Ducted Wind Turbine 'Breaking' Betz Limit?

[bwprototyping]

I'm skeptical, the company Ducted Wind Turbines (ducted wind dot com) is claiming Cp's of up to and exceeding 0.9


Their calcs look like they're based on rotor area, even though the ducts pull in wind power from a large area. Should this be a consideration? Or are there special ways to look at power converted in ducted designs?

[MattM]

You couldn't get more than the Betz limit of the area inside the duct no matter how big your rotor.  The air in the duct has to speed up, which would require energy added to air.  The air should go around the outside of a duct faster than through the duct, which is why these things are magical thinking.  Maybe it works different if the duct is at an angle to the wind?

[makenzie71]

I always try to give incredible things the benefit of the doubt because without consideration there would be no progression.  Think of all the impossible things we have today just because someone said “that's not impossible”.

If a ship can sail against the wind, i have to consider that this device might similarly do brain breaking things through some clever manipulation.

Improbable, but i would not consider it impossible

[JW]

Think of all the impossible things we have today just because someone said “that's not impossible”.

Thats not possible in this situation,,,   Its like saying we can go faster than the speed of light.

In the ensuing centuries, a number of other scientists worked to determine the speed of light and, using improved techniques, came up with increasingly accurate calculations. French physicist Hippolyte Fizeau is credited with making the first non-astronomical measurement, in 1849, using a method that involved sending light through a rotating toothed wheel then reflecting it back with a mirror located a significant distance away. One of the first precise calculations of light's velocity was made in the 1920s by American physicist Albert Michelson, who carried out his research in the mountains of Southern California using an eight-sided rotating mirror apparatus. In 1983, an international commission on weights and measures set the speed of light in a vacuum at the calculation we use today: 299,792,458 meters per second (186,282 miles per second)—a speed that could circle the equator 7.5 times in a single second.

(186,282 miles per second)—a speed that could circle the equator 7.5 times in a single second.

-edit correction

[SparWeb]

Their calcs look like they're based on rotor area, even though the ducts pull in wind power from a large area.

Yes.  Rotor area becomes duct inlet area, and that's easy to increase by 50% with some contraptions.  That correction is the only thing you need to do, and it usually fixes their mistake.  Unless, they made a mistake elsewhere, then, god help them they're just making up fairy tales.
Ducts add mass and complications to a machine that already works.  If you really want more power, make the blades longer.

[makenzie71]

Thats not possible in this situation,,,   Its like saying we can go faster than the speed of light.

Prove, definitively, for all eternity, that we can't

If someone claims to be able to achieve the impossible, I will always be willing to hear them out.

[JW]

Just off of my head. Will TSR be a factor used in these type of measurements.

[SparWeb]

Don't get me started!!   

Careful dude.  Ya I know you're just yanking JW's chain. 

[SparWeb]

JW,
You can work out the basic efficiency of any wind turbine without needing to know the TSR.
You can go deeper into the other performance calc's and use TSR to set other things, but it's not a factor in the Betz limit.

[JW]

  makenzie71

https://www.fieldlines.com/index.php?topic=140024.0

I've used Alton's wind calculator to find that a 3 foot prop with a TSR of 3 gives a pretty good match although it wont cutin until around 13.5MPH winds.

on the link look at replys #1 #6  by FLUX

JW

[bwprototyping]

Their calcs look like they're based on rotor area, even though the ducts pull in wind power from a large area.

Yes.  Rotor area becomes duct inlet area, and that's easy to increase by 50% with some contraptions.  That correction is the only thing you need to do, and it usually fixes their mistake.  Unless, they made a mistake elsewhere, then, god help them they're just making up fairy tales.
Ducts add mass and complications to a machine that already works.  If you really want more power, make the blades longer.

Thanks Spar, that's kinda what I was thinkin. I haven't done the math to check if the Cp increased proportionally with duct area in their calcs, but it seems like that may be the case.

I may have to side with makenzie on this one  although I don't think it's PROBABLE that the Betz limit can be busted, I feel like it's not quite comparable to the constant speed of light in a vacuum.

[JW]

Likes this reply

[topspeed]

I'm skeptical, the company Ducted Wind Turbines (ducted wind dot com) is claiming Cp's of up to and exceeding 0.9


Their calcs look like they're based on rotor area, even though the ducts pull in wind power from a large area. Should this be a consideration? Or are there special ways to look at power converted in ducted designs?

I found no such a claim there.

