DIFFUSER AUGMENTED WIND TURBINES

[Usman]

This posting is to discuss the a very widely argued issue but so far no verdict has been made. The subject is:

 "does a diffuser really augments or amplifies wind into a given turbine in such a manner so that 100-300% increase in power output could be achieved?"

Diffuser Augmented wind turbines exits from as back as 1970s and may also be known as:

1. -DAT (Diffuser Augmented Turbines).
   
2. -Shrouded Wind Turbines.
   
3. -Fairing Wind Turbines.
   
4. -Wind Amplified Wind Turbines.
   
5. -Ducted Wind Turbines.
   
6. -Static Pressure Wind Turbines
   
7. -Venturi-effect Wind Turbines
   
8. -Vortex Wind Turbine
   
9. -Tunnel Effect wind Turbines
   
10. -Wind Amplifies Platforms
    
    

Over the past few years, I have been trying to come up with a space-efficient wind turbine concept that is smaller in size and could generate more watts for a given swept-area, no matter by using a diffuser, or any other means. Such turbines could also be logistically more efficient and more suitable to developing countries.

In the worst case, if a given shroud could increase power factor by 1 or 100%, than it would be he most cost effective in it's class i.e. a 100% more power turbine would also cost 80-100% more in price as well as logistics, whereas a shroud could be fabricated and added onto a given turbine at less than 30% of those costs!

In this discussion, I would like the participants to ignore the yaw error factor, as that it a valid claim against DAWT. My conclusion is that a DAWT needs to have a motorized r active yaw control rather than passive (or tail vane) due to the heavy diffuser may not be able to align itself with the wind, especially at low wind speeds and may noticed struggling to position itself in high winds due to high inertia of due to the added diffuser.

Mostly, people and research workers look into the closure of Vortec in New Zealand that was trying to build a 3.5MW DAWT. A few say that the company went bust due to not being able to prove it's power gain claims.

On the other hand, some research in Europe emphasizes that the diffuser needs as much design efforts as a designing a blade in order to be efficient, rather than the conventional concept of a simple ring-alike shroud.

The research initiates the need to develop an airfoil.

-A match between rotor diameter and the diffuser depth

-A match between rated rpm Vs. clearance in between the shroud internal surface and the blade tips.

-Optimizing the rotor position within the diffuser as the rotor would only be most efficient at a certain depth within the diffuser.

-The Shape of the Shroud, simple ducted, flower like, funnel like etc.

-A few research papers even indicate a need to develop new blades specifically for DAWT, disposing off the concept that a shroud could increase the output of any given wind turbine.

I would appreciate an input into this subject, by giving pout opinions on the credibility of DAWT, design notes and suggestions, sharing experience, and the reason why DAWT have not seen the commercial success as the convention HAWT have.

A few links to this:

http://www.wind-works.org/articles/vortec.htmlVorect enclosure not a surprise

http://www.ifb.uni-stuttgart.de/~doerner/diffuser.html

http://www.wind-works.org/articles/vort_july97.html

http://www.enflo-windtec.ch/

http://www.iit.edu/~ipro307f/business/plan.html

http://findarticles.com/p/articles/mi_qn4155/is_20030611/ai_n12506077/pg_1

[Countryboy]

Without a doubt diffusers will increase turbine output.

The question is not whether or not diffusers concentrate energy, but whether or not a diffuser can be added to a turbine at a justifiable cost.

The Alta Mont Pass in California is an example of a natural diffuser.  It is a natural venturi formed by mountains.  The mountains force the wind to funnel through the mountain pass with increased windspeed.  By placing wind turbines in the pass, the turbines are able to harvest a much greater amount of energy than they would be able to harvest if the mountains were not present.

It would not be impossible to put a 10 foot diameter shroud around a 4 foot diameter turbine to concentrate wind energy.  But, it is impractical.  The tower would need to be much stronger to handle the wind resistance from the 10 foot shroud.  The added mass of the shroud would make it yaw much slower.  Yes, your 4 foot turbine would outproduce a 4 foot turbine without a shroud flying beside it - but your 4 foot turbine would take up the space needed by a 10 foot turbine, and your 4 foot shrouded turbine would perform poorly compared to a 10 foot HAWT.

If you have a natural diffuser to focus wind energy, such as a mountain pass, by all means take advantage of that opportunity.  If you have to build a diffuser - you will be better off making longer blades.

[electrondady1]

i think the verdict is in.

look around, both home made and commercially manufactured

units have for the most part chosen to not use a shroud for hawts.

for all the reasons country boy has mentioned .

 it may be a viable method to increase output for a vawt depending on location and prevailing wind.

