Off-grid house

[clv101]

I'm building a house this year and it'll be off-grid. For power I have 5.67 kW of solar panels. I'm still thinking about the system design, so far I'm thinking like this:

21 panels in 7 strings of 3.
2x Victron MPPT charge controllers (2 'cos of the size of the array and not wanting to put all the eggs in one basket if one dies).
48v lead acid battery store with a total capacity of around 30kWh (625Ah), 15kWh usable.
A 5kW 230v inverter and a 1.2kW inverter, powering separate consumer units/circuits.
48v DC emersion heater (~1kW) for when batteries are full.

The thinking behind the two inverters is that the small one will be on 24/7, run most sockets (fridge/freezer, water pump, IT...) and lighting. The larger one will only be switched on manually when needed (power tools, oven, microwave, induction hob...). The reason for not just using a big inverter 24/7 is their high (~35W) power consumption.

Lighting alternative: I'm also toying with the idea of having a separate dedicated system for lighting. Taking one of the 7 strings (810Wp), separate charge controller and separate 12V battery store and just using this system to run 12V DC lighting. This is attractive from a redundancy point of view, not relying on main battery store of inverters. But in a full house set up, voltage drop is still an issue even with 3mm wire (anything larger is a real pain in switches and fittings). 24V might be an option, but the availability of decent/affordable bulbs is worse than at 12VDC or 230VAC.

The batteries are the most annoying/expensive part but li-ion solutions don't seem quite ready for off-grid setups, most are still very expensive or rely on a grid connection. They also feature more complex electronics than old-school lead acids.

On a typical summer's day we should be looking at something like 20kWh which is just crazy - can't hope to store summer generation so it's a 'use it or lose it' situation. What would you do with ~5kW on tap for several hours a day in the summer?

[vtsnowedin]

I'm building a house this year and it'll be off-grid. For power I have 5.67 kW of solar panels. I'm still thinking about the system design, so far I'm thinking like this:....
..........
What would you do with ~5kW on tap for several hours a day in the summer?

AC for the house if you need it.

[adam2]

12 volts is rapidly becoming very last year !
Until recently I recommended 12 volts versus 24 volts for exactly the reasons that you give regarding greater availability of 12 volt lamps and other equipment.

However 24 volt LED lamps are now very widely available in numerous styles. Many are multi voltage.

Are you already committed to 48 volts ? If not, then I would consider a 24 volt only system.
Lighting, refrigeration, and some small appliances at 24 volts, with inverters for 24 volt input.
A 5KW inverter for 24 volt input will need about 250 amps, entirely doable if the inverter is close to the batteries.

24 volt lighting circuits are very manageable. Without any detailed calculation I might be inclined to go for a "standard circuit" in 2.5mm cable from a 10 amp fuse. 6 such circuits would be generous and if only part loaded would still be several hundred watts of high efficiency lighting.

That would be enough for about 100 lamps !

[clv101]

Okay, I hear what you're saying about 24V lights, that certainly would help with the voltage drop. Maybe a dedicated 24V system off one string of panels would be best. Cheaper charge controller and easier cabling.

I really do need 48V on the battery store due to the size of the array. 5.67kW at 24V would need a 240A MPPT charge controller! They don't exist, so I'd be looking at three in parallel. That just gets a bit silly.

[clv101]

I'm building a house this year and it'll be off-grid. For power I have 5.67 kW of solar panels. I'm still thinking about the system design, so far I'm thinking like this:....
..........
What would you do with ~5kW on tap for several hours a day in the summer?

AC for the house if you need it.

With a very thermally efficient house, in Wales, air-con really isn't needed!

[BritDownUnder]

Seasonal heat storage but evacuated tubes would be a better idea for this.
A tropical greenhouse linked to the seasonal heat storage.
An electric car/lawnmower/powertools/chainsaw. On weekends I use a lot of electric gardening tools during the mid morning just as output is rising and before it gets too hot. I also have a garden shredder which uses 2kW and can be placed under an awning and used during the midday and hot weather to shred all the cuttings I have created earlier in the morning.
Try to do most work during these hours assuming you are at home.
Give your power away or run a mobil battery charging service.

My wife is at home all day so I encourage her to do all the laundry/cooking around midday (she is an understanding sort). We can also use a lot on aircon/heat pumping at appropriate times.

Amazing how you can change your behaviour.

[adam2]

Despite the substantial charging currents involved at 24 volts, I would still be inclined to go for a 24 volt system since this gives far more flexibility regarding lighting and possibly other appliances.

Ultra high efficiency DC fridges and freezers are readily available for 24 volts.
24 volt electric kettles are a lot more use than 12 volt ones. 24 volt microwave ovens and even very small conventional ovens are available.

A 3 wire DC system is another possibility, so as to give 24 volts for lighting and refrigeration, with 48 volts for the inverters. Such a system adds complexity but the choice of voltages and the reduced currents can be most attractive.
For a given cable size, a 3 wire system allows FOUR times the load to be supplied for the same percentage voltage drop, an important factor if long cable runs to outbuildings are required.

I do not favour lighting from an inverter since a single point of failure is thereby added and extra losses result.

[adam2]

Regarding the summer surplus of electricity, have you considered a cool store for root crops, some fruits, beer and wine.

A basement store on or under the north side of the house, well insulated and designed to be naturally as cool as possible.
Such a facility could be much improved by the addition of a small electric cooling unit that runs whenever surplus electricity is available.

A basement cool store needs to be well insulated from heated areas or the warm outside air, but it does not normally need insulation from the surrounding earth. Stone or concrete walls in thermal contact with the subsoil add vast thermal mass.

