is 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 2.2 shows the energy production of a 10 MW project for a range of long-term annual mean wind speeds.  

Table 2.2 Sensitivity of wind farm energy production to annual mean wind speed, Garrad Hassan

Wind speed (m/s) Wind speed normalised to 6 m/s
Energy production of 10 MW wind farm
Energy production normalised to 6 m/s site
Capital cost normalised to 6 m/s site
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

Note 1:  Assumes typical turbine performance, air density of 1.225 kg/m3, total losses of 12 % 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%, the energy production increases by 134%.  This range of speeds would be typical of Bavariaat the low end, and hill-top locations in Scotlandor Irelandat 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 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% change in wind speed might result in a 2% change in energy, whereas for a high wind speed site the value might be only 1.5%.  Table 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.  Nevertheless, Table 2.2 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 is, in financial terms for both debt and equity, an enormous difference.

In summary, the single most important characteristic of a wind farm site is the wind speed.  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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