Very large off-grid planning considerations - timing

If you're traveling to the build site from another home, what about a traveling battery?

Put a powerpro in a truck bed or trailer with an inverter, charge at home and power the site while you're there.

It's really the PV mounting and wiring that turns into a time suck imo. But it could go faster for a very effective DIYer, someone who can trench, conduit, bury in two days, or dig and pour concrete footers in another two days. I am not that guy. Each big step takes me a leisurely weekend.

Hedges said:

Sunny Island for split-phase or 3x for 3 Phase

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Is it unheard of to implement a 3-phase solution for an off-grid solar setup? Load balancing between phases might be tricky with a handful of large current loads intermittently used. (I'm also not aware of many residential products offered in 3-phase, so balancing individual legs would be more important?)
That said, I am installing a 3-phase Mitsu multi-split AC and a 3-phase oven next week as part of an unrelated foreign project (so I'm curious about 3-phase options for off-grid solar)

jnmit12 said:

I (will) have poured concrete roofs with a TPO layer on top (belt and suspenders for a DIY job). Solar 'boots' will be mechanically fastened to the concrete roof deck, with welded TPO boots surrounding them. Something of a direct attachement / ballast hybrid in that the mounts are firmly attached, but there are not penetrations through the roof itself. I've not done a TPO roof before, but have some contacts for help.

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You designing one of them modern monstrosity box houses huh? Jk though, I would happily take one.

jnmit12 said:

wondering if ventilation plan might be useful

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Oh, you definitely need ventilation and humidity control or you’ve built a concrete cave with mold and mildew that you’ll never see the end of.

hwy17 said:

If you're traveling to the build site from another home, what about a traveling battery?

Put a powerpro in a truck bed or trailer with an inverter, charge at home and power the site while you're there.

It's really the PV mounting and wiring that turns into a time suck imo. But it could go faster for a very effective DIYer, someone who can trench, conduit, bury in two days, or dig and pour concrete footers in another two days. I am not that guy. Each big step takes me a leisurely weekend.

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Thought about this, but turned into a cheapskate... I have several small generators for drills, saws etc, and a dedicated inverter generator for clean power (Starlink sitting outside, drill batteries). I have a handful of guys doing the hard work and I keep them supplied with materials and instructions. I'll do most of the electrical work with help on repetitive tasks. I suppose I could make a temporary traveling battery from the cells I need to buy anyway? (no orphaned batteries). I drive 1 hr each way, several times a week... Would make sense - thank you!

wpns said:

Oh, you definitely need ventilation and humidity control or you’ve built a concrete cave with mold and mildew that you’ll never see the end of.

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thank you. I have humidity management (though we are in a very dry area - more water would be nice!) Outside of humidity/temperature management, do you think ventilation is necessary for battery room? I will use an ERV for the larger house, so I can easily add an outlet to the battery room to constantly exhaust the air.

hwy17 said:

You designing one of them modern monstrosity box houses huh? Jk though, I would happily take one.

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100%... haha. (But it is largely hidden from view)

In my experience, large concrete rooms with no ventilation tend to suck the carbon dioxide out of the air to cure the concrete, and it feels hard to breathe in such a room. This can last for years. I’ll take your word for how dry it is, but note that moisture goes right through concrete, so waterproofing and vapor barriers are still important.

Oh are you designing with a separate battery shed 10ft or more away from the house? They're all the rage nowadays, build it into your plans if you can.

jnmit12 said:

Thank you. I was considering this topic yesterday. My domestic breaker and manifolds will sit in a basement mechanical room. Batteries and RO water system will sit directly below this, in a -2 level basement. All 12" concrete with concrete suspended floor decks etc. Basically mechanical rooms are concrete vaults. Steady temp, & gravity floor drains. It fire suppression is low priority, wondering if ventilation plan might be useful?

