Science: Energy: Water
Marine Propeller, marine turbine   (+4, -3)  [vote for, against]
Why should offshore tidal plants look like wind turbines?

Modern wind turbines have just two slender blades (or, at most, three). In this respect they are very similar to propeller-driven aircraft, as they have been optimised for operation in air.

Marine current turbines, developed to produce power from sea currents, look almost identical to their landborne counterparts. The sole difference is that the propeller diameter is smaller to reflect the higher density (and thus greater energy availability) of water.

However, ships' propellers have been carefully optimised for operation in water. They typically have five broad, well-swept blades and operate at relatively low rpms. Logically, this should be the optimum configuration for a marine current turbine also.

I propose developing such a system to extract energy from sea currents.

(Note that marine current turbines are totally distinct from tidal barrier schemes which, in effect, use large dams and conventional hydropower turbines to generate electricity)
-- david_scothern, Oct 05 2004

Turbine Selection http://www.canren.g...sp?CaID=4&PgID=1154
see pic of propellor turbine and chart of head and flow speed [afinehowdoyoudo, Dec 01 2008]

Small propellor turbine http://www.tfe.se/
[afinehowdoyoudo, Dec 01 2008]

Big propellor turbine http://www.guardian.../mar/31/tidal.power
[coprocephalous, Dec 01 2008]

Because of course people who build these multi-million dollar projects love to spend the money without doing any hydrodynamic modelling first.

Why should a shape that's efficient at putting energy *into* water to push a weight of x tonnes forward at y knots, be efficient at extracting energy *from* a slow moving body of water?

I love these "My hunch beats your team of post-doc mathematicians and their supercomputer" posts.
-- BunsenHoneydew, Oct 05 2004


hey, Doctor Honeydew, no need to get shirty. Have you considered that marine current turbine technology is very much in its infancy, with just two test sites operational in the world (Bristol Channel, UK and another site off the Norwegian coast)? Furthermore, the systems are directly descended from wind turbines. Research into the question would reveal suggestions that a greater number of blades might be advantageous.

I am in fact about to begin my fourth year project as part of a mechanical engineering degree, and am to focus on some aspect of MCT technology. I was hoping to stimulate discussion, and maybe get some pointers.

It struck me that the propeller used to put energy into air efficiently (light aircraft) is very similar in form to the propeller used to take energy out again (wind turbine), even though their operating speeds are different. Marine propellers turn at very low revs and relatively low speeds - a ship may travel perhaps fifteen knots. Fifteen knots, coincidentally, is a good speed for operating an MCT. A container ship running on a big diesel has a powerplant of the same order of magnitude as the output of a turbine.

So, similar power, similar conditions, yet the two designs are totally different. Can you give a more enlightening answer than "It's like that so it must be right"?
-- david_scothern, Oct 05 2004


//Fifteen knots, coincidentally, is a good speed for operating an MCT//

I thought they were meant to be fixed to the seabed...

I suspect that the marine current turbine design differs from marine propellers because they have different duties. The propeller has to transfer as much energy as it can in a compact unit of given diameter, and is not primarily concerned with maximum energy efficiency. It is best in this case to use a short, fat blade. The MCT on the other hand solely has to extract the energy with maximum efficiency, resulting in a long thin blade.

Anyway, shouldn't you sponging, lazy, good-for-nothing students be in the pub or something, instead of thinking about your chosen subject of study?
-- suctionpad, Oct 05 2004


That's a believable explanation. An aircraft in flight isn't in danger of catching hidden rocks, etc etc etc, and so a slender, efficient prop is appropriate to the task. I'll go do some more research, but that does sound convincing.
-- david_scothern, Oct 06 2004


And they work in different ways - marine propeller is basically a "screw" which drives the water backwards. An aeronautical propeller is more like an angled aerofoil/wing, and as such you get max efficiency (i. smallest Reynold's Number I think) when chord is fat, width small, and wing is long - i.e. wind turbine shape. Different tools doing different things. You need to go and read a bunch of fluid dynamics books - I read some once but it was far too long ago for me to add any more knowledge to the argument.
-- goff, Oct 06 2004


I'd just like to add that it would be wrong to generalise about either aircraft or marine propellors. The variation in diameter and pitch of these, especially marine, propellors is quite wide. Also, what about the blades of a jet engine, these are obviously made to perform at very high revs, would they're design be worth taking a look at?
-- etherman, Oct 06 2004


Wind turbines use few blades because high blade numbers causes aerodynamic interactions between blades. As a result, a turbine similar to a jet fan would not be very efficient.

<edit> Following a meeting with my project supervisor, we're going to investigate three-bladed turbines. They pose problems in wind applications as balance suffers as the bottom blade passes through the wind gradient / boundary layer, but this may not be significant in marine applications.
-- david_scothern, Oct 06 2004


here's a thought (though probably not a good one). What about the good old fashioned waterwheel. or in this case comething more akin to a paddlesteamer unit but in reverse. Large rectangular blades on an axle. The upper half could be sheilded from the flow with a cowling and the lower half exposed to the current. ???
-- etherman, Oct 06 2004


[d_s] I thought 3-bladed wind turbines were the most common design.

If you're looking for suggestions to investigate, what about lift-type vertical axis MCT's? I heard that vertical axis lift turbines had problems with resonance, and with stability in high winds, but this might not be an issue subsea. Just a thought - I haven't looked at any of the numbers.
-- suctionpad, Oct 06 2004


I thought from thie title that this idea was going to be regenerative braking for boats.
-- caspian, Jan 11 2006


It's late, my tired eyes saw "regenerative braking for bats".
-- normzone, Jan 11 2006


Or, "regenerative baking for brats?"

Sorry...
-- csea, Jan 11 2006


Apologies for my crankiness two years ago. How are the studies progressing [david]?

I wonder if a neutrally bouyant, horizontal axis machine anchored at the axis from a long line would eliminate the interference from the mast.
-- BunsenHoneydew, Jul 29 2006


/I love these "My hunch beats your team of post-doc mathematicians and their supercomputer" posts/

I totally love them!
-- bungston, Jul 29 2006


Well [Bunsen] in honesty, we never did get very far, but got a reasonable mark and graduated, so I guess it was OK.

The idea of trailing the turbine from a mast seems to make sense, although you'd need to be able to counteract the torque exerted on the generator by the blades. Maybe the structure could resemble a gallows, so the generator was still rigidly mounted to the mast, but further downstream of it to eliminate interference.
-- david_scothern, Jul 29 2006


For low head hydropower e.g. tidal power it seems that the propellor turbine or Kaplan turbine are the standard choices. Most of those seem to have wide blades, looking quite a lot like boat propellors. (+)
-- afinehowdoyoudo, Dec 01 2008



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