Alternator for Low Wind Speed Wind Turbine.

[MechaMZ]

Hi

I've a wind turbine which have to run under a very low wind speed, from 2-3m/s.

Currently, I'm using Envirotek V20, the result is around 1.5V-2.5V, and current is 0.3mA or nearly zero.

My aim is to achieve 50W.

Any suggestion for the alternator which I should use?
p/s: without aided by any gear arrangement.

thanks alot.

 

[russ_watters]

Are you sure you have 50W of available wind energy at 2-3m/s? The alternator's size, unless it is absurdly large, doesn't affect the output power much.

 

[LURCH]

Along with what Russ said; if the windspeed is that slow, I don't think any change of alternator is going to give you the Watage you're trying for. The only thing I can think o ffor increasing output is increasing input. Since you can't incrase the wind's speed, I suppose this will mean increasing the size of your airfoil blades. Catching more wind at the same speed will increase the energy that is being input into your system.

 

[MechaMZ]

Along with what Russ said; if the windspeed is that slow, I don't think any change of alternator is going to give you the Watage you're trying for. The only thing I can think o ffor increasing output is increasing input. Since you can't incrase the wind's speed, I suppose this will mean increasing the size of your airfoil blades. Catching more wind at the same speed will increase the energy that is being input into your system.

think this is a good idea..

I will try out with an experiment..

thanks alot..

 

[russ_watters]

What is the size of the turbine? We can help calculate the available energy and likely capture.

 

[MechaMZ]

What is the size of the turbine? We can help calculate the available energy and likely capture.

I'm using this currently..
www.envirotekpower.co.uk/V20info.html

but I'm going to redesign the turbine blade..
currently, I have few designs. they are most likely same as these examples
www.peswiki.com/index.php/Image:Statoeolien_95x95.jpg
www.peswiki.com/index.php/Image:Helix_Wind_95x95.jpg
www.hi-energy.com.tw/selection-6-english.htm[/URL]
[url]www.oregonwind.com/[/url]

just not sure which can really function good in low wind speed, 2m/s-3m/s
I think I have to run a few test on them..

by the way, you are so kind, really thanks :)
and I'm very appreciate..

p/s: sry for the urls..

 

[MechaMZ]

Besides change the size of wind blades, Do you think it's possible to generate 50W with a gear system?

my v20 now is able to achieve 165rpm @ 4.3m/s, and the current produce is almost 0! (even the wind speed reach 8m/s)

do you think just because the alternator is a lousy one?

many thanks:)

 

[russ_watters]

More gearing will only hurt you - it doesn't amplify energy and you lose energy from friction.

According to the site, that turbine only generates 3W (.25A @12V) at 4.5m/s using a turbine with a cross sectional area of .18 m^2. I don't have time to run through the energy calculation for the wind right now, but my gut tells me that if theirs can only generate 3W, you have no hope of generating 50W with a turbine that small.

 

[MechaMZ]

More gearing will only hurt you - it doesn't amplify energy and you lose energy from friction.

According to the site, that turbine only generates 3W (.25A @12V) at 4.5m/s using a turbine with a cross sectional area of .18 m^2. I don't have time to run through the energy calculation for the wind right now, but my gut tells me that if theirs can only generate 3W, you have no hope of generating 50W with a turbine that small.

how about with only 2 gears, as to increase the rpm of alternator.

wind turbine with a bigger gear, and small gear for alternator. So my alternator able to turn more round as wind turbine gear turns one round.

or I can let alternators connected in series since my project can involves more than 1 wind turbine and alternator.

thanks alot

 

[russ_watters]

Well, you could certainly run 20 of them in parallel, or hook 20 up to a common alternator.

But again, just changing the gear ratio does not change the energy you can generate. You'll increase the rpm, but decrease the torque.

 

[russ_watters]

The wind energy available to one of those turbines, by the way, at 4.5 m/s is 8 watts. That's based on a kinetic energy calculation: 1/2 mv^2 = .5*(4.5*.18*1.29)*4.5^2 = 8. A total system efficiency of just under 40% is pretty good, so I wouldn't expect you to be able to beat their performance.

1.29 kg/m^3 is the density of air.

 

[MechaMZ]

this fact is really disappoint me :(

but how about increase the wind turbine size and with the help of gear ratio?

thanks again

 

[MechaMZ]

I found this design is able to generate around 200 watts @ 10mph, 4.4704 m/s.

unfortunately it is horizontal wind turbine.

and it's able to generate 200W @ 4.4704 m/s, because it is horizontal?

