wind properties vs. altitude

It is important to understand that the properties of wind are very different at different altitudes. Near the earth wind is greatly affected by ground features which affect speed, consistency and changes in direction whereas at the altitude of 30,500 feet the speeds are much greater and the flow much more uniform. This may be seen dramatically on wind maps sliced by altitude on the Unisys website. Note particularly the very different wind speed legends, (0 to >100, 0 to <30, both in knots), at the bottom of these maps for the most recent noon hour. In addition to the large difference in wind speeds, note the much greater variety in wind direction in the surface map.

The subject of turbulence is also very different at high altitude than it is just above the ground. Ground friction and features not only result in low wind speeds near the earth’s surface, but also cause wind shear and heat differences which are a constant source of problems resulting in component fatigue for tower mounted wind turbines. FEGs fly above these problems, but must cope with other turbulence problems at high altitude. The two sets of problems which engineers must face are very different.

Helen Chadwick of De Montfort University in the UK is well known for her course in wind energy capture technology. The first paragraph in the “Wind Energy Training Course” reads as follows:

“The Planetary Boundary Layer

The strongest, steadiest and most persistent winds occur in bands at the jetstream level some 10km above the earth’s surface. Unfortunately present technology has not overcome the problems of building 10km high wind turbines! Wind turbines are limited to the lowest few metres of the atmosphere. At these heights the wind is directly affected by the surface, through friction. Wind speeds are thus lower. This layer of the atmosphere is known as the planetary boundary layer.

The planetary boundary layer grows throughout the day as the thermal heating increases. It varies in size from a few hundred meters at night to as high as 2 km on the most convective days. The transfer of momentum, heat and moisture, between the atmosphere and surface, take place within this layer. The processes occurring in the planetary boundary layer include convection, rotation and fully turbulent flow. It is amongst the most complex phenomena of fluid dynamics. A good understanding of the processes occurring in this layer is essential both for optimum siting of the wind turbine and for an understanding of the loadings that the structure will be subjected to.”

As may be seen, the course article then goes on to describe the problems engineers working on tower mounted wind turbines face in coping with the wind shear and turbulence problem which are to be expected where these tower based wind turbines are located, that is, at the very bottom of this Planetary Boundary Layer, usually called the “Surface Layer”.

The characteristics of the “Geostrophic Layer”, in which Flying Electric Generators will normally operate, and which starts above the Planetary Boundary Layer, are quite different, and, as the course article indicates, statistically much better.

While turbulence does occur here also, normally, this body of air is stable, moving steadily, resulting in correspondingly steady forces on the FEG rotors and steady power generation. When turbulence does occur, the FEGs may be moved to a different altitude, or wait out the problem protected on the ground if turbulence is great at all altitudes, such as in storms.

Nevertheless, FEGs must be capable of handling the much less frequent types of turbulence which do occur at high altitude. This subject is covered in the section “Flying Electric Generators”.

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