The estimated potential of wind power currently stands at 3600TW1, much higher than the 15TW required worldwide on a daily basis. Nevertheless, wind power is limited at low altitudes, as estimates show an exploitable share of only 72TW2 , in-land at altitudes below 100m. This is only 2% of the entire potential, as power density is much higher at greater altitude (see Figure 1). Up until now electricity production from wind has been derived exclusively from wind turbines, using only low altitude winds which suffer from low speeds and an inherent intermittent nature (Figure 2) associated with the surface boundary layer and natural terrain orography.
The use of high altitude winds, although presenting a tremendous challenge from an engineering point of view, is capable of providing a solution for both problems mentioned above: low wind speed and wind intermittency. Additionally it carries with it a paradigm change vs. the current rationale which drives wind turbine installation: the best locations for a high altitude wind installation are not directly connected with any orographic expression but solely with the globe latitude at which the equipment will be installed. The reason is that, above 1000-2000m the prevalent winds are geostrophic winds, commanded solely by the earth�s latitude and the season of the year (stronger blowing in the winter of each of the hemispheres).
Figure 3 shows the wind energy density available with the altitude around the world in the months of December to February. The altitude is plotted in the two vertical axes (meters on the left axis, feet on the right axis), while the latitude is plotted on the horizontal axis (the southern hemisphere is in the left half of the figure, the northern hemisphere is in the right half, and the equator in the center). The wind energy density gradient is plotted in contour lines with colored areas between them, ranging from white (very low energy density) to deep purple (very high energy density). In this graph a shaded rectangular area highlights the latitude range from southern to northern Europe. It clearly shows that Europe is located in a latitude range where wind energy is available with high density levels, in particular the southernmost European countries, which are located in latitudes where very high wind energy densities are available at altitudes ranging from 4000 to 15000m.
As can be seen, even at an altitude of 2500m the wind power density can be roughly 5-10 times higher than the average wind power density available for a large wind turbine. The mathematical reason is evident from the equation of available energy present in Figure 3: the formula for available energy depends directly on the density of air (?) and on the cube of the wind speed (�3). This means that, if the average wind speed doubles in going from an altitude of 100m to 2500m, the wind power density will go up roughly by a factor of six5. The factor of 8 (23) which would normally be applied due to the wind speed increase should be reduced by 25% due to lower air density at the higher altitude. Thus an overall increase factor of six is achieved.
Apart from low altitude wind limitations, one is also confronted with limitations of the current technology. Chief amongst them are installation requirements, as current technology wind turbines are installed solely on land or in shallow waters. Also, for turbine towers to continue to grow in power it is required that they continue to grow in size. As the rotor diameter gets larger, the cost grows nearly exponentially (see Figure 4). Since tower weight increases with the cube of height7 a practical limit exists, with today�s technology and materials, as complexity and cost become prohibitive above 12-15MW.
The arguments above raise the following issues:
� Should current existing wind power systems with their inherent benefits of a lower risk approach and possible short term profitability continue to be developed?
� Should the focus be on a revolutionary (as opposed to an evolutionary) path in which wind will no longer be viewed as a scarce resource, thus dispensing with concerns for the efficiency of the operating systems, but on one that presents challenging engineering and research problems in an initial phase but which will be capable of yielding much higher capacity factors and power densities per unit of swept area in 5-10 years?
It is the consortium�s belief that the latter path, that of a revolutionary development should be undertaken. The priorities set by this consortium are clearly to: focus on high altitude systems for enhanced power densities and for a system in which the operating height can be changed in order to attain the optimum operating speeds required to yield an increased capacity factor. Both of these features call for the elimination of the use of a fixed tower, with the added benefit of system portability, reduced installation costs and easier installation over water with open potential for deep off-shore installation. These advantages have already been recognized by the internet giant Google which in 2007 acknowledged publicly that it foresaw only two renewable energy technologies as having the potential to produce energy at competitive prices with coal fuelled power plants. High altitude wind without fixed towers is one of the two technologies supported by the initiative Re‹C (renewable energy cheaper than coal) which has already invested over $10Mn in US based companies.
1 Source: Saul Griffith (Makani Power)
2 Source: Stanford University
3 Source: Makani Power4 Source: Makani Power
5 Example given for 7,5m/s of wind speed at turbine height vs. 15m/s of wind speed at 2500m of altitude.
6 Source: Laurence Livermore Lab. calculations based on European Centre for Medium-Range Weather Forecasts data.
7 Source: Wikipedia
8 Source: Fingersh, M. Hand, and A. Laxson, Wind Turbine Design Cost and Scaling Model, NREL 2006
