Published on April 7th, 2014 | by Mike Barnard83
Vertical Axis Wind Turbines: Great In 1890, Also-rans In 2014
April 7th, 2014 by Mike Barnard
Vertical axis wind turbines (VAWT) continue to get attention, press and R&D funding. Antagonists of mainstream wind generation continually point at them as if they were a superior technology. People perpetually re-invent them and believe that they have found something new and exciting. However, they are undeserving of any significant attention, are an inferior technology and definitely aren’t new. Outside of a couple of niches, they are more of a distraction from deployment of effective utility-scale, horizontal axis wind turbines (HAWT) than anything else.
Simple drag vertical-axis wind turbines have a long history, and a version was used as the first electrical generating wind turbine ever, built by Professor James Blyth of Scotland to power his holiday home in 1887.
Even then, however, the advantages of horizontal axis wind turbines were being explored, as Poul la Cour of Denmark built the first HAWT with aerodynamic blades in 1891.
Fast forward 130 years, and there are about 300,000 utility scale HAWTs with a capacity of over 300 GW in over 100 countries around the world. There are exactly zero utility scale vertical axis wind turbines operating today, and yet the myth of their superiority persists.
Despite variances, there are really only two major types of vertical axis wind turbines:
- Savonius-style turbines are basic drag machines. They typically have rounded paddles which catch wind in the cup and shed it on the rounded front, allowing the difference in drag to rotate the turbine. They have been around for centuries in one form or another, but Savonius got his name to stick to them in 1922.
- Darrieus-style vertical axis wind turbines have aerodynamic blades which fly through the wind on their power strokes as they rotate around a shaft. Once again, origins of this are older than Darrieus himself, but he did create the patent in 1931.
The physics behind the superior generation capabilities of HAWTs over VAWTs is fairly explanatory with a couple of straightforward graphics. The graphic on the left shows that HAWT blades are always flying in the cleanest air possible, and are turned to face directly into the wind. Wake eddies do not interfere with generation of the device but dissipate downwind. The blades are always flying at optimal angle to catch the force of the wind in the swept area.
VAWT blades, by comparison on the right are in their optimal angle to the wind in only a small portion of their entire span, and their blades fly through air that they have made turbulent the majority of the time. That is of course for the more efficient of the two styles of vertical axis wind generators, the Darrieus-style ones. The Savonious-style ones actually push air out of the way on the return of the paddle, even more severely limiting potential generation.
The graph below is from the 2006 book by E. Hau., Wind Turbines: Fundamentals, Technologies, Application, Economics. Springer. Germany. 2006. It shows relative potential (and actual) generation efficiencies for various types of wind generators, with three-bladed HAWTs unsurprisingly at in the most advantageous position. It’s actually a much older diagram than that, but merely an obvious statement to include in textbooks.
As you look at the devices, you can also fairly quickly see that for a given swept area, more material is required for a VAWT than a HAWT. Combined with the requirement for VAWTs to have a larger swept area to achieve the same generation, this drives up material costs as well. This reduces the economics of VAWTs further.
So given that physics, testing, economics and experience clearly show HAWTs to be the superior choice, why do VAWT proponents and others still get excited over them? Well, a lot of it has to do with a negative reaction to the dominant, mainstream technology, and myths that get spread, as well as a misunderstanding of the compromises involved in wind generation. What follows is a collection of debunkings of the most common differentiators VAWT proponents claim for their technologies.
1. Utility-scale wind farms are set up where winds are relatively stable, so catching the wind from any direction isn’t a particular advantage
Proponents claim that VAWTs catch the wind from any angle, making them more effective than HAWTs. Catching shifting winds and eddies from any direction is only an advantage in small-scale situations such as urban or rooftop settings where the wind resource is so low and turbulent that potential generation is also heavily constrained. Horizontal-axis wind turbines (HAWTs) are set up in areas with steady winds and the blades are above ground turbulence that causes variable eddies. Wind testing is performed to establish the wind resource, and generally rooftops and low-to-the-ground implementations of any technology result in very poor generation potential.
Let’s take the Crissy Field Small VAWT Demonstration in The Presidio in San Francisco as a case study. The three types of vertical axis devices are tiny, with a combined peak generation of 6.8 KW, which is virtually non-existent compared to a single utility scale horizontal axis wind turbine which generally start at 1500 KW and go up to 5000 KW, 220 to 725 times the capacity. Given that they are low to the ground in substandard winds, their capacity factors will likely be lower than the typical 35% capacity factors (lower end) for modern wind turbines, more likely in the 20% maximum range given lower reliability. As such, a single utility scale wind turbine will likely generate 386 to 1,286 times more electricity annually than all five Crissy Field devices. As capacity factors for Category 1 wind resources now are in the 47% range and wind farms in the USA and Brazil are regularly experiencing 50%, it’s likely even worse.
2. HAWT masts don’t collapse due to lateral stress
Proponents and inventors claim that VAWTs produce less stress on the mast. This is engineering and economics. There have only been a couple of instances of HAWT mast failures in the history of wind generation; the engineering is very sound. This isn’t a problem that needs a different solution.
