Windmill or Wind Turbine- powered boats: how many are out there, and are they viable?

bil
It is a matter of best for what.

I have experimented with speeds around 10:1 and they can do serious damage even on a light blade 1.2m long.

If you specify what you want to achieve I can look at various options.

A fixed turbine has the objective of extracting the maximum amount of energy from the air stream. A turbine on a boat going to windward needs to have high mechanical efficiency. These are quite different requirements.

I have never looked closely to see if there is any optimum for tip speed. There might be. Other factors might also need to be considered such as the amount of noise.

Rick W

 

Rick
Mounting high up would solve the problem of danger wouldn't it? (as long as birds keep away! ) Can you explain how the fixed turbine is different from the boat turbine going to windward? What is it that drives the turbine round, is it the lift from the blades? - sorry to ask so many questions!

 

Mounting it high up might be good on land but it increases the heeling moment on a boat so not reliable.

When you are going to windward the propulsion system on the boat has to overcome the force on the turbine due to the wind as well as the drag on the hull. So if it takes a lot of force to generate the power from the turbine then you will not have less power to drive the boat. Most of the power will be used just holding the turbine into the wind.

On land the turbine is not being moved so the force is only limited by the strength of the structure.

Rick W

 

Ok Thanks Rick. So obviously, the best thing would be to try to reduce the force on the turbine somehow and allow more power to be used to move forward. It seems to be a battle between the wind and the turbine I guess the power to move the boat comes from the rotation of the turbine: so are there any ways to increase the rotational force and reduce the windforce on the turbine?

 

With turbines that are used on the land you will see mention of Coefficient of Power or Cp and Betz Limit. The Betz limit is a derived physical limit for the proportion of power you can extract from an unducted airstream. This fundamental limit is 59%. It means you can never recover 100% of the energy from the stream. It is the important factor for a fixed turbine. It is not mechanical efficiency.


With a moving turbine on a boat you want maximum efficiency. The power input is the near field velocity of the airstream onto the blades times the force on the blades. The power output is the shaft torque times the shaft rpm. With good design you can get mechanical efficiency up around 90%. However to achieve this the Cp will only be 20 to 30% depending on the operating conditions.

By getting the maximum efficiency possible you will get exactly what you asked. It is achieved by having the best possible blade section, blade shape and blade angle of attack over its span for the specified operating conditions. As far as I know there is no simple way to optimise these things so it is an iterative design process, making small changes, until you can find no way of improving.

In summary be wary of turbines that have high Cp because you will find they will not have the best efficiency. Turbine designers are focused on Cp not efficiency as none are made for moving unless you look at the Aeolus Racers:
http://www.youtube.com/watch?v=B7h6ZGOsa-Y&feature=related
The shroud is primarily for protection on these. I expect it would have a negative impact on efficiency.

Rick W

 

Rick

You're getting a little ahead of me here . I'm new to this and I need some help. I hope you can clear up a few points in my mind.
I think a windturbine blade is an aerofoil right? Therefore, it's purpose is to make "lift" - you did not mention how this lift is converted into rotation. Am I right in saying that some "lift" is used to turn the rotor and some creates the "backwards" force you mention before, that tends to slow the boat when trying to get into the wind? I was wondering what proportion of the lift is being used to turn the rotor and what proportion is pushing the boat back. I'm also assuming here that the lift is created at 90 degrees to the aerofoil surface. Is that correct?

 

bil
The angles change across the blade. The foil is twisted or pitched to suit the angle of the velocity vector at any radial point. So you get a component of drag force as well as rotating force from the lift force on the blade.

I have attached an image of a section of the blade near the tip of the turbine. It shows the things required to analyse what is going on.

The usual theory is based on dividing the blade into a number of radial elements like this having some finite width. For each element you determine the forces in the way I have shown.

The wind velocity is constant but the radial velocity increases as you move outward on the blade. You need to determine the angle of the resultant velocity vector at each element. You then determine the angle of attack for the blade at that point and from this you use foil lift and drag coefficients to determine the lift and drag force. These forces are then resolved normal to the axis and perpendicular to the axis. The same needs to be done for the drag component but this is only a tiny fraction of the lift force if the foil is a good shape.

So the torque produced by this element is the lift rotating component minus the drag rotating component (too small to show in the sketch) times the radial distance.

The angle of the blade closer to the hub will be more in line with the axis. So the rotating component of the lift force is bigger than the thrust component but it acts on a smaller radius.

Does this make it any clearer or more confused. It helps if you sit down with pen and paper and work with some real numbers for the turbine radius, turbine rpm and wind speed. Draw some velocity vectors.

I use JavaFoil to determine the performance of various foils. It is as good as any wind tunnel test data. You can get very high lift to drag ratios maybe 100 or more but the blades may not be strong enough.

There are also other factors that have to be considered but they are secondary to understanding what is going on.

