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All about Wind Energy

February 8, 2013
Paul Gipe

Vertical Axis Wind Turbine Revival?

Note: The following is adapted from my 2009 book Wind Energy Basics Revised: A Guide to Home- and Community-scale Wind Energy Systems. I am reprinting it here because of the constant re-invention of Darrieus or Vertical Axis Wind Turbines (VAWTs) and the presentation of these devices as though nothing like them has gone before. I've written about VAWTs in all my books, but I've never shown one on the cover.

Since Wind Energy Basics was first published in 1999, there has been an explosion of interest in new vertical-axis wind turbines. In that edition, the conclusion illustrating one photograph was stark--"Practically no one is working with vertical axis wind turbines today." The text went even further when describing the difficulties that "new" or "novel" designs face.

"Even promising designs from legitimate manufacturers encounter enough technical and financial problems that only a few survive. Darrieus or "eggbeater" turbines are one example. Sleek and relatively simple, they promised lower costs and greater reliability than conventional wind turbines. But they didn't deliver.

Darrieus designs failed first in the demanding small turbine market. They couldn't compete. Then they failed in the commercial or wind farm market. Today only a few Darrieus turbines remain standing, and soon these will all be removed for scrap."

Harsh? Yes. It was a conclusion based on bitter disappointment in the Darrieus wind turbines of the day, especially the derelict FloWind Darrieus turbines once littering California's windy passes.

Have times—and technology—changed? Possibly. That interest and experimentation in new Darrieus technology is high can be clearly seen in the dozens of Web sites devoted to various vertical-axis designs. Why this is so is much less clear.

Not everyone is thrilled at the VAWT renaissance. "The driving force (behind the revival of VAWTs)," says Ian Woofenden, "is ignorance of past failures, and arrogance about overcoming the problems" inherent in the designs. Woofenden, an editor who lives with conventional small wind turbines off-the-grid, points out that VAWTs are not new. He admonishes that "anyone designing a new one (VAWT) should do their homework first to find out what past designers learned."

As Woofenden notes, few of the "new" designs are in fact new at all. Many of the designs touted as new have been around for decades if not generations. The exceptions are the few Darrieus turbines, such as the Turby, the Quiet Revolution, and the Marc Twister that use curvilinear blades. These designs have not been seen before. Whether they are revolutionary or not, or whether they are better than preceding Darrieus designs, only time and field experience will tell.

Another of the "show me it works" skeptics is Mick Sagrillo. And when he says "show me it works," he means show him that it produces electricity in kilowatt-hours—not just instantaneous power. After all, it is wind energy in kilowatt-hours that we're after, not power in kilowatts.

Sagrillo grumbles that videos of VAWTs "spinning" can be found on One famously shows men installing a purported wind turbine on the roof of a building to dramatic music and immediately at the climax the "turbine" starts spinning. Indeed, spinning is the operative word because the rotor of this "turbine" wasn't connected to anything, no generator, no gearbox, no drive train at all. People are "dazzled by the motion," says Sagrillo in despair. Most viewers never notice that the "turbines" are not doing anything productive, only "spinning". As Sagrillo pithily notes, "if that's all you want, buy a whirligig for $12 and be done with it."

VAWT skeptics, such as Woofenden and Sagrillo, hold inventors of conventional wind turbines to the same standards as they do vertical-axis designers. They don't single out vertical-axis designs solely because they use a vertical axis, as some VAWT proponents charge. As Jon Powers, a Tehachapi windsmith with decades of operational experience on conventional wind turbines, notes, if inventors develop a non-traditional wind turbine, they should prove it, commercialize it, and deploy it in the thousands. Then it, too, will become the new conventional thinking. And, he could add, everyone, skeptics included, would applaud their success.

For whatever reason, many VAWT designers are prone to more hyperbole than most other wind turbine designers. Some claim their turbines will produce at less cost and with less impact on the environment than conventional wind turbines. Maybe such claims are due to widespread ignorance of VAWT technology or its long history. Maybe vertical-axis designers imagine that their turbines don't suffer the limitations of "all those other ordinary wind turbines". Certainly it's easier to visualize how VAWTs, like cup anemometers, work than how those thin spindly blades on conventional wind turbines can be so efficient at extracting the energy in the wind.

