Windtamer Hype Needs Trimmed: Betz Beaten Yet Again Raises Questions

By Paul Gipe

Poor Albert Betz, the German physicist must be turning over in his grave. He’s been beaten yet again.

Seems like it is becoming almost a weekly occurrence now that some inventor or another announces they’ve beaten the Betz limit for the aerodynamic performance of wind turbines. The latest is the so-called Windtamer from Livonia, New York.

Betz, following in the footsteps of his British colleague Frederick Lanchester, calculated that the theoretical maximum energy a wind turbine rotor can extract from the wind is 59% of the energy in the windstream.

Update May 20, 2010:Windtamer inventor replaced as chief technology officer and chief finance officer. See Adeeb Saba assumes responsibility for product development at Windtamer Corporation.

Windtamer says they not only beat Betz but they will produce twice as much as a conventional wind turbine. However, a quick scan of the “report” justifying Windtamer’s claim reveals that the turbine’s performance is not any better than all those wind turbines that have gone before.

But first, the famous press release.

Betz Beating Press Release

In a December 2, 2009 press release, Windtamer Corporation bragged that their ducted wind turbine broke the Betz limit based on a report from a local university.

The good news is that they knew of Betz. That’s better than most inventors and startup companies in the small wind sector. Unfortunately, that’s where the good news stops.

Windtamer’s press release said the university found that their turbine reached efficiencies of 60% to 80% versus Betz’s theoretical limit of 59.3%. If true, it would be a breakthrough. Most conventional small wind turbines rarely reach half of the Betz limit in actual operation.

Naturally, since Windtamer had broken the Betz limit there’s no holding them back, and they just piled on one claim after another. The release claimed that their turbine would produce 2.1 to 2.5 times what a conventional wind turbine of the same size would produce.

Disclosure: I sold Bergey wind turbines in Pennsylvania during the early 1980s. Actually, I didn’t sell many of them, but I did sell a few. Bergey wind turbines are “conventional” wind turbines.

Windtamer further crowed that because their wind turbine doesn’t need to furl or reduce power it will continue to produce more power as wind speed continues to increase. This is the wind industry’s equivalent of the claim by nuclear power proponents in the 1950s that nuclear energy would be “too cheap to meter”.

Of course, how this miracle device works in high winds is never explained on their web site. Companies that have designed their wind turbines to rely solely on “aerodynamic stall,” or “dynamic braking,” or simply big massive brakes in high winds have all passed into history.

As Danish wind turbine owners learned three decades ago, all wind turbines must have an aerodynamic means of controlling the rotor when all else fails– because it does. This has become an iron clad rule of wind turbine design and those who don’t abide by it pay a severe price eventually. Often it is their customers who pay.

The Windtamer worked so well in high winds that it burned up the university’s load bank. That should be a tip off that they can’t control the little devil in high winds.

But Windtamer wasn’t done yet, that one press release just kept going on. Not only did Windtamer beat Betz, require no overspeed controls, but you can use it on very short towers-because it’s so efficient, remember-4 meters or 7 meters (13 ft or 24 ft) tall. Yes, put it right there among the shrubs and trees so no one will see it. Mick Sagrillo, of the “taller the tower the better the power” fame, is just going to love this claim. I can hear him sputtering now.

Naturally, the turbine is “safer than many turbines currently in use” and “whisper quiet” and you can mount it on your roof. What’s not to like?

The secret to this machine is its “fluid-driven, vacuum-enhanced generator.” When you hear mumbo jumbo techno-babble like that it’s time to head for the exits.

Windtamer is also bird and bat friendly of course. What new sooper-dooper wind turbine isn’t? Conveniently, none of these promoters ever provide any evidence of such claims.

And–this is always my favorite claim of inventors–the Windtamer “begins turning in winds of less than 2 mph.” Wow, that’s the lowest yet. To top that, the next invention will have to begin turning in no wind. Try to top that!

