A number of posts have described extremely large wind machines that use flow accelerating structures. The circular wind dams are a good example. Since these machines are able to concentrate the flow of a very very large “streamtube”, you might wonder what the cut in wind speed would be. Seems like power could be produced from the smallest breeze. This is a very interesting characteristic, since it seems to suggest an improvement in the most glaring deficiency of wind energy – its low capacity factor. High altitude ideas like the Sustainable Skyscraper may provide an even greater increase in capacity factor, since average wind speed increases with altitude. I don’t crunch any numbers for this blog, but that doesn’t matter since I’d have no idea how to approach the problem of quantifying this improvement anyway. But it certainly seems like an issue that is worth a thorough investigation, particularly when you consider that the high capital cost of the fixed (non-turbine) part of a piece of aeroarchitecture may lead one to conclude that these ideas are not economically competitive. I think it would be a mistake to jump to that conclusion before developing an understanding of the capacity factor issue.
Another good reason for investigating the capacity issue is that it may impact the optimum scale of some of the machines here. Consider for example the circular wind dam. If this machine becomes so large that many rotors (say 10) are dispersed about the perimeter of the dam, then you might wonder whether the advantages of scale have been exhausted. And in this case you might draw the conclusion that building several smaller dams instead of one big one would do just as well. So I am just suggesting that the effects of scale on capacity factor should be examined before arriving at this conclusion. (Can you tell that I am biased toward thinking that capacity factor will be proportional to scale? I realize that this is probably not true. But is it true up to a point… or what?)