Do VAWTs work? Of course they do. This isn't in question. Can they produce useable quantities of electricity? Yes, the record in California is clear: Darrieus wind turbines generated millions of kilowatt-hours for nearly a decade. Can they compete with conventional wind turbines? Perhaps. Are there specialized markets for which VAWTs may be ideally suited? Possibly. But by definition, a "specialized market" is a limited market. Do we need wind turbines for specialized markets? Yes, we'll need all the renewable energy we can find in the coming decades. Do VAWTs offer a panacea of limitless renewable energy without the drawbacks of conventional wind turbines? Unlikely.

VAWT proponents sometimes claim there's little or no evidence that their designs won't do what they promise. Maybe so, but that begs the question. The burden of proof lies with the inventors to prove that their claims are true. As critics of new-fangled inventions point out, it's impossible to prove that they won't work. Students in Logic 101 learn that you can't prove a negative. There's always a reason, an excuse really, why many new, earth-saving inventions don't work quite as well as advertised and are then quickly forgotten.

Here, then, are some of the arguments VAWT proponents use to contrast their technology with that of conventional wind turbines. Note that the arguments and the following responses can be applied equally well to any proposed wind turbine that is advertised as a technological breakthrough.

They are simpler.They are more reliable.They are less costlyThey are more efficient.They are more cost effective.They don't kill as many birds.They are less noisy.

Simpler: Yes, some VAWTs are simpler than conventional wind turbines. However, some are more complex. In the end, VAWT designs often trade one form of complexity for another.

More reliable: Possibly, but unlikely. In nearly all cases, proponents have no field experience to support such a claim. Often the claim is based only on a wish, not on real performance. With the exception of Windside, the Finnish manufacturer of S-rotors, few manufacturers of VAWTs today have any operating experience. None have performance data in the public domain where independent analysts can gauge the reliability of their design.

Less costly: They may indeed be cheaper than conventional wind turbines. But if the turbine doesn't work at all, doesn't work well, or doesn't work for long, it's no bargain. In one North American case, the turbine was far more costly than the conventional wind turbines it was said to replace.

More efficient: Calculation of efficiency from wind tunnel or truck tests says very little about how a wind turbine will operate in real winds. Even where a wind turbine is markedly more efficient than another, reliability is a far more critical parameter. A wind turbine may be efficient, but if it is not working reliably, it will produce little or no electricity.

Cost-effective: Whether a wind turbine is a good buy or not is a function of its installed cost, the amount of energy it generates, and the cost of operating and maintaining it. Modern VAWTs may be a good buy relative to other wind turbines, but there is so little data on their performance that no one can say for sure.

Safe for birds: This is simply an unfounded claim. Very few studies have been done on birds and modern VAWTs because there are so few VAWTs in operation. Nearly all studies to date have been conducted on large commercial wind turbines. These studies find that the number of birds killed by wind turbines is primarily a function of turbine size. A big wind turbine will kill proportionally more birds than a small wind turbine—of any configuration. Proponents argue that there's no evidence that modern VAWTs kill birds at all. While technically true, the reason for this paradox is that there are no studies on modern VAWTs and birds. Moreover, nearly all modern VAWTs are extremely small, and the likelihood of a small wind turbine—of any configuration--killing any bird is, therefore, very small.

Less noisy: This is one claim that may have real merit. The blades on modern VAWTs may move through the air at much lower speeds than blades on conventional wind turbines. The lower blade speeds often translates into lower noise emissions than those from conventional wind turbines. Unfortunately, there is very little field experience, and even less publicly available data, to verify this assertion.

VAWT design, like the design of any other wind turbine, is a series of tradeoffs. For each plus there is a minus. We can't evaluate design elements in isolation. We must consider the wind turbine as complete package.

For example, the lower blade speed of modern VAWTs is a design element, a byproduct of which is lower noise emissions. However, notes Jim Tangler, a retired aeronautical engineer, VAWTs derive more of their power from torque than conventional wind turbines because they spin at lower speeds. To handle the greater torque, the blades and their supports must be stronger. This results in greater mass, and hence, often greater cost than for similar components of a conventional wind turbine. Further, says Tangler, the blades of VAWTs only operate at optimum aerodynamic performance over a small portion of their carousel path, and they typically use less efficient symmetrical airfoils than the cambered (asymmetrical) airfoils used on conventional wind turbines. All in all, says Tangler, VAWTs of the 1980s weighed more and were less efficient than conventional turbines of the period.

