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Open Wind Lens

by 4ndy, published

Open Wind Lens by 4ndy Aug 4, 2012


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OWL is a prototyping platform for ducted/cowled Horizontal-Axis Wind Turbines, which can deliver increased efficiency-per-area, blade safety and reduced noise.
Starting documentation at: http://opensourceecology.org/wiki/Shrouded_wind_turbine

These types of turbines tend to show an increase in power output of 2-5x for a given blade-swept-area, depending how well the shroud is designed. (See open paper: http://www.mdpi.com/1996-1073/3/4/634 and 2.5m 5kW-rated-each results of this study http://en.wikipedia.org/wiki/File:Windlens1.jpg) A shroud also tends to cut down blade-tip vortices, which are typically the single largest cause of noise and downwind turbulence in wind turbines, since it induces a wind-tunnel-like environment where an almost '2D' flow pattern can form around the blades without spilling around the end as much. (http://en.wikipedia.org/wiki/Wingtip_vortices)
A couple of safety considerations with wind turbines can be improved by a shroud/cowling; firstly in the highly unlikely event of a blade failing, any fragments would hit the cowling before anything else, so there would be significantly less danger to any passers-by, and the popularly overstated danger of birds hitting turbine blades is further lessened by having a stator in front. I have kept the current design to a roughly 1m diameter, since this should give a good safety margin of strength to wooden blades and make construction easily manageable for a very small team.

An idea that I've been sitting on and trying to figure out the details of for too long (a couple of years now), but still needs a fair bit of design work.
I would like the turbine to run in as wide an operational envelope of wind-speeds as possible, in order to take advantage of intermittent storm winds that frequent a site that I will be testing it on. As such the present design does not furl out of the wind with increasing speed, and has a heat-sink applied to the stator coils in order to prevent overheating in strong winds. This has yet to be tested though and should be regarded as stupid/reckless/unsafe until it can be verified by testing. I may also design an alternative off-centre mounting that should allow the system to furl out of very high winds.

The mounting pole is just there to give an idea of how to compatibly mount this to a tower. Having such a long pole directly from the horizontal axis would not be recommended in practice, and the turbine should be mounted by a relatively short stub at the top including some kind of yaw bearing. The structure is presently designed to accept up to a 35mm diameter pole, and I am considering the use of 25mm bicycle headset bearings in conjunction with thick-plate threaded steel tubes, or possibly sand-casting a mounting stub out of aluminium.

My design work has been inspired by:
Hugh Piggott's "Wind Turbine Recipe Book" 2009 metric edition; http://www.scoraigwind.com/
FloDesign's concept designs http://www.youtube.com/watch?v=WB5CawKfE2M (might as well mute this video as it is full of marketing BS)
...and of course the many designs of gas turbine engines that presently power most of the world's large passenger aircraft and military aircraft, which also inspired FloDesign's engineers to do their work.

The aerofoil profiles were achieved using Divahar Jayaraman's MATLAB script: http://www.mathworks.com/matlabcentral/fileexchange/19915-naca-4-digit-airfoil-generator under BSD License.
I tried editing it to output scaled data, but it seems that simply copy-pasting output vectors into OpenOffice Calc and then saving as fixed-column-width .CSV gives cleaner data that is easier to import to SolidWorks.
I used NACA designation 0020 for the stator blades and 9415 for the wind-lens. The stator is fitted with symmetric blades in such a way that its angle of attack can be altered to 0, 4, 8 or 12 degrees in either direction (shown at 4 in picture) depending on which way you want to cut your rotor blades to spin, and I have designed the wind lens foils to extrude at 12 degrees angle of attack.
If I was any good at CFD then I could optimise this more, but I'm hoping someone can help me with that.


I would greatly appreciate the help of anyone experienced in Computational Fluid Dynamics who could help me to optimise the contours of this design, but I am happy to test it empirically.

