When I became interested in sustainable energy systems, I wanted to understand the nature of wind as an energy source. An early step was to create wind speed distribution diagrams for several hundred locations around the world taking data from a wide variety of sources such as aerodromes, weather ships, offshore buoys, radiosondes and small weather stations located in backyards. The principal conclusion from this exercise was the wide variation in both average wind speed and its distribution.
This post is based on my own research and has not been reviewed by anyone else, thus it should be treated with caution as there may be other and more appropriate interpretations of the same data. Another caveat is that much wind data is collected at a standard height of 10m in open terrain to assist ground operations. Thus the bulk of readily available data describes air flow at the base of the boundary layer but the ideal location for wind turbines is as far above this zone as possible. There may also be gross generalizations.
The variation of wind speed with location is apparent in daily life. In the coastal town where I live, the wind maybe blowing smoothly at 10 m/s on the seafront whilst in the town it will eddy around buildings appearing as long period gusts struggling to make 8 m/s even on the roof of a multi story car park and in residential areas away from the sea, there may even be still or light air. For a brief period in my youth, I was a merchant seaman and had ample opportunity to study wind and waves during lookout duty, still air was rare, usually there was a steady breeze with accompanying waves gave the the ship varying amounts of motion. In winter, gales with speeds of more than 20 m/s had an energy seldom seen onshore. The energy of wind is proportional to the cube of its speed, thus, a steady stream of air with a velocity of 10 m/s has eight times more energy than one with a speed of 5 m/s, hence the destructive nature of storms where the wind speed exceeds 25 m/s.
The average wind speed over Western Europe at a height of approximately 800m is typically in the range 10 - 12 m/s, this increases to the north and decreases to the east. At this altitude the influence of terrain and surface cover is small and whilst there are variations, it is reasonable to assume that this is close to the top of the planetary boundary layer over low lying ground. Wind speeds at this altitude are measured by weather balloons. The variation at the surface is much greater. On mountain tops, the average speed can be similar to that observed by weather balloons, at the other extreme are sheltered urban locations where the average speed maybe less than 2.5 m/s which is far too low for the economic operation of wind turbines. In open inland locations the average speed might be 5 m/s, whilst on a downland ridge it might rise to 6 m/s. A few miles offshore, the range could be 6 - 9 m/s. From the work done to date, there is an inference that average onshore wind speed decreases with increasing distance from the coast, but no obvious correlation has been obtained.
This work suggests that, from an energy harvesting perspective, coastal and offshore locations offer significant advantages resulting from higher and smoother wind flow. Against, these must be set the higher costs of offshore operation and construction. Most offshore wind farms are located in shallow water with the turbines mounted on a single pile, but there is growing interest in floating support structures such as the semi-submersible platform.
The attraction of semi-submersible platforms is that they minimize the motion caused by waves which is the reason they are extensively used by the offshore oil and gas industry drilling operations. They are also economically attractive for two reasons. Firstly the installation cost is largely independent of water depth and secondly because the construction can be completed at the building yard before the completed unit is towed to the installation site.
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