 

[MechaMZ]

Dear Russ,

I've found this, which is able to generate 150 Watts in a 10 mph wind!

Wow!

 

[MechaMZ]

the link is http://www.stiltman.com/html/wind_power.html

 

[russ_watters]

Dear Russ,

I've found this, which is able to generate 150 Watts in a 10 mph wind!

Wow!

That person think's he's found a revolutionary new way to capture wind, but he hasn't. His own tests should tell him that:

We averaged 20 mph winds, which spun the wind turbine at 120 to 130 rpm, which translates to speed of circumference of the 4 ft dia. sail-disk of 17 - 18 mph, which is 85-90% of the wind speed, spinning the bicycle wheel on high gear at 52/14 = 3.7 x 130 = 481 rpm x 27” = 47 mph. There was not enough torque to turn the powerful bicycle motor (with it’s 1.25” roller and much magnetic and frictional resistance at that speed ~ when we geared down it could almost balance the friction of the motor, but we didn’t generate much power. [emphasis added]

 

[russ_watters]

but how about increase the wind turbine size and with the help of gear ratio?

Yes, the physical size of the turbine is the key factor in determining how much power it can generate.

 

[russ_watters]

and it's able to generate 200W @ 4.4704 m/s, because it is horizontal?

No, it generates more power because it is bigger.

 

[MechaMZ]

dear russ, could you recommend some alternators that are suitable for low wind speed?
because I'm trying to find those can generate well current & voltage under low wind speed.

because this is the first priority then turbine design and gear system.

 

[Redbelly98]

The wind energy available to one of those turbines, by the way, at 4.5 m/s is 8 watts. That's based on a kinetic energy calculation: 1/2 mv^2 = .5*(4.5*.18*1.29)*4.5^2 = 8. A total system efficiency of just under 40% is pretty good, so I wouldn't expect you to be able to beat their performance.

1.29 kg/m^3 is the density of air.

Russ, is that 8 watt figure per square meter, or for the area of Mecha's particular turbine, or some other area?

I agree that increasing power must come from using larger turbine blades.

 

[russ_watters]

Russ, is that 8 watt figure per square meter, or for the area of Mecha's particular turbine, or some other area?

Just that particular turbine - the .18 in the equation is the cross sectional area of the turbine in square meters.

 

[russ_watters]

dear russ, could you recommend some alternators that are suitable for low wind speed?

Sorry, that I really can't help you with except to say that I'd try to match the input and output power to each other to minimize loss. So that would mean basing the selection on the maximum expected load/wind, not the low end.

because this is the first priority then turbine design and gear system.

The difficulty here is that all of these parameters are interdependent. "Low wind speed" is not a property of an alternator, it is a property of a turbine. So my design process would go like this:

1. Determine performance requirements (peak watts, minimum watts, min/max associated wind speed).
2. Select an alternator that covers the entire range of power required.
3. Select/design a turbine system that is capable of providing the required performance (remembering to include inefficiency factors).
4. Select/design a gearing system to transfer the power between the turbine and alternator.

 

[Isarmann]

My research on this subject led to an inescapable conclusion: The only way to get the kind of efficiency desired is to build the alternator yourself.

There is basically no possibility of buying an 'off-the-shelf' alternator that doesn't make tradeoffs you're not interested in.

An alternator's efficiency can be improved in several ways, but none of them are very likely to have been found worthwhile for commercial uses. Using the strongest magnets possible is a big one (rare Earth magnets), designing the rotor and stator to be as close together as your methods permit is another, using three-phase power (more efficient generation) is a biggie... and biggest of all; if you design an alternator with a specific turning speed, friction load, and output in mind, you can get much further along than any 'savings' in time or effort that would accrue from buying your alternator 'as-is'.

I can see how you might think alternator design and building is a daunting task, but I assure you, it's worth it... and probably not as difficult as you might first think.

Let me put it this way: If you try everything else suggested and still see only minor improvements/ negligible output, then how reasonable would it sound to suddenly achieve what you wanted at the cost of 'merely' building an alternator? Put in a thousand man-hours with nothing to show for it and I'll wager that even a "herculean" 100-hour-effort will seem like nothing, if that's the price for success.

There are useful data on this subject available; let me know if you're interested.

 

[MechaMZ]

My research on this subject led to an inescapable conclusion: The only way to get the kind of efficiency desired is to build the alternator yourself.

