3rd party testing of Archimedian spiral wind turbine shows Cp of 0.29 for 1m

[oneirondreamer]

Hello Diy wind,

I've ranted about this wind turbine being vapourware in the past, and so I'm eating crow.   I'm not suggesting that it's better than my turbine, however it's interesting in that it doesn't use "lift", and I believe at the 1m size, is more effective at collecting energy than a "lift" based conventional HAWT or VAWT.   Does anyone have reports of a 1m diameter HAWT or VAWT that exceeds Cp 0.29? as reported by independent testing parties?    I'd love to see them.   I can't even find the NREL tests of the Mariah Windspire (Darius), which I think peaked at Cp 0.17 but in normal wind ran closer to Cp 0.11. 

Clearly Cp isn't everything, and a good small HAWT, might be of lower Cp, but use less material to build and install, or be much more durable for the same build, and therefore be a better choice.   This design however may have a lot of room for optimization.    Unlike the lift based VAWT's and HAWT's which have had millions of dollars and thousands of hours of research embedded. 

I hope to be out testing again in the next few days..    It's -12 here today, and I don't want to get frozen into the creek :-)

https://www.google.com/url?sa=t&source=web&rct=j&url=https%3A%2F%2Fwww.mdpi.com%2F1996-1073%2F12%2F24%2F4624%2Fpdf&ved=2ahUKEwib4pW3g7TnAhU_QkEAHYiZBuMQFjARegQIBBAB&usg=AOvVaw2pZqWs_owj4Tn4wHQis3kk&cshid=1580686788089&fbclid=IwAR3BOqZaTu6EF6xkr8q43JQC0ibnQV5w2xJCuiWGpLuR2ZzP93dp8khBTQI

[Adriaan Kragten]

In autumn 1979 we have tested a 1.5 m diameter scale model of the 6-bladed CWD 2740 rotor (D = 2.74 m) with 10 % cambered blades in the open wind tunnel of the University of Delft. This rotor had a maximum Cp of 0.39 at an optimum tip speed ratio of 2 which was also the design tip speed ratio. The rotor was measured for many different blade configurations and also for a whole range of yaw angles. The measurements are given in report R 408 S of the University of Technology Eindhoven, written by the American student Marc Schumack under the guidance of Jos Beurskens and myself. The report has 58 pages but it is no longer available and scanning it is too much work. However, I will make a summary of the most important part of it. If it is ready I will make a post about it.

I think that the Archimedian spiral turbine makes use of mostly the lift component acting on the spiral and that therefore it can have a rather high maximum Cp. But as the rotor isn't designed according to the aerodynamic theory, the maximum Cp must be lower than for a well designed HAWT.

[oneirondreamer]

Thank you very much Adrian,

I would like very much to see any part of the report you can find time to scan.   

It's interesting that according the the CFD work in the report I attached, there is no significant energy captured from conventional "lift"forces.   There are no high velocity "lift" zones.   

I think this is a boundary layer turbine, not a lift based turbine, so rather than distort a fast flow of air with a small surface, this uses the drag, or stickiness of air dragging along a surface to distort a flow, more like a Flettner rotor.   

I am not saying that I think this is any better than a conventional HAWT, of a similar size, on the contrary, because the conventional HAWT can be made from less material, will have a higher TSR so use a less expensive alternator, and is simpler to set up a yaw system to reduce high wind loads on towers and mounts, the conventional HAWT seems like a clear winner.   

I wonder though if it might be improved by being centerless?    I suspect the centre mostly contributes to negative drag.
 

In autumn 1979 we have tested a 1.5 m diameter scale model of the 6-bladed CWD 2740 rotor (D = 2.74 m) with 10 % cambered blades in the open wind tunnel of the University of Delft. This rotor had a maximum Cp of 0.39 at an optimum tip speed ratio of 2 which was also the design tip speed ratio. The rotor was measured for many different blade configurations and also for a whole range of yaw angles. The measurements are given in report R 408 S of the University of Technology Eindhoven, written by the American student Marc Schumack under the guidance of Jos Beurskens and myself. The report has 58 pages but it is no longer available and scanning it is too much work. However, I will make a summary of the most important part of it. If it is ready I will make a post about it.

