A tornado not only twists the air, it also sucks it upwards. As air is drawn into an eddy, it swirls faster and faster the nearer it gets to the center, but it also gets thinner. In fact, if air were better trained in theoretical aerodynamics, there would be no air at all in the center of the vortex, and this no air at all would have an infinite velocity. Fortunately, real air has friction and other properties which prevent it from qualifying as an ideal gas and behaving exactly as the textbooks would like. But it comes close: Near the center of the vortex there is very little air, i.e., the air has very low pressure. The outside air rushes into this partial vacuum by the only way open to it, the bottom of the vortex tube, and as it swirls upward, it draws in more air.
The upward draft is concentrated into a small cross-section (which is what makes tornadoes so deadly in a small spot at a given time), and its velocity is determined not by the general wind conditions around the tornado, but by the difference in pressure between the outside air and the vacuum in the center of the tornado tube. Put a small wind turbine in this upward draft, and you have the ideal wind power generator; all you need is a tornado.
So let's twist a little twister, said Yen, and took a vertical metal tube with slots on the windward side (actually slots all round, but the others are closed by movable vanes). As the wind blows into the slots, it is deflected and spirals upward, forming a well-behaved little tornado inside the pipe. A wind turbine with small diameter is then placed under the pipe at the top of an inverted funnel leading the air into the pipe. In the full-scale version, the pipe would be a tall tower, secured by anchoring cables.
The wind blowing over the upper end of the pipe blows away the top of the vortex, restoring atmospheric pressure above the pipe and maintaining low pressure inside it. This slows the wind above the pipe, so that energy is fed into the system by winds that never come into contact with any mechanical structures. Moreover, the only moving part of the arrangement is the turbine, which is not in mechanical contact with the pipe and is mounted quite low above the around. It is not even exposed to the incoming winds. All of which eliminates the vibrations, stresses and mechanical instabilities that kill the design of an orthodox windmill before it can reach respectable power levels.
Yen speculates that he can get as much as 1 MW from a turbine only about 6 ft in diameter, the (fixed) pipe above the turbine perhaps 200 ft tall and 70 feet across. He even envisions single units of several hundred MW, which seems overenthusiastic, but even 1 MW would be impressive; certainly more so than a $3,000 windmill that will, at times, run a coffee percolator.
[More: Science, 10/17/75]
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Vol. 3, No. 4
Newsletter: Access to Energy Newsletter Archive Volume: Volume 3 Issue/No.: Vol. 3, No. 4 Date: December 01, 1975 11:16 AM Title: Selective Morality
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