On 09.05.19 10:24, Dave Cole wrote:
Erik,
Do you have a blog going on your build?
Now that's an idea. All I've started is the seeds of an article for
"Owner Builder" magazine - the editor was interested when we last spoke.
I'd be very interested in your solar and battery setup for your off-grid
home.
The existing home, from the 1950's & extended, only has a little 2 kW
petrol generator. The new roof which will carry the solar arrays goes up
in June, if the framing carpenters turn up on time. (Just off the phone
to 'em half an hour ago.) There's 6 or 7 kW of equator-facing panels,
but the west-facing hipped roof can only take 9 panels, so only 2.5 kW
or so - but still enough to keep pace with a modest aircon.
The best trick for allowing high power consumption straight from the
arrays, yet limiting battery charge rate to permissible maximum, is to
use a hybrid inverter - they're beginning to become more available now.
The Redflow ZnBr battery has a limited max charge rate (44A), and pretty
much any other does too, e.g. 20A/100AH of capacity for LiFePO4. The hybrid
inverter looks after that while delivering to load first.
I like the Redflow, as it's a long-life unit, unkillable by 100%
discharge. It does though need that once a fortnight to regenerate, so
it can be handy to have another battery. For off-grid, just one 10 kWh
battery is maybe enough for one occupant, but a second is great for
visitors from the city. But the reflow is about A$14k (US$10k), so I've
even been looking at old technology like NiFe. They're also robust, but
can drink a lot of distilled water, emit quite a bit of hydrogen, and
put out a bit of mist. About 80% energy recovery is common for a lot of
battery chemistries, these included. Li-Ion, or better LiFePO4, are
better efficiency-wise, but cycle life on deep discharge is less. Do
your machining in sunlight, and only run lights, computers, tv, and a
microwaved egg sanger at night, then they'll do well enough, I reckon -
certainly long enough for a better technology to reach a better price.
We know from laptops that Li-Ion loses capacity with age. The ZnBr unit
is claimed to retain capacity, just losing efficiency. If a few (cheap)
extra panels are put in the array(s), then that's pretty much covered.
One thing - the hybrid inverter should have two MPPT string inputs - one
for each PV array, as their voltage at max power will never be equal,
given widely divergent orientation.
You must have a substantial setup to be able to run your AC off your battery
bank.
I'm hoping to be able to get the roof up in time to qualify for a
current A$5k government rebate on batteries. Systems with only 6.3 kWh
battery capacity are selling well here for on-grid customers. I'd like
twice that, as without that, running the mill for hours after half a week
of overcast winter days could mean arcing the generator up for a charging
burst. But a litre of petrol now and then is a darn sight cheaper than a
big battery.
What do you do for domestic water? A deep well?
For 55 years it's just been rainwater tanks. I'll be putting in another
90,000 litres of tanks, to catch it when it does come. There'll be
nearly 400 m² of roof all up. (But yes, years ago in a big drought it
really was bathe the baby in a bucket of water, use that to wash her
clothes, then wash the lino floor with it, then put it on the few garden
plants you could keep going.) The concern is that a tight rain deficit
might be shaping up to be the new norm for SE Australia, not improving
at all while we go from the current 1°C warming, through 1.5°C maybe as
early as 2030, to 2°C a bit later. (Accelerated warming at the poles
does not bear looking at, unless you've had a stiff drink first.)
The neighbour had a drill rig in before easter, to drill for stock
water. It was expected around 70 feet down. I haven't heard the results.
That's a finite resource though, only buying limited time. Slabs of
India is hundreds of feet down now, and heading for a brick wall. More
roof, more tanks, and live on what precipitates is more viable for
longer.
Erik
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