That said, VAWTs could be cost-effective and even more-cost effective than conventional wind turbines if they were cheap and reliable enough. That is ultimately the test, the Holy Grail for modern VAWTs—or wind turbines of any stripe.

Modern Small VAWTs

Because of scale effects, it is unlikely that the small wind turbines of the modern VAWT revival will approach the historical performance of FloWind's large Darrieus wind turbines, the most successful VAWTs ever built.

Many manufacturers of the small VAWTs of the vertical-axis revival, like FloWind before them, often rate the power output of their turbines at a much higher level than conventional wind turbines. This can be seen in the rotor loading of modern VAWTs that are about double that of conventional turbines.

The characteristic loading of conventional small wind turbines is about 250 W/m². For example, the Bergey Excel has a rotor loading of 260 W/m² while that of Southwest Windpower's Skystream is 166 W/m². Southwest Windpower's Air 403 was notorious for its high rotor loading of 373 W/m². Fortunately, they cut in half the rating of the Air Breeze, the newest model in the Air series, to only 186 W/m², a much more realistic value.

The high rotor loading of many modern VAWTs doesn't mean that the wind turbine is capable of generating more electricity than a conventional wind turbine of the same swept area. It simply means that the wind turbine uses a much larger generator relative to the area swept by the wind turbine rotor. Remember that it is the swept area that is the prime determinant of how much energy a wind turbine will capture—not the size of the generator.

However, high rotor loading does suggest that a VAWT manufacturer may be overstating expected performance. Rotor loading greater than 300 W/m² for small wind turbines should be viewed with skepticism, whether on the over-hyped Air 403 or on a small VAWT.

Another way to look at rotor loading is to consider a Standard Rated Power of 200 W/m². Many--though certainly not all--VAWTs have rated their turbines at twice the Standard Power Rating. PacWind, a notorious promoter of a straight-bladed VAWT touted by Hollywood celebrities, had a power rating of 500 W/m². PacWind rated their Delta I at 2 kW, when in all likelihood it was less than a 1 kW turbine. Worse yet was Vertica, which rated its VAWT at 3 kW, or more than 800 W/m² for a turbine that like PacWind's Delta I, was more realistically labeled something less than 1 kW.

Note that the performance at rated power and the projected loading at rated power in the table are based on manufacturers' advertised characteristics. That a manufacturer claims a conversion efficiency of 30% and rotor loading of anything greater than 200 W/m² does not guarantee that the wind turbine will actually deliver such performance. In fact, very few small wind turbines--of any configuration--have reliably delivered such performance.

Small VAWT Designs

During the 1970s and early 1980s, several designs for vertical-axis wind turbines with straight blades emerged. These H-rotors used articulating blades that changed pitch as they moved around the carousel path. Articulating H-rotors never performed as advertised and were mechanically unreliable.

The recent VAWT revival has seen the introduction of several new small H-rotors that use fixed blades. These rotors use blades that are fixed in pitch like those of Darrieus turbines. Two examples are turbines made by the Canadian company, Cleanfield, and California's PacWind. In contrast to earlier articulating VAWTs, these turbines are simpler. Unfortunately, there is little to no information on how these turbines perform in the field.

As with conventional wind turbines, new vertical-axis designs should have some aerodynamic means of protecting the rotor, should the normal braking system fail. Some Darrieus turbines of the 1970s and 1980s used air brakes, some (FloWind) did not, and relied solely on their mechanical brakes.

In conclusion, modern VAWTs may or may not work reliably, may or may not deliver the performance promised, and may or may not be more cost-effective than conventional wind turbines. As Dr. Robert Gasch told his engineering students at TU-Berlin, until a wind turbine is built, installed, operated, the performance measured, and the results published, no one should pay the slightest attention to the product or its proclaimed benefits. This is sound advice for any wind turbine, period.

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