There is basically no possibility of buying an 'off-the-shelf' alternator that doesn't make tradeoffs you're not interested in.

An alternator's efficiency can be improved in several ways, but none of them are very likely to have been found worthwhile for commercial uses. Using the strongest magnets possible is a big one (rare Earth magnets), designing the rotor and stator to be as close together as your methods permit is another, using three-phase power (more efficient generation) is a biggie... and biggest of all; if you design an alternator with a specific turning speed, friction load, and output in mind, you can get much further along than any 'savings' in time or effort that would accrue from buying your alternator 'as-is'.

I can see how you might think alternator design and building is a daunting task, but I assure you, it's worth it... and probably not as difficult as you might first think.

Let me put it this way: If you try everything else suggested and still see only minor improvements/ negligible output, then how reasonable would it sound to suddenly achieve what you wanted at the cost of 'merely' building an alternator? Put in a thousand man-hours with nothing to show for it and I'll wager that even a "herculean" 100-hour-effort will seem like nothing, if that's the price for success.

There are useful data on this subject available; let me know if you're interested.

Ya, I'm interested. Could you tell me about it?

 

[Isarmann]

Sure!

A few sites:

[disallowed by forum; think I have one more post to go]

That last link is to actual instructions on the subject, and all these resources offer possible sources for rare-earth magnets to use.

There is valuable exposition explaining the advantages of building your own alternators; but they also discuss the best to use if you simply must buy.

The how-to instructions also feature a very clear explanation of how to wind an alternator for three-phase power--- I think that what can be learned here is applicable even in parts; say, if you'd like to rewind a purchased alternator, or replace its magnets, to increase efficiency without having to make the entire thing.

The 'otherpower' guy has been at it for some years now--- I think it is his site that I first saw a few years ago which turned me on to this whole idea. All the resources listed are very much focused on real results and real output; there's very little 'armchair engineering' going on here. I know it can be very disappointing to do a lot of work for little-to-no results; these people will give you a solid foundation and real-world results to base your designs and work upon.

Hope it's helpful.

(I will have to try again, the forum wouldn't allow the URL's to show)

 

[Isarmann]

Have to get to fifteen posts...

 

[Isarmann]

Okay, I think they'll make it now--- here are the links:

http://www.scoraigwind.com/

http://www.otherpower.com/

http://windstuffnow.com/main/alt_from_scratch.htm

Incidentally, I ran across a table which seems to show that there's little chance of getting usable power from a 2-3 mph wind; I think that's how this topic was started...

I did some work on a variable-geometry Darrius wind turbine a few years ago... if you must try to make use of such a low wind, a turbine of this style, made of ultra-lightweight materials (I'm thinking composites here) may be the only practical way to go.

Darrius turbines have interesting features, including the fact that they can produce torque capable of bending the driveshaft--- so if you are stuck with the "gearing to match your windspeed" idea, this may work to produce power that (even at low rpm's) you can gear up to the speed needed for the alternator.

 

[MechaMZ]

Hi russ,

I've found an alternator, could you kindly help me take a look?
it called " Futurenergy Permanent Magnet Generators", I think it should be the one that I'm looking for.

here is the link, http://energistar.com

thank you very much

 

[Redbelly98]

Those generators require 9 mph winds to generate the 50 Watts you require.

See their table here:
http://energistar.com/FE1012U%20(404).pdf

 

[MechaMZ]

Those generators require 9 mph winds to generate the 50 Watts you require.

See their table here:
http://energistar.com/FE1012U%20(404).pdf

but it's for horizontal wind turbine, I'm wondering how about my vertical wind turbine

 

[MechaMZ]

but as what they state here, http://energistar.com/FE1012U%20(404).pdf

the alternator able to generate 53W with 260rpm(4m/s), but with a horizontal wind turbine.

 

[ank_gl]

MechaMZ, You need to understand that no matter what kind of alternator you use, you can't produce energy which is not there, because you are not "producing" energy, merely transforming wind energy to electrical energy.
Its the turbine size you should be worried about, maybe blade profile etc etc

 

[MechaMZ]

MechaMZ, You need to understand that no matter what kind of alternator you use, you can't produce energy which is not there, because you are not "producing" energy, merely transforming wind energy to electrical energy.
Its the turbine size you should be worried about, maybe blade profile etc etc

ya, thanks for remind, i knw tht we can't create energy.
just because I want to reduce the losses as least as i could.

any suggestion for the gear type?