It should be obvious, but often isn’t to the uninitiated-and that includes politicians, and state bureaucrats doling out subsidy money–that a wind turbine with its rotor spinning may not necessarily be producing any electricity. “Spinning” isn’t the same as “generating”. As Mick Sagrillo likes to say, “if you want something spinning, buy a whirligig.”

Of course, Windtamer’s two tiny test turbines and one “mobile” unit have been up only a year so there’s no real data to say such claims are truthful or not.

Other shrouded turbines such as Wind Cube, Jetstream, Vortec, Enflo–the list continues to grow-have come and mostly have gone before Windtamer’s most recent run at ducted turbine technology.

The hype around Windtamer reminds one of Jetstream, a 10 kW ducted turbine “invention” from Ohio in the 1990s. Jetstream caused a flurry of interest among those looking for something “too good to be true” as the Post Service warns. Fortunately, Jetstream disappeared quickly without causing too much damage.

The Facts Please

Windtamer installed two self-described 1.5 kW prototypes at Clarkson University’s Center for Sustainable Energy Systems in mid 2008. The turbines were to be tested by Professor Ken Visser of the university’s Dept. of Mechanical and Aerospace Engineering.

Controversial, or certainly unusual, turbine installations are nothing new to Clarkson University, located in Potsdam, New York. A generation ago, Clarkson was known for mounting an Alcoa Darrieus VAWT on a farm silo at the school’s test site. The university’s web site shows photos of the Alcoa machine still attached to the silo. The turbine probably hasn’t run for three decades now.

Dr. Visser has continued in the Clarkson tradition with his investigations of contra-rotating VAWTs (Vertical-axis Wind Turbines) and contra-rotating HAWTs (Horizontal-axis Wind Turbines), and various ducted turbines of which Windtamer is but one.

Windtamer has posted report 11 in a series of private, for hire, studies by Dr. Visser on their web site. The beating Betz press release was loosely based on Dr. Visser’s Report 11. I say loosely because the report doesn’t really support the claims in the press release.

Let me repeat that so it is clear: The report doesn’t really support the claims made in the press release.

The reports are private commercial property written under a non-disclosure agreement between Dr. Visser and Windtamer. For this reason, the report or its critical chart, are not reproduced here. You’ll have to go to their web site and see them for yourself.

There are no specifications on the Windtamer product on the company’s web site even though the company is selling the “turbines” and raising money. The only publicly available technical information is the report by Dr. Visser dated November 2009.

The Betz Beating Report

The report, titled Wind Tamer Turbine Performance Report, is subtitled 1 kW-15 kW Performance Predictions. It is the latter that the report covers: estimates of potential generation. Of the 23-page report, only two pages cover the measurements purporting to show that the design beats the Betz limit. To say that the report’s documentation of the “test results” is sketchy is more than an understatement.

In the report’s introduction, Dr. Visser makes a point of comparing the Windtamer to “open rotor” turbines such as Bergey’s XL.1. The photos used by Dr. Visser are of Bergey Windpower’s 1 kW model, which Dr. Visser modified in one of his experiments to use six blades. To add insult to injury one of the technical documents circulated by Windtamer used a spreadsheet created by Mike Bergey–that he makes freely available-to show Windtamer’s outsize performance.

Bergey shouldn’t feel alone. The same product literature also takes aim at GE by comparing the projected performance of a yet to be developed 15 kW model with GE’s real 1.5 MW model. (Yes, it’s true. You can’t make this stuff up.)


Most significant is the mistitling of the report. It is not a performance report. It is a report on the projection of performance based on Clarkson University’s theoretical computer models. It is not a report on the measurements of Clarkson’s tests.


There is no evidence in the “report” that the turbine(s) were tested to accepted international testing standards for small wind turbines. The inventor, Jerry Brock, says they will test the turbine to international standards. While that may be so, they haven’t done so yet-at least that has been reported publicly. Meanwhile they claim to have beaten the Betz limit and are selling the product.


The report’s summary (page 3), includes several key phrases that explain the report and the subsequent press release.

Performance claims are based on the “rotor” area, not on the diffuser area. Projections are based on an “assumption” of an augmentation factor of 2.