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Venting LFP cells can produce hydrogen and there was a UL test report that showed that basically a 5kWh rack battery could produce enough undesirable gas to fill a large two car garage with vapors. Having active ventilation or a sensor based activation (smoke, heat detector etc) could certainly reduce risk of a fire. Based on a recent fire thread make sure you have ionizing smoke detectors in the selection to catch fires which are smouldering but not producing big particles.

hwy17 said:

Oh are you designing with a separate battery shed 10ft or more away from the house? They're all the rage nowadays, build it into your plans if you can.

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Brucey said:

Venting LFP cells can produce hydrogen and there was a UL test report that showed that basically a 5kWh rack battery could produce enough undesirable gas to fill a large two car garage with vapors. Having active ventilation or a sensor based activation (smoke, heat detector etc) could certsinky reduce risk of a fire. Based on a recent fire thread make sure you have ionizing smoke detectors in the selectiob to catch fires which are smouldering but not producing big particles.

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Thanks. I will certainly follow this. Much of what I will end up with will be make-it-up-as-go... I added a battery room underground, but I did not consider separating it from the main structure. As this point, I think ventilation and fire detection/suppression might be easier than adding another structure?

hwy17 said:

Oh are you designing with a separate battery shed 10ft or more away from the house? They're all the rage nowadays, build it into your plans if you can.

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Brucey said:

Venting LFP cells can produce hydrogen and there was a UL test report that showed that basically a 5kWh rack battery could produce enough undesirable gas to fill a large two car garage with vapors. Having active ventilation or a sensor based activation (smoke, heat detector etc) could certsinky reduce risk of a fire. Based on a recent fire thread make sure you have ionizing smoke detectors in the selectiob to catch fires which are smouldering but not producing big particles.

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Thanks. I will certainly follow this. Much of what I will end up with will be make-it-up-as-go... I added a battery room underground, but I did not consider separating it from the main structure. As this point, I think ventilation and fire detection/suppression might be easier than adding another structure? This is my targeted storage environment. Concrete on all sides.

jnmit12 said:

Is it unheard of to implement a 3-phase solution for an off-grid solar setup? Load balancing between phases might be tricky with a handful of large current loads intermittently used. (I'm also not aware of many residential products offered in 3-phase, so balancing individual legs would be more important?)
That said, I am installing a 3-phase Mitsu multi-split AC and a 3-phase oven next week as part of an unrelated foreign project (so I'm curious about 3-phase options for off-grid solar)

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Just a desire of us guys who like industrial equipment, good for motors to have high starting torque. Also last for ever without so much as needing capacitor replacement.

Several battery or hybrid inverters support connecting 3 (in some cases 2) inverters for 3-phase including Sunny island, Schneider, Sol-Ark, Rosie, Outback, quite a few more I think. Mostly I see 120/208Y, not 120/240V high-leg delta. Some are 277/480V.

When I thought I could get 3-phase replacement for my split-phase residential service affordably, I looked up 3-phase AC units. Didn't see any as inexpensive as split-phase, think they started around $5k vs. $2k.

It is common to feed 3-phase to apartment buildings and condo complexes. Each unit gets 2 legs for 120/208V, L1/N/L2, L2/N/L3, etc. so wired just like 120/240V split phase, but lower 208V for large loads.

Implemented with three 120V inverters, load balancing isn't required but of course you want to distribute reasonably evenly to utilize all available power. I think 3-phase starts to care about imbalanced voltage when going through transformers or into motors.

I tried using a 3-phase Sunny TriPower 277/480V GT PV inverter with 3-phase 120/208Y Sunny Islands, and the transformers cause some problems. In Europe with 240/416Y or other voltage close to that it would work better, no transformer.
Not many 120/208Y GT PV inverters to better to stick with single phase inverters supporting 208V & 240V if you do this.

True, easier to balance multiple 240V loads on 120/240V split phase.

I do use a VFD with 2HP pool pump, but I've found non-PF corrected VFD upsets GT PV inverters.
Either need PF corrected VFD, or use DC coupled or hybrid inverter.