 

[ank_gl]

just because I want to reduce the losses as least as i could.

any suggestion for the gear type?

I am afraid the more you add gearboxes, more you are going to increase losses. Keep in mind, smaller the number of units, more is the efficiency.
Only way i see to increase the output is to increase the blade area, and its profile. I think Mech_Engineer already said this.

MechaMZ, do you know that a wind turbine in very 'windy' conditions is feathered(AoA is made close to zero) to reduce the speed below a minimum safe level??

 

[MechaMZ]

a wind turbine in very 'windy' conditions is feathered(AoA is made close to zero) to reduce the speed below a minimum safe level??

not very understand, could you further explain?

btw, now my rpm is around 160 at 4m/s, I'd have to increase it to 260. So I think gearbox is needed. of course I will increase the blade size as well :)

but what are the factors actually will affect the rpm?

 

[ank_gl]

btw, now my rpm is around 160 at 4m/s, I'd have to increase it to 260. So I think gearbox is needed. of course I will increase the blade size as well :)

No No, you are still doing the same mistake, increasing the speed won't do you any good, as the torque will be decreased. Alternator demands both torque and rpm(remember power = torque*angular velocity), it is this energy that will be converted to electrical energy.

not very understand, could you further explain?

A wind turbine works in varying environmental conditions, so if it were to operate at a constant blade pitch, its rpm would vary, but you already know that power generated has a fixed frequency, so the rpm of the turbine needs to be controlled, to give a preset frequency of AC produced.
The rpm is controlled by what is known as feathering, ie, the pitch of the blade is varied as to give a defined rpm for the unit. That is why during very windy condition, blades are almost parallel to the flow, and during calm environment, they are almost perpendicular to the wind.

I haven't actually seen a wind turbine, so I really don't know that much, just a bit of theory. Someone else may be able to describe it better i guess

 

[Isarmann]

The description is more-or-less on-target, but if you've not seen a wind turbine feather itself, well, there are differences. Only the extremely large (50kW-to-1MW) wind turbines actually feather their blades like a variable-pitch aircraft propeller does--- that kind of complex gearing is never used (at least, that I've seen) on the smaller wind turbines we're talking about. With these smaller ones, the way that they feather is to move the entire propeller disc off-angle to the wind. The tail always follows the wind, of course, so some kind of hinge arrangement allows the turbine head and blades to fall off in one direction; off the tail's axis, as it starts to overspeed.

This whole discussion in the last few comments keeps making me think that vertical-axis turbines are most appropriate to your concerns--- they're much more torquey than horizontal styles, and it's fundamentally easier to vary blade pitch, especially in orientation to the wind... I think they're more efficient overall; but for you, they're definitely more tolerant of gearing.

It sounds like you already have something built, though, eh? A lot of this type of discussion, I've found, will say things that are true but more idealized than neccesary... Although it's possible to make a mistake that will keep the whole thing from functioning at all, it should be said that a lot of things (like gearing, for example) while not perfect, will work, to a greater or lesser degree... it just depends on what kind of issues you're willing to 'take the loss' on, and which you want to design around and prevent.

 

[ank_gl]

No again, you are still missing the point.
What I wanted MechaMZ to do, is, this "feathering", "windy" thing in reverse. Increase the AoA(angle of attack equivalent to decrease in pitch).

Come to think of it, I am having a hard time imagining the pitch for a vertical axis wind turbine, how does one describe the pitch of a VAWT??

I actually have seen how feathering works, in my fluid systems lab, & variable stator vanes of the LPC(low pressure compressor) on V2500Jet Engine. I only meant i haven't seen a wind turbine in action.

Isarmann, I didn't get this, "The tail always follows the wind, of course, so some kind of hinge arrangement allows the turbine head and blades to fall off in one direction; off the tail's axis, as it starts to overspeed."
Could you please explain??

 

[Isarmann]

I just meant that the head, with the turbine blades attached to it, bends off-axis from the wind-- as little as 30 degrees will spill the wind from the blades; they all stall. Look at it this way: if you can't vary the pitch of an individual blade, how do you turn that blade edge-on to the wind? Tilt the entire disc, and all of the blades at once-- their pitch doesn't change, but they'll be facing edge-on to the wind.

 

[Redbelly98]

btw, now my rpm is around 160 at 4m/s, I'd have to increase it to 260. So I think gearbox is needed. of course I will increase the blade size as well :)

but what are the factors actually will affect the rpm?

