Wind Turbine Hawt vs Vawt

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Why aren’t vertical-axis wind turbines more popular? 2013/02/23 · by Mike Barnard · in Smart Grid Some groups and individuals tout alternative technologies for wind generation, especially vertical- as opposed to the horizontal-axis wind turbines normally used. They make a variety of claims about power generation, noise, bird friendliness and others that in their minds make it a superior technology. So why aren’t vertical-axis wind turbines (VAWT) in their various forms more broadly used? Short Answer: Vertical-axis wind turbines trade one set of compromises for another. At this point, no one has been able to make the balance work out in their favour except in niche roles. And, frankly, they are solving non-existent problems. Long Answer: It’s worth starting with some examples of the various forms of vertical axis wind turbines out there. Here are a couple of examples of refined vertical-axis wind turbines with aerodynamic blades:

There are less efficient and refined wind generators as well, based on very old principles such as the Savonius design (named for a Finnish engineer who created a variant of it in 1922), examples of which are shown below.

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.

Here, for comparison, is a set of offshore, highly refined, triblade, horizontal-axis wind turbines (HAWT):

Their primary characteristics are three aerodynamic blades facing the wind and rotating around a horizontal axis. VAWT Proponents Overstate Advantages 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 and inventors 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. Horizontal-axis wind turbines (HAWTs) are set up in areas with steady winds and the blades are well above ground turbulence that causes variable eddies. VAWTs’ advantage is only in niche environments. [2]

Coast wind farm courtesy of Green is Sexy 2. VAWT blades are rarely at an optimal angle to the wind or in clean air, so they can never be as efficient as a triblade HAWT and won’t generate more electricity Proponents and inventors claim that VAWTs generate more electricity than HAWTs. Point 1 above dispels this, but what else comes into play? Energy generation is a factor of surface area of blades exposed to wind plus aerodynamics of the blades (plus other factors of diminishing returns). 4.5 MW HAWTS have massive spans with lots of blade surface catching wind, very aerodynamic blades (with variable pitch and surface area along their length to account for varying speeds) and the three-blade standard allows relatively clean air for them to spin through as the wind carries blade eddies downwind before the next blade passes through. VAWTs on the other hand have extreme difficulty achieving the same balance of surface area, aerodynamics and ‘clean’ air for best laminar flow. The majority of the time the blades are not presenting optimal surface area to the oncoming wind, but rather very sub-optimal surface areas. Only the blade in the very front is in clean air; as the blades rotate, they pass into very turbulent air with much lower efficiency of aerodynamics. The odds that a VAWT of equivalent area will generate as much electricity as a triblade HAWT are virtually nil, and they typically will use more material to generate less electricity resulting in an even lower lifecycle cost of electricity. [2], [5]

3. HAWTs almost never collapse due to lateral stress, and VAWTs typically have very asymmetrical front and rear stresses on their bearings Proponents and inventors claim that VAWTs produce less stress on the stalk. This is engineering and economics. HAWTs collapse extremely rarely; the engineering is very sound and the full-lifecycle cost analyses show that they are actually the fastest payback form of electrical generation in the world. At present, VAWTs don’t generate enough electricity that the full-lifecycle accounting shows them to be advantageous on a cost or materials basis over HAWTs. What problem are they solving? [1], [5] 4. HAWTs have been getting quieter as they get larger, and modern wind farms generate 10 times the electricity with less total noise than older wind farms Proponents and inventors claim that VAWTs are quieter. This is unproven actually. The first larger scale, 2 MW VAWT prototype is just being constructed; noise characteristics are undefined as yet. And of course, HAWT refinements continue to keep noise emissions at the same level or lower despite massive increases in size; a 4.5 MW wind turbine is only a dB or two noisier than a 1.5 MW wind turbine, which is much quieter than older 600 kW wind turbines. And 4.5 MW wind turbines are spaced much further apart than smaller wind turbines, so the total environmental noise is actually much less for significantly greater power output. Wind energy is actually remarkable in that it is actually quieter the bigger the generation factor. This is not true for any other from of electrical generation. VAWTs aren’t proven to be quieter and are competing with technology which is already very quiet. [3] 5. 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. 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.

