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Many people seem to complain about large wind farms. Instead of all that, there are many cities with all their tall buildings that actually 'magnify' the wind or augment it. Even corridors through a building can be a virtual wind tunnel. Why not put a lot of medium size wind-to-energy devices or small ones all over a city, wherever there is a known spot or area that augments the wind because of buildings or corridors etc.? (I even saw an old Popular Science magazine with a sci-fi style picture of a huge building with a big fan to generate energy built right in it..)

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  • $\begingroup$ Having answered this, I just realised that it's on Earth Science. I'm recommending its migration to sustainability. $\endgroup$ – Semidiurnal Simon Jan 14 '16 at 9:34
  • $\begingroup$ people complain about new stuff, especially if it is visible. they'll just get used to it eventually. $\endgroup$ – njzk2 Jan 14 '16 at 19:04
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This has been tried to some extent; Strata in London was a skyscraper that was built with three wind turbines at the top, with a deliberate design to funnel wind into them. There are other examples.

Picture of Strata, from Wikimedia Commons

Siting renewable energy generation in cities is very attractive, partly because many people feel that the "industrial" feel fits better there, but also because it locates sources of generation close to sources demand. For some other technologies, such as photovoltaics, this is indeed a good approach. However, it doesn't tend to work out that way for wind.

There are three main disadvantages to urban wind:

  1. "Dirty air" : Not what it literally sounds like (that's also true in cities, but wind turbines don't mind). The wind that is funnelled through, around and above buildings is sometimes accelerated, but it is also made turbulent, and wind turbines do not like turbulent flow. Turbulence increases the unwanted forces that turbines experience (i.e. forces that do not help them make electricity), and makes them less efficient.

  2. Economies of scale: The amount of power that a wind turbine can generate, in a given wind speed, is proportional to the square of its diameter. Which is to say, size matters. In the countryside, onshore rotors commonly reach 80m diameter (offshore are much larger). In cities, this size would not be practical or acceptable. That means that a much larger number of turbines would be needed to generate as much power, which would be more expensive and have more visual impact.

  3. It isn't all that windy: The power that a turbine of a given diameter can generate is proportional to the cube of the wind speed. Because of this, small differences in wind speed are very important to wind farms. Most cities simply do not experience such high windspeeds as rural hilltops do.

A fourth reason may be public acceptance; while people may view a few token turbines on rooftops as a good thing, the quantity needed to make a substantial contribution to national demand would drastically change a city's skyline, and might meet with disapproval similar to many rural wind farms. However, I have no evidence for this.

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    $\begingroup$ I'll mention Green Park in Reading that put a huge turbine in the middle of a glass-n-steel business park. Controversial at the time of construction, now quite accepted. And it hardly ruins the view as the skyline is already ruined by a bunch of glass-n-steel office blocks! Ironically it does not power the offices, but instead feeds into the national grid. $\endgroup$ – gbjbaanb Jan 14 '16 at 15:51
  • $\begingroup$ Maybe a lot of small wind turbines at specific places, not just at the top of buildings, could 'working together generate a fair amount of power. Cities like Chicago air known for their 'windy' quality; maybe energy could be made from this artificially accelerated wind. $\endgroup$ – 201044 Jan 18 '16 at 18:03
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There's just not that much kinetic energy in the air within the urban canopy layer; much of what there is, is in the form of turbulence, rather than the laminar flow that turbines harness.

Furthermore, air is very low density, which means that to get a decent amount of energy out of it, we need lots of effective rotor area, and even with the funnelling effect of buildings, it's not economically viable to get the rotor area required. A lot of small-to-medium sized devices within the urban canopy layer would still produce very little energy, and would cost a lot.

Furthermore, although the opposition to rural turbines is very noisy, it's also (in almost all the world) a small minority.

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