Air has mass. Moving air has both mass and velocity. Wind is moving air and therefore wind contains kinetic energy. The higher the velocity of the wind the greater the kinetic energy it contains.
Consider a D400 wind turbine. Its air blades have a diameter of 1.1 metres which sweep out an area of about 1 square metre. Assume the wind is blowing at a velocity of 5 m/s (10knots) and then visualise this as a cylinder of air 1.1 metre diameter and 5 metre long in front of the air rotor. Air weighs about 1.2 kg per cubic metre at sea level so our ‘cylinder’ of air weighs 6kg. This ‘weight’ of air passes through the air rotor each second, and as it does so, a proportion of the kinetic energy it contains is extracted by the D400’s air rotor. The velocity of the wind is the critical element in this process because higher winds pass a greater mass of air through the air rotor each second (imagine an ever longer cylinder of air!). Because the amount of kinetic energy varies as the square of its speed, this results in a cubic relationship between wind speed and available energy. The effects of this are dramatic. For example there is almost 75% more energy available in a 12 mph wind than at 10mph. Just as available energy rises quickly with increasing velocity, so it diminishes rapidly at low wind speeds.
There is very little useful energy in wind speeds below 6mph (3m/s). The physical laws which define wind energy mean that it is absolutely vital to site wind turbines where there is a good wind resource. Therefore an objective assessment of the wind resource available at a given site is the first and most essential step for anyone considering investing in wind energy. Measuring instantaneous wind speeds is not very helpful. What is required is measurement of wind speeds over time. This necessitates a dedicated wind data logger which will measure and sample the wind speed, continually recording data in the form of 60 second, or five minute, averages. Ideally such data logging should continue for 12 months or longer. With 12 months’ worth of data, an annual average wind speed can be readily calculated.
Although wind measurement over time is the ideal, in many situations it is neither practical nor economic proposition. This is particularly true where the proposed wind system is small and the capital costs modest. In this situation, an assessment of wind speed can be made by referring to existing published wind speed data bases. Most countries have wind speed data collected by their meteorological and aviation services. Data can be presented in the form of wind maps or annual average wind speeds for post code areas. In the UK, NOABL wind data is available. However these NOABL values are derived mathematically and take no account of topography or land type, i.e. rural or urban. As a result NOABL typically overstates wind speeds in urban areas. More recently, data bases have been produced which factor for urban or rural sites and these can be accessed at www.est.org. Such data provides a very useful starting point in assessing a location as a potential wind site. The higher the quoted annual average wind speed the better. Values below 4m/s are not encouraging and suggest that yields from a system installed at that location are likely to disappoint. In general coastal locations and high ground offer the best wind sites. Any open rural site is likely to be superior to an urban site.
Mount height is always a significant factor on performance. The higher a turbine is mounted the more productive it is likely to be. At any location the closer you get to the surface of the earth, the lower the wind speed. This is a result of the friction between the earth’s surface and the obstacles upon it, which slows the air flow in relation to the air above.
This phenomenon is known as wind shear, and, as the diagram shows, air stream velocity increases rapidly with height above the surface. The nature of the surface is also a significant factor. Smooth surfaces such as water or grasslands slow the air stream less than woodland or dense urban development. Even a single obstacle up wind or downwind of the turbine can have a very negative effect on turbine performance because, as air flows around it, turbulence is introduced. To visualise this, consider water flowing around a rock in a shallow stream. The eddies and swirls in the wake of the rock also occur in air in the wake of a tall tree or building. A turbulent wind stream greatly reduces the efficiency of any turbine. Therefore it is important to install turbines at a site with an open aspect that offers the cleanest air flow possible. This explains a further advantage of mounting wind turbines at height, in order to take them out of turbulence and into the faster air.
Although using data logged anemometers is the best way to assess a wind site, much can learned from simply looking at the location from a wind energy perspective. In pre-industrial Britain there were over 250,000 wind mills, and these provided an essential source of mechanised power.
Millwrights of that era would camp out at a proposed mill site, and observe the wind patterns over a period of time using their experience and judgement to decide how good or bad the site was likely to be. There are also clues in the vegetation. The growth of trees and bushes is effected by the wind. Symmetrical growth suggests light winds whereas ‘flagged’ trees display stronger growth 180 degrees away from the prevailing wind direction. The more marked the flagging, the stronger the winds. At very high wind sites, trees permanently lean away from the prevailing wind. This is known as ‘throwing’ and in extremis the trees will be ‘carpeted’ i.e. growing virtually parallel to the ground.