Very large off-grid planning considerations - timing

[1201]

With a Cerbo or other VenusOS device there's two settings I know of.

You can choose which shunt to use for soc:
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You can also select the device that controls DVCC whether that be a battery BMS or you can disable BMS control (I don't have a comms BMS so it doesn't show as an available option for me). So for example if you did have battery comms but didn't want the battery to decide when it should charge etc.
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Thanks! I thought this would be possible with a venus device but I wasn't sure. If I can use a smart shunt for all soc based control I'm good to go. Hard to beat that since almost everyone has a smart shunt already.

 

[jnmit12]

Our off-grid place sits on 940 acres, some miles off pavement. Our area has zero utilities. The local phone company surprised us with the offer of fiber optic internet. We jumped on it. Our driveway is 3/4 of a mile long. They dug that trench for zero dollars.

Since we had put an extra 2” conduit from electronics shed to house, we had them connect the two buildings with fiber optic cable. 135 ft between buildings in our case.

Thank you. Thats an impressive offer from teleco! I'm using starlink, which is so much better than the wired service we have elsewhere. In my case, I'm truly off-grid (no address even...) I put in extra 2" runs for future what-ifs, which already paid off yesterday with decision to add a vault. I am adding a couple more 2" runs and 4" for AirCon lineset overhead before backfill.

Got the idea yesterday...Digging the vault today and pouring floor tomorrow... I have 3-4 guys helping me full time, and they just laughed and rolled their eyes when I explained we would be adding more underground concrete...

Planning to be about 8x12, as this vault will only house batteries and related electrical. AC loads and other mechanical items will remain in the original 2-level mechanical room. Mini-split head will primarily address humidity concerns as this area will have a different drainage profile compared to main structure.

 

[jnmit12]

It's always good to focus $/W. Nothing wrong there. Just keep in mind that white shiny TPO is perfect for bifacials and they can give you something like +15% to +30% additional power compared to monopanels depending installation. It's all about getting reflected light to backside of bipanels so installation matters. They like to be elevated and loosely installed and they don't mind being little steeper angle than ideal as the more frontside production you lose because "wrong" tilt angle the more you gain from the backside.

In here mono and bipanels seem to be equally priced now, so I'd go BI if I had your situation.

I need to work out the mounting geometry, but it definitely looks like I can benefit from some TPO reflection, if only on a portion of the array. The house is dug into a cliff, so I don't have a good spot for the array other than on the roof. Even though the roof is huge, I see that 40k pv will take a lot of space, and cheaper, low watt/sqft panels may require placement on a larger area than intended. Ideal Southern orientation might be difficult to achieve, so I might end of with extra panels to compensate.

Aircon and heatpumps will also sit behind parapet walls, so I want to do a good job hiding everything from view.

 

[jnmit12]

The primary way to stop electrical fires is cut the power. Power is coming from the batteries, so try to stop current by disconnecting load.

Another thread reports a fire involving 16s7p battery. Not certain the cause, but the 7 strings of cells were paralleled through fuses only, no relay or BMS FETs to disconnect. That would be a good addition. Idea is to stop over-charging or over-discharging before there is catastrophic failure, and be able to operate with a string out of the circuit if cells imbalanced.

House burned down

So, after 4 to 5 years of working flawless, the 100 kwh lifepo4 battery caught fire at night and burned our house down. Luckily our family just made in out on time. Inspection of the fire expert revealed that a melt fuse melted, and created an arc between the 2 points wich ultimatly started the...

diysolarforum.com

Heat and smoke alarms could be used to trigger shutdown.

LiFePO4 vents below auto-ignition temperature, but gasses can be ignited by a spark. So want contactors and fuses of an explosion-proof type. And want gasses vented outside, so they won't built up and enter house when door opened. Forced air with fan would be good.

thank you the thoughts.
This should be easy enough to implement. I'm burying plenty of additional conduit for communication / aircon / AC feed etc. Direct ventilation out doors is also achievable in this location. Imagine a ~100ft tunnel leading to daylight from a lower basement. I've built the first 50ft, so I will simple add this vault at the midpoint and then continue with the tunnel another 50ft later. Pretty good separation from the main structure, but easy to plumb for connections. (I'm just glad my silly tunnel idea is now pragmatic and useful!)

 

[Brucey]

Thanks! I thought this would be possible with a venus device but I wasn't sure. If I can use a smart shunt for all soc based control I'm good to go. Hard to beat that since almost everyone has a smart shunt already.

No problem, I have the following assorted batteries all connected together and using smart shunt to track soc, works fine.

2*24V 100Ah djlb
2*24V 100Ah ecoworthy
1*48V 50Ah ecoworthy
4*12V 100Ah Redodo minis

 

[Browse]

That lower basement room would make a great wine room or fallout shelter.

 

[Brucey]

That lower basement room would make a great wine room or fallout shelter.

Perfect combo, wine and fallout.

 

[SilverbackMP]

thank you the thoughts.
This should be easy enough to implement. I'm burying plenty of additional conduit for communication / aircon / AC feed etc. Direct ventilation out doors is also achievable in this location. Imagine a ~100ft tunnel leading to daylight from a lower basement. I've built the first 50ft, so I will simple add this vault at the midpoint and then continue with the tunnel another 50ft later. Pretty good separation from the main structure, but easy to plumb for connections. (I'm just glad my silly tunnel idea is now pragmatic and useful!)

Colin Furze? Is that you?

lol man, living the dream. Something that I’ve wanted for a long time too.

I’ve worked in two underground bunkers carved into granite. I don’t have that geology in my current (and likely forever) residence.

 

[jnmit12]

Perfect combo, wine and fallout.

Have the former built already... Hoping to avoid the later!

 

[jnmit12]

Colin Furze? Is that you?

lol man, living the dream. Something that I’ve wanted for a long time too.

I’ve worked in two underground bunkers carved into granite. I don’t have that geology in my current (and likely forever) residence.

Had to look him up - looks nearly the same, but I'm less space constrained (and not digging by hand!). Thanks to the collective guidance here, I have an 8x15 room dug and formed for morning concrete. Added overhead conduit for Aircon lineset and for DC wiring. AC will go in existing conduit underfloor. Its nice not having to conform to plans (or even to make a plan ahead of time!)

We have relatively soft limestone, at least compared to granite. I have a big hammer for the big excavator, so it wasn't a big deal for the bunker. The battery room was easy - just caliche and clay to dig.

 

[shadowmaker]

I have 3-4 guys helping me full time, and they just laughed and rolled their eyes when I explained we would be adding more underground concrete...

Sorry for that.