A suitable electric cooling unit will also lower the humidity.

[clv101]

That's a good idea Adam. We are building a 'pantry' on the north wall, outside the thermal envelope of the house, but active cooling with surplus summer might be useful.

[Pepperman]

I'm not sure about the idea of having a dedicated string and battery set up just for lighting. Better to keep it as one as you could well end up wishing that extra 15% was available to your main battery. (ETA: although I suppose you could wire it so that you can switch that string between the two batteries).

ETA: thinking about it though...if you're going to have the small inverter on 24/7 then you may as well go with 230V lighting because you can then go for pendant lighting with GLS type bulbs rather than recessed MR16s or similar. It is quite nice to have a DC lighting circuit in an off grid context though for when your inverter goes pop...

If there are compelling reasons to go with a 48V battery, why not fit a buck converter on the lighting circuit to step it down to 24V? They can be pretty efficient (at least 85% I beilieve) and your lighting loads should be pretty modest so the inefficiency shouldn't have a big impact. You'll need some sort of regulation on the lighting circuit anyway to avoid cooking the LEDs when your batteries are charging so why not let the buck converter handle it?

What size hot water tank are you going for? Have you considered going for a proper thermal store?

Potentially one more reason to go with 48V is that by the time you're ready to replace your first battery bank you'll hopefully be able to slip a couple of these in at a fairly reasonable cost with no need to reconfigure your electrics.

[Pepperman]

If there's a source of waste sawdust near you, maybe you could run a solar powered heat log press during the summer? It's almost a form of seasonal storage

ETA: Or get in to the ice distribution game? I like the idea of solar powered ice...

[adam2]

I can not recommend powering most lighting from an inverter since extra losses are introduced and a single point of failure added.
Similar arguments apply to powering the lighting from a DC/DC converter, single point of failure added.

LED lamps have improved a great deal and there is no question of any form of voltage regulation being needed.
At least two popular makes are designed to work correctly at from 10 volts up to 30 volts.
A nominal 24 volt system is unlikely to exceed 30 volts even on charge.

LED lamps are available not just in MR16 style but also in styles resembling standard 230 volt incandescent lamps.

Lamps for 48/50 volts do exist but only in a very limited choice.
Also, light switches for that voltage will be expensive specials and probably pig-ugly.
For 24 volts I would be inclined to use standard switches as are sold everywhere at low prices.

All this points to 24 volt lighting circuits and probably also to 24 volts for refrigeration, circulating pumps and ventilating fans.

If a simple 2 wire 24 volt DC supply is not desired, then I would consider a 3 wire system to give both 24 volts and 48 volts. This adds complexity but only marginal extra costs.

[Pepperman]

Yeah we've got 10V-30V LEDs in our boat. They work well but don't give a great quality of light. Being able to take advantage of the really great conventional domestic lamps that are out there is worth it I reckon. YMMV though. Buck converters aren't expensive so you could have a bunch in stock that would see you through the life of your batteries no problem.

You've mentioned this three wire thing before but I haven't followed it up. Do you have any good links that explain it?

[adam2]

A 3 wire DC system was popular back in the days of DC mains supplies since it partialy overcame the limited distance over which DC at domestic supply voltages could be distributed.

The upper limit of voltage for domestic purposes was about 250 volts for reasons of safety and availability of lamps. 250 volts was rather low for distribution.
A 3 wire system could be installed that allowed consumers to be supplied at 240 volts but with distribution at 480 volts..
The mains in the street contained three conductors, an earthed center wire, a positive outer and a negative outer.
The center wire was at about earth voltage.
The negative outer was about 240 volts negative to earth.
The positive outer was about 240 volts positive to earth.

Each house was connected to the center wire and either the positive outer OR the negative outer.
Each consumer therefore received a 240 volt supply, some houses receiving a negative voltage to earth to earth and others a positive voltage.
Note that the average loading on the positive and negative outers was well balanced and that therefore the center wire carried very little current and could be of reduced size thereby saving copper.
Large loads could be designed for 480 volts and connected between outers.


The same principle can be applied to an off grid home.
Consider a 48 volt battery bank.
Make a connection to the center of the battery string and earth it.
You now have 24 volts between the positive end of the battery (the positive outer) and the center wire and 24 volt lamps may be connected thus.
24 volts is likewise available between the negative outer (negative end of battery) and the center wire, more 24 volt lamps may be connected thus.

The whole 48 volt battery may be charged like any other 48 volt battery, thereby keeping charging currents more manageable than the same KW at 24 volts.
Large loads such as inverters, water heaters and large motors should be specified for 48 volt input and connected between outers.

The lighting and small appliance load should be divided as equally as possible between outers so as to avoid un-equal discharge of the two halves of the battery.

[adam2]

Battery balancing on a 3 wire system.

The loads on the positive and negative outers should so far as possible be equal.
In practice some inequality is unavoidable and would result in one side of the battery becoming overcharged and the other half becoming undercharged.

Various means exist to overcome this.

The best way is probably to install a couple of PV modules, complete with 24 volt charge controller for each half of the battery.
The great majority of the charging input is still at 48 volts to the whole battery.
If one side of the battery reaches full charge before the other, then the 24 volt charge controller on the that side will cease charging, whilst the other side will still be charging.

Another approach is to install a solid state battery balancer, Victron and others sell these.

The most basic solution is to connect one well used lighting circuit via a changeover switch such that it may use the positive OR the negative outer as needed.
Each day observe the voltage of each side of the battery and switch this lighting circuit to load the higher voltage side.