The Scoop on Vertical Axis Wind Turbines, Part II

| 5/1/2009 3:43:11 PM

Tags: Dan Chiras, wind, renewable energy,

In my last blog, I noted that vertical axis wind turbines (VAWTs) leave much to be desired. My point was that these designs, which have been around for thousands of years, just haven’t panned out. Buyer beware: they’re not all they’re cracked up to be (See An Open Letter-To inventors of Vertical Axis Wind Turbines and Rooftop Wind 'Technology Breakthroughs'.) I wasn’t trying to discourage people from tinkering with them, but I was trying to warn folks who think they’re going to be making a wise investment from buying one.

Many of you posted comments to the contrary, either accusing me of stomping on the dreams and aspirations of young inventors or simply not knowing what I was talking about. Some readers accused me of making baseless claims without any science to back up my assertions.  Let me provide some more information on VAWTs.

While many modern VAWT inventors show videos of their turbines spinning, which convince news organizations and potential buyers of their value, it’s not spinning blades that matter. What matters is energy output. Because wind speeds are low at ground level, VAWTs won’t produce much energy — nowhere near as much useful energy as a well placed horizontal axis wind turbine. That’s why horizontal axis wind turbines are the technology of choice for most applications. 


Wind Speed Chart 

The main reason that wind speed is so low at ground level is ground drag. Ground drag is caused by friction when air flows across a surface. Friction is the force that resists movement of one material against another.

When wind flows across land or water, friction dramatically reduces the speed with which wind flows over land.  Ground drag due to friction varies considerably, depending on the texture or roughness of the surface. The rougher or more irregular the surface, the greater friction.  As a result, air flowing across the surface of a lake encounters less friction than air flowing over a meadow. Air flowing over a meadow encounters less friction than air flowing over a forest.

Thomas Loeber Jr.
7/8/2012 3:59:33 AM

I think this article alters evidence to support a theory. HAWTs have serious short-comings too, orientation, the blades need to have a fair amount of mass which means momentum is a significant factor, a surface is moved back into opposing wind subtracting from the output efficiency, a counter-rotating generator cannot be used to full potential due to the need to make one set of opposite spinning blades smaller for orientation. VAWTs don't have a problem with orientation. So far implementations of VAWT schemes have used relatively strong wind area surfaces that incorporate a fair amount of mass, return a significant surface into opposing wind that subtracts from efficiency, and their size makes a counter-rotating generator impractical. What if all these problems could be solved by a specific sort of hybrid between VAWTs and HAWTs. I think that exists in the Hotine vane assembly, as depicted in the video you can see of a Tinkertoy(R) model of a single vane assembly on Youtube. I've tested a dual vane assembly system that spun quickly, smoothly and quietly with about 6 mph wind that was constantly changing direction. That was with the Tinkertoys(R) again, only their small stock vanes, no bearings and just pressed assembly. It did not destroy itself as so-called flapping vane VAWTs have done in the past, shaking themselves to bits. As far as I can tell the Hotine scheme allows the use of sails for the wind capturing surfaces, immediate orientation to wind coming from any direction, maybe more than 90% feathering of the opposing surface returning into the wind, full use of a counter-rotating generator which can double output, and the ability to be relatively small, built using home workshops, wont need dedicated towers and can be put in those places where the wind is naturally focused easily, removed for maintenance easily, incorporate protecting bird cages easily, allow the use of cone deflectors to double effective wind hitting the vanes from any direction. The patent on the Hotine vanes has expired. Another person who has claimed to come up with the idea, you can find via a search for "the chopper human wisdom" with an animation, released the idea into the public domain. I don't think you can patent the idea of using a counter-rotating generator. The best idea will be extremely cheap and easy to build anywhere and not conducive to making any one person controlling the technology some money.

William Holladay
6/14/2009 8:34:17 PM

U guys are almost getting it. T. Boone Pickens is opposed to VAWT - he has put his money on HAWT. cogitate that in the discussion of VAWT vs HAWT. William Holladay ps advocate of VAWT

Steven Kay
5/22/2009 3:42:28 AM

The torque distortion and stress of a vertical turbine is easily eliminated by multiple collectors so fashioned as to turn in opposite directions. The actual directional stress is less easier to resolve. A spiral vane angled into the wind on accumalator side will be angled away from it on the opposite returning side. This is used in ultra high rise structures to decrease wind induced stresses. However I doubt any design is cost efficient at 80 ft since the surface area is great even if wind coefficiency is only 20 or so percent drag. Severe weather condictions would make the towers necessary heavy construction and subsiquential costs prohibitive. Lowering the turbine seems likely to be as great of an expense as well as a logicistal nightmare.

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