The power in the wind increases with wind speed cubed. If you design a machine to survive 90 mph running at full power it will do nothing in light winds. If you design it to work well in low winds it can't hope to be left working at full power in high winds, the alternator will not be able to load it and will burn out. The inability to load the blades will result in high speed and eventual destruction.
Furling is needed to protect both cases. Furling works best to protect the alternator and if the alternator is powerful then furling to protect it saves everything. If the alternator is the weaker link you need to be much more careful about the furling as the blades will be running clear of stall and you have far more power to get rid of for the same wind speed. Furling can still work but the things need to be set up differently.
With some types of alternator that reactance limit you won't damage the alternator and then you need to set the furling as if the alternator was unloaded and that is more tricky but possible. This type of machine is intrinsically safe. Normal machines when set up properly are safe on load. With loss of load the furling may not be adequate to protect the blades so don't let the load come off.
If you use a highly efficient alternator the thing will increase load so quickly with increasing rpm that the blades will be held at virtually constant speed and will stall. The power produced will not rise much above cut in and it may never need furling for protection but the performance will be dreadful.
To keep the prop efficiency at maximum the blade speed needs to track wind speed and unless you have some clever scheme the only way you can achieve this is to reduce the electrical efficiency ( in the alternator or the wiring to it).
If you start with a cut in at 7 mph and you rely on electrical inefficiency to let the speed rise you keep the prop efficiency up but you loose power from the electrical losses. It is a tricky compromise to get the best overall result from the gain in prop efficiency and the loss from the electrical side.
The best starting point is to start with the blades running as fast as possible at cut in consistent with producing useful power. ( tsr above prop design figure). The tsr will fall rapidly as the alternator loads the prop in higher winds. At 3 times cut in wind speed the ideal prop will be running at 3 times cut in speed but it will still not be hard stalled if you let it drop to twice cut in speed. Any significant loading beyond this will stall it and power will drop rapidly.
That is why I suggest you aim for about 50% electrical efficiency in wind speeds about 3 times cut in. At this point you are seeing very useful power and without a costly oversized alternator you will need to be furling soon anyway. By getting a run at it at the bottom end with a high tsr at cut in you reach the stage that you need to think about furling at something like 25mph. These wind speeds are not met often in normal sites and you loose very little energy capture by furling at this speed.
Elsewhere here I have dealt with the design to achieve this compromise, you can predict the alternator output into a battery at different speeds from the cut in speed and the winding resistance but unfortunately this information is scattered throughout many replies and I can't link you directly to it.
The only thing that is not easily predicted is the stator temperature rise. I find that others seem to survive with about 50% efficiency at furling point but the temperatures will be high and you are looking at class H winding conditions and the potting materials are not class H but with the duty cycle of wind power it seems to work.
Flux
