Dear Mr Rawles,
Mr J’s submission makes interesting and useful reading, however I would
like to add a few observations on the subject to possibly aid the decision
making/wallets of people who are building/upgrading photovoltaics systems.
- Batteries: if at all possible get used golf cart batteries that have
been properly maintained. Around here they get recycled for a $25 core
charge which is easy to beat. My deep cell batteries are now 4-5 years old
(2 seasons in various golf carts and 2.5 years in my battery bank. They
are in like new condition as they take really well to charging with
photovoltaic panels. The biggest enemy of deep cell batteries is repeated
fast discharge / too deep discharge cycles. The smaller your bank the more
likely it is that your batteries will be subject to these.
3 batteries delivering 300Ah may sound like a lot of power but isn’t. If
you are pulling a 500W load @ 85% efficiency, the batteries must deliver
(500/.85)/12.5 = 47A. That is 16A each, while their safe discharge rate
(C20) is about 6A (114/20). If the wiring in the bank is not ideal, 1
battery may even have to provide half the load (see last year’s submission
on photovoltaics for details). This is the real reason why batteries don’t
last in small banks. I know this sucks but if you think Wal-Mart won’t be
there to provide cheap replacement batteries in 2 years, one needs to pony
up now and get as big a battery bank as one can afford. On the bright
side: a basement full of batteries beats getting bailed in ….
- 24 volt systems have certain advantages as stated in the article.
However when used with photovoltaic panels you will need to create a
serial-parallel setup of panels to charge them. This isn’t a big deal for
a static installation but less than ideal for small portable systems.
Personally I decided to go with 12V on all systems because I don’t want to
deal with multiple voltages. Furthermore 12V tools, lighting, etc. are
plentiful but 24V or 48V units are harder to come by.
- Though its true that larger panels are cheaper per Watt; they also are
less mobile which may or may not be a problem depending on how you plan to
ride out the storm. My setup includes 95W, 60W and 15W panels so I have
more options. Even 1W panels are useful for charging phones, rechargeable
batteries, trickle charging car batteries, etc. Our lawnmower uses a 5 yr
old motorcycle battery for starting. Last year we had some starting
problems with that battery toward the end of the season, so this year I
hooked it up to a 1W panel that was shaded by a roof overhang. Never had a
problem all season and, to my surprise, at the end of the season the
battery showed a resting voltage of 12.8V. Don’t overlook these little
things – you need reliability first and foremost.
- Mr. J. got a pure sine wave inverter which is great if you have the money
for it, but it isn’t really needed unless you run sensitive electronics
like medical equipment. Power tools and consumer electronics can handle
dirty power supplies up to a point. Having said that, I agree with Mr. J.
that its quite possible for your power tools to sound ‘raw’ and feel
underpowered. I believe this is due to the size of the inverter being
used; most likely the number/size of its capacitors is inadequate to
sufficiently smooth out the inverter’s stepping at higher load factors. I
have 300W, 1000W, 2000W and 2500W inverters at my disposal. The 1000W unit
really causes trouble with medium size power tools (rated @ 500-600W) but
on the 2000/2500W units, the same tools sound/behave like they are running
on power line supply. So, for the same money I prefer to get twice the size
inverter even if its modified sine wave.
- Another note on inverters: though its always good to be careful with
your wiring, there is no need to get obsessed about it. The inverter’s
micro controller monitors the outputs and will shutdown the inverter at
any sign of trouble like shorts. And it reacts a lot faster than a
standard fuse or circuit breaker. This is a key difference between
inverters and generators or power line supplies where only passive
components stand between you and possible disaster. Sticking to the
manufacturer’s installation guidelines should be sufficient.
- My system is stand-alone so I don’t need transfer switches and the like,
but I will add my voice to JWR’s note: be very careful with male-to-male
wires and other un-standardized solutions. You do not want to come home to
find your precious inverter has blown its mosfets (or did something far
worse) because someone plugged that cable in the wrong outlet. The second
danger is that you will make that mistake yourself because after being
half awake for a few nights you will not be thinking as clearly as you
like to believe. How seriously do I take this? Some of my circuits are
monitored by micro controllers. If I flip a switch the controller will go
through a checklist to see if my request can be executed under the
circumstances without causing harm to the system, if not I just get a
blinking light.
- I can see someone topping up their batteries with a car’s alternator in
a pinch. However using a 14.4V alternator to run an inverter to run a
battery charger to charge a 12.6V battery doesn’t sound particularly
efficient. Expect to lose 15%+ on each step. Why not buy the heaviest
starter cables you can find and make a direct battery to battery
connection?
The thing is that being inefficient is not a big deal when gas is
plentiful. Once you are going through your last tank, a person may feel
different about it. If at all possible try to take such scenarios into
consideration when designing your system’s layout.
At the end of the day you are going to find something will limit your
system. The question is: can you live with that bottleneck? If the answer
is ‘no’; well, there is still time to do something about it.
I hope my notes will be of help for some of you. – D.P.
Article source: http://www.survivalblog.com/2013/11/letter-re-alternative-power-systems.html