Rick W

 

Thanks Rick
That's a very good diagram. I'm getting there.
Just to clarify, in the diagram, the blade moves from bottom to top of page.
If the boat was going directly into the wind, the fore and aft axis of the boat would be coincidental with the turbine axis with the bows on the left?

 

You have it. My rough sketch was good enough.

As I stated before. It helps to put some realistic numbers to it.

I use 20 elements for either turbine or a propeller blade design. There is not much value in going to more elements.

There has been one fellow on the forum who did a full CFD of a propeller and was getting results close to me. My method is semi-empirical using well developed methods so i was impressed that he could get similar results. If you had something like this you could make an adaptive optimisation that could work automatically. I do some automatic optimisation but it is still tedious.

I do not know how the shape of the big wind turbine blades are designed.

Rick W.

 

Hi Rick

Yes, I can appreciate that the vectors will change at the different spanwise locations of the blade if it is twisted to allow a constant angle of attack along its length.
I don't know at which particular location your diagram is - quite far out I should think since the Lift Rotating Component is quite a bit smaller than the Lift Thrust Component.
In the direct upwind case this is obviously a bad thing since the Lift Thrust Component is acting in pulling the boat in the wrong direction. I'm assuming that this negative result is overcome by the radial mechanical advantage (leverage or gearing) applied to the Lift Rotating Component as it is connected to the drive means of the boat?

 

The blades do not necessarily work at a constant angle of attack over the entire length of the blade. You aim to set the angle of attack at different values to get optimum performance. Also the angle of attack will increase as the turbine is loaded in any given windspeed. There is a peak power level for that given windspeed.

The component of the lift that is acting against the travel is larger than the component radially but you need to also consider the work that two forces are doing.

The work done against the axial component is applied against boat speed while the work done by the radial component is applied at the radial speed at that point.

If I am in a boat doing 3m/s (say 6kts) and the wind speed is 6m/s the following situation could exist.

The apparent wind speed is 9m/s. The near field air velocity is 8m/s - this is because the turbine is taking energy from the air by slowing it down. Looking out toward the tip of the blade, assuming the tip speed ratio is 10, the air velocity at the tip is 80m/s. So the ratio of axial component of the lift force to radial component could be as high as 80/3 and the blade element is still producing net power.

You will see from the above that you can go to very high tip speed ratio and still get net power but the ability to make the blades accurately and to have the stiffness to hold these angles limits what is possible.

Rick W

 

Rick

Thanks for all this information.
Did you address my question about leverage and gearing? Maybe you did, but not in those terms.
I guess that if the blades of the wind-turbine can be moving so fast and the boat so much slower then they have a high-mechanical advantage does that make sense? This is so different from a normal sailing boat where the sail always must move at the same speed as the boat. On a normal sailing boat a force on the sail is a force on the whole boat, but with this setup a force on the turbine blade, through the gearing or leverage, is magnified to the boat (at the expense of distance travelled of course).

Bil

 

I did address it but not directly. It is a function of the windspeed and boatspeed.

In the example I gave the windspeed onto the blades was 8m/s and the boat speed was 3m/s.

You would also aim to have a high efficiency propeller so it has very little slip.

Hence if the turbine and propeller work on a common shaft then the pitch ratio is 8 to 3 - meaning the turbine will advance 2.7 times faster through the air than the propeller through the water. This is the gearing. It is crucial to the whole thing working.

In practice the best ratio is around 1.6 but it is unlikely this would be self starting. You would need to be able to adjust the gearing or alter the pitch. Very similar to trimming sails as you pick up speed.

If the gearing is the other way around then the water blades become the turbine and the air blades become the propeller. In this case the boat sails down wind rather than upwind. It is possible to exceed the windspeed going downwind but this is a whole other story.

Rick W

 

Rick

Thanks for all your advice. It's certainly educated me about this. I will think further and let you know if I have any questions. I was hoping some other people might join in this debate but they haven't so far.

Bil

 

Bil
This a quite a tired topic.

Most people take a while to get over the fact that it is possible to power directly into the wind with a turbine. Once this is sorted out you start to think about the practicality and it requires a lot of development.

Just quietly I might be able to demonstrate something this weekend. It is not a particularly complete design as I will use bits I have here right now. If it works OK I will post some clips. It requires the right wind. Not enough and it does not go. Too much and the transmission will fail.

The really hard concept to grasp is sailing directly downwind faster than the wind. That was probably the hottest topic on this forum and resulted in some hectic debate. It has been hotly debated on many forums.

I started the DDWFTTW thread to try to give a less polluted account but it also attracted quite a storm:
http://www.boatdesign.net/forums/propulsion/ddwfttw-directly-downwind-faster-than-wind-25527.html

It is much more difficult to achieve DDWFTTW than directly into the wind but it has been demonstrated on land. Doing it with a boat is a real challenge but I have proposed some design solutions. They would be very difficult to build and it would have no practical purpose.

Rick W