Off-grid house

[vtsnowedin]

I'm surprised that after forty years of people seeking to move off grid there are not known proven designs with their advantages and disadvantages well documented in the literature and online postings.
Of course old postings from the 80's would not mention LED lights or cell phone apps but I would think the basic BTUs and KWHs in and out of off grid homes has been fully researched and experimented with by now.

[fuzzy]

It is a very much a niche area in the UK with our lower light levels. The official suppliers [those who have the legal paperwork arranged by lobbyists to our kak-handed leaders] will charge a fortune, and, like those supposed gurus of our gas appliances and house wiring, they probably don't know much or have limited legal sources.

The dude who wrote the chapter on renewable installations for the US electrical code commented at the time that he was not legally qualified to work on his own solar installation, which I think sums it up.

If you want to bespoke it Chris, after a bit of thought, I would:

Use 3 heating elements - assuming there are solar tanks with enough flanges for connections

For a 4.5kW source thats 4.5/7=.64

So minimum element =640w, next = 640 x2, next =640 x 4

That's 3 solid state relays and binary 3 bit switching from 0 to 7. That gives you 8 power levels from 0W to 640 x 7 = 4.5kW with all 3 elements engaged.

You need to know the solar panel volts for maximum power [just guessing but maybe 3/4 of open circuit volts]. You then need to monitor this and switch elements on/off if the volts goes lower or higher.

In a sane world, you could ring up your local 6th form or FE college and get some bright kids to design it for free, but this is the world where it's legal to sell imported crap of unknown manufacture, as long as it has this weeks BS 'euro CE paperwork' or whatever lawyerfest is this weeks rules.

[fuzzy]

Thinking more, I would use 2 water tanks. The 1st with a 2.5kW element flowing into the 2nd with the 640w and 1280w. You may need to store a lot of energy and the lower power elements will be on more so may warm the water enough for washing up, preheating the 'cold' supply to a washing machine, or just about warm enough for a strip wash if everything else fails. Unless there is someone who can usefully make elements as required, you may need to dismantle existing 240v immersion elements and reassemble with thicker [lower supply voltage] constantan wire:

https://www.wires.co.uk/acatalog/cn_bare.html

[adam2]

Immersion heater elements are available in a range of wattages, but attempting to modify one is virtually impossible.
The coiled resistance wire is not accessible, it is surrounded by tightly packed powdered insulating material and sealed in a metal tube.

[adam2]

Not sure how you are going to reliably switch very heavy currents.

Would something like this work: http://uk.farnell.com/crydom/d06d100/ss ... dp/1213166

I think that you will need a higher voltage solid state relay than that example.
48 volts is the recommended operating voltage, with 60 volts being the absolute limit. Too close for comfort IMHO.

[Mr. Fox]

Chris,

Personally I think you're on the right track: Keep it DC, 3 x 1.5kW and I like your ideas on switching using what you've got (the two MPPT box relays with staggered settings plus the battery monitor - built in redundancy )...

I take it you're not using the 'load' outputs on the charge controllers at all (max 70A each)?

Do you have a system diagram showing what's connected to what and what talks to what and how (if at all)? Photos? (might help if suggesting layout, etc)

Are you certain that the 'relay hi volt' setting is independent of everything else the MPPT does (like 'stopping charging' - I can't find a manual for your model on their website)?

What is the equalisation charge set to (as in: what is the absolute max voltage you'll ever see on the battery terminals)?

Finally - in the interests of keeping costs down, how 'homebrew' are you comfortable getting WRT the SSRs?

[clv101]

Chris,

Personally I think you're on the right track: Keep it DC, 3 x 1.5kW and I like your ideas on switching using what you've got (the two MPPT box relays with staggered settings plus the battery monitor - built in redundancy )...

I take it you're not using the 'load' outputs on the charge controllers at all (max 70A each)?

Not sure what you mean here, of course I'm using the 70A output?

