At 05:55 AM 18/07/2010, Shawn wrote:
>With all the discussion about battery and charger efficiencies, a
>question occurred to me about a smaller scale project I've been
>pondering. Given a boat where all power (motor & amenities) is
>drawn at 12v, wouldn't it theoretically be more efficient if a
>backup generator were a dedicated 12v unit akin to running a car
>alternator and voltage regulator? It seems (on face value) that
>going from a 12v generator to a 12v controller to a 12v bank to 12v
>loads would reduce at least some of the controller/charger losses.
G'day Shawn, All
12V alternators are notoriously inefficient, typically in the order
of 65% (mechanical energy in vs electrical energy out). Alternators
are designed to be cheap to mass-produce, and in their typical
application the inefficiency and amount of waste heat will not
matter. Where you are coupling up an engine to some kind of
generator, automotive alternators are an attractive "cheap, quick and
dirty" option, fine if its' just a now-and-then need, but if it's a
significant part of your generating mix, I'd be looking for alternatives.
A brushed permag motor being run over-speed or a three phase permag
alternator will give much higher efficiency, in the order of 80% to
90% (with claims of higher that I've seen but unsure as to the
validity of the claims).
If you use a brushed permag motor, the motor can be the starter for
the ICE as well (although it is a bit savage and hard on the motor so
better to be a little careful and let there be plenty of voltage drop
on starting, to keep the peaks lower).
If you can use a water-cooled ICE and use the cooling water to heat
something you can use, you'll be catching about 70% of the available
fuel energy (the ICE wastes about 70% as heat) as well as the
electrical energy.
If you are chasing fuel efficeiency, you need to match the ICE peak
performance to the generator peak performance - if the ICE peak
efficiency is at 3500RPM, and the generator is at 6000RPM you may be
better off in speeding up by a toothed belt (the efficiency gain in
speeding up may be greater than the efficiency losses of the belts).
Best efficiency is direct-drive, where the generator is matched to
the ICE by design.
The area of efficiency is a great big can'o'worms that the best we
mortals can do is choose between paying a premium for something
designed and fully spec'd, vs taking a risk and doing the best we can
by being aware of what we're trying to achieve. Its' a trade-off of
the fuel cost vs savings by DIY-ing, unique to each persons' needs.
Regards
[Technik] James
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