Wind power at 30,000 feet is what the start-up Sky WindPower is proposing. They would use clusters of Flying Electric Generators (FEGs), on the end of a current carrying tether, in the jet stream at 15,000 to 30,000 feet. The company has done wind tunnel tests and low altitude tests to prove their idea. They are hoping to build a 200 kw model, flying at 15,000 ft, somewhere in a remote area of the U.S.
They envision a commercial FEG will have four or eight rotors each generating 2.5 MW. Clusters of FEG's could provide as much power as need for a given site.
The FEG would fly up into the sky, with its rotors powered by electricity off the grid, pulling up its tether. Once it at its desired altitude it would change the pitch on its rotors and start generating power from the wind. GPS technology would be used to assure that the rotorcraft stays within a few feet both horizontally and vertically of where it is programed to be and a computer would control the rotorcraft's attitude, i.e. pitch, roll and yaw.
The amount of power that you can produce in a wind turbine varies as the cube of the velocity and linearly as the density. So although the density decreases with an increase in altitude, the increase in velocity that you gain with higher altitudes more than makes up for the decrease in density. This further explains the advantage that FEG's have flying at high altitudes and allows the rotors to be smaller in diameter. The wind speed, in addition to being higher, is more uniform.
It is much more steady, blowing at high, useful velocities a much greater percent of the time than do winds at ground level. This gives FEGs the advantage of having a higher capacity factor. Capacity factor is the percentage of energy actually captured relative to what would be captured if the wind turbines were operating at full capacity all the time. Ground based sites that can produce a capacity factor of 35% are hard to find. Capacity factors in the jet stream range from about 70% in the southern parts of the U.S to over 90% in the north. At a capacity factor of 90%, FEGs could become the nation's cheapest source of electricity, with an estimated cost per kilowatt hour of less than 2 cents, about half the price of coal.
Also the wind tends to blow from one direction and when it changes direction it changes slowly. The air is free from turbulence caused by friction and ground protuberances, but is subject to high altitude turbulence. This is moderated by the ability of the FEG to sway on its tether rather than being attached to a ridged tower. The FEG can move up and down on its tether the same way an airplane does to avoid turbulence.
The use of tethers to position objects in the sky is not unknown. Balloons tethered at altitudes up to 15,000 feet exist now at fifteen sites along the southern borders of the United States carrying radar equipment to detect illegal flights from the south trying to smuggle drugs. These sites are shown on all aeronautical charts, as restricted spaces, and are well known to pilots. By reserving less than one four hundredth (0.4%) of U.S. air space, located at relatively remote locations, not on airway routes, all the nations electrical energy needs could be met. The strength to weight ratio of new tether materials has improved over time so much that tethers now available are no longer too heavy to be held up by flying energy generating devices at the needed high altitudes. Tether technology is not simple, but a number of vendors now compete in this field selling primarily to the military and NASA. The FEG would be able to moved up or down on its tether to seek the best wind conditions or to move out of excessively turbulent air space.
Sky WindPower, San Diego, CA, http://www.skywindpower.com
Windmills in the Sky, Popular Science, September, 2005
Windmills in the Sky, Wired News, 4/6/05
Flying windmills, Urban Renaissance Institute, 3/19/05 (free registration required)