3. Modern wind farms generate a lot more electricity more quietly
Proponents and inventors claim that VAWTs are quieter. This is unproven actually, but the argument is that lower blade speeds results in lower noise, turning a problem into a virtue. There have only been two utility-scale VAWT farms or individual VAWTs built, a small farm of eggbeaters around 100 KW each and a massive 4 MW eggbeater in Canada. Neither lasted more than a decade in production, and no real noise measurements exist.
HAWT refinements continue to keep noise emissions at the same level or lower despite massive increases in size. Let’s take a Vesta V47 660 KW device. It has rated noise emissions of 102 dB. Compare that to a modern wind turbine of 2.5 MW capacity, the GE Brilliant 2.5 – 120 with it’s sound rating of 106 dB. That is a perceptible increase in sound, and as a result, setbacks from homes are typically further resulting in the same noise levels. But the comparison is worth taking further. 660 KW devices typically had 20-25% capacity factors, while modern wind turbines are in the 35-50% range. Larger wind turbines are also spread further apart to avoid wake interference. A comparison of single devices shows that the modern wind turbine generates about 5.5 to 8.5 times as much electricity in a year for a slight increase in noise emissions, but it’s wind farms that are important, not individual machines.
Assuming a historical wind farm of one hundred 660 KW devices and typical spacing of ten diameters downwind and four diameters sidewind, this would spread over roughly five kilometres by two kilometres. A wind farm of only 16 modern GE devices would exist in the same space with perhaps 30% more generation over a year. That’s sixteen 106 dB noise generators vs a hundred 102 dB noise generators for a significant reduction in overall noise near the wind farm, and on a per MW basis as well.
4. To generate the same electricity, VAWTs would have to be as tall as HAWTs, so visual impact will be virtually identical
Proponents and inventors claim that VAWTs have lower visual impact. This is only true for smaller wind generators, closer to the ground, which would be true for smaller HAWTs as well. This is trading off making it less conspicuous for making it less productive which isn’t particularly useful. To make them economic for grid-scale generation, they would have to be so big that they would still be very, very visible. As pointed out earlier, they would required a larger swept area to achieve equal generation. VAWTs only really lower visual impact in some cases as some designs are sculptural objects that happen to move, but this is also a very subjective point.
5. Shadow flicker is only a problem at dawn or dusk for few minutes a handful of weeks out of the year for any given home near a wind farm
Proponents and inventors claim that VAWTs don’t generate shadow flicker that is problematic, as that is a factor of the triblade design. Shadow flicker is a vastly over-stated concern. It will only occur for a few minutes at sunrise or sunset for a week or two twice a year at some residences near wind farms. HAWT rotation is too slow to cause epileptic seizures (and there are design studies and standards to ensure that this is true). Wind farms typically try to assess their impact on local dwellings and adjust where possible, and all three major wind farm siting tools — WindPro, WindFarm and Windfarmer — includes shadow flicker modelling. Anti-wind advocates drum it up, but you pretty much have to be looking for things to hate about wind turbines to think it’s a problem. VAWTs sufficient in scale to generate utility levels of electricity will still be hated by NIMBYs.
6. If all fossil fuel generation were replaced with HAWT wind farms, million fewer birds would die annually
Proponents and inventors claim that VAWTs will kill fewer birds than HAWTs. As HAWT bird mortality rates are typically vastly overstated and are much less than fossil fuel generation, lighted windows, cats, transmission lines, cars and many other sources of avian mortality, this is a straw man argument. As VAWTs scaled up for utility generation capacity have not been assessed for avian mortality, it’s a straw man argument without merit, similar to the noise problem. The table below is from just one of the cross-generation form studies of wildlife impacts, this one from New York State. It points out that of the major utility-scale forms of generation, wind energy is better than most existing forms in virtually every category for birds and other wildlife.
For context, in the USA roughly one in 86,000 birds are killed annually by wind turbines.
New research is interesting, not promising
Jon Dabiri is the only researcher finding new and interesting things with VAWTs today, as he assesses interactions of more closely spaced VAWTs to see if fish schooling vortex studies apply to Darrieus-style wind turbines. He has so far found that his theory of putting counter-rotating VAWTs in more closely spaced arrays is resulting in greater generation than they would be projected to have by themselves or in more widely space arrays. However, based on a flawed underlying assumption of 100% use of land by mainstream wind farms, he’s vastly overstating the significance of his findings. His work might result in new areas where it is economical to put arrays of VAWTs, but it is very unlikely to disrupt the mainstream wind industry.
Offshore wind generation has recently been raised as the new hope for a niche for large scale wind turbines, however assessment of all of the efforts in this space recently found only stalled research projects and small scale prototypes for remote site generation needs.
In summary, vertical axis wind turbines are historically interesting, but irrelevant to utility-scale generation and insignificant as a wedge against global warming. Continued attention to them merely distracts from the much more useful effort of deploying horizontal axis wind turbines, and is used as a wedge by anti-wind campaigners.
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