I saw an article on ecogeek.org which talked about the possibility of harnessing winds at high altitudes. At high altitudes of around 30,000 feet, where the jet stream lies, the wind power density is every high (10 kW/m2 on average). This is about ten times greater than at ground level. As well, the wind blows much more steadily than at ground level. Researchers estimate these high-altitude winds blow around 95% of the time, making them very reliable for power generation.
Aircraft frequently ride the jet streams because it reduces trip time and fuel consumption.
Research published by the Carnegie Institution of Washington and California State University shows that high altitude winds can power the world 100 times over. The title of the report is “Global Assessment of High-Altitude Wind Power”.
It’s estimated that current designs for high-altitude wind generators can generate 40 MW of electricity.
Clearly, the potential is there, but the problem is of an obvious practical nature: How to practically harness that energy for use back on earth?
30,000 feet is about 9 km — that’s a very long transmission/support cable to be tethered to the ground from the wind generator. Plus the weight of it (since it would be made of metal) would be great. For example, if a 9 km long steel cable of 1 cm diameter were used, it would weigh almost 6 tons.
The greatest force acting on the cable would be at the top since it has to support the weight of the entire cable below.
Mathematically speaking, cable selection is largely independent of diameter (in terms of strength). So there is no “optimal” diameter when determining the required cable strength. For instance, a larger diameter cable would be stronger but would have to support its greater weight, so beefing up the cable is not necessarily advantageous. Thus, proper cable selection is primarily based on material type.
The basic criteria to consider when determining the proper cable, is flexibility and strength. As it turns out, the cable would have to be made of very strong material, and would likely have to be specially designed. But even the strongest cables would probably be maxed out at the top, due to the sheer weight of the cable hanging underneath. Furthermore, there would be significant “tugging” force on the cable when the wind is blowing. This would stress the cable even more.
And then there’s the issue of how to keep the whole system aloft.
My thinking is that the only reasonable way to keep it aloft would be with large balloons which could perhaps be filled with helium, using (perhaps) electric powered rotors (propellers), for position control.
Using Archimedes buoyancy principle, I estimate that, at 30,000 feet (taking into account the density of air at this altitude and the density of helium), it would take one spherical helium-filled balloon with a diameter of around 30-40 meters, to hold up the 1 cm diameter 9 km long steel cable.
An alternative is to tether much smaller balloons all along the length of the cable. And at the top, you would have separate balloons to hold up the wind generator. The figure below illustrates this concept.
This reduces the need for several really large balloons at the top holding everything up, and reduces the stress on the cable, since it’s essentially “held up” at locations all along its length.
However, I still think this is a “pie in the sky” idea (no pun intended). From a construction and maintenance standpoint, it’s just so difficult and impractical to achieve, and possibly dangerous, to have so much heavy hardware just dangling from the sky.
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