I think that the Archimedian spiral turbine makes use of mostly the lift component acting on the spiral and that therefore it can have a rather high maximum Cp. But as the rotor isn't designed according to the aerodynamic theory, the maximum Cp must be lower than for a well designed HAWT.

[Adriaan Kragten]

I have studied the report about the Archimedian spiral wind turbine mentioned in the link. They have developed a theory and tested a model in real wind. They found a good accordance in between the predicted output based on an estimated Cp-lambda curve, a generator for which the characteristics are known and the generated electrical power in real wind. So the used theory gives a good result.

However, I don't understand how this can be a drag machine, at least if the normal definition of lift and drag is used. In chapter 3 of my report KD 35, lift and drag are explained, first for an airfoil in the wind tunnel and next for an airfoil of a rotating blade. The lift is the aerodynamic force perpendicular to the relative wind W and the drag is the aerodynamic force in the direction of the relative wind W. Both forces can be resolved into a component in the direction of the blade rotation and a component perpendicular to the rotor plane. The component of both forces in the direction of rotation is given by formula 4.13. If the lift component in the direction of rotation is larger than the drag component against the direction of rotation, a certain positive torque is supplied and in the case the machine is called a lift machine. The component of lift and drag perpendicular to the rotor plane is given by formula 4.14 and this component is called the rotor thrust.

A wind turbine which has Flettner rotors as blades, is also a lift machine. The rotating cylinders have a boundary layer which is also rotating and this boundary layer makes that the wind flow becomes asymmetrical when it flows along the cylinder. Therefore lift is created and the component of this lift in the direction of rotation supplies the torque. However, to rotate the cylinders, power is needed. So the power generated by the rotor must be reduced by the power needed to rotate the cylinders. Apart from lift, a rotating cylinder will also have a lot of drag which is much larger than for a real airfoil. So these two effects make that the maximum Cp of a wind turbine which has Flettner rotors as blades, is rather low.

For a pure drag machine, only drag and no lift is generated. For such a machine, the blade is moving in the direction of the wind like the sail of a sail boat which sailing in the direction of the wind. A pure drag machine is explained in my report KD 416. Certain VAWT's are partly a lift and partly a drag machine. The clearest example is the rotating blade turbine which is described in report KD 417 and for which a blade is mainly a lift machine for about 300° of a revolution and mainly a drag machine for about 60° of a revolution. But I think that a Savonious rotor has also a part of a revolution for which it is mainly a lift machine.

Now I go back to the Archimedian spiral turbine. Assume that the rotor stands still. In this case the relative wind W is perpendicular to the rotor plane. The drag is therefore also perpendicular to the rotor plane and supplies no torque. Only the lift supplies the torque. This is the same condition as the condition for which any normal HAWT supplies the starting torque (see KD 35 figure 6.4). So for this condition the Archimedian spiral turbine is certainly a lift machine.

If the rotor starts rotating, a blade gets its own tangential speed and this makes that now the relative wind is no longer perpendicular to the rotor plane. To become a drag machine, the drag component in the direction of rotation must be larger than the lift component against the direction of rotation. I see no possible flow pattern through the rotor for which this is the case. So I believe that this Archimedian spiral turbine is a lift machine. It doesn't mean that one has made mistakes in the theory the estimate the Cp-lambda curve but that only one has used the wrong name. Another indication is, that if it was a pure drag machine, it could never have a maximum Cp of 0.29.

[MattM]

The Archimedian spiral wind turbine works by influencing the curvature of the air, and anything trapped anywhere within the wake gets dragged in tow.  It really should have more application in water turbines, because it requires funneling of the flow through a controlled set of vanes.  The Archimedian spiral wind turbines don't seem to get the energy where they do from the water turbine, so I cannot wrap my mind around it being any more remarkable than an axial wind turbine.

