Ocean Current Power Generation

Well seeing as this was intended for currents of speeds of 4m/s or less I dont' think cavitation is really any issue. It is only a free stream.

And as you say that 70% of the lift is generated by the low pressure zone on the top. This still doesn't explain why the high pressure zone appears to be on the inside of the venturi....

I guess I'm still confused. Thanks for you help though

StephenT

I don't see any pressure information on the diagram. Perhaps I am reading it wrong.

A water velocity in excess of 3m/s will cause cavitation, so you'll either need to increase the diameter of the throat or reduce the flow.

The point of lowest pressure in a venturi is in the throat. Since it enlarges in diameter past that point, the fluid will slow down and increase in pressure as a result.

A water velocity in excess of 3m/s will cause cavitation,

I might modify that to say could cause cavitation, but will probably not. In my speed record attempt sailboat, at 32 knots (17m/s) there was no evidence of cavitation on even on the low pressure side of the hydrofoil, where the flow is faster yet. On hydrofoil-supported boats, cavitation is usually not an issue until speeds get up to 45 knots or so. It is at about those design speeds where people start to use supercavitating sections.

1. I didn't know that a venturi will work in water. Please 'splain this to me Lucy. And where has it been used?

2. First sentence is in line w/ Bernouli Principal. Second sentence is almost right, except that the difference is 'created' behind the trailing edge by a double vortex, one large and a smaller one behind that. Both vortexes are quite seperated from the wing but serve as 'drivers' for the airflow. That's why the trim is so important on an airplane, and why most Marconi rigged mains have a leech cord to adjust the trailing edge of the sail. It's those vortexes that will make or break the airflow over/under the foil.

Hey, my first opportunity to be a nitpicker!

Carl

1. They can be used to draw chemicals out for dosing and mixing with the water, as in softener regeneration. They are also commonly used to draw in air to aerate a jacuzzi or pond.
2. Been more than 6 years since I last touched an aircraft so I may be a bit rusty on my aerodynamics, but the basic principle still applies.

"1. I didn't know that a venturi will work in water. Please 'splain this to me Lucy. And where has it been used?"

Bernoulli's Principal is based on fluid flow. Gas and liquid react alike, except that because liquid does not compress as easily as gas one would expect subtle differences. But Bernoulli's formulas upon which a venturi operates apply for both gas and fluid.

Check out http://en.wikipedia.org/wiki/Bernoulli's_equation and notice the "compressable" and "incompressable" flow equations.

GA.

The pressure available in sea currents is very low. The surfaces will be large to concentrate enough energy at the turbine. If you want more pressure, you need a dam of some sort.

Still, this is much better than wind mills as far as energy available per surface unit and the power flow is more consistent than with wind.

Hi there stephent,

I am not a mechanical engineer but in response to your enquiry I would say this:

re. 1) I believe the point of this design is to NOT constrict the flow. It opens up widely to produce a low pressure behind it and therefore a higher differential pressure and therefore a higher flow through the duct.

re. 2) Aerofoils produce lift by creating a low pressure zone above them NOT high pressure below them. (it would seem you are thinking of kites) In this case however I think the rings are just there to let in just enough water to limit turbulence.

It looks like a good idea especialy (as is suggested in the article) in places where the water isn't deep enough for a larger turbines but it seems to miss an important consideration. The size of the duct is as big as the turbine it would replace UNLESS (this is the part they missed) the duct isn't circular! If the diagram is a top view but the side view is straight then you could get a lot more power out of the small turbine while keeping the duct close to the bottom.

All this aside, I don't see any reason why this would be any better than a duct with a large inlet and a small outlet, but as I said, I am not a mechanical engineer.

Gordie.

Hello Stephent,

Your venturi is only a venturi in part and thus part of the confusion. Asymmetric airfoil is the label used. While approximating a venturi you only have a 2 dimensional flow. The stream bed and stream surface are fairly parallel.

As for the small frontal area you could build a larger one but unless you funnel the whole stream or segregate a portion of the stream you will quickly max out on the frontal pressure wave and the water would just go around. The pressure on the face of the turbine is caused by impulse. The mass of the water changing direction. The lift from the pressure difference is the low pressure keeping the mass from pushing the back with the same pressure.

Cavitation at high speed destroys the low pressure. While you may encounter turbulent flow I doubt cavitation will be an issue at these pressures and speeds.

Pick up a good book on fluid dynamics. If you can do scalar functions the math will be fairly strait forward.

If you want a brain bender try a Kline lifting body.

United States Patent 4606519

Abstract:

"An airfoil having improved aerodynamic characteristics incorporates a leading edge (13) and a trailing edge (14) longitudinally displaced therefrom. A continuous lower surface (21), defining the lower camber of the airfoil, extends from the leading edge (13) to the trailing edge (14). The upper surface of the airfoil incorporates a first upper surface (22) extending rearwardly from the leading edge (13) and terminating in an offset (20), and at least a second upper surface (23) extending rearwardly therefrom. The first upper surface (22) defines a first upper camber portion of the airfoil and the second upper surface (23) defines a second upper camber portion thereof."

