this is why i reference the white paper over at microcogen.info
there is so much misconception out there as it relates to the use of auto alternators, and the paper clears up
most of it fairly well.
basically there is no reason that a 12volt alternator can't be used to charge at 24 (28.8vdc) or 48 (57.6vdc) provided
you have suitable rectifiers and drive it at the required rpm.
what is not widely know or appreciated is you can get much more power and efficiency from such an alternator
the wrong thing to do from an output and efficiency standpoint is to buy an automotive 24volt alternator, the efficiency
will be similar to a 12volt alternator charging a 12volt battery.
the issue comes down to a set of compromises that the auto engineers must make that we do not, that being they
have to have an alternator that will produce significant amounts of power at engine idle all the way up through cruising speeds
that sets the design up to be a jack of all trades and master of none.
what is outlined in the white paper is the specific application of an alternator running at a specific design speed, and with a specific
high side output , and quite frankly we don't care if it produces "any" power at engine idle (unless of course that is the design speed
of the genset).
in the example given a typical heavy truck alternator the prestolite/leece neville 110-555jho which has an oem spec of 160amps cold
at 14.2volts at ~6000rpm. when the alternator is hot it will derate to approx 130amps give or take. the output therefore when hot
is approx 1850 watts.
the very same alternator being managed with the balmar controller driven at 4850rpm, will charge a 24volt bank at 28.8vdc at 100amps
hot, and do it all day long. however if used as a truck alternator it would produce no power at engine idle, but who cares? the engine
is set to run at a specific speed which be virtue of pulley ratio allows the engine to run at 1800rpm and the alternator to run at 4850rpm.
therefore the reapplied alternator is good for 2880watts.
(its also fair to note this alternator is good for 8krpm continuous.)
so, in stock trim doing what it was designed to do, and considering the compromises taken the alternator produces 1850 watts, reapplied
and controlled by the balmar it will produce 2880watts, that is an increase of ~56% in power.
the interesting thing is the efficiency goes from approx 54% charging into the 12volt battery all the way up to 80% (+/- 1%)
that is an increase of approx 26 percentage points.
now if we were to just go out and buy the typical 24volt counterpart of the oem 12volt unit, we would get the 28.8 without much trouble, but
we would get approx 1/2 the hot rated amps or in this case 65amps, which brings us interestingly right back to ~1850watts and 54% efficiency, but
we get to have a good charge rate at engine idle, but that is not important to us in this application.
now using the same alternator (12volt nominal) and replacing the rectifiers with higher PIV, non avalanche diodes, and running it at 48volt nominal
it will produce approx 54volts and 80amps (this is the limit at 4850rpm, which is what i am hard ratio'd at with my trigenerator), this is not quite enough voltage and the current would also go to closer to 100amps without issues if i were to turn the speed up to approx 6500rpm.
even at 54 volts and 80amps that works out to approx 4300watts output, which is well over twice the output power (232%) and the efficiency improves to a bit over 80%.
as for the balmar controllers, they are expensive, at around 350bucks retail, but can be found for under 200 on ebay from time to time, also the xantrex units will do the same job and are somewhat less expensive.
anyway that is the short story, if you want more, go check it out.
currently i am working with a leece neville alternator with a simple modification of the stator, going from delta to wye, adding water cooling to the stator, its goal is 130-150amps at 57.6volts with an efficiency of over 80%, the alternator started out life as a 12volt nominal unit, but has had
the rectifiers changed, the stator reconnected, control via balmar, and running at ~6krpm. field power at ~50-60watts max.
even at 130amps and 57.6 volts that works out to approx 7500watts, and that is a serious amount of power from a 31 lb alternator.
to answer your first question, yes running the 12volt alternator at half output will be much more efficient that running it at or near full rated output, heat is the big killer of efficiency in my opinion, and is borne out in testing.
bob g