Roy has covered this pretty well.
With wind power you have to be very careful when you think about efficiency. With conventional engine driven generators it makes perfect sense to aim for the highest possible efficiency compatible with cost.
Wind has peculiar difficulties and an alternator having the highest efficiency at full load may not be the best choice. Wind turbines spend virtually no time at maximum power and you do far better optimising for good performance on the common winds.
To make use of light winds you need an alternator that is efficient at low power and that presents serious problems from the turbine point of view. Ideally the prop speed needs to track wind speed and a high efficiency alternator wants to run at constant speed, with large increase in current when tied to a fixed battery voltage. To keep prop efficiency up you need an alternator with rising speed characteristics and that implies a high internal resistance ( low efficiency). The general compromise is to drop to 50% alternator efficiency at 3 times cut in wind speed. Loosing half the power in the alternator as heat is a challenge which you can deal with by careful attention to cooling and heat transfer up to a certain point and beyond that you need to limit prop power by furling or other means.
The other option is to keep a high efficiency alternator and produce the losses externally in some form of resistor. This cures the heating problem but comes at an enormous cost in terms of magnet and copper so again you have to compromise.
Far better is to adopt a scheme where the alternator is run at maximum efficiency and the increase in prop speed is obtained by some other means such as an electronic converter to match the rising alternator volts to the fixed battery volts. This solves most of the problems but is not a solution that many will adopt. Eventually we shall see units that are designed for this in the same way as the mppt converters used for solar but at present it is a build it yourself option and not for general use.
If power out in high winds is the main requirement then the iron cored alternators are easiest to cool. They will not equal the axial air gap machines in low winds. How far you can improve the cooling of the axial design depends on your ingenuity and even more on local climate and the protection needed. In the end the power gained in high winds is not often used effectively with battery systems as the batteries are usually fully charged under high wind conditions so as long as you can control the output within the limitations of stator temperature there is little point in worrying.
For heating the low winds are less important and the alternator is best matched more towards the higher winds and then if you track the heating load to keep on the peak of the prop curve you will run the alternator efficiency a lot higher.
With grid tie some form of electronic interface is inevitable ( except induction machines) and the converter will incorporate some approximation to cube law tracking and stator temperature is not a big issue.
I seem to have explained in a very long winded way what Roy covered so well but it may help your understanding of the problems.
Flux
