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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.

Interestingly, friction slows wind speed to a height of about 1,650 feet above the Earth’s surface. However, the greatest effects are closest to ground — the first 60 feet (20 meters) above the ground over a relatively flat, smooth surface. 

The effect of friction is quite profound. A 20-mile-per-hour wind measured at 1,000 feet above the surface of the ground covered with grasses, flows at 5 miles per hour at 10 feet — where most VAWTs are located. It then increases progressively until it breaks loose from the influence of the ground drag or friction at around 80 feet.

Because the effects of friction decrease with height above the surface of the Earth, savvy installers typically mount their wind machines on towers 80 to 120 feet (24 to 37 meters). This removes them from the influence of energy-robbing ground drag. At these heights, the winds are substantially stronger than they are near the ground.

Savvy installers also avoid suburban and urban environments because the surface texture is quite rough in these areas. Trees and buildings dramatically lower annual average wind speeds. They also create a lot of turbulence and eddies — pockets of relatively dead air. Place any turbine in this environment and you can expect significantly reduced energy production.

I like the looks of a lot of VAWTs and wish inventors success, but don’t lose track of the fact that there’s very little energy in ground-level winds during a given year. In fact, a recently published study of wind turbines mounted on buildings in England showed that many turbines failed miserably when it came to energy output. Average wind speed at these levels were just too low to produce a significant amount of electricity. The inverters in many of these systems consumed more energy than the wind turbines produced in a year. See the results of the Warwick Wind Trials.

When you invest in a wind turbine, you want energy output. It’s not spinning blades, but electricity that most of us want — and lots of it — to make our investment worth the while.

Power available from the wind is a function of the wind speed cubed. If the average annual wind speed is low, which it is at ground level or even on the roofs of homes, you just won’t get much energy from a turbine — any kind of turbine. (See Rooftop Wind-Determining Your Resource.) Mounting a turbine at ground level places it in much weaker winds at most locations. It is a bit like mounting solar panels in the shade! NREL’s wind energy expert Jim Green says, “For a given swept area, VAWTs just don’t extract quite as much wind energy as a well-designed HAWT.” 

I’d be happy to be proven wrong. Show me the data that indicates that a ground-level or even a roof-top turbine of any sort produces a sizeable amount of electricity during a year!

It is important to note that years of experience with VAWTS has been rather discouraging, to say the least. “Hundreds of commercial VAWTs were installed in California in the late 1980s and early 1990s,” according to energy consultant Bob Aram. “They all failed and were removed from service. These were not experimental units, but production units.”

 

 



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Post a comment below.

 

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 holladay@ymail.com 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.

Jason Giesen
5/18/2009 3:27:40 PM
The significance of the output can not be determined without addressing the cost. A home built, home maintained VAWT can easily be more cost effective than a HAWT that you have to buy and pay someone else to install and maintain. The point should be that everyone who endeavors to conserve and/or create energy needs to do what they can in the most cost effective and responsible means possible.

Hytekhik_1
5/16/2009 1:32:25 PM
It would seem plausible to put a huge Savonius Rotor VAWT on top of an 80-120 foot tower simply to remove the gyroscopic effect of having to turn a HAWT to follow the wind, and also to occupy much less space between the towers.

cathrynm@whale-mail.com
5/16/2009 10:02:16 AM
Science and scientists throughout the years have proven themselves resistant to and even refuse to consider alternatives to their own closely-held beliefs. Vertical axis wind power has fallen behind because, just like gas-guzzling cars versus hybrid/electric vehicles, due to the aforementioned resistance. That does NOT mean it is, as stated, less economically viable or the output negligible for the monetary input. Both the websites below provide excellent data sheets on their vertical axis wind turbines, the second one should provide ALL the information needed to effectively disprove the majority of what is mentioned in this blog. As the saying goes, you have to see with better eyes, and sometimes, you must set aside your own strongly-held beliefs to truly see something and evaluate it without prejudice. http://www.urbangreenenergy.com/turbine.php?id=4 http://www.quietrevolution.co.uk/

JD Polk
5/16/2009 9:08:07 AM
Does anyone remenber the Vertical axis turbines about 15 or 18 years ago @ EPCOT in orlando. these were the most efficient I have ever seen... they looked like a telephone pole with holes drilled in them in such a way no matter wich way the wind blew from they only spun one way? they were about 20 to 30 feet tall and were actually low to the ground in front of the entrance to Tommorrow land. please let me know...SolarmanJD@DCemail.com

Gryphon327
5/15/2009 10:01:34 PM
When I read the first article about VAWTs I decided to see what I could come up with to increase efficiency. The idea I had was to create vanes that folded. This allowed the side leading into the wind to be a much smaller profile than that moving with the wind. This would reduce drag, but the downside being the wear or friction involved in the constant opening and closing of the vanes. Just an idea that might help someone inventive.

Keith Hallam_1
5/15/2009 5:30:21 PM
Hi Dan. I, along with many others I guess, have no problem with your info on ground drag. I think what is more important what you don't say. So, is a 'H' more efficient at converting wind into useable energy than a 'V', under the same conditions? I can't begin to work out how to compare two such dissimilar machines. A 'H' has a circular surface area but little power is generated nearer the roots of the blades. A 'V' has a regular surface area but half of it is moving towards the wind force. In the end, perhaps it is just a cost comparison? What is the total cost of a 'H' compared to a 'V' for the same useable energy output under the same conditions??

MJ _1
5/15/2009 9:35:44 AM
Here's your requested different vawt to disrupt your security... www.aerotecture.com including the data on harvested energy among the links as well as photos of installations with history and interesting heights. Nice urban potential on commercial rooftops. MJ

eric_3
5/15/2009 8:44:08 AM
This is not meant as a detraction by any means, but I have found that different situations abound(surprise), and that wind speeds may be very high near the ground. The information on the drag coefficient near the ground is correct, but as with most things there are exceptions. For example, my home is located in the 'high plains' on the top of a hill that is about 6000' above sea level. We average about 30 mph winds at the surface fairly consistantly and often much higher speed winds are present. In combination with the drag, there is also the build up of pressure when a large air mass is pressed up against a hill which creates an artificial wind 'concentrator'. In this type of situation, a VAWT would be very logical. I do prefer the HAWT though, and am in the process of building one that will sit less than 20' off the ground due to extreme wind speeds that exist higher. So, certain situations can exist, but as with most things, generalities are the rule, not the exceptions that we argumentative humans often focus on. Enjoy your day, and happy living!

Daniel Skidds_1
5/13/2009 4:14:19 PM
Whats not to say that a vertical could be placed on a pole. And I would have to believe that a pole mounted vert would have to have less torque on that pole based on design, in theory---Im not a math whiz. But I would agree on your arguement on lower wind speed closer to the ground, but you make your point as that only horizonals could be placed on poles much higher off the ground.

Dan Bossenbroek_1
5/10/2009 11:17:03 PM
Thanks for the update Dan. It sure would be great if they worked. Dan Bossenbroek







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