I am kind of confused about the efficiency. The efficiency of what?
Let's just say I'm not too impressed with most of the homebrew book designs when it comes to properly matching a generator to available shaft power. Most of them involve gluing the biggest neo magnets possible to the rotors so they can use less turns, decide on the number of turns based on cut-in wind speed, then stuff the biggest two-in-hand (or more) winding into it that will fit, wiring the thing three phase wye.
That's not the way to build wind turbines if you want one that performs.
--
Chris
Chris is right, efficiency of the whole system is not the same as the efficiency of the alternator. There are many different efficiencies to consider. Best electrical efficiency, best blade efficiency, most efficient use of money, most efficient use of magnets, and so on and on.
The homebrew design books that I have published, and the Dans' one is of the same era, are based on a time when NdFeB (neo) magnets were the obviously most cost-effective solution, and they do offer high electrical efficiency. They still offer a good solution, but there is going to be a cheaper one based on ferrites (maybe Chris can publish one that can be built in a back shed?) as the price trend continues.
The problem with high electrical efficiency is that you end up with a machine that has a low range of speed. Once you have reached cut in speed then it doesn't take a lot more rpm to reach full power because you only need those extra rpm to overcome losses. So the lower the losses, the less variation in speed above the cutin. Blades, on the other hand, want to run over a wide range of speeds in proportion to the speed of the wind which varies from 3 -12 m/s in some cases (7- 26 mph). So they would like the rpm to increase by a factor of 3-4 times so as to keep their best tip speed ratio.
Here is where the discussions of increasing the air gap to prevent stall and of putting resistance in the line to keep the blades running in stronger winds start to emerge. Blades that run too slowly in stronger winds can perform poorly (although it's nice and quiet). The best solution for efficiency (overall) is to use some kind of MPPT controller such as Flux has developed or would be available if you use a grid-tie inverter. By varying the voltage the alternator works at you can have the best of both worlds (low electrical losses and happy blades). But for those of us who are suspicious of black boxes and like to connect the rectifier to the battery, we have a dilemma. Higher electrical efficiency means lower blade efficiency at some windspeed. You can only tune the machine for a narrow band of windspeeds if you keep electrical resistance low.
OK so if we know that electrical losses can be 'good' where does that lead us? The neo alternator is compact and higher losses will make it burn. So in that case using an inefficient line can help the blades or you can even use heaters in series with the battery charger. I do this in some cases. The heat may be useful. it's certainly better than having it in the stator. And it allows the voltage and speed to rise so that the blades run faster in stronger winds so avoiding stall. The best solution for you is going to depend on your circumstances in so many ways to do with line length, battery voltage, priorities and so forth. and maybe if you expect the recipes and the homebrew book to guarantee a simple single solution you will be let down.
Using ferrites does help the blades because they do tend to be less efficient, so they have to be built bigger enough which allows them to dissipate more heat. So things are turning around, and lower electrical efficiency does not seem so bad. You can use single phase and you can use dual stators, simply so as to make the machine bigger and less efficient than it needs to be be, and actually do well because the blades will thrive on the variation in rpm that this produces. OK so you can get a wind turbine that works really well and I respect Chris for doing this and for stimulating our minds at the same time. His speed increase chain drive gives a big advantage too, and I am not sure how many homebrew guys will take on that chain drive approach any more than they would wish to tackly flux's MPPT controller.
For me I prefer to make the most efficient use of material where possible and that's why I bang on about a single stator and 3-phase windings.. I am thinking hard about how the low price of ferrites plays into that philosophy and hoping not to lose the good braking effects you get with neos, but I look forward to getting away from the corrosion issues. I have plenty of neos on the shelf which is are not helping me to be innovative yet, but I see the way forward. Whatever the design I will seek the most cost-effective use of materials, and if there is power to be burned in reducing the electrical efficiency I will try to use that to reduce the cost of the wiring line or to heat a heater in my house rather than to heat the stator.
Chris is still perfecting his ferrite magnet turbine and we'll continue to watch in awe. And one day there'll be a simpler version available as a homebrew manual.
However