Poor power factor also reduces efficiency of your system.

hwy17 said:

Oh are you designing with a separate battery shed 10ft or more away from the house? They're all the rage nowadays, build it into your plans if you can.

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I have a brief window of opportunity to build a somewhat detached battery storeroom before I backfill my main basement. It would be an underground concrete box, connected to the main structure by a concrete tunnel... I have a free 2" schedule 40 conduit unassigned as well. It would not sit below the main structure - but would be connected by ~60ft of horizonal tunnel...

Should inverter/charger/optimizer etc be located closer to the batteries, or closer to the PV array? Main structure would house the PV panels and the consumption side breaker panel. Battery room could host anything but the domestic breaker panel and the PV array. In this scenario, it would be more difficult to condition the air in the battery room, but it would remain warm year round.

Wires from charge controller to battery should be as short as possible, for best voltage regulation.
Wires from PV array to charge controller can be very long. Doesn't matter too much if voltage drop > 3%, > 5%, or even higher. It's only watts lost.

If battery is 48V and AC is 120/240V, inverter should be close to battery. If battery is 400V then it can be farther, making AC run shorter.

Seal the passage from battery room to house, fire door, fire stop in conduit. Vent the room to outside (or vent in case of pressure or fire.) But keep climate comfortable.

jnmit12 said:

Thank you. I was considering this topic yesterday. My domestic breaker and manifolds will sit in a basement mechanical room. Batteries and RO water system will sit directly below this, in a -2 level basement. All 12" concrete with concrete suspended floor decks etc. Basically mechanical rooms are concrete vaults. Steady temp, & gravity floor drains. It fire suppression is low priority, wondering if ventilation plan might be useful?

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So you don't plan separate garage/tool shed? Everything is incorporated in one building? That's ok and underground concrete vault would do with fire stop and venting outside like @Hedges already said. Also keep in mind that there's a lot of heat produced (inverters, not so much LFP) and climate control is needed to keep temperature below +25C. Being underground in hot (and cold) climate is wise, but I bet it's not enough to fight this much energy. Keeping things around +20C will make your system to last much longer (electronics and LFP chemistry).

jnmit12 said:

Should inverter/charger/optimizer etc be located closer to the batteries, or closer to the PV array?

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Just keep PV voltages high and use proper cabling. This way inverter can be long way from PV. My array is 100m (330ft) away from my solar corner which is 100m from my main panel (3-phase AC). Each PV string has ~11kWp, >700V and 4x 4AWG aluminum underground cable to keep losses down. AC underground cable is 2AWG aluminum.

If you are going to use TPO roofing go with bifacials. It'll give you around 20% more production when installed over white surroundings. In hot climate try to find panels with Pmax temp coefficient less than -0,31%/C.

jnmit12 said:

Should inverter/charger/optimizer etc be located closer to the batteries, or closer to the PV array?

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Inverter, batteries, and charge controller pretty much must be colocated. PV DC can travel. 240v AC can travel. 48v DC does not travel.

100-500 ft from battery to array no problem. 100ft from inverter to house no problem just maybe $1000 of copper. Battery to inverter though, should be less than 10ft.

hwy17 said:

100ft from inverter to house no problem just maybe $1000 of copper.

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Or $600 worth of aluminum. At least in here.

Thank you for the guidance! I will go to the site now and dig for a battery vault this week. As luck would have it, I only built half of my tunnel, so i will simply build a small room midway, then continue the tunnel. Since I already have extra large conduit under the floor, it will be tidy. (The tunnel was a 10ft deep trench for conduit, drain etc, so I put a roof on it instead of backfiling - no good reason to have it other than the novelty)

I've just run 4/0 cable to my seperate barn, so I can get a little more for my AC connection back to the primary breaker panel. Won't have any voltage drop over that distance.

You can have batteries in the vault, wires through to electronics on other side of the door.
Lithium batteries don't produce much heat (except when burning) but electronics does.

We've also seen pictures of nice installations with batteries on the floor and inverters on the wall above. Batteries burned, and took out the electronics.