Open Wind Lens
by 4ndy, published
OWL is a prototyping platform for ducted/cowled Horizontal-Axis Wind Turbines, which can deliver increased efficiency-per-area, blade safety and reduced noise. http://en.wikipedia.org/wiki/Wind_lens 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.
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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