The crude short version is it looks something like this and travels to the left. The lift is up.

Brad

I may be wrong but I think a simple misinterputation of the diagram is happening. The Wikipedia document states "it can be seen that a down stream low pressure (shown by the gradient lines)". So the pitch of these blue/green gradient lines show the pressure level normalized to ambient condition. The negative slope gradient lines are downstream and to the left of the venturi. Taking this in mind the media flow direction is from right to left. So the venturi inlet is of a larger diameter than the outlet.

I dont' think it is a misinturpretation. There are numbers on each of the airfoils and it starts on the left.

The explanation that the throat cannot be too restrictive due to there being little momentum driving the fluid into the venturi in the first place makes sense to me. If the throat were too restrictive, the water would likely "pile up" in front and be pushed around especially if there is a turbine there.

I agree with the comment that a larger turbine the side of the shroud would ideal and make more power than a smaller shrouded turbiner BUT my point was that if you had a stream of say .75m/s that could not be harnessed by regular turbines due to the low speed then perhaps you could use a venturi to increase the velocity to a high enough degree to be used by a turbine.

I say this becaues I am mostly interested in ocean current power generation and not tidal power generation. Also I should point out that the idea is *not* to create a dam. Dams flood land, destory habitats, and are only available in very limited locations.

Tidal currents reach velocities as high as 10m/s whereas ocean currents only reach roughly 2m/s (the Gulf Stream) and less than 1m/s is common.

The benefit to harnessing ocean currents vs tidal currents:

1) The power is nearly constant whereas tidal power varies everyday (it even switches direction)

2) If you can harness the power of low speed currents then there are far more locations for turbines

So the point was to increase the potential zones where turbines may work efficiently.

Educational! GA

Thank you very much for your input. You have cleared up quite a few things here. I especially like your BB analysis.

So you're saying the airfoil has a low pressure zone just after the leading edge? That would explain the flow being drawn into the venturi.

"This is the truth, the whole truth, and nothing but the truth (provided you ignore the parts I made up , misrepresented, over-simplified, over-complicated, or left out)."

hahahaha

So you're saying the airfoil has a low pressure zone just after the leading edge?

(As I'm writing this I'm thinking that it would be really nice if CR4 had a sketching capability.)

In the picture above, there is wingy looking thing. Then there is a graph of the pressures on the upper side of the wing, and a graph of the pressures on the lower surface. These two graph lines must meet at the leading edge and trailing edge. The wing picture serves as the indicator of how the pressures are distributed from leading edge to trailing edge.

The pressure scale is reversed from the usual, in that the pressure is lower as you go up. The horizontal line is the zero point.

You can see a couple things about the pressure distribution. (This would be, BTW of a wing at perhaps 10 degrees angle of attack). There is about twice as much low pressure area on the top of the wing as there is high pressure on the bottom. (Thus, it is the upper surface of a wing, or the leeward surface of a sail, which does most of the work.) On both the upper side and lower side, most of the pressures making lift are concentrated near the leading edge. This is especially true on the upper (low pressure side), with a strong peak just behind the leading edge. In most wings the total lift created on the first 25% of chord equals the lift created on the last 75%. (So if you wanted to create a "balanced" rudder [one having very light or no "feel"] for your sailboat, you would put the pivot point near 25%.)

If you think about flying, you might see a problem with this pressure distribution. If there is any angle of attack at all, the wing would want to flip around: if the pilot raised the nose a tiny bit, the wing would suddenly want to raise it a lot. The Wright Brothers contribution had much more to do with how to make a plane stable and controllable than how to make a wing that creates lift -- which was pretty well known many years before.

GA again... Now if you could just create a few hundred more sketches for a wide range of velocities and AOA....

Chris

You may wish to look at this low-velocity hydro generation scheme.

http://www.vortexhydroenergy.com/html/links.html

It depends upon turbulence in the water, which can always be created by installing hardware the right distance upstream of the vortex device.

I did not read every line of commentary- however it appears that one aspect has been overlooked. That would be that the angle of attack- for straight line surfaces particularly can be adjusted- if a secondary plane (surface) is a attached at a distance- the primary plane can produce/generate a more consistant result/flow. This same type of mechanism can help eleviate the catastrophic crashes seen in boat races.. those that flip over (front over the back). A secondary (other than the boat hull) contact going through the upper regions of the water surface can be linked to a wing at the front of the craft to help keep the proper angle of attack as it speeds across the water. Thanks Carlos

"The choices you make might be mistakes but it's never too late to turn around." Johnny Lang- musician

I have in fact read about the VIVACE technology. It is very interesting. One concern I have is with the low relative movements there is a large chance for large marine plant life build up. This would affect the output and would be a large maintenance cost.

The movement shown in the sample video seemed pretty substantial to me, at least a few times the diameter of the cylinder if not several.

Gordie.