Connecting the turbine to a generator will slow down the rpm.

Using a gearbox to increase the rpm is like shifting to a higher gear on a bicycle. And, connecting the turbine to a generator is like riding your bicycle up a hill. If you go to too high a gear, you will not be able to pedal up the hill, and actual rpm becomes zero. Likewise, using a gearbox to increase rpm could actually make the thing run slower (or even stop) when you attach the generator.

There will be an optimum gear for a given wind speed.

edit added:
Given the low winds you are working with (2-3 m/s or 4-7 mph), you would require a relatively low gearing, compared to having the same turbine in say a 15 m/s wind.

 

[taylaron]

simple friction or a transmission. VAWT design

A VAWT designed around a helix is commercially built by "helix wind" [as mentioned previously] they have a youtube video at:
the great thing about this design is that it works in any wind direction; no need for a tail.
personally, I think that the helix wind is a beautiful piece of art and science. something that i would like to have on my roof (one that doesn't cost $6,500 to get...like this one)
notice the size of the alternator they have on the base of it in the movie... I know this thing is big but i didn't expect an alternator that size on it... any ideas?
Room for a transmission?

some sort of simple transmission work regarding this problem of having too much speed? I am not talking about the complexity and size of a car engine; just a simple transmission like that on a bike.

I just watched an episode of How its made on TV, they were making electric wind generators (HAWT) and to solve this problem they had a device that is mounted on the axle of the drive shaft and when the shaft is spinning too fast, the increase in centrifugal force activates a spring mechanism which increases the friction on the drive shaft and the alternator head.
something like this could be crudely made.

the helix wind could be simply made by using pvc pipe and some rip-stop nylon; on top of making your own alternator.
on the movie it almost looks like the "fins" on the turbine are a stack of the same fin, just mounted in a way that it forms a helix. you can see the gaps of light in the movie if you look closely. it appears that the fins are mounted on 2 metal bars that go up the vertical axis of the generator, to form a helix it looks like they are bent in a way that the fins rotate along the horisontal axis while moving up on the drive shaft.

----------------------
simply cutting a HUGE piece of PVC pipe or an oil drum in half down the long axis, and sticking them together to form a VAWT is a simple yet effective way to do this. much less work than building a sail etc...
youtube it... they have how-to videos about it.
along with part 2.
although with this design, you would have to stack 2 sets on top of each other perpendicular to each other in order for it to work with any wind direction effectively.

 

[MechaMZ]

No again, you are still missing the point.
What I wanted MechaMZ to do, is, this "feathering", "windy" thing in reverse. Increase the AoA(angle of attack equivalent to decrease in pitch).

Come to think of it, I am having a hard time imagining the pitch for a vertical axis wind turbine, how does one describe the pitch of a VAWT??

I actually have seen how feathering works, in my fluid systems lab, & variable stator vanes of the LPC(low pressure compressor) on V2500Jet Engine. I only meant i haven't seen a wind turbine in action.

Isarmann, I didn't get this, "The tail always follows the wind, of course, so some kind of hinge arrangement allows the turbine head and blades to fall off in one direction; off the tail's axis, as it starts to overspeed."
Could you please explain??

thanks, I will take note for this. By the way, for the shaft connection between turbine and alternator, I should use gear directly or belt such as timing belt?

 

[MechaMZ]

Hi

I've a wind turbine which have to run under a very low wind speed, from 2-3m/s.

Currently, I'm using Envirotek V20, the result is around 1.5V-2.5V, and current is 0.3mA or nearly zero.

My aim is to achieve 50W.

Any suggestion for the alternator which I should use?
p/s: without aided by any gear arrangement.

thanks alot.

I made a mistake, should be generate 50W in 12hours.

 

[taylaron]

I made a mistake, should be generate 50W in 12hours.

with how much surface area facing the wind and how many turbines do you plan to use?
i would suggest you use deep cycle NiCd batteries for your storage unit.
vertical wind turbines are great (highly recommended) try making one out of pvc pipes and rip-stop nylon. you will have to make a chamber in which to soften the pvc pipe so you can bend them into the desired form for the fins. i will post pictures of the one I'm building currently later.
i would also suggest using a belt system depending on the alternator you use and how much torque from the wind you're getting. you don't want to 'burn out' your alternator (if you can...) from going too fast. i'll also see if i can upload some CAD drawings for reference.

good luck

 

[taylaron]

commercial wind turbines

Does anyone here think that it would be possible to mass produce a wind turbine that could be mounted to your roof for mainly suburban areas?
great designs like that of the Helix Wind http://www.helixwind.com/en/ are perfect, but the cost of 6,500 dollars isn't helping any

Am i correct in saying that this is the kind of modification that has to be kept in mind when designing the house you intend to supply energy to? because of the storage, conversion, AC-DC, and all that; and that it is the contractor companies that should be making this transition effort?

