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Offgrid Solar - distributed DC microgrid

So I was writing this as a reply to The_Beave in his offgrid thread, but realised it should probably be its own thing.

Here, I can literally go to jail for working on anything over 40v without doing a long apprenticeship first (or paying someone who has, to pretend they did the work). It almost never happens - unless someone doesn't like your politics, or a scapegoat is needed.

So while SWIM may or may not have wired up 415v 3-phase industrial machinery perfectly safely at work (where if work gets raided he can adopt a blank look and claim he doesn't know how that came to be :-p), he's not going to do that where he sleeps.

Government-subsidised solar systems here are exclusively the grid-tied kind. Grid goes down, they go down. And fairly overpriced because of the expense and bother of maintaining certification to qualify for those subsidies.

So I've started experimenting with essentially the opposite architecture to The_Beave's - building a multiple-source, multiple-load DC microgrid. The idea is to keep it legal but offload some usage to solar, both to save $$$ and to have a backup for when our grid starts failing harder. And it will keep working even if some components fail (always right at the worst possible time).

What I've done so far that works:

Sources:

- 24v 250W nominal solar panel x 4

- 24v 5A AC-to-DC power supply

Infrastructure:

- 10A and 20A twin-core DC cabling

- Rail-mounted DC circuit breakers for each source (safety first!)

- portable capacitor bank ( 6 x 4.7 uF) with three automotive sockets for inputs and outputs (this allows electric motors to spin up without the voltage sagging to zero).

- multi-socket junction boxes with internal shared busses, designed to take cables with automotive plugs. I mod the plugs with 20A ceramic fuses.

Loads:

- many DeWalt power tools, the older style with minimum electronics, modded with automotive sockets where the batteries once slotted in.

- evaporative airconditioner, all original electronics removed, 24v nominal electric gear motor now driving the fan and a 24v peristaltic pump moving the water.

- seat heaters, resistive, automotive kind

- Fans, 24v nominal, computer kind but larger

- 19v 5A step-down transformer: charges laptops and 20v nominal battery packs if the cases are opened.

- camping 12v stuff - kettle, ministove, air pumps

- automotive USB chargers, many of these, working well.

- 12v dc-to-dc power supply, runs led lighting in my shed.

Things planned:

- huge old 24v camping fridge (going to need a bigger capacitor bank!)

- little 2kg mains-voltage washing machine I purchased to convert. Going in my under-construction offgrid container cabin.

- little 24v truck air conditioner, also for cabin.

- DC ATX power supply for a desktop computer. Looks underpowered and under-filtered, previous one blew up. Going to mod it with stepped-down input and possibly a secondary 12v supply.

A friend is adamant I should have converted all the automotive plugs and sockets to Andersons.

Your thoughts?

#diy #solar #offgrid #prepping

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Discussion

Low Information Voter 2y ago

Also under construction:

- low voltage cutoffs, the Aliexpress ones are cheap, just mounted it in a PVC case

- LiPO4 battery pack, 28v nominal. I have the cells, but haven't found the right BMS. Might need to KiCAD one and build it myself.

[horror movie theme plays]

The Beave 2y ago

Andersons are nice but not at all my favorite.

That's a really cool setup, and very viable! I like it a lot!

The Bird 2y ago

This is something that I want to learn more about especially building a battery bank back up

Low Information Voter 2y ago

Re battery banks:

My original version had a solar charge controller managing a lithium polymer battery bank, feeding an inverter.

The low voltage cutoff function... didn't. Dead lithium cells in the morning.

Version #2 had a better solar charge controller with lead-acid cells and the inverter. Worked okay for months until the solar charge controller developed a short.

(But before that the 2000W inverter blew up the first time I tried to use a small AC power drill. It was 5x overrated for the drill. Some people say 7x overrated is enough, others say 12x. I gave up and went all-DC for tools.)

Unlike The_Beav, my plans haven't been surviving first contact! :-p

I've charged a number of battery packs since then with my system, but haven't wired any in as online backups.

I'll try to keep everyone posted on how my Mk III battery pack goes!

inventor 2y ago

Nice job!

First of all: congratulations. Your work is admirable and I would love taking a look at it, there's a lot to learn from it.

Now the bad part: why do you give a fuck about laws? I will never understand.

40v? seriously? What happens if you use a 12v battery to power a Van de Graaff generator that produces 100,000v??? It's so fucking ridiculous!

Low Information Voter 2y ago

You're absolutely right, it is ridiculous and wasteful, designed to pander to some people's profits and to other people's lazy, fearful ignorance. It also makes our society far more brittle, as the supply of certified electricians is inelastic but the demand for them very unpredictable.

So about typical for our government, at all levels.

But having once worked as a crisis counsellor, no way Jose do I want to be vulnerable to jackbooted petty autocrats stealing my stuff or jail me like what some of my clients went through. Or friends in their younger years. I have a middle-class accent, a decent grounding in law, and a lawyer's number in my phone, but a time may come when that's not enough. So if I can plan a way to jump, dive or roll through a legal loophole without it costing me too much, I do it.

I sent excited traffic-stop cops home with sad faces three times during COVID lockdown, because my paperwork was in order. Essential Worker, KMA!

(But if I lived out in the sticks like my sister and BIL, I'd care a whole lot less. I'm in Melbourne).

inventor 2y ago

Arghf... Melbourne! lol Sorry.

I'll tell you what I would do (if I had the money, of course.)

Underground bunker.

Although, if I had the money, I would probably move to the desert or some forgotten island.

My dream.

But yeah, be careful with those NPC's, you're surrounded there, take care.

Low Information Voter 2y ago

All of those sound like good plans! One day. Meanwhile, we learn skills :)

White The Crow 2y ago

Hi.

For the plugs I can recomand XT60 they are good till 60A so 20A shouldn’t be a problem.

Do you have a battery ? Or doing directly dc solar to the load (otherwise I do not understand what the capacitors are good for)

With solar-mppt-battery-load you can store power and the solar is mor efficient. This is more true for grid down situation.

Battery wise old car batteries work fine especially if you get them for free. Or if you like the task use Li-Ion. These can be scavenged form all modern battery appliances (laptop, e-bike, scooter,…)

Maybe have an option to go from dc to ac. This could be tied in as the regular ac during grid down.

Low Information Voter 2y ago

Thanks Crow!

Direct to the load, but I do have a number of battery packs I can charge off this system.

Solar-mppt-battery-load was my earlier setup. It gained a couple of percent in efficiency, at the cost of greater complexity and multiple single points of failure.

I learned that the first thing I needed to do when designing an intermittent power system is to kill the Little Tin God (efficiency) and bury him in the foundations.

Solar panels are very cheap, single points of failure are very expensive.

XT60s I shall investigate, thanks for the tip :-)

White The Crow 2y ago

Yes complexity is increasing.

I do have something like a star configuration. So all solar feeds the battery and then I have the loads connected to the battery.

If you like the distributed system you can use buck converter instead of mppt. Not as efficient as mppt but they are cheaper.

Also if you don’t like centralized battery you can hock up battery’s with buck boost converter (24v to 24v) or buck converter for 24v-12v like in a dasy chain.

For me relatability is the highest priority.