In article
,
" wrote:
periods. I plan to use solar and wind but may have to rely on a
generator for some periods. The generator is the part that has me
stumped. Tho the generator will run off renewable fuels, I'm unsure
whether to run a large generator making AC for a short time, or run a
smaller generator making DC power for a long time. I had considered
an engine small enough to drive generators the size of those found in
wind generators. Any help or direction would be appreciated. Thanks,
Paul
Well, first off, price out everything (not just the big obvious
components). I did, and given that I'm running a shop and house, not
merely a house with no tools (as is typically assumed) my system cost
for doing a good job off grid came to $25K, minimum (parts only, me
doing all the labor). When I started, the power company was tossing
about similar figures, but I revisited once it came time to spend money
(no borrowing, took a few years, during which I rehashed what and how to
do it off-grid a lot), and once I get done filling the trench, will be
connected to grid at 400 amps for about $5K less, in the end, as opposed
to off-grid at about 50 amps and having to do a little dance of what
runs when to make tools work. That includes hiring in a master
electrician to keep the power company happy with it - given that I've
done a lot of wiring on the other side of the box, but never a service
entrance, that kept me happy too.
You want a charge rate of at least C/20 (and perhaps greater) your
battery bank size, so don't go too low on the "tiny little thing." ie,
if you have an 800 amp-hour bank, you want a minumum of 40 amps DC
available to charge it. You get issues with the acid stratifying if the
charge is too slow, among other reasons - some of the reasons touted
seem a bit funky, but the practical fact is that rules of thumb come
into being because of actual behavior, even when we may not have the
correct understanding of the underlying cause, so don't go too far down
into tiny charging land or your battery will not be happy.
Depending on what inverter you use, there can be a lot less trouble with
a fuel-DC DC-Inverter-AC setup, as most inverters use the same parts for
charging the batteries as they do for making DC into AC, so an AC
generator (usually) has to be able to run all your loads and provide
power to charge batteries, while a DC generator can be sized to charge
the bank, without having to be big enough to run your largest loads (if
they come on, the generator will contribute, but the battery will be
discharging while they are on, if the generator is not that big). You
also miss one step of less-than-perfect-efficiency conversion from fuel
to charged battery. You may well still want an AC generator set for
further backup, but it can be a little less robust than you'll want if
it's your primary, perhaps. Also, a good DC generator setup can actually
be throttled by its controller when producing less than full power
rather than running at constant speed as typical AC generators do to
make 60 (or 50) Hz. AC generators that don't run constant speed are
actually DC generators with an inverter as part of the generator. On the
third hand, unless you have a lot of spare power from the system, it
might make more sense for something like an air compressor to simply be
hung off the diesel directly rather than powered by electricity in an
off-grid setting. And then you can get into antique machine tools and
lineshafts also driven by diesel.
If your system is still open to design, 48VDC offers many advantages,
not least of which is surplus cell-phone equipment, such as 48V DC
diesel generator sets...
If you are in a cold climate, use liquid-cooled generator and make use
of the engine waste heat in the building heat/hot water - maximum return
on your fuel money.
I don't know what groups you have been trying - I have not seen anything
in alt.energy.homepower or alt.solar.photovoltaic, or if I have the
subject line has not caught my eye amid the spam and trolls. The
homepower magazine website is also worth browsing through. There's a
pretty good webforum (I don't really like them, but traffic in the
newsgroups is down with servers being dropped by so many providers) at
http://www.wind-sun.com/ForumVB/index.php
Though I have not really been there since my project flipped from
off-grid to on-grid. Xantrex has one that's useless, Outback has one
that's sort-of OK, but this one is better (with a bit less of a
one-maker slant to it.)
--
Cats, coffee, chocolate...vices to live by