Ahh.. controversy... I love it!
Great list - good points. Very nice that you kept it all so positive!! Although I would say that sometimes I don't veiw it quite as simple. Lots here for a good discussion though! Perhaps Im just in the mood to ramble and state some of the obvious... but here are my thoughts...
"Axial vs Radial alternators"
Lets remember that both axial, and radial alternators could have laminates.. or air cores. I actually saw a really neat 'brake drum' style wind turbine once that had a the coils cast in a cylender, with magnets on the outer diameter and a steel ring on the inner diameter to return the flux.
* axial+: no 'core', so easier startup
yes... this could apply either way though and an axial machine could have (and often does have) laminates.
* axial+: airgap is adjustable
Yes.. I agree! That is a handy feature!
* axial+: ultra simple construction, fairly light weight
utra simple construction is nice, and I think often times the choice is based upon available resources. I think just about anyone can build an axial flux machine and this is probably their most attractive feature.
* radial+: easy, use an existing industrial motor and add some magnets
Yes, that is handy - although I think, to get good results and a good match for the prop you have to start with the 'right' motor... and like anything else, starting with something new thats not been done before may yield slightly unpredictable results.
* radial+: small cross section
I would say that depends. Some radial designs are still quite thin with large diameter to keep the magnets moving faster and fit more wire in.
* radial+: high power levels achievable
hmm... Im not sure either has an advantage in this area. There have been some very powerful axial flux alternators built in very large wind turbines.
And... in favor of the 'laminated' machines, lets not forget that they do require a lot less magnetic material. I think perhaps that is their greatest advantage. There are also differences in the power curve for laminated machines I think. Once laminates start approaching saturation... then full power is reached. I think this can prevent some laminated machines from stopping nicely when shorted out so in some cases some other type of brake might be required. (some other type of brake is
probably not unwise on any wind turbine)
"rotating case vs stationary case (radial alternators only)"
* rotating case+: centrifugal force keeps magnets in place instead of tossing them off the rotor
* rotating case+: lots of space for copper
* rotating case+: closest approach of magnets is where field is densest
* stationary case+: more rugged construction (bearings on both sides of the main shaft)
* stationary case+: cheaper laminations, and easier to get
* stationary case+: use existing industrial motor frame
All good points. I'd also think there is advantage to the rotating case in that the magnets are moving faster, and there is room for more magnetic material for a given alternator diameter. Another advantage of a rotating case is the nice large rotating front of the alternator which makes a natural blade hub. With a shaft - I think you probably wind up with a more expensive hub, but it's hard to say...
"fixed pitch vs variable pitch"
* fixed pitch+: much simpler hub construction
* fixed pitch+: cheaper to construct
* variable pitch+: in theory a higher efficiency
I wonder if thats true... I don't know from experience or anything, but it seems debatable. Once it starts to feather.. or furl - of course it supposed to become inefficient, but till then... isnt your pitch angle fairly fixed in either case? During normal operation I wouldnt think feathering the blades slightly one way or the other would affect efficiency that much. But again - I dont really know about this from experience, just being a bit skeptical.
* variable pitch+: less wind loading in very high winds
Do you mean on the tower? I wonder if that's true as well...
* variable pitch+: higher survival winds
I think that could be debated as well... again, I don't really know - but nowhere have I read that this is the case and don't really see why it should be.
* variable pitch+: does not need a dumpload
Yes - you explained that to me, I can see the advantage of being able to let the machine free-spin. I guess in theory a side furling machine protects itself too while free-spinning, but they do run much faster and it's scary. According to Victor, Bergy claims their machines can be left free spinning - again though, it seems scary, I wonder if its good practice to do that with any wind turbine, variable pitch hub or not. I have no experience with variable pitch hubs, hopefully that will change one day.
To me, one other obvious issue with variable pitch machines - it's bound to cost a lot more to build, and there will be a lot more expensive moving parts to wear out. There is surely an economy of scale here. It would be fairly insane to build a variable pitch hub for a 8' machine, but perhaps on a 16' machine it starts making sense. Maintenance and repair is bound to be expensive over time though in my opinion.
"HAWT vs VAWT"
... hmm - this one is so controversial Im not even sure I want to comment on it! ;=)
* HAWT+: better vibration stability than VAWT
* HAWT+: self starting
Some VAWT designs are self starting.
* HAWT+: well understood, lots of time & money invested
(VAWT-: fairly well understood, gobs of time and money wasted) (sorry.. just kidding here - actually, I think there has been lots of good research into vertical axes designs and there are some serious problems which seem difficult and complicated to overcome)
* HAWT+: easier to put on a tower to get into 'good' winds (most VAWT's sit on the ground)
* VAWT+: does not need to be 'aimed' at the wind
* VAWT+: does not need a tail
* VAWT+: in theory very high efficiency
If we stay on the positive side of looking at things - I think we should mention that that HAWT's are fairly easy (and there are several reliable approaches) to control in high winds. VAWT's have a problem with this. HAWT machines also suffer from much smaller fatigue forces on the blades.
* VAWT+: has it's alternator low to the ground
"AC machines vs DC machines"
* AC+: power is easily transformable
yes, that's the biggie.
* AC+: longer distance between machine & consumer possible
Only because it's easily transformable though... distance otherwise has only to do with voltage and if the voltage is equal, over distance.. then DC wins. But it's the fact that we can transform AC that makes it desirable.
* DC+: does not need a rectifier, so no power lost there
Yes - although at higher voltages than 12, I think thats almost a non-issue.
On the favorable side of AC also - we should remember it's a lot easier to build an alternator than it is a dynamo no matter how we do it.
"permanent magnet vs excited field alternators"
* permanent magnet+: does not need field current or a mechanism to switch it on/off
Yes.. and for small wind turbines, I think the field current required by a machine with an electro-magnetic field would be a killer. Perm. magnet machines will be much more efficient in low winds, especially small ones.
Fun post - brings up all the fun arguments we like to have ;-)
I can see this one might get 100 comments or more!!!