Also if you know much about electrical engineering and alternators, have a look at this thread and see if you can help me figure out how to not make this thing burn out with high power: https://groups.google.com/d/topic/ose-europe/oWP-7kAHeVU/discussion

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Phenomenal design gang! as far as noise goes, it will much more quiet than expectations!- Kids nowadays- they think a clear explanation of great science is advertizing! Johnny

I woud like to print it ... but is this not a final form? Has anyone tried to print it?

There are still a bunch of changes that need to be made, e.g. a better shape for the 'lens', fewer supporting bars from the hub, and a hub design optimised for metal casting. I have been collecting materials to prototype this, but progress has been slow.

Update v0.5:

  • Realised STEP format supports assemblies, so dumped the old solidworks e-drawings files in favour of full STEP assemblies, which you can open and measure in FreeCAD :)
  • Scaled stator blades down to 80mm chord, total PLA needed for 1-perim 7.5% rect-infill parts for stator blades and shroud fell from 322m to 257m :D
  • Looked up thrust bearings since the axial force on regular shielded ball bearings would probably wear them out too quickly. The rotor assembly now uses 1x 51100 and 1x 6000ZZ bearings.
  • Moved the shroud back by 5mm.
  • Changed suggested alu. plates to all be 3mm thick for simplicity and rigidity.
  • Changed suggested heatsink plate from holed edge to bent-over.
  • Tested shroud in Autodesk Falcon beta 4 and then 5 - this indicated that at 60mph (100km/h or 27m/s) winds that we get here whenever a storm passes by, the frame would need to support a load of about 8N or 3N per stator...?? I think this at least indicates to me that this lazy-man's analysis may not be very accurate, but at least gives me some confidence that a plastic hub might hold together.
  • Front hub design strengthened anyway, and I'm still considering casting one in aluminium.
  • Having wear & tear problems on my Mendel's Z-axis made worse by trying out thing 9864 :/ ...at least the nuts and studding should be cheap to replace.

Next to do:

  • Fix printer and test a hub or symmetric slice of it to destruction.
  • Design a proper mounting stub for the top of a tower.
  • Write up a Bill of Materials.
  • Finish part sourcing and make a prototype!

I used slic3r gcode generation to calculate probable plastic cost of building a shroud for this. With my 2.9mm PLA filament, 0.4mm nozzle, 0.3mm layer height, 1 perimeter, 2 solid-infill layers, 7.5% rectilinear infill and assuming each part is printed individually with a 1-line skirt at the start:

With 12 stators each having 7x50mm sections (84 in total) and the shroud using 12 corresponding joints with 24 spanning sections, the stator/shroud assembly used up 338m of filament. That is quite expensive for me, since the cheapest PLA supplier I can find in the UK doesn't even carry big 2.3kg reels that are typical on the continent, while current VAT makes a nasty dent.

By making the stator segments 70mm long, so that there would be 5x7cm instead of 7x5, this dropped to 329m (just under 3kg of PLA); not an amazing saving, but there may be other places to save some more.

Having tried all the infill styles that slic3r currently has to offer, and comparing 5% and 7.5% infill for most of them, I reason that the small weight saving that can be gained by using less than 7.5% infill might not be worth the strength loss, since most infill patterns then don't span lines b
etween all the holes and the outer walls in more than one direction, which is important for rigidity when a moment is applied to some bolt/pin/dowel/whatever seated in a hole in a plastic part.
Using only 1 perimeter around the foils makes the biggest difference in material saving, but I have ye
t to test and see what surface finish is like when they are printed like this; I was not far from test-printing a part like that when the heating resistor burned out on my nearly-new hot end.

Seriously, wtf? How does a wire-wound resistor burn out in normal operation? So I'm currently trying to carve it out of its little silicone-sealed nest in order to replace it. Such is the life of RepRap.

By changing the stator segments so that there are separate joint and spanning parts with slightly different holes, that estimate for the whole shroud fell a bit more to 322m. I'll be including those separate parts in the next version, since although it adds a miniscule bit more complexity to the part list, the construction makes more sense this way and smaller dowels can be used towards the trailing edge.

I am following your project.

