Bladeless Wind Turbine Too Good To Be True?

December 12, 2013 by  
Filed under Green Energy News

Here’s Saphon’s promotional video for its prototype system “The Saphonian,” linked in that 2012 Treehugger piece and making the rounds afresh this week:

Those are some strong claims. Saphon’s video says the company’s turbines are twice as efficient as traditional bladed turbines, without defining how that efficiency is calculated. It claims to have incorporated a power storage mechanism, but doesn’t describe it.

Saphon’s website also claims its turbines pose less visual impact than their competitors:

The non-rotating sail-shaped body of the Saphonian remains virtually stable upwind. It could therefore be used as billboard [sic] which allows a better integration into the neighbouring landscape, either rural or urban.

We’ll just let that claim go for now.

And most startling of all to anyone who’s ever geeked out over wind turbine specs, Saphon claims its design isn’t subject to Betz’ law, a century-old law of physics that describes a limit to the amount of energy any turbine can extract from the wind. But the video doesn’t say how.

The video doesn’t even really describe how the turbine works, but ReWire did a moment’s worth of digging and found some video of The Saphonian in action:

According to a patent the company filed in 2010, the disk is attached to three hydraulic cylinders on the windward side of the generator housing. As the disk wobbles as shown in the second video, it pushes pistons into the cylinders in a repeating cycle, and the resulting pressure in the cylinders’ hydraulic fluid turns a hydraulic motor attached to a generator.

In theory, there’s no reason such a design shouldn’t work to generate some power. But the claim that this design is more efficient than standard bladed turbines is eyebrow-raising. Conventional wind turbines directly harness the wind’s mechanical energy to spin a generator. If ReWire understands the Saphonian’s design correctly, it’s adding at least one step to the process: the wind’s mechanical energy pressurizes a fluid which then turns a turbine.

Due to the pesky Second Law of Thermodynamics, every time you change energy from one form to another, you lose some of it as waste heat. By adding at least one step to the process of turning wind into electrical power, the Saphonian sets itself at a disadvantage to conventional designs. That disadvantage would need to be overcome somehow. That’s entirely possible, but Saphon is silent on just how they tackled the problem.

The company is also frustratingly silent on how it’s achieved the capability of storing the power its disks generate.

Most troublingly from an amateur engineering standpoint is the claim that the technology escapes the limitations of Betz’ law, which says that a maximally efficient wind turbine of any design can never capture more than 59 percent of the energy in the wind blowing on it.

Betz’ law seems abstract, but it’s actually pretty easy to understand the basics. Wind is moving air. The more energy the wind has, the faster the air is moving. If a wind turbine were to take all the energy out of the wind, the air would stop moving after it hit the turbine. If it did that, the wind turbine would have a pool of still air right behind it. The wind in front of the turbine would slow and stop, because there’d be no place behind the turbine for it to go with the still air blocking its way. And if the wind stopped, the turbine wouldn’t generate power.

In 1919, German physicist Albert Betz calculated the ways in which slower air flows downwind of a wind turbine, and the degree to which that slower air robs energy from the wind upwind of the turbine. He determined that under ideal circumstances, a perfect wind turbine can’t extract more than 16/27 of the energy in the wind blowing past it. That figure, more easily remembered as 59.3 percent, is called Betz’ coefficient.

That “perfect wind turbine” would have an infinite number of infinitely thin blades with no friction. Real world materials reduce that efficiency significantly. But the thing to remember is that the design of the turbine is irrelevant: Betz’ law comes from the way air moves, and in order to escape it you’d have to redesign the air.

There is one technical out: if you add a scoop or funnel to catch more air in your turbine, you can extract more energy. But you’re not evading Betz’ law that way: you’re just expanding the “catchment” area of your turbine, and thus the amount of wind energy your turbine can try to convert.

It looks like Saphon’s bladeless turbine features some aspects of a funnel, with its more or less bowl-shaped design. That might be why they make the claim about evading Betz’ law. It’s hard to tell from the information the company provides.

The late Carl Sagan was fond of a catchphrase that seems relevant here: “extraordinary claims require extraordinary evidence.” (The well-read Sagan probably borrowed and updated the idea from Laplace, who wrote “The weight of evidence for an extraordinary claim must be proportioned to its strangeness.”) Saphon makes some extraordinary claims here, but it’s short on the requisite evidence to back them up.

We’re not saying Saphon’s wind turbine won’t work. The company isn’t giving us enough to go on to judge the merits of its claims. There’s a chance the company is playing its valuable Intellectual Property close to its vest because it works exactly as advertised, and its investors are all going to get fabulously rich while saving the planet. And there’s a chance that Saphon is wrong.

Saphon isn’t alone in making extravagant claims about its technology. But the recent popularity of its video is a great reminder of an unfortunate fact. When someone promises to solve most of the problems with an existing technology and doesn’t really explain how they’ll do it, it’s time to raise the skeptical alarm.

Especially if we really want it to be true.

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