another idea

[Rick Willoughby]

Originally Posted by MPraamsma

Rick,

Just wanted to clarify what I meant by the buildup of the axial forces in a wind turbine blade as the tip velocity increases. In my diagram I show a typical foil at various radial positions along the blade, and the pitch angle at each point. All sections are considered moving with a zero degree angle of attack to the relative wind for simplicity. To keep it simple, we resolve the basic lift and drag, which at zero degrees pitch align along the tangential and axial axes perfectly, into a resultant force. Higher up on the blade, at 30 degrees for instance, it is clear to see that the resultant, when projected on the tangential and axial axes (green vectors), is transfering force increasingly into the axial direction. At 60 degrees, the axial component has swamped the tangential, even though the magnitude of the tangential vector may be many times larger than at 30 degrees. In a stationary wind turbine this is of little to no consequence, other than requiring a very robust support pylon, since in that situation the entire earth is the anchor. However, in a wind powered vessel the anchor consists of the thrust created by the underwater propeller, which in turn is derived from the output of the wind turbine. This explains some of the disappointing performance of upwind runs of turbines, and needs to be addressed if they are to take advantage of the greater energy available directly windward.

MP
I take all that into account in the numbers I gave you. I determine best tip speed for your device will be around 6:1 in apparent wind 0f 10m/s. This results in the best performance.

Rick W

[Rick Willoughby]

With regard to tip speed, did you determine the tip speed of the model vehicle. That blade seems quite aggressive.


For your prop I considered rpm from 200 to 900 in 10m/s apparent wind. 700rpm was similar to 800.

Anyhow you will have some data next week. I will be interested to see how it goes.

Rick W

[Rick Willoughby]

Originally Posted by MPraamsma

Rick,

Your concern about the duct drag is a little pessimistic. I once made a soapbox car for my daughter years ago that had a monstrous venturi duct as the body, with a frontal area 8 sq. ft., and it performed as well as tiny little cars that had about 1 sq. ft. area. A properly designed venturi is a net zero device, where the forces of flow are balanced between the inlet and diffuser. In effect, the car I made was a giant Laval Nozzle similar to a rocket engine. I have added some diagrams a pictures to illustrate what this contraption looked like. Also, the flow plot shows the conditions in the duct at the point that the throat starts to go sonic. Notice that the external flow is almost completely undisturbed.

I usually make an observation from previous experience and then look to check my observation with some numbers.

When I checked the ducted turbine with my turbine model I was getting funny numbers that were hard to believe.

Anyhow I found the attached paper that backs up my numbers. The interesting aspect is that it is possible to exceed the Betz Limit in the example given in this paper:
http://www.ibpsa.org/proceedings/BS2...3_0407_414.pdf
I am not sure if the same result is possible in a turbine isolated from other interference. My model may not have enough detail about far field conditions to give the right result.

I compared a 1m diameter ducted turbine with the 1.5m open turbine. The air velocity over the turbine was doubled. This improves the blade performance even though the diameter and area are smaller.

What I am not accounting correctly for is the inlet pressure due to velocity reduction through the ducting at the turbine. This will add pressure to the inlet area and will increase effective drag. THis has nothing to do with the aerodynamics of the ducting in free flow conditions. I expect the ideal shape of the ducting to be different to what you showed in the original sketch because there will be reduction in the air velocity exiting compared with that entering.

So the fact that the turbine gets into a better operating regime at lower air flows may help overall. It is worthy of further analysis.

Rick

[MPraamsma]

Originally Posted by Rick Willoughby

I usually make an observation from previous experience and then look to check my observation with some numbers.

When I checked the ducted turbine with my turbine model I was getting funny numbers that were hard to believe.

Anyhow I found the attached paper that backs up my numbers. The interesting aspect is that it is possible to exceed the Betz Limit in the example given in this paper:
http://www.ibpsa.org/proceedings/BS2...3_0407_414.pdf
I am not sure if the same result is possible in a turbine isolated from other interference. My model may not have enough detail about far field conditions to give the right result.

I compared a 1m diameter ducted turbine with the 1.5m open turbine. The air velocity over the turbine was doubled. This improves the blade performance even though the diameter and area are smaller.

What I am not accounting correctly for is the inlet pressure due to velocity reduction through the ducting at the turbine. This will add pressure to the inlet area and will increase effective drag. THis has nothing to do with the aerodynamics of the ducting in free flow conditions. I expect the ideal shape of the ducting to be different to what you showed in the original sketch because there will be reduction in the air velocity exiting compared with that entering.

So the fact that the turbine gets into a better operating regime at lower air flows may help overall. It is worthy of further analysis.