Great progress so far! Keep it up!

That's encouraging to hear, Loki, but if you could be so kind as to spare my RepRap from your divine trickery, I would be very grateful.

Update v0.4:

  • Assembly is more-or-less complete, but there are plenty of things to tweak for greater strength and efficiency.

  • The suggested alternator design is laid out with 9 stator coils and 12 magnets, using 3x3 series-wired blocks of coils, in order to produce a 3-phase AC output. A simple diagram supplied shows how that occurs, as whenever any magnet is directly aligned with a stator coil, two others in the series block simultaneously see a magnet face, whilst the other two blocks are at different points of their AC cycle. When a 3-phase AC supply is rectified into DC, it gives a much cleaner output. A 3-phase rectifier can be mounted on metal parts of the tower as a convenient heat-sink while saving some wire. I hope that the effort I went to in representing the stator wiring in 3D can help someone - all the diagrams I've seen looked a bit confusing to me with the number of overlapping lines, so I didn't want to reproduce something 2D-crippled like that.

  • The nosecone now has a slot through which an insulated earth-wire can be guided to place a lightning-rod at the front. This would be preferrable since earthing the tower itself would allow a lightning bolt to pass through plastic parts and the axle in order to reach the tower, which could do serious damage and even present a fire hazard. I have read someone elsewhere suggesting that 3-core cable be used for turbines in order to bring an earth point to the top of a tower, but I think it would be better to have the earth wire separate, in order to prevent any strike from inducing a nasty surge in the DC supply.

  • The tail section now has 8 vanes, which I hope can aid in stabilising airflow behind the turbine. It may be possible to mount an additional diffuser floating behind the rotor, in a similar way to the main wind-lens foil fitting, in order to further reduce noise and turbulence downwind of the turbine, which may allow them to be erected effectively at tighter spacing.

  • I increased the number of wiring slots through the hub sections, just to make it easier to mount the alternator, should something need to be fitted in a strange way.

Update v0.4 (continued):

  • Although Hugh Piggott has suggested embedding magnets in thermosetting resin in one of his 'wind turbine recipe' books that I read, which I will not use due to their un-recyclability etc. Therefore, I propose a magnet-mounting disk made out of laminations of sheet aluminium, which should be easy to assemble and even dis-assemble using bolts, and could be sealed around the edges with lead-free solder, linseed oil, pitch and/or beeswax.

  • On that note, if anyone wants to build something like this close to the coast, and like me can't afford lots of stainless steel components, it would be a good idea to apply lots of linseed oil or similar to not only exposed components but practically all ferrous parts. Also I know of a small company in England called 'Green Oil', who produce more optimised vegetable-derived bike lubricants that currently protect both my bike and my reprap quite well.

  • The simple placeholder rotor blades from before have been replaced by an aerofoil showing what can actually be achieved by shaving strips off a 1x4 board with a hand-plane and then sanding it up. Any suggestions for a more efficient profile are more than welcome, as I've only just managed to throw this together after figuring out SolidWorks' lofted-extrusion feature by practicing on my outrigger design (the blades wouldn't have such a nasty corner at the root of course, but SW's filleting feature couldn't cope when I tried to smooth it out lazily).

Unless I can find some hardworking genius who has already found a way to accurately model the stress-strain characteristics of FFF-infilled objects in FEA, I think I will be performing some destructive testing on printed hub sections. I have doubts as to whether even PLA will be able to support th
e stress of high gale-force winds pulling the shroud back, and if it can't then I will probably make the front hub a cast aluminium part, or in some way augment its strength with sheet metal.
Knowing what force/moment to test the hub with would be easier if someone could guide me through calculating
the probable drag on the shroud for various wind speeds. Again, calling all aero majors; the Navier-Stokes equations baffle me. If you can figure that out single-handedly, even better.