It should not be necessary to parse reports such as this. Careful readers will see the gist of the report quickly. Most of the Internet hypsters that circulate only the press release will never read the report, and even when some do they will simply scan it and not catch the subtleties.

First, Betz’s theory is based on the area of the windstream intercepted by the wind turbine rotor. It is a misinterpretation of the Betz limit to describe it only in tems of “rotor area”. The correct interpretation, and the only one used by professionals in the wind energy field, is “intercept area”.

For ducted, augmented, diffuser, or otherwise shrouded turbines, it is the shroud that determines the intercept area, not the rotor. This is fundamental. Newbies to wind energy always get caught in this trap: confusing rotor area with intercept area of a big shroud.

Second, if you “assume” an augmentation of 2, you will get twice the generation of a conventional turbine. Duh. That’s not rocket science.

Self Regulation

In describing the Windtamer design (page 5), the report notes that “there is no need to furl the turbine in high winds as the design self regulates the power to the rated design output”. If this is true, why then does Windtamer boast that it burned up the university’s load bank in high winds.

Claims that a wind turbine “self regulates” without any mechanical, electrical, or aerodynamic means to do so is a recipe for disaster. There have been many wind turbine manufacturers who have made this claim through the years. None exist today.

Betz Beating Chart

The pièce de résistance is the chart (page 7) showing the tests on the “mobile test rig”. The accompanying text explains that the data “above indicated that the Windtamer is capable” of performance better than the Betz limit. This statement only applies to the instantaneous measurements recorded and is carefully worded.

It should be obvious to any reader that the same data also shows that the instantaneous measurements show performance far below the Betz limit as well. That’s not mentioned in the text.

Most damning, however, is the data for one-minute averages. This data clearly shows the turbine performing right at the Betz limit.

Someone not accustomed to measuring, recording, and analyzing scatter plots of wind turbine data like this could be excused if they say, “well it’s not better than Betz, but it is as good as Betz, and that’s a breakthrough in itself”.

Now remember, what is being measured here by the Clarkson researchers: the coefficient of performance of the “rotor” only. The performance data is only compared to the rotor inside the shroud and not the shroud and it is the shroud that intercepts the wind.

A trained eye will see that the 1-minute averages are clusterd around a Cp of 60% with a rotor ½ that of the shroud diameter. The performance is actually about half that portrayed in the chart, let’s say 30%. Thus, the Windtamer is as good as a conventional “open rotor” wind turbine, but certainly no better.

Performance Projections (Calculations)

The report very clearly states in the section on the turbine’s geometry (page 9) that the “rotor diameter, which is used for the performance estimates, is smaller than the shroud opening.” That should be the end of any outsize claims for the “design” right there.

Generator Rating

Despite all the hype in the press release, the stated “power rating” in the report (again page 9) is what it should be for any conventional wind turbine with a shroud that doubles the intercept area of the wind turbine.

The 52-inch model (1.3 meter diameter rotor only) is rated at 1 kW. The standard power rating of a wind turbine with an area swept by a 2.6 rotor-or shroud–is 1.1 kW.

The 77-inch model (1.96 meter diameter rotor only) is rated at 2.5 kW. The standard power rating of a wind turbine with an intercept area swept by a 3.9 m rotor-or shroud–is 2.4 kW.


The report lists two references from this web site yet doesn’t actually cite them in the report. But they also list a report by Trevor Nash on the Vortec 7 ducted wind turbine in New Zealand. Any due diligence officer worth their salt will read that report and subsequent reports on the failure, some would say scam, of Vortec 7 and conclude that diffuser-augmented designs are long on hype but short on results.

Peer Review

A lot of the questions around the “report” would have been cleared up if it had been peer reviewed.


The Wind Tamer is an experimental turbine. It is not a commercial product and its claims of performance are overstated and not substantiated.

Moreover, the design is not unique, startling, or extraordinary. Windtamer appears to be a garden variety diffuser-augmented turbine.

The test results to date show that the turbine’s performance is no better than a conventional wind turbine with the same area intercepting the windstream.