Rick

Rick,

Actually, the shape of the duct I showed is not that far from what would be a practical configuration, other than the exact dimensions and proportions. The inlet in this case would ideally be a simple tubular duct so that the flow is directed outward as far as possible so that it intercepts the blades as far away from the axis as feasible. The flow at the throat has less static pressure, but enormous ram pressure. In the ideal nozzle this flow moves at Mach 1, and represents the maximum kinetic energy of the airstream. Once past the turbine, it must expand back to it's natural state, minus the momentum removed by the turbine. This requires the diffuser to be slightly larger than the inlet to draw the air through (so as not to cause a stall), which is known as diffuser augmentation. The central body can be the almost bloated torpedo shape shown, because it forms part of the venturi wall anyway, and you need plenty of room to house alternators and other mechanical stuff. Also, some form of stators at the inlet would direct the flow more normal to the underside of the turbine blades, effectively allowing better pitch angles. It is well known that ducts are not subject to the Betz limit.

[ancient kayaker]

On a slight tangent to the discussion so far, most of the argument for a wind turbine powered boat is based on its advantage in narrow waters when headed directly upwind compared with a conventional sailboat which must be repeatedly tacked.

I have also read claims that a wind turbine powered boat can continue to "beat the wind" on a run if its speed is first made to exceed the windspeed under power, after the power is cut. Has anyone tried this? It would be easier to demonstrate with a wind turbine powered land vehicle than a boat, just give it a good shove downwind, but would still be valid for demonstration purposes. There seems to be no theoretical reason why it would not work.

I wonder if a wind turbine powered boat is actually more efficient than regular sailboat in a variety of wind and water conditions on various courses or if the advantage is limited to upwind (and maybe downwind). By efficient I mean faster in terms of distance made good in the desired course. I suspect a well-handled sailboat would have the advantage on a reach, due to the lack of transmission loss.

Presumably for a sailboat without auxilliary power upwind performance is generally more important unless sailing the trades.

[MPraamsma]

AK

As a sailor of a conventional ketch rig, I am very skeptical of faster than the downwind wind idea. When I go downwind I am impressed by the calm that surrounds my boat, like nothing is happening and no wind exists, almost no sensation of speed whatever. All I am aware of at that point is the massive drag on my hull, and little power noticeable in the wind. I personally think that all the normal sailing positions from a close reach to a run are well served by regular sails, which should still always be part of a sailing vessel, it is the directly to wind situation that needs a better solution, as tacking can often be tiresome as well as dangerous, and certainly time consuming.

My goal (as an engineer) is to figure out the secret to extracting maximum power from the wind when at it's maximum potency. As I check the web, it is clear that little work has been done on moving wind turbines, which in principle should be the most efficient way to extract wind power.

[MPraamsma]

..it seems we have been beaten to the punch...

[ancient kayaker]

Originally Posted by MPraamsma

AK

As a sailor of a conventional ketch rig, I am very skeptical of faster than the downwind wind idea. When I go downwind I am impressed by the calm that surrounds my boat, like nothing is happening and no wind exists, almost no sensation of speed whatever. All I am aware of at that point is the massive drag on my hull, and little power noticeable in the wind. I personally think that all the normal sailing positions from a close reach to a run are well served by regular sails, which should still always be part of a sailing vessel, it is the directly to wind situation that needs a better solution, as tacking can often be tiresome as well as dangerous, and certainly time consuming.

My goal (as an engineer) is to figure out the secret to extracting maximum power from the wind when at it's maximum potency. As I check the web, it is clear that little work has been done on moving wind turbines, which in principle should be the most efficient way to extract wind power.

Agree, except your skepticism of the faster than the downwind wind idea.
I've heard of conservation of energy and "you can't get something for nothing" by the way. What is claimed to happen is, the waterscrew extracts energy that is used to drive the airscrew; overall the energy to drive the boat comes from the air which is moving in the direction of the boat albeit slower. Yes, it sounds screwy (sorry, I've a weakness for puns) but the math seems to work. Conservation of energy is not violated.

[MPraamsma]

AK
I'm not saying it is impossible, just that I'd like to see a working model, however crude.

[Rick Willoughby]

Originally Posted by MPraamsma

AK

As a sailor of a conventional ketch rig, I am very skeptical of faster than the downwind wind idea. When I go downwind I am impressed by the calm that surrounds my boat, like nothing is happening and no wind exists, almost no sensation of speed whatever. All I am aware of at that point is the massive drag on my hull, and little power noticeable in the wind. I personally think that all the normal sailing positions from a close reach to a run are well served by regular sails, which should still always be part of a sailing vessel, it is the directly to wind situation that needs a better solution, as tacking can often be tiresome as well as dangerous, and certainly time consuming.

My goal (as an engineer) is to figure out the secret to extracting maximum power from the wind when at it's maximum potency. As I check the web, it is clear that little work has been done on moving wind turbines, which in principle should be the most efficient way to extract wind power.

It is the same principle as sailing directly into the wind but you use a water turbine and an air propeller. The gearing has to be reversed.

It requires quite a large propeller to work for the reasons outlined earlier regarding low Re# airfoils.

