Hi Rainwulf,
Maybe you'd find this (rather old) discussion interesting:
https://www.fieldlines.com/index.php/topic,127288.0.htmlFlux built the discussion and details slowly, but very thoroughly examined buck-boost options for making the match.
When I read your post from Saturday I got a little sidetracked by the term "active PFC" but now I see you really wanted to talk about DC buck/boost.
I have not personally delved into the MPPT world, either as a user of off-the-shelf equipment or as a DIY builder.
Regular hard-wired connections through rectifiers don't suffer from too much inefficiency, and their chief advantage is being dead simple and reliable. In this scheme, the EMF generated by the alternator grows linearly with speed. In high winds this EMF can be many times the voltage at the battery. When rectified with a 3-phase rectifier, the whole electrical system is clamped to the battery voltage. Roughly speaking:
I = (EMF - Vbatt - Vf) / Z
In words, that says the current equals the difference between the generator electromotive force and the battery voltage minus the rectifier forward voltage drop, divided by the total system impedance. In practice, the equation needs adjustment to account for the AC EMF and the DC battery voltage - it's just to give you an idea.
There is a time near "cut-in" where the EMF is only barely higher than the battery voltage, and the "short pulses" do happen as you say, but the current is very low. Once the EMF rises, then there's plenty of delta-V over the battery voltage for a majority of the sine for current to flow most of the time, and this situation gets set up at RPM not much higher than cut-in. From there, the AC waveform stays pretty "square" as RPM increases. Different mills respond differently, and mine is a horrible mess, but in the Axial-flux machines they are neat and tidy. One major difference between wind turbines and solar panels is that solar energy always tops out at about 1000 W/m^2 and many solar panels start to produce a trickle at 100 W/m^2 or so - a factor of 10x. The power in the wind rises as the cube of the wind speed, so for a turbine that cuts-in at 10mph wind and furls at 40mph, the range of power it has to accept is nearly 100x.
The big advantage you get from MPPT or a DIY buck or boost controller is that you don't have to get the blades perfectly matched to the alternator, and in fact you can do other tricks that allow you to improve even a well-matched set of blades and alternator. That comes from the difference in power curve between passive electrical machines and wind power. They never do match; you can only get them close. With MPPT, you can make them much much closer, keeping the turbine running at peak blade efficiency at a wider range of wind speeds. Without it, you can only pick one wind speed to optimize, and accept less than optimal matching at other wind speeds. This one feature is what offers a 50% increase in energy yield from a MPPT-controlled turbine.
The electronic scheme that the MPPT controller uses is not, I don't think, like power factor correction (PFC). I believe units like the Outback use high-frequency switching - much higher than the turbine's generator frequency - to manage the load. The generator may be giving 60-100Hz but the controller is switching at many kHz, and sampling the turbine output frequency/voltage frequently to sense the turbine conditions to be regulated. I believe it's closer to your last suggestion
"...you could simply shunt regulate the alternator using high frequency PWM." but I don't believe it's simple. In a wind turbine, there is no condition where a disconnect is permissible. Any scheme used for regulation has to fail-safe. This can be done with a shunt regulator, and with the controller shut down or failed the load remains connected to the turbine.