On Monday 16 April 2018 06:29:12 Erik Christiansen wrote:
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.
I think grandpa's hot water was pressurized, it was a copper coil in the
firebox of grandma's wood fired Monarch cookstove. The water was pumped
up to a water tanks of 5k gallons on a 25 foot tower so the gravity fed
pressure wasn't very high. Fed by several hundred acres of riverbottom
forest. Ice from the coon river was covered with sawdust from his
sawmill that made all the building materials needed although the ice
house itself was hollow tile. Several tons of river ice was cut and
hauled up to the ice house in Jan/Feb, and he had ice for the icebox all
summer and into a foot of snow on the ground in late fall.
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.
Or fire up the 20kw nat gas fed standby out back of my garage/shop.
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.
Which makes way more sense.
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 the datasheet.
They could also be used as a heat sink, for night heating I'd assume.
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 the year.
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
washing out?
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
there's sun.
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 mind ATM.
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.
Erik
(Headed out to the farm on Wednesday, to push this thing along a
little bit. Have to have a chat with the building surveyor.)
Thanks Erik. Some of the above I've not heard about till now, like the
salt water battery. And its sounding like costs are coming down, just
not fast enough to do me a lot of good.