Do you have a system diagram showing what's connected to what and what talks to what and how (if at all)? Photos? (might help if suggesting layout, etc)

Here's an early schematic, minus the fuses:

Are you certain that the 'relay hi volt' setting is independent of everything else the MPPT does (like 'stopping charging' - I can't find a manual for your model on their website)?

Pretty certain. Manual: link

What is the equalisation charge set to (as in: what is the absolute max voltage you'll ever see on the battery terminals)?

Equalisation will be at 64.8V, but I don't have to set to do so automatically. I'll do it manually maybe twice a year.

Finally - in the interests of keeping costs down, how 'homebrew' are you comfortable getting WRT the SSRs?

Never used them before - but they looks pretty straightforward to wire up.

[Mr. Fox]

Not sure what you mean here, of course I'm using the 70A output?

Apologies, I'd assumed that the MPPT had separate PV, BATT and LOAD terminals, but I see now that it's just PV and BATT on that model... (I'm used to Steca stuff).

So how is automatic load disconnect achieved on low BATT voltage? Does each inverter / voltage converter do it itself? Or is the box above the battery in the diagram a contactor controlled by the BMV-702? Is there CANBUS connection between MPPT & BMV?

Pretty certain. Manual: link

Cheers... Duh, I didn't spot the dropdown datasheet/manuals thing.

Equalisation will be at 64.8V, but I don't have to set to do so automatically. I'll do it manually maybe twice a year.

So manually pulling the dump load (water heater) fuses twice a year to protect the (60v max) SSR wouldn't be much of a problem. If Farnell say it's good for 60v, I'd be inclined to believe them.

Finally - in the interests of keeping costs down, how 'homebrew' are you comfortable getting WRT the SSRs?

Never used them before - but they looks pretty straightforward to wire up.

What I was hinting at here was 'rolling your own' SSRs with optoisolators and big FETs - certainly not a straightforward as shelling out for the plug-n-play SSRs.

Another couple of questions that occurred (probably daft and probably answered elsewhere):

Is anything else feeding the thermal store (Solar thermal, back boiler...)?

How much scalding-hot water do you really want in the summer?

A rough calculation suggests to me that 3.5kW will heat 360L of water from 20 degs C to boiling in around 10 hours... bare in mind that on day 2 it's not going to start at 20 degs C!

You're going to need some sort of thermostat control for the upper temp limits on the thermal store - if you let it run up to the cut-out on the heater elements, you'll have to poke about re-setting them every day (assuming they're not self-resetting).

I'm not trying to put you off doing it, but it's a lot of additional complexity and a fair expense to go to to get a couple of days of v. hot water when you probably don't need it, and once the thermal store is above the cut-out temp, the systems back to 'wasting' power anyway.

Talk about 'first world problems': "The house is so good it doesn't need much energy, I can't get rid of the excess!"

My standard go-to solution for OGPV dump loads is usually a dehumidifier or two on a 'special' socket, as crappy housing stock built by smurfs is the norm for my area... finding dehumidifiers that can be switched via their supply is becoming a problem, though (computers in everything)... even then, looking at the data shows they'll only use ~10 - 20% of the excess. Always the problem in the UK... size the system for winter, get 10x the required power in summer... short of tilting the planet slightly on it's axis, there's not a lot we can do.

[Pepperman]

Why not go for a low powered DC immersion for late autumn to early spring and then use AC immersions for the rest of the year? In summer you'll have plenty of power left over to run your high power inverter.

[kenneal - lagger]

One possible use for excess power during the summer would be to put soil heating cables 1.8 to 2.4 metres in the ground underneath the house. Use these in the summer to heat the ground and by the time the winter comes the heat pulse produced has worked its way up the soil profile and starts to heat the house from underneath. A lack of underfloor insulation helps and some perimeter insulation also helps keep the heat where you want it. The depth depends on the soil's heat transfer properties but isn't too critical.

I do appreciate that this is something that probably wouldn't be any use for Chris now as he has probably got an insulated floor slab but it could be useful to install on older houses using a mole system and backfilling the moled cables. Insulation could also be installed around the house to keep the heat where required.