If you have the ability to continuously adjust gearing through a transmission or variable pitch you can get optimum performance in any direction relative to the wind including going directly into the wind and faster than the wind down wind.

100 years ago sailors had trouble understanding you could have VMG 90 degrees either side of the wind direction. Today most people have difficulty wrapping their brain around the idea of using wind energy going directly into the wind. Very few people can understand going down wind faster than the wind but it can be done. It is a matter of getting the foils right and the gearing between them right.

There is a demonstration of a land vehicle about twice the size of the one you built accelerating down wind. Generally all these things are easier to do on land because you have a nice firm medium to react on but water is not too bad if you have a low drag hull and a nice size prop.

I can provide the conditions for open or ducted propellers and open turbines. I have not considered closed turbine until you raised it.

Rick W

[MPraamsma]

Rick,

(BTW I am an old Aussi way back. Lived in Sydney from 1950-1960. Originally from Holland.)

The ideal closed turbine has a small annular throat with lots of tiny stubby blades on a very large radius from the axle. This plus some form of fixed vortex vanes in the inlet to creat a swirl in the direction of rotation.

[MPraamsma]

Rick,

I cleaned the diagram of the ducted system up a little to represent a more realistic configuration. Also, I wanted to expound some of the advantages of this type of system for use on sailing vessels. As you know, my pet peeve on the standard open bladed design is the massive axial component of thrust that develops as a consequence of the pitch angle approaching 90 degrees at the tips of spinning props. This component is highly undesireable from the standpoint of a vessel mounted wind generator that is used to power the vessel's motion through the water, and everything possible must be done to eliminate this force component. Conventional props cannot solve this issue unless they have very low tip speed ratios, and this means very low flows over the prop foils, and therefore low output. This can however be solved by changing two properties of the flow field, the effective angle of attack on the blades, and the speed of the flow field. These two properties can be modified by using a fixed stator that introduces a twist into the flow, and accelerating the flow by passing it through a venturi duct. A properly engineered venturi can accelerate the flow from 100 MPH to the speed of sound, which is around 550 MPH! Since ducted turbine blades are short, this speed acts over the entire blade surface! Once the flow has established, it has enormous inertia, since most of the potential energy of the air has been converted into kinetic energy at the throat, which keeps the flow moving along quite nicely. Once up and running, the venturi acts like a giant vacuum cleaner, drawing air into the inlet from well ahead of it. The air (exclusive of the real wind) is actually moving before the inlet arrives, because the speed of sound is so much higher than the vessel speed.

Some other advantages:

• All moving parts are inside a shroud
  Much smaller size
  Can be built to withstand huge RPMs safely
  Operates at speeds compatible with high performance PM alternators
  RPMs are independent of the wind speed
  Very small blade twist
  Blades can be extremely rugged
  Gyroscopic forces easier to control
  Can be built in complementary pairs that cancel gyroscopic forces so that pointing effort is low
  Can be sealed off from the elements with covers
  Ducts are simple tubes
  Stator acts as unobtrusive support structure for turbine
  
  
  
  

I could add a few more, but I think you get the idea.

[MPraamsma]

This website describes a turbine with some of the ideas I have raised earlier for the more efficient production of mechanical output.

http://peswiki.com/index.php/Directo...urbine#Patents

Notice the use of a fixed stator at the inlet to create a swirled inflow that strikes the blades at a much higher angle of attack.

[MPraamsma]

Rick,

Just wanted to keep you updated as to the progress of the bicycle trailer project. I attached some pics, and a small video of it in operation after I got it all assembled. So far we tested it behind a bike for structural and mechanical integrity, but no wind this weekend as fate would have it. We towed it up to about 20 MPH and that blade really cranks! Without the trailer we can still only get the bike up to about 22 MPH. Since we can't test it in the desert this weekend I will make a small vane steerable front wheel this week that can be detached so that it resembles the little model only bigger. I had to keep the chain off for the pictures, because any slight breeze wants to make it take off.

Michael

[Rick Willoughby]

Michael
That is really nice work.

If you can set this up to sit low so you minimise your windage using a small front wheel for steering it should give impressive performance. I believe this a better option than setting it behind a bike.

You could find a small decline to get it going if the wind is not very strong. Or just kick it along to get some speed. In strong wind it should be trying to take off on its own.

This will be a very useful test bed. You could fit bike gearing to it as well.

I would appreciate an approximate pitch angle and blade profile at about 5 points down the blade if you can provide that.

With the one I made I was staggered by the power it generated and the blade deflection under load. It was 2.2m in diameter. The blades used aluminium flat bar as a former with balsa for shaping and then all glassed over the balsa. The section was constant MA409 foil with 100mm chord. The blades were not a work of art but demonstrated that they need to be well designed to cater for the potential loads in a good breeze.

Your blades may flex a little more than you would like but hopefully they will not break.

Rick