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High-Speed Alternator for OWL HAWT
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    October 2012
    Re-posted and updated from OSEE google group:
    I've been looking into electrical requirements for the ducted HAWT that I'm working on, and I am a bit concerned about getting blade speeds and amperage ratings right so that I don't end up with a giant fireball on the end of a pole.
    I'd like to have a purely drag-oriented (blades always downwind of the tower) wind turbine since
    this simplifies construction and strength concerns when there are high-speed gusts, as a shroud won't have to deal with potential vibrations from furling side-on to the wind, however at the same time it raises potential heat-dissipation issues if the alternator is capable of harnessing much of the power from high wind speeds. I know it can be done because it has been field-tested a few times in Japan, for instance this system at Kyushu University, where this open paper notes that this prototype had a 2.5m rotor diameter and was rated to 5000W, while an earlier prototype in the paper with a 0.7m rotor diameter and a (proportionately) much larger duct was apparently rated at 500W.

    So far I've been taking most of my electrical and rotor-blade advice from Hugh Piggott's "Wind Turbine Recipe Book" (2009 version), where he put forward a set of scalable plans for un-ducted turbines with rotor diameters ranging from 1.2m to 4.2m with alternators rated from 200W to 1000W respectively (he vaguely rates anything over 3.6m as 1kW), with a design Tip Speed Ratio (air velocity past blade tips in relation to free-stream wind speed) of 5 to 7 respectively.
    In that book, 9-coil 24V stator windings for 1.8m and 2.4m turbines (rated 350W and 700W respectively) are suggested as 110 turns and 45 doubled-up turns of 1.5mm diameter copper wire (the latter being equivalent to 90 turns of 2mm, suggested to 'double up' in order to avoid 'clumsy wire sizes'). I want to make a similar 24V stator for a 0.9m diameter ducted prototype, but with the coil mounting more squashed in diameter (and so possibly slightly less efficient). I have drawn up a rough representation here of how I think stator coils could be mounted, using an ABS casing pressed against an aluminium plate as a heat-sink - cut with small parts poking out into the passing airflow to help dissipate heat.
    My question to any electrical engineers is: what sort of current rating might be safely achievable in such a setting if I were to use something like 2mm diameter wire? I ask because I was checking this handbook-sourced guide on AWG ratings, and the quoted 'conservative' maximum ratings sound a bit confusing. I just got hold of a big cheap pack of N35 20x3mm neodymium disk magnets to test this with, after reading that lower-flux ones have greater thermal and mechanical stability. These things are still strong and difficult to handle though.

    This is especially important where I want to test out a wind turbine because here on the north coast of Scotland we occasionally get storms blow across that can bring wind speeds up to around 50mph (80.45km/h, 22.35m/s) and gusts over 60mph (96.54km/h, 26.82 m/s). If you run a calculation on theoretical kinetic energy in that wind, P = (density/2)*area*(windspeed^3), then this turns out as max theoretical power = (1.2kg/m^3)/2 * ( (3.14*0.45^2)m^2 * (26.82m/s)^3) = 7360W.
    If I assume the turbine is a mere 25% efficient, but the shroud doubled its power to 50%, that would still be a theoretical 3.6kW that could hit the alternator with a strong gust of wind. At 50mph that drops to 4259W and 50% at 2129W. To not put too fine a point on it, that's some crazy shit right there.
    I know Japan is meant to have quite a similar climate to Scotland (with the addition of some more crazy shit like seismic waves that flood nuclear plants), but I'm guessing the turbines at Kyushu Uni either didn't see those kinds of speeds due to being on the coast that faces China (this coast faces the North Atlantic), or they had some clever way of managing high-power events that isn't mentioned in their paper.

    While Hugh Piggott explicitly stated that his turbines (including blade profile) are meant to run at a TSR of 5-7, at a designed optimal wind-speed of 10m/s with a cut-in speed of around 3m/s, his machines furl away from high winds by means of an off-centre mounting, so I don't know what blade performance would be like at ridiculously high wind speeds.
    So, my question to any aeronautical engineers is: how might (0.45m long, 0.1m root-chord, 20% thickness and a few degrees of camber only on the upper / rear-facing side) turbine blades behave in such ridiculous winds? Would they be likely to stall and not harness all the power potentially available to them, or might they spin so fast that the leading edge would need a coating to protect the wood from eroding? Could you suggest any particular blade profile, root angle of attack and amount of twist & taper for this spec?
    Also, can anyone substantiate this un-cited statement on Wikipedia? "Wind turbines developed over the last 50 years have almost universally used either two or three blades. Aerodynamic efficiency increases with number of blades but with diminishing return. Increasing the number of blades from one to two yields a six percent increase in aerodynamic efficiency, whereas increasing the blade count from two to three yields only an additional three percent in efficiency. Further increasing the blade count yields minimal improvements in aerodynamic efficiency and sacrifices too much in blade stiffness as the blades become thinner."
    Because if that is the case then I am inclined to try a 5-blade rotor over a 3-blade one for such a small-diameter prototype, since strength concerns and increase in cost are both trivial at this size. The lowering in starting windspeed that comes with more blades might not matter though if I can build a decent diffuser.

    I would be grateful if anyone in OSE who isn't able to answer my questions might forward them to anyone sympathetic to our goals who is more able to help. If I can't get any sound advice on this within a couple of weeks, then I'll probably default to trying 2mm diameter stator wire, Piggott's rough blade profile, whack a 100A-rated rectifier between it and a heater, and hope for the best (while actively observing and ready to shove a not-so-figurative spanner in the works if it's going wrong).
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  • I've been a bit busy with other projects lately to move further on this, and I'm going to be away for the next couple of weeks, but then I'll actually go ahead and get that 2mm copper if I can't get some advice on this, because all the online searching that I've done hasn't turned up any useful information on heat dissipation in stator coils yet, whether theoretical or empirical.

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