I've read of it probably ten or fifteen years ago being done using heated air from conservatories but using heater cables shouldn't be a lot different. A higher input temperature from the cables would be the only difference but shouldn't be a problem.

Funny what you remember!

[clv101]

Why not go for a low powered DC immersion for late autumn to early spring and then use AC immersions for the rest of the year? In summer you'll have plenty of power left over to run your high power inverter.

Whilst 'on average' there is more excess energy in summer, in practice there are plenty of days even in Dec and Jan when there's excess and days in mid-summer then it's dark and rainy.

[adam2]

Don't forget the merits of what might be called virtual electricity storage.
That is not storing the energy itself, but storing the results of use of that energy.
Examples include, postponing laundry until a day with plentiful electricity. It is almost certainly cheaper to store dirty laundry, within reason, than to store energy and use the washing machine on demand.
Cutting and splitting firewood with electric tools when power is plentiful makes a lot of sense.
Grinding grain with an electric mill and store the flour.
Cutting up garden waste to better compost it with an electric shredder makes good use of surplus electricity.
If you have a lawn, mow this with an electric mower whilst power is plentiful.
"Spring cleaning" can be done at any season, but if much use of a vacuum cleaner or floor polisher is involved do it when there is plenty of PV.
Cooking foods for freezer stock.
If you enjoy ice in drinks, make this when power is plentiful. The energy cost of STORING ice is zero, but the energy used in MAKING ice is significant.
Buy an electric barbeque and enjoy this when power supplies permit.
IIRC, I have already suggested a north facing cool store, with good design this will naturally maintain a fairly low temperature, but the addition of a small electric cooling unit* will help a lot in sunny weather.

A little thought on the lines of the above will no doubt find other ways of using surplus electricity to add to comfort, or save money, or reduce of other fuels.

*the temperature required is a bit low for a standard air conditioner, consider a cellar cooler as used in a public house.

[vtsnowedin]

Don't forget the merits of what might be called virtual electricity storage.......
................
The energy cost of STORING ice is zero, but the energy used in MAKING ice is significant.

I have to nit pick you on that one. Even the best built freezer or even an ice house has some loses to heat absorption so it will take some energy to keep your ice frozen or you will have to tolerate volume losses due to melt water draining away. They used to ship pond-ice blocks from New England to New Orleans in wooden sailing ships and lose a third to a half of the volume during the passage.

Buy an electric barbeque and enjoy this when power supplies permit.

Or barbecue over a hardwood fire after it has burned down to coals.
Mow the lawn by grazing it with a couple of lambs all summer then eat the lambs.
Let the yard waste just sit in it's compost pile an extra year and let the microbes and worms do the chipping for you.
Patience Grasshopper.

[adam2]

Presuming that a freezer is already in use, then the energy cost of storing ice IS zero.
The energy used by a freezer wont increase because ice, or indeed anything else, has been stored in it.

There may in fact be a slight DECREASE in energy used by increasing the volume of goods stored in a freezer (less cold air to escape and be replaced with warm air when the door is opened)

[vtsnowedin]

Presuming that a freezer is already in use, then the energy cost of storing ice IS zero.
The energy used by a freezer wont increase because ice, or indeed anything else, has been stored in it.

There may in fact be a slight DECREASE in energy used by increasing the volume of goods stored in a freezer (less cold air to escape and be replaced with warm air when the door is opened)

That is a stretch too far. While the energy required to keep ice frozen or any food that is frozen for that matter is low it most certainly is not zero. It is true that most of the energy used by a freezer is used to freeze the warm food you place in it and the warm air you let in when you open the door, but even if you filled it to the brim with ice and locked the door you could not unplug it without it eventually thawing out. The time required for that would depend on the temperature of the air against the outer skin of the freezer, the amount and quality of the insulation between inner and outer skins and the volume of ice inside. The most efficient freezers are the chest type kept well filled and placed in the coolest corner of the house.
Ever see an ice machine without a plug?