[bwprototyping]

I'm skeptical, the company Ducted Wind Turbines (ducted wind dot com) is claiming Cp's of up to and exceeding 0.9


Their calcs look like they're based on rotor area, even though the ducts pull in wind power from a large area. Should this be a consideration? Or are there special ways to look at power converted in ducted designs?

I found no such a claim there.

Check their most recent technical publication in Publications & Presentations

[makenzie71]

Check their most recent technical publication in Publications & Presentations

You got a link?  Because I'm having a hard time finding it, too.

[bwprototyping]

can't post external links, but here is the site (it's in the Our Technology section):

[makenzie71]

https://www.wind-energ-sci.net/3/919/2018/

I'll take a looksee when i get back home

[MagnetJuice]

This Ducted turbine was "invented" by Kenneth Visser. He has a patent on it.

He has authored and co-authored several papers on it.

Anyone interested in doing more research about this turbine can find the papers here:

https://www.researchgate.net/profile/Kenneth_Visser

Ed

[JW]

@ bwprototyping

This will go away after 20 posts (post external links) The reason for doing this is to thorite spammer's.

JW

[bwprototyping]

*patent pending 

Yea the link limit is totally understandable  haha

And don't get me wrong, I'm not trying to negate the turbine or invalidate the company. I was just wondering how the Betz limit applied to the duct design.

[makenzie71]

This Ducted turbine was "invented" by Kenneth Visser. He has a patent on it.

He has authored and co-authored several papers on it.

Anyone interested in doing more research about this turbine can find the papers here:

https://www.researchgate.net/profile/Kenneth_Visser

Ed

https://www.youtube.com/watch?v=EXxA-RkwuRY

Essentially it's just forcing wind from a larger area across the blades.  They're claiming it makes their 2.5m turbine more efficient but they're omitting that it's no longer an 2.5m turbine.  The duct is a static “blade” and is part of the swept area.  They're numbers are still impressive, but less so when you consider it's actually a 3 meter wheel.

It is, however, a pretty neat way to power out of small, readily available, cheap, high rpm generators.  I bet they have trashy tracking, though

[bwprototyping]

The duct is a static “blade” and is part of the swept area.

Hey, that's a pretty cool way to think about it! Makes sense

[SparWeb]

It is, however, a pretty neat way to power out of small, readily available, cheap, high rpm generators.  I bet they have trashy tracking, though

Yeah, and heavier, and more complicated, and, and, and.  It can hide some of the noise coming from fast blades.  Maybe.  Or maybe project noise like a trumpet...

If you believe a wind turbine should be a pretty toy in your yard to look at, or prove to your friends that you are virtuous, then ducts are great.

If you want electricity for a low cost, it's a waste.

[makenzie71]

Yeah, and heavier, and more complicated, and, and, and.  It can hide some of the noise coming from fast blades.  Maybe.  Or maybe project noise like a trumpet...

If you believe a wind turbine should be a pretty toy in your yard to look at, or prove to your friends that you are virtuous, then ducts are great.

If you want electricity for a low cost, it's a waste.

I don't really know how to respond to this considering the only thing i said it could probably do was provide a little more rpm for smaller generators.  I never said anything about complexity, noise, aesthetics, virtue, or cost...

[SparWeb]

That's my job: be grouchy.  Ask anyone here

Visser's video has a lot of interesting stuff in it.  Cheers for including the Bergey data and it looks OK.
Their own gets better output than the Bergey but that's no surprise since the XL-1 is a 30-year-old generator design.

The next thing they do makes things a little fuzzy.  They plot the Betz limit line, which is fine, except that's a "input" power line.  As long as you can keep the inputs and outputs straight, you're all right.
@9 m/s Betz = 1300 W available (or in other words 59.3% of 2.2 kW = 1300 W

We can compare the two turbines - before the duct:

Bergey = 700 W ouput / 1300W input = 54% blades and generator efficiency

Visser, no duct = 920 W / 1300W = 70% blades and generator efficiency

So the basic turbine that Visser's using (in a wind tunnel) does better than Bergey's (in natural wind).  So let's say Visser's machine is just as good as a Bergey XL-1.

Visser, Ducted = 1900 W / 1300W = 146% blades and generator efficiency

With the duct on, the output improves dramatically.  Visser doesn't say that the blades turn faster, or that the generator may be operated with different electrical load, so assuming the generator or the load are not changed, then the blades must be turning about 20% faster.

