 |
|
|
|
|
|
 |
|
For the EBN feature article this month I spent weeks learning about building-integrated wind. I'm a huge fan of wind energy in general, and the idea of putting wind turbines on top of buildings — or actually integrating them into the architecture of buildings — was really appealing. Why not generate the energy right where it's needed, and by putting turbines on top of buildings wouldn't you be getting them up higher where it's windier? What a cool idea.
Unfortunately, as I point out in this month's feature article, "The Folly of Building-Integrated Wind," it's actually pretty hard to get wind turbines to perform well on buildings and, even if you can, the economics are not very good. A huge challenge is noise and vibration. Spinning things tend to generate noise and vibration, and that can be a big problem when people are occupying the building those turbines are mounted on. I went from being open-minded about the practicality of building-integrated wind to believing that it's usually a pretty dumb idea.
Another big drawback to building-integrated wind is that even though it's often windy on top of buildings, that wind tends to be quite turbulent. It's twisting around and not nearly as effective for wind turbines as laminar flow.
But a lot of rooftop wind turbines are being installed — how are they working?
It turns out that it's really difficult to find actual data on how rooftop or integrated wind turbines perform. You would think that information would be fairly available — after all, electric meters aren't that expensive. But wind turbine manufacturers seem reluctant to share that information; so do building owners. Getting hold of actual performance information on real projects was like pulling teeth.
I did find some data, however, and it's not a pretty picture. In one year-long study of rooftop wind turbines in the U.K., the average "capacity factor" was found to be less than 1% — while 10% to 30% capacity factors are typical for commercial wind farms.
This is not to say that there aren't some really well-designed, functional, attractive wind turbines on the market. There are. Probably the most thoroughly engineered product is the AVX1000 from AeroVironment, a California company made famous by its human- and solar-powered planes, General Motor's EV1 car, and the revolution in unmanned military planes. 
Aerovironment's 1-kilowatt turbine is specifically designed to harvest the accelerated wind at the parapets of commercial buildings. There's a row of 20 of these on a MassPort office building at Logan Airport in Boston. The AVX1000 is elegant and it works — but from my analysis, it's not as cost-effective as building-integrated photovoltaics (PV).
Vertical-axis wind turbines look even cooler than the more traditional, horizontal-axis machines, and they are usually quieter. But their efficiency is usually quite a bit lower.
We're seeing more and more green buildings that include wind turbines, and this worries me. These turbines aren't cheap; some of the vertical-axis turbines sell for $30,000 to $40,000. If they end up not performing as they are supposed to, it's going to give the green buildings they're installed on — and the green building industry — a black eye.
The mainstream media loves to cover green buildings that aren't operating as well as expected. If you have highly visible building-mounted wind turbines that just sit there failing to spin, or if it comes out in USA Today that these turbines have a dreadfully poor economic return, green building could take a hit.
What do you think? Use the comments field if you have an opinion on building-integrated wind. I'd love to hear your views.
Alex Wilson is the executive editor of Environmental Building News.
Comments
I had assumed that the vertical axis machines (whether mounted vertically or hoizontally) would work pretty well in urban areas with turbulent conditions. If this does not hold true, then I wonder what the benefit of this device is.
Perhaps a better building integrated wind application might be a solar updraft tower, where a turbine is placed at the top of a high air shaft. The air inside is heated and allowed to rise; the top of the shaft is pinched to accelerate wind speed. Green buildings like the Commerzbank already include an internal thermal chimney to induce ventilation; this application could be combined with the updraft tower. Internalizing the wind energy production would eliminate turbulence as a factor, even lack of wind speed.
One minor correction: There are many types of VAWT, and some function much better in turbulent winds than others. But the lift-type VAWT that do function reasonably well are not yet well known. They make of use of independent, passive blade pitching (very cheap and simple to do, yet accurate); each blade can quickly adjust itself to any change in the velocity of its apparent wind (the wind that each blade "feels").
Fixed-blade VAWT, lift-type, can suffer a relatively large drop in power in turbulent winds. N. C. K. Pawsey, in his online Ph.D. thesis calculated a comparative 28% drop in power for fixed blades in response to turbulence as compared to a 10% to 12% drop for passive, variable pitch blades whose pitch is controlled by what I refer to as a "centrifugal pendlum" type of control spring. The reason is that the passive, variable pitch blades don't stall, whereas fixed-blades do stall during part of their obit and/or over part of the blades. I hope to post information on my future website.
Thanks for helping everyone to do a reality check.
As you correctly say, wind turbines are very sensitive to turbulence, but perhaps new innovations are needed to better use of real life wind conditions. For example trees can absorb large amounts of the kinetic energy of the wind, but is largely wasted when it is transfered to a small amount of heat.
On the subject of innovative wind turbines, I saw that a novel roof turbine called the "RidgeBlade" turbine just won €500,000 in an eco friendly challenge run by the lottery in Holland. It was designed by a former Rolls Royce engineer who said it makes use of the wind flow over roofs. So one would assume that it works better than the conventional turbine, but no performance data has been announced yet.
|
|
|
|
|
|
|
|
|
Since I was trained as an engineer, I always tell them it's a stupid idea, for some very fundamental reasons. For example, I think you should have mentioned two key flaws in the case for wind on buildings, based on the simple physics of wind power: wind power is directly proportional to the swept area of the turbine and to the CUBE of the wind speed.
There is little swept area to work with on buildings AND most buildings (and cities) are not built in very windy places, for a good reason: a place with 10 to 12 miles per hour average annual wind speed (minimum conditions for an economically successful wind farm) is a very uncomfortable place to live.
Cities are in river valleys, not on ridgetops for a good reason (beyond access to water); they're more protected from wind. So, wind power on buildings loses out for two fundamental reasons: no swept area and no sustained wind speed of any consequence. That's why the capacity factors are so low.
As for wind speed increasing with height, the increase only goes as the 1/7-power of the height, so to get an increase of 40% in wind speed from say 10 feet above the ground (i.e., an increase equal to the square root of two), you'd have to increase the height by a power of 3.5 (or 30,000 feet).
As Paul Gipe (an old friend) says, a thoroughly bad idea, for all kinds of reasons.