[Adriaan Kragten]

If the Archimedian spiral turbine would be a drag machine, the blades should move in the direction of the wind, at least during a part of a revolution. However, they move perpendicular to the wind if the rotor axis is in line with the wind direction. So it must be a lift machine whatever strange shape the blades may have. They claim a maximum Cp of 0.29 at an optimum tip speed ratio of about 2.1. To realize this, the rotor blades must have a very complex shape which is difficult to manufacture. The solidity (the total blade area divided by the swept area) is much higher than 1, so a lot of material is needed for a small swept area. If this rotor is compared to the rotor which is described in my recent report KD 696 and which has a maximum Cp of 0.39 at an optimum tip speed ratio of 2, it will be much more expensive if it has the same rotor diameter.

Another big disadvantage is that the rotor also works as vane. It is claimed that this is an advantage but because the rotor works as a vane, it will be impossible to turn it out of the wind at high wind speeds. So it isn't possible to limit the rotational speed and the rotor thrust at high wind speeds. So to my opinion this is again a hopeless design which will never win the competition with a well designed normal HAWT.

[MagnetJuice]

The Archimedes turbine is an interesting design. It looks a bit complex for do-it-yourselfers, but I am sure that some brave souls would like to experiment with it.

[oneirondreamer]

However, I don't understand how this can be a drag machine, at least if the normal definition of lift and drag is used. In chapter 3 of my report KD 35, lift and drag are explained, first for an airfoil in the wind tunnel and next for an airfoil of a rotating blade. The lift is the aerodynamic force perpendicular to the relative wind W and the drag is the aerodynamic force in the direction of the relative wind W. Both forces can be resolved into a component in the direction of the blade rotation and a component perpendicular to the rotor plane. The component of both forces in the direction of rotation is given by formula 4.13. If the lift component in the direction of rotation is larger than the drag component against the direction of rotation, a certain positive torque is supplied and in the case the machine is called a lift machine. The component of lift and drag perpendicular to the rotor plane is given by formula 4.14 and this component is called the rotor thrust.

It seems to me that Drag is used as a term in two different ways.   One way, the way I think you are reffering too, comes from the wind tunnel engineering world,  is the sum of all the forces that push it backwards in a wind tunnel, with the second paired term being lift, the sum of all the forces that push it upwards.   

I think most engineers would agree that both of these summed forces, lift, and drag, are the result of more than one effect.   

The "drag" for example is a combination of viscous skin friction, frontal area, and shape, "lift" is the result of action of the flow over both the upper and lower surfaces (yes in level flight it's mostly upper, but foils, other than propellors are not always in level flight).   

So a while a Flettner rotor creates "lift", it does so by mechanically creating a rotating flow field via drag, skin friction.     

It's really a semantical argument, but it's useful to understand that "lift" and "drag" are not really forces themselves, but rather the sums of various forces.   I fall into the NASA and CFD camp of understanding air flow, and in general the recognition now, is that all turbines blades should be considered as flow turning devices and energy will be available relative to the flow turned.    It's all about turning the flow, which the low pressure area above an airfoil is perfect for in high Reynolds number systems.   In a high Reynolds number airfoil system, where kinetic energy dominates the flow, lift is generated by a small area having lots of interactions with molecules, in a low Reynolds number system "lift" must be generated by a larger surface area having lots of interactions.   

[Adriaan Kragten]

It is true that there is only one aerodynamic force working on an airfoil and this aerodynamic force can be caused by several reasons. But the component of this force in the direction of the wind speed is called drag and the component perpendicular to the wind speed is called lift. This definition is generally accepted for any body also if it isn't a real airfoil. For a rotating body you have to take the direction of the relative wind speed.

Wind turbines are normally divided into two categories using the lift component or using the drag component. To me it is very confusing to call your wind turbine a drag machine when in reality, it makes use of the lift component. It is about the same confusion if someone calls the rotor thrust "drag" and says that a HAWT is therefore a drag machine. This Archimedian spiral turbine is also that big that the aerodynamic force will mainly be caused by the curve of the relative wind speed when the wind flows in between the blades and not by the friction of the boundary layer. So the airfoil drag will not have a significant influence on the flow pattern. So the influence of the boundary layer isn't that important that you can call it a drag machine because of this reason.

[MattM]

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