It took me a while to figure out what I'm looking at in the video.  Watching the turbine run, notice that the fans aren't turned on.  The turbine is being driven by some other source.  Given the lighting in the building, it seems to be open behind the camera, so there may be a breeze blowing into the building.  So what we're watching is probably not a data-collection test, just something caught on camera. 

The duct is upwind of the turbine, and the blades trail down-wind of the tower.  The open mouth of the duct is 10.5 feet diameter, making the mouth 87 square feet, instead of 50 square feet for an 8-foot rotor swept area.  The ratio is 1.74.

Comparing Visser's turbine with and without the duct:  1900/920 = 2.1
That's a better ratio than just comparing the duct vs. swept areas.  Well done on that point.

Next I want to see Visser's turbine out "in the wild" and see how it does with a bit of turbulence.

[SparWeb]

He was still working on it in 2016, and continued testing until 2018:

“We don't just have the drag of the rotor, we have the drag of the wind duct and we have to account for these areas in the structural loading on the tower and foundation. We can generate twice as much energy, but if it costs twice as much, than we haven't gained anything. We might as well put up two turbines.”

https://nexus-ny.org/nyas-podcast-ducted-turbines-international/

[Adriaan Kragten]

The Betz limit is only valid for a wind turbine turning in open air. A duct of the correct geometry can increase the mass flow through the rotor but only if the duct itself is much larger than the rotor. The largest diameter of the duct must be at the back side and the duct must have a nice rounded geometry at the front side. The wind speed outside the duct increases at the back side of the duct and this increase of the wind speed results in a decrease of the pressure. This decrease of the pressure results a larger pressure difference over the rotor plane and this larger pressure difference increases the mass flow through the rotor. So it is possible to get a maximum Cp of the rotor larger than the Betz limit if the rotor radius R is used to determine the Cp. However, for this increase of the Cp, a complex duct is needed. The positive effect of the higher Cp is generally completely neutralized by the extra costs of the duct.

[kevbo]

...If you really want more power, make the blades longer.

This.  Longer blades are almost always cheaper and easier than all the proposed efficiency enhancements.

Engineers are trained to always be thinking about improving efficiency.   When the energy source is free, that can lead down many silly paths.   The only thing that matters with wind turbines is TOTAL cost of ownership vs. KWH generated.   Total cost of ownership has to include not only the inital cost, but any financing, tower requirements, maintenance, insurance, liability, infrastructure (power lines, substations) and disposal costs.

[neilho]

This particular debate has been going on for more than 40 years. And the answer is the same, every time.

Output being equal, the increase in duct cost is much more than the cost of longer blades.

[Ungrounded Lightning Rod]

This particular debate has been going on for more than 40 years. And the answer is the same, every time.

Output being equal, the increase in duct cost is much more than the cost of longer blades.

You can get most of the effect of a duct with a wingtip.  The main thing a duct (not shaped like a funnel) does is keep the air from running around the tip from the high pressure to the low pressure side, partially depowering the outer part (say a foot or so) of the blade.  The "extra power" (NOT above Betz - just closer to it) is power that would have been lost in the tip vortex as it is blown away downwind.

That's what "wing keels" on a sailboat are about:  The bottom of a blade keel is mostly about keeping the water from running around the end, so putting the "wing" there does the same thing.  This lets you get your side-force from a shorter keel, which lets you go into shallower water.  (Downsides:  Less resistance to heeling, because the force is closer to the center of bouyancy.  And list reduces the effectiveness of the keel more, too, as more of it "hides behind the hull" when the boat heels.)

Wingtips on a mill, however, haven't proven cost-efficient.  I'm not sure whether that's because they add enough drag to compensate for the powering or because they're structurally more of a pain (and cost) to build than just making the blade longer, or some combination of drag and cheapth.

Note that things like a duct, shaped like an airfoil, can also affect air a little beyond the physically swept area, making the effective blade radius a tad longer.  In principle you might get a slight boost compared to applying Betz computations to the physically swept area.  In practice I haven't seen any situation where the gain overcomes other losses.

Their showing of just over Betz for their basic rotor without the duct looks like another issue:  In free air the upwind pressure of the blade's drag on the non-supersonic flow has already started expanding the airflow before it actually gets to the plane of the blade.  In a wind tunnel the nearby walls act like another duct, reducing that spreading.

I bet they don't beat Betz if you count the cross-sectional area of the wind tunnel as the swept area.  B-)