6. 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 problem. It will only occur for a few minutes at sunrise or sunset for a week or two twice a year at 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. [4],[8]

7. 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 the utility generation capacity have not been built or compared for avian mortality, it’s a straw man argument without merit, similar to the noise problem. [7] Horizontal-Axis Wind Turbines have additional disadvantages 1. VAWTs typically aren’t high enough off of the ground The major innovation that has maximized wind turbine capacity factors and generating ability is that they have been getting taller. Laminar flow physics slows the wind down the closer you come to the ground (which I’ve experienced viscerally while paragliding). VAWT designs have the blades much closer to the ground than HAWTs, so they are losing significant amounts of wind velocity. In order to overcome this, they have to be much higher up, and lose most of their purported advantages. [2]

Eggbeater VAWT guy-wired to ground 2. HAWTs work very well There have been vertical and horizontal axes wind turbines around for decades. There are about 240,000 horizontal axis wind turbines generating electricity in the world today and they are now operating at 35%-47% capacity factors because they are a proven, tested and constantly refined technology. [1], [6], [10]

3. HAWTs have economies of scale The corollary to HAWTs being the winning technology for all of the reasons listed is that there are very well optimized and integrated supply chains that allow specialization for different wind conditions and good pricing. If VAWTs were actually solving a significant problem, there would be incentive to create subsidies to shift the market, but they don’t actually solve sufficient problems to make them worth subsidizing. [1] There is one model of VAWT that has a specific claimed advantage in that it self-stalls at 27 meters per second, allowing it to be used for small wind generation in areas with frequent very high winds. Of course, utility-scale HAWTs have control systems that automatically brake and feather the blades in these circumstances without an problem whatsoever, but

small-wind HAWTs typically do not have these control systems. A niche, but worth considering. [9] Testing by the Sandia Lab confirms all of the above in side-by-side tests of VAWTs and HAWTs, finding that VAWTs will produce 15-25% less electricity for the same swept area with likely higher costs. [11] References: [1] How effective are wind turbines compared to other sources of energy? [2] The wind doesn’t blow all the time. Why doesn’t this make wind power ineffective? [3] Humans evolved with infrasound; is there any truth to health concerns about it? [4] Wind farms don’t make people sick, so why the complaints? [5] Quora: What are the parameters involved in making a wind turbine blade? [6] Wind turbines pay back total environmental ‘debt’ in under six months [7] How significant is bird and bat mortality due to wind turbines? [8] UK Shadow Flicker Evidence [9] http://re-innovations.co.uk [10] http://www.gwec.net/global-figures/wind-in-numbers/ [11] http://www.windworks.org/cms/index.php?id=43&tx_ttnews%5Btt_news%5D=2220&cHash=b55932dfebd04 9ab01c1d968670afc1e See also: 1. http://www.windworks.org/cms/fileadmin/user_upload/Files/Jim_Tangler_VAWTs_AWEA2000b.pdf 2. Are airborne wind turbines a plausible source of cheap clean energy? 3. Quora: Why is ocean energy used so seldom? 4. http://www.motherearthnews.com/Renewable-Energy/2008-02-01/Wind-PowerHorizontal-and-Vertical-Axis-Wind-Turbines.aspx 5. http://news.nationalgeographic.com/news/energy/2012/08/120820-helix-windcollapse/

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2 Comments

1. Jota Efe · 2013/02/23 - 04:52 · Reply→ This is a well though out, complete overview. Thanks for that. I will say though, that for small, inexpensive “home” wind turbines, the tradeoffs are much harder to balance out — brake systems are not as sophisticated on HAWTs, and the noise levels are crazy loud for those small whiny turbines. They are also not as optimally placed for constant wind direction. That is probably where the VAWTs are most appropriate — the second home in cottage country that’s unpredictably windy…

1. Mike Barnard · 2013/02/24 - 05:25 · Reply→ I agree. But in that case small wind generation is almost never going to be cheaper than just getting electricity from the grid. Small wind generation isn’t particularly economic unless you are off the grid and willing to live with significant compromises. My general recommendation is to invest heavily in insulation and conservation before generation for homes.

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