 

[conntaxman]

Glade to find this site.Im bilding a vawt, wind turbine.the alternator is a two rotor with 12 magnets on each rotor and a stator with 9 coils.The coils will have 200 windings of number 14 magnet wire.The turbine will have 3 blades ,each blade with 2880 sq inches. I am hoping for 12 volts at about 200 rpm in about a 3 to 5 mph wind. I have a pic of it in another fourm at
http://www.instructables.com/id/DIY-1000-watt-wind-turbine/?comments=all&sort=OLDEST#comments
I didn't build the coils yet because I Broke my leg about 12 weeks ago.Not smart.
Would like some comments on the coil size / rpm
Tks
Johnny

 

[WindGenerator]

If you want a viable wind turbine and not a hobby project then use a real windmill, like horizontal three blade turbine. I DO understand that the vertical captures the imagination of folks trying to do things different and better, but with a vertical your backside is creating drag. You also cannot as effectively apply the force of leverage to the shaft like a normal windmill can. Think of the force at the shaft with a long blade in a stiff wind.. x 3 blades. You can't get this with vertical, that's why there are no giant vertical windmill farms anywhere.

Atleast not yet. Just like anyone else, I look at all the vertical designs too. I think.. must be some way, and maybe there is. But you must know that vertical turbines are nothing new, one that works well, that would be new. Pound for pound $ for $ the good old horizontal design is best if you want performance in a turbine.

 

[Heaven Net]

My research on this subject led to an inescapable conclusion: The only way to get the kind of efficiency desired is to build the alternator yourself.

There is basically no possibility of buying an 'off-the-shelf' alternator that doesn't make tradeoffs you're not interested in.

An alternator's efficiency can be improved in several ways, but none of them are very likely to have been found worthwhile for commercial uses. Using the strongest magnets possible is a big one (rare Earth magnets), designing the rotor and stator to be as close together as your methods permit is another, using three-phase power (more efficient generation) is a biggie... and biggest of all; if you design an alternator with a specific turning speed, friction load, and output in mind, you can get much further along than any 'savings' in time or effort that would accrue from buying your alternator 'as-is'.

I can see how you might think alternator design and building is a daunting task, but I assure you, it's worth it... and probably not as difficult as you might first think.

Let me put it this way: If you try everything else suggested and still see only minor improvements/ negligible output, then how reasonable would it sound to suddenly achieve what you wanted at the cost of 'merely' building an alternator? Put in a thousand man-hours with nothing to show for it and I'll wager that even a "herculean" 100-hour-effort will seem like nothing, if that's the price for success.

There are useful data on this subject available; let me know if you're interested.

 

[Bob S]

I read that the Envirotek V20 VAWT Savonius rotor is 30 cm wide (diameter) by 90 cm high. It is rated at 8 watts for 2 to 3 mps wind speed.
The basic formula for the achievable power from a VAWT is
W =(1/2) βρAv³ watts
where β = Betz max theoretical efficiency (0.593), ρ = density of air = 1.2 Kg/m³, A= area = 0.27 m², and v = velocity (m/sec).
Max efficiency for Savonius rotor is about 20% (with a very narrow blade-tip speed window).
Plugging in numbers, I get 0.88 watts at 3 m/sec wind speed. Do you get a different number?
The recommended rotor tip speed is about 1 times wind speed, so at 3 m/sec wind speed, the rotor should turn at 20 radians per sec or 190 rpm. Do you agree with this?
I presume the alternator you want should be very efficient at this rpm, so you want lots of poles. Will the alternator need a thrust bearing to support the rotor load (weight)? How much thrust?

α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ ς σ τ υ φ χ ψ ω

 

[mgb_phys]

so you want lots of poles. Will the alternator need a thrust bearing to support the rotor load (weight)? How much thrust?

Favorite technique seems to be 20-30 small high-strength magnets around the hub of a car wheel bearing (usually a Volvo) and a lot of home wound coils.