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Comment by Workaccount2 | Hacker News Reader

>Also, the internal resistance depends entirely on how many cells you have.

Right, from the one study I can find, commercial flow batteries have about 10-20x the internal resistance of a lithium ion battery, so the match the power and energy capabilities of a single li-ion cell you would need a liter of electrolyte and about 30 (!) cells (3 for voltage x 10 for power).

And that is for a commercial quality flow battery. And lithium ion batteries are wholesale in the $2 a piece range.

I'm not trying to say flow batteries are stupid or dumb, but their use cases are going to be very limited without some huge breakthroughs that will probably dramatically increase the complexity too.

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> I'm not trying to say flow batteries are stupid or dumb, but their use cases are going to be very limited without some huge breakthroughs that will probably dramatically increase the complexity too.

The largest use case is going to be grid scale storage, and for that one a bunch of dumb tanks and a bank of cells are far easier to handle and less risky than a bunch of li-ion cells that can go into runaway for whatever reason.

The thing is that there are like 50 other options for energy storage, it's not just lithium-ion that flow batteries have to out compete. Everything from electrolyzed hydrogen, to hot sand, to air pressure tanks, to sodium-ion and zinc air and LiFePo batteries.

Flow batteries are cool because the storage element is extremely easy to scale. But its not even that great because you also need to scale the amount of cells dramatically to make it useful outside of edge cases. At which point it probably makes more sense to just use another storage mechanism.

Their use cases would become extremely relevant to people trying to rebuild society from scratch, I'd imagine. This device is so simple you could probably build one in the woods with nothing but a basic survival kit given a year or two alone.

So that's something. Learning how to build one from scratch seems worthwhile, much like learning to build a radio from scratch

Unless you're flying a drone, you don't need to match the power and energy capabilities of lithium ion.

On top of that, you don't have to match the internal resistance to match power. If you have plenty of material to absorb the heat, then you can tolerate more percentage points of loss.

In particular, while lithium ion batteries can be built to sacrifice discharge rate for a bit of extra capacity, something like a 3C discharge rate is easy enough to reach. And if your use case is powering a building for several hours, you might only need a .2C discharge rate. That would mean lithium ion as a technology is 15x overqualified, and a flow battery that gives you 10x less power would still be overqualified.

I see what you are saying, in reality the right energy storage is very application dependent. The crux of my argument is that I cannot think of many applications where a commercial grade flow battery is the best choice, much less a single application where a DIY flow battery is the best choice.

My main point is that while it definitely has to compete on price, it doesn't have to compete on price while also making you buy ten times as many cells.

Many elements of being the "best choice" are thresholds. Excess performance doesn't make it better. Price is extremely important, but power density is not so important for most use cases. So if it's even slightly cheaper, expect to see a lot of it.

> application where a DIY flow battery is the best choice

Christmas present for a battery enthusiast? :D

>The crux of my argument is that I cannot think of many applications where a commercial grade flow battery is the best choice

I think the argument against that is that we've spent tens of thousands of person-hours a millions of dollars more researching, developing, improving, and refining lithium-ion batteries than we have flow-batteries, and the purpose of the project in OP is to make it easier to tinker with flow-batteries.

Think of how much better lithium-ion batteries have gotten since we first started using them. We went from "You simply cannot achieve the energy density to propel (tethered) cars without fossil fuels" to "Oh wait we have electric cars now." in like a decade, and in the decade since them, lithium ion batteries have improved in every metric by at least an order of magnitude. We simply don't know how good flow-batteries can get, because we haven't tried, and it's silly to say "why bother trying, they're not very good right now" when we've just seen how much of a difference it makes to invest into this sort of technology.

1 liter of electrolyte is nothing for a flow battery, the smallest scale they become a serious competitor is ~10m3 (10,000L) tanks which are ~(7 foot X 7 foot X 7 foot).

Start taking GWh of storage and lithium ion technology gets really expensive and has a lot of associated risks. Flow batteries on the other hand don’t need to worry about a single cell failure resulting in a fire which then spreads.

The amount of electrolyte doesn't scale the available power though, only the available energy.

Energy storage is the problem that needs a solution, e.g., storage from summer to winter.

Annual and long-tail storage are both problems that need solving, and fuel cells do look like a possible solution. But it's not clear at all what the winner will be for those applications.

Even hydrogen is competitive here. IMO, more competitive than that battery chemistry on the article.

Fuel cells are a possible solution (especially if one can build a dual mode electrolyzer/fuel cell), but combined cycle power plants burning hydrogen would also work, if only at very large scale (but then economical hydrogen storage likely requires large scale).

Seasonal energy storage is the last thing that needs to be addressed in the storage hierarchy. There are very high-value targets starting on very low time frames.

Seasonal energy storage is the last thing, but that also means it's the last caveat, as far as I can tell. If we figure out seasonal energy storage, that's it. Solar just wins. There's no further need for fossil fuels. We're done.

I guess there's still aircraft.

That just means you get to scale it independently.

While they both need to scale the amount of DC<>AC inverters based on peak power demand. If you want to discharge over 16 hours you’re using 2% of lithium ion’s peak power output and need a huge mess of wiring to move power from each internal cell to that inverter + complex battery packs with individual electronics cooling etc.

Flow batteries on the other hand can use a single pump (+ redundancy) and fat pipe to supply a huge array of ion-exchange membranes which then sit next to the inverters.

A flow battery isn't going to burst into extremely high temperature flames in a self-sustaining not easily extinguishable fire that also spews toxic fumes in mass quantities.

I'll take a basement (or garage) with a flow battery over lithium ion ANY day of the week if I want battery backup for my house.

You would not use a laptop-style lithium battery for battery backup for your home. You would use LFP. It does not have the burst-into-flame problem you are thinking of.

At some point I expect we will look back at the era when we used really flammable electrolytes and laugh about how wild that was. I bet we are not that far from it being just a memory.

PowerWalls use NMC cells. And the majority of "diy powerwall" builds are reclaimed 18650 cells.

There's been a rumor for a while that Powerwall 3 is LFP, but I don't think anyone has actual confirmation on that.

I've seen occasional 18650 builds for DIY powerwalls but the vast majority are using prismatic LFP cells. Way easier to wire up