On 15.04.18 19:40, Gene Heskett wrote:
> Greetings all;
> I see Banggood has a 500 watt wind thingy for under $200 that net folks
> are calling professional grade stuff. I am looking to put up an
> anerometer to record the wind speeds, one of those 3 cups designs that
> isn't direction sensitive, and one of these card computers to log what
> it sees so as to get an idea who many of these 500 watt things I'd need
> to keep a bank of truck batteries topped up while running the house,
> including the AC.
> Has anyone else walked this trail, or do I have to start with a machete?
Gene, my off-grid build has so far only passed planning approval, and
I've just embarked on building approval. It is based on wood-stove
heating plus 7¹ kW of PV panels on a 30° roof, not quite matching my 38°
south latitude. Battery requirements are reduced by trying to use the
aircon(s) only/mostly when it's hot & sunny, and using a heat pump for
non-winter water heating, as that uses only a quarter of the power of
direct water heating, taking the other 3/4 of the energy from the air.
The secret is to run dishwasher, washing machine, lathe, milling machine,
anything you can in the middle of the day, powered directly from the PV
array, not the battery.
In winter, water heating is by an in-flue SS double-skinned tubular unit.
(Hot water system vented, so it's not a pressure vessel.) Fuel supplied
by 2 sq. km. of forest on the property.
Batteries: Even deep-cycle LA are not worth the biscuit. LiFePO4 are
better than plain Li-Ion for fixed installation, being _much_ less
flammable, and more robust. The zinc-bromide flow battery from Redflow
is more robust still. Not only can it be left for years on a shelf
either charged or 100% dead flat, but it needs to to be run dead flat
every few weeks to keep it young. It is a SS box on top of a 190 L tank
full of water & chemicals, so it'll tend to put a fire out if that melts
the tank. A vent tube to outdoors is advised, as in extremis there could
be a smell of bromine, possibly.
It was originally sold as the ZBM2 for about $7k for a 10 kWh unit with
inbuilt long-life BLDC pumps and controller. Now it comes in a
consumer-friendly lockable outdoors enclosure and what-not for around
$12k installed. I've been umming and aahing about a second one for long
dark winter weeks, but I live alone, and it's cheaper by several country
miles to just arc up the petrol generator the year that happens.
Its one weakness is that you can only charge it at 42A (~ 2.5 kW), which
isn't very compatible with a DC-couples system, i.e.
PV Array -> MPPT charger -> Battery -> Battery inverter -> 220v
To fully utilise the array output, we need to swallow up to 7 kW. The
Redflow is better suited to an AC-coupled system:
PV Array -> Solar/Battery inverter -> 220v -> AC charger -> Battery
Then you can use much of the 7 kW directly, storing only what's not
used. There are though two units (with small losses) between array and
battery. But most systems on the market are built this way.
There are some saltwater batteries on the market, but they are enormous,
heavy, and have an even lower permissible charge rate, on my reading of
OK, supercapacitors are coming, and they will annihilate the chemical
batteries, but the first ones are not as good as the ones still in the
labs, so I'll be reevaluating when I have a roof up, towards the end of
Wind? The consumer generators provide piffling amounts of power, are
disproportionately expensive compared to solar, and die in the arse as
there isn't enough wind enough of the time. That fusion reactor in the
sky still delivers a useful bit even in light overcast, and who needs to
run the washing machine when the weather's no good for hanging the
In my new build, lighting will all be LED, run from DC. The inverter
won't need to be running for that. The plan is to make some of the light
fittings. I need something to build in the two workshops which make up
1/3 of the build. (Also have to find time to finish the design of a
networkable LED dimmer (3 channel), and send off for some boards to be
made. The firmware needs a fair bit of work too. The light switches are
to be neat clicky pushbuttons. The last build, 30 years ago, was as
owner-builder. This time I'm paying a builder, so I _hope_ to find time
for the fiddly bits, despite having to prepare the old build for sale.)
I'm also looking at a camping fridge with phase-change material inside
the insulation. That only needs power for a couple of hours in the
morning, and again in the afternoon, so does not load the batteries if
Cooking is microwave and induction hotplate - not a complete cooktop.
I get out of bed late enough so that will rarely draw on the batteries.
Double glazing reduces heat loss and gain, and might help if you have to
run the aircon after sunset. An MPPT charger or solar inverter which
takes two array strings allows you to put one of them on a westerly
roof, to catch more late afternoon power to help there.
Hope there's something useful in these muddy footprints from where I'm
headed. There's doubtless stuff I've forgotten, but that's what's top of
¹ Some reckon 12 kW is the go for off-grid, but they usually have a wife
and kids. I could go to 9 kW with 330 W panels instead of 250 W.
(Headed out to the farm on Wednesday, to push this thing along a little
bit. Have to have a chat with the building surveyor.)
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