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re: blade design for low wind speed
13 may 2002
anthony matonak wrote:
>it seems to me that we've got two questions here.
>a) how to design wind turbines to operate at very low wind speeds.
>i imagine there are people out there that know more than enough about
>designing airfoils and blades that this isn't too tough a problem.
>making them economically is probably a question of tradeoffs though.
>i imagine that since the power density of air is much less at lower
>wind speeds then the wind turbines would have to be significantly
>larger to produce the same amount of power.
paul gipe's wind power book says wind power density is 0.05472v^3 w/m^2,
where v is in mph, and the best rotors achieve 40% efficiency (vs the 60%
betz limit)... 90% efficiencies for the transmission, generator, and power
conversion make the wind power density 0.01596v^3 w/m^2, ie 8 w/m^2 at 8
mph, or 122 m^2 of swept area, eg a 40' circle with 4 130 ft^2 sailblades,
like a dutch windmill.
>b) how to design a low-rpm generator that is highly efficient, small,
>cheap and lightweight.
>since there isn't a lot of power in low wind speeds in the first place
>it would make sense that the less power lost in the generator itself
>the better, and more economical, the entire unit will be.
n might say we could be happy with a cheap generator with 60% efficiency.
>...smaller, lighter and cheaper... it seems to me that you're not
>going to get all these things in one package very easily.
not at low rpm. recall the 15 hp 200 rpm steam engines with 20' flywheels?
>i imagine that the easiest and most cost effective (though perhaps not
>most efficient) method is to use some form of gearing.
belt drives can be more than 90% efficient. then again, we might use 4
20' sailblades with standard 4' windmills at the ends and connect them
in series, with slip rings or a rotating transformer at the center.
>either that or take h.p.'s brake drum style generator and turn it
>inside out. create a ring generator that consists of a ring with
>attached magnets around the outside circumference of, say, a
>horizontal axis wind turbines blades and attach a matching, fixed,
>ring of coils. you would probably need a lot of bearings and some
>way for the two rings to be fixed to each other while still allowing
>flexing and motion of the entire structure. no doubt this is not an
>insurmountable task but the finished product would not be either
>small or a standard generator.
that was done in the 70s. a 1 kw generator might weigh a ton.
is there an electrical way to do this with a standard generator?
nick
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