Alongside testing and tweaking parts, I'll now try to produce some decent documentation for this system. Currentl
y this will probably be a sub-section of the wiki http://opensourceecology.org/wiki/Shrouded_wind_turbinehttp://opensourceecology.org/w... but may go on its own page if development along these lines includes enough forks to warrant moving it. I've been considering publishing a PDF manual on here, and may still do that, but wiki pag
es are just so nice for making small updates.

Update v0.3:

  • Designed a very crude tail-cone, which will be smoothed out later. I might increase the number of blades on it.

  • Added locating pins/dowels to the whole assembly, represented by 25mm-long bits of M8 studding. These holes could be made larger/smaller, but below 8mm diameter I probably wouldn't want to use wooden dowel, though I could have my mind changed by strength-testing.

  • Altered how a mounting pole fits into the structure, putting it completely inside the nosecone, thereby simplifying construction by eliminating unique stator and shroud parts. The nosecone now locks in place with the front hub using a locating pin/dowel, in case washer friction is not enough to prevent the stator-shroud assembly from spinning if stator blades have an angle of attack.

  • Added a slot in the hubs for wiring to come from the alternator through to the front, where it can exit the turbine safely via the mounting pole.

Next up will be for me to design the stator part of the alternator; put simply, a disk to mount copper coils on, for magnets to swing past, something like this: http://www.thingiverse.com/thing:1589http://www.thingiverse.com/thi... but a little more complicated so that coils can be clamped down securely to an aluminium plate as a

Printable Alternator

Grate idea first of all, might not be so loud if you study what apple did to make there fan for the new mac book thay said thay changed the fan blades to make Quieter

and for the The mounting pole i would make it arrowdynamic with a shape to let the wind go through.

whith that many stator blades interacting with 5 rotor blades, I doubt this will be anything but loud? Is there any design philosophy behind that many elements?

Perhaps a little curve to the startors will help reduce sudden pressure changes seen by the rotor blades?

I don't know what RPM you are aiming for, but your generator should turn quite quick?

I've replied to these concerns in the description, but if it interests you then you should go and read into some relevant aerodynamics.

The alternator will be of a similar design to that in Hugh Piggott's well-proven wind turbines, which operate at high-RPM with no gearbox, only I don't intend to use any thermosetting resins due to their unrecyclability and in the case of epoxy its harmful breakdown products.

Be as defensive as you like. Or rather, don't.

So you think that this skirt will reduce noise, and then introduce 8 stators and 5 blades. That's about 40 pressure drops per revolution * say 1000 rpm = 666Hz. This thing will be a screaming banshee if it is ever to produce any electricity.

Neat project 4ndy.

JelleBoomstra - A hundred years ago when I was a young whimpersnapper working for Rolls-Royce I was involved somewhat in compressor design for one of their older engines - probably the Avon. The compressor on a gas turbine is orientated exactly in such a fashion and by modeling the vectors of the air as it goes from stage to stage you determine the correct angles of the rotors and stators to get the velocity and hence pressure change you require. Clearly in a gas turbine the noise from the compressor is literally "in the noise" compared to that due to the rapid expansion and velocity of the hot gases going through the turbine but it is not a given I don't think that stators interacting with rotors necessarily will be noisy.
That said I am a wind power skeptic given its poor power density, economics and particularly its inability to provide baseload without an available efficient and economic energy storage technology. Still it keeps a lot of engineers busy and who knows someone might yet crack the baseload dilemma.

That depends on the gap between the stator and rotor, how much pressure recovery happens in there, stator angle of attack, resulting pressure
amp; direction changes, and yes, the number of elements. This is why I'd like to model and/or test the thing.

Having the stator blades induce an initial rotation in the airflow isn't at all necessary, and if a lot of noise had to be traded off just for a small power gain, then I wouldn't do that, but again this is the whole reason why it needs to be tested and optimised.

I don't understand how you thought I was being defensive, and I also didn't say that I think the skirt 'will' reduce noise, but since other such systems have, it could do, hence, hopefully not putting too fine a point on it: testing. I also didn't say there had to be 5 rotor blades, and there are
12 stator blades shown in this initial incomplete design so I don't even know where you got 8 from.