Batteries for home solar storage
MALCOLM TAYLOR
| Posted in General Questions on
When this has come up in the past there was some advice around to the usefulness of providing a fire resistant mechanical room or enclosure for them. If they are LPF batteries is this still necessary, as I understand they are pretty stable?
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Replies
Malcolm,
I've been in the battery world for a long time and I would not install a big battery in a house without some protection. Each 30kWh of battery stores enough energy equivalent to 1 gallon of gas. Something goes off, that energy has to go somewhere, no matter the chemistry, Not Good Things ™ will happen.
LFP will be less energetic than other chemistries but you are still dealing with a flammable electrolyte and a lot of electrical energy.
Akos,
Thanks, that was my understanding too, but I've seen some chatter about the LFP being stable enough that it wasn't a problem.
They are more stable, but they are still storing a lot of energy in a relatively small space. I like to say "energy is like an angry little genie, and he doesn't care who or what he hurts on his way out of the bottle". Building out a fire rated battery room isn't difficult, and doesn't cost much -- just a layer of 5/8" type X drywall -- so it's cheap insurance. I recommend building it as a 1 hour room, which is a regular 2x4 studwall with a single layer of 5/8" type X drywall on both sides, and put a heat sensor in there tied into an alarm system inside the house. The general idea here is to give the occupants of the house as much notice as possible of a problem (the heat sensor in the room tied into a linked fire alarm inside the home is ideal), and the fire rated room is there to buy the occupants enough time to get out of the house safely. Total cost for all this stuff during the intial build is likely only in the several hundred dollar range, so nothing crazy, and you don't need any exotic materials, either. Do remember that the ceiling also needs to be fire rated, so the same drywall rules up there too.
To Patrick in Post #3: The issue is the time over which the energy is released. This is calculated in joules usually. The total available fault current for a residential service is rarely going to exceed around 8,000 amps, and usually will be signifantly less than that. If we cheat a little to make the math easy, and we assume no volt drop and one second of delivered energy (more than enough to pop a fuse under short circuit conditions), that's 1.92 MJ of energy (one joule is equal to one watt*second). In reality, the actual deliverable energy will be less than that. That 30 kWh battery pack dumping into one second is about 1.8 MJ, so about the same, but it's in an enclosed space, and it has fuel interspersed with it (the battery's electrolyte, plate, and cell jar materials). The biggest issue is that it's contained in a small package, which tends to make the destructive effects worse.
As a practical matter, I have been in the immediate vincinity of both a battery exploside and arc faults on large 480v commercial 3 phase power systems. The 480v systems tend to flash, smell like ozone, and take a notch out of the busbar or lug where the arc occurs. It's all very exciting, but done in an instant, usually with minimal damage to the facility and zero damage to personnel.
The battery explosion was due to an internal arc within a cell, which lit of a stoichiometric mix of oxygen and hydrogen within the cell, rupturing the battery, and going off like an explosion spraying both the dilute sulphyric acid electrolyte and parts of the battery jar and electrode materials around. I have a bit of hearing damage in my right ear from that. There was a decent size chemical mix mess to clean up (baking soda and time to clean up), and acid etch damage to the surrounding metal surfaces. This was also very exciting, but had lasting damage to both the facility and the personnel (me, in this case).
So having personal experience with both potential issues, I strongly advocate that ALL battery systems be installed with at least SOME measure of safety in mind. It is inexpensive and relatively easy to build a small fire-rated battery room to contain any battery issues, and it's easy to put a heat sensor in that has a remote annunciator too. This stuff only needs to work ONE TIME to be worth it, maybe saving the lives of your family. Maybe that sounds dramatic, but it really is a possibility, even though pretty unlikely, and the protection is cheap, easy, and requires nearly zero maintenance so it's really worth it to do.
BTW, I personally am not a fan of home battery storage generally. Everyone I know of who works with batteries hates the things. The newer lithium technoligies are a lot better in some ways than the lead acid and lead calcium batteries routinely used in the telecom industry, but they are all energy stuffed into a small space, which always comes with some inherent dangers. It is prudent to be respectful of the things at all times, which includes putting in reasonable safety measures to deal with potential failures.
Bill
I agree with treating these energy sources with care and safety in mind. But what are the pathological failure modes of LFP? I think it's well understood that NMC Lithium ion is inherently dangerous.
The reason I frame it this way is that most homes have a nearly endless source of energy flowing into them in some way as the power company will often specify a fault current of thousands of amps.
Even "just" 1,000 amps at 240 V gets you to 30 kWh in 7 1/2 minutes. I suspect a lot of things would have burned up by that point, but hopefully you take my point.
In many areas, there is no overcurrent protection device *outside* the home, so this full fault current is technically available inside the home in a worst case scenario.
The issue is inside each cell. You have a flammable electrolyte plus a source of energy packed tightly together. That is fine if everything is Ok. Say somebody puts a screw by accident through a cell (this does happen) or the cell develops an internal short, that energy is released. Usually if it is a single cell, it isn't much energy and in most cases the cell popcorns and that is about it.
If however it spreads to the next cell then the next, you can see where you run into trouble. You can design around most failure modes but not all so this scenario is unlikely but still can happen.
Your house is fused at the main breaker, most codes only allow a small length of unfused run inside the house. The power feed to the house is fused at the main transformer. If something shorts, one of the fuses will pop stopping the power flow. There is lots of fusing outside each cell but nothing inside the jellyroll (the active bits that make the cell) itself.
You don’t get to change the laws of physics.
Transporting potential energy from on location to another is risky. and storing up large amounts of potential energy is even more dangerous. Pipes leak transmission lines start fires tank farm blow up and reactors melt down batteries fail.
Pipelines don’t go under homes, transmission line don’t go over homes, no one lives within 100 yards of a tank farm or a reactor.
Living in a building with huge amounts of potential energy is crazy. I don’t care if it is suspended weights, compressed springs, spinning flywheels, compressed gasses, molten rock or chemical energy.
No matter how you slice it every battery is stored potential energy stored in chemistry. There will always be a failure mode. Some batteries will be safer than others but the risk will always be present one maybe risker than the next. One is only safe by comparison to the other not in absolute terms.
The only thing we know for sure about any new technology is that we do not know everything about it yet. The advantages are mostly clear the disadvantages not so much.
The smart move is not to sleep in the building with big batteries.
Walta