Draft PPS 18: Renewable Energy
Annex 1 Wind Energy: Wind Resource
A28. The energy produced by a wind turbine depends on the strength of the wind to which it is exposed. The simplest indicator of the wind resource available at a given location is the annual mean wind speed at the site (usually given at the hub height of the turbine). A machine located on a site which has an annual mean wind speed of 6 metres per second will typically produce only half as much energy as the same machine on a site where the annual wind speed is 8 metres per second.
A29. For any given location the wind speed rises with elevation above the ground due to wind shear. The degree of wind shear (the rate at which the wind speed increases when moving vertically away from the ground) is dependent on the surrounding ground conditions; the higher the surrounding obstructions (e.g. vegetation or buildings) the greater the wind shear produced. Due to this, raising the hub height of the turbines, by mounting them on taller towers, can increase the energy capture at any given site. Current hub heights available to developers are between 50-100m.
A30. As well as affecting the wind shear, surrounding obstacles such as woodlands and buildings will increase the turbulence in the wind. Higher turbulence levels in the wind adversely affect wind turbine performance and life expectancy and, as such, developers will look to position turbines as far away from obstacles as is practicable. Again, the use of taller towers can ameliorate this effect by placing the rotor in less disturbed air.
A31. Assessing whether a particular site will harness wind power satisfactorily entails using historical meteorological data (available from the Meteorological Office) and information derived from anemometers placed on site. Anemometer masts are normally required on a site for at least 12 months; the longer measurements are taken the better the predictions will be. The measurements from the anemometers help to determine whether or not a candidate site is suitable and, if it is, the measurements help to determine the best position for the wind turbines within the site’s boundary. The masts should be approximately as tall as the hub height of the planned turbine. However, often when the mast is erected it is not known either if the site is suitable for wind farming or which turbine type would be most suitable. Masts are usually 25-60m tall. Planning permission is required to erect a temporary anemometer mast.
A32. The mean wind speed at hub height (along with the statistical distribution of predicted wind speeds about this mean and the wind turbines used) will determine the energy captured at a site. The simplest way of expressing the energy capture at a site is by use of the Capacity Factor.
A33. This can be expressed alternatively as the actual energy generated by a wind turbine over the course of 1 year divided by the energy that would have been generated by a wind turbine over the course of 1 year had the wind been consistently blowing at speeds between rated and cut-out (typically 12-25m/s). Capacity factors in the UK may generally fall anywhere between 0.2 and 0.5, with 0.3 being typical in the UK.