[finnsawyer]

You've been around a while, so you should be aware of some of my comments concerning this phenomenon.  If you're not, you could review my comments since I came on board.  I have specific ideas concerning what could be done and have dropped hints from time to time, leaving it up to an interested party to do their homework if they were truly interested.  In any case, after the alternator debacle, I decided to only treat this area in a general fashion.  If you have specific questions, I will try to answer them.  That the phenomena is real is indicated any time one walks between two buildings when the wind is blowing.  Heck, the difference in walking at three miles per hour with or against the wind is easily apparent.  Why do they call it a diffuser if it is concentrating the air flow?  In any case the addition of an "air management device" will necessitate redesigning the blades.  You need a "system" approach.    

[electrondady1]

if you are looking to build a product for urban use, then perhaps

a funnel like shroud, made to face into the wind, would allow you to use a very small prop and run at higher revs.

might be noisy.

one thing that has been discussed here , is the use of a bullet shaped diffuser .

this installed within the shroud to deflect air to the tips of the blades.

 

[vawtman]

In my early wind turbine days i tested a backward incline fan which is sorta ducted.

It seemed to me that the wind would rather go around it than through it.Path of least resistance i guess.

 Just my experience.

 Mark

[electrondady1]

i really don't want to help design a comercial product for usman .

i've got my own fish to fry.

but vman is probably right.

only a specific amount of air can be forced to through the back aperture.

before the velosity would be lost.

thats fluid dynamics i quess,  and hard to resist.

man, i love science.

 

[adobejoe]

A while back I saw some company was trying to sell a dome type of unit, where the tower of the turbine goes through the top/center with the turbine above.  The idea being to concentrate the wind.  Looked kind of strange, and I question how it would hold up over time.  Seemed a gimmick to me, but who knows?

AdobeJoe

[Usman]

I just came across a website while doing some search on Water/Sea/Tidal Turbinee. Britih governments are supporting any proejcts realted in the field of tidal power. As water and wind, both are classified as fluids and share same physics principals in thermodynamics/aerodynamics, the website also reinforces the fact of more power production by a rotor in a ducted structure.

But more surprisingly, it illustartes in an animation, that DAWT may be more yaw error- tolerant than bare turbines, a phenomenon usually opposed to date! Please have a look at these two links.

Thanks.

http://www.lunarenergy.co.uk/productOverview.htm

http://www.lunarenergy.co.uk/duct.htm

[scottsAI]

Hello Usman,

Yes it will work, I too have designed a hydro VENTURI DUCT.

Nothing new here.

Then study the cost. The math is simple.

Boost the wind speed 25% output goes to 195%, you think WOW!

Then realize the area is a square function and could achieve the same effect by extending the blades diameter 40% or each blade 20%, just as the duct well have to be.

So the question comes down to which is cheaper?

Lunar energy made a big deal of off axis benefit, I say so what.

Allow the turbine to rotate and follow the flow, then it stays at 100%!! lower cost than the duct, don't you agree?

Wind during a storm is turbulent, the duct will be exposed to high wind off axis, the duct can turn just so fast, the duct looks like it has more surface area than the blades, thus more stress to contend with on the tower. If you can find a valley that limits the direction of the wind and use the duct to some benefit.

There is some practical limit to the wind speed boost, will leave the math to you.

A low wind area with a fixed wind direction, then maybe ducting will be worth it.

Have fun,

Scott.

[finnsawyer]

"only a specific amount of air can be forced to through the back aperture."

So, what is the limit?  As far as I know, there is none.  All you have to do is get the air moving faster.  Perhaps the problem is related to the turbine, rather than the aperture.  In the case of water moving through a turbine it is clear that as the water gives up energy and slows down you must give it more room to move in order to ultimately remove it from the turbine.  And it seems all water turbines satisfy that requirement.  Presumably an air turbine must operate in a similar fashion.  So, the air flow must expand as it passes through the blade plane and gives up energy.  For a turbine in free air there is no problem.  If one continues the "diffuser housing" past the blade plane then one has interfered with the natural process that removes the slower moving exhaust air.  So, perhaps the answer is to cut off the housing at the front of the blade plane.  I don't believe that has been done.  We've gone around on these issues before with no clear conclusion coming forth.  The slowdown of the air need not be that great.  Even at the Betz Limit it would only be 26%.  This would require a radially spreading of only 16%.  

[finnsawyer]

I'd replace the bullet shaped central diffuser with a hemispherical one 30% to 50% the diameter of the mill and forget the shroud.  The thing is, when fluid passes around a sphere it speeds up by 50% which means a 337% increase in potential power.  Of course, that is just along the surface of the sphere.  The speed up of the air decreases with distance from the sphere.  Nevertheless, by properly matching the blade twist to this speed up of the air, one could theoretically get 15 to 22 percent more power from the mill.  The actual figure could be greater since one is smoothing out the otherwise turbulent air flow around the center of the mill, which is probably one of the factors that limit mill efficiencies to 35% or so.  Also, such a design would allow the mill to track the wind.  One also would find the working length of the blades to be less for the same diameter of mill.

What is done, works, and what isn't done, won't work.  Right?  How many times has that being proved wrong in the past?  For those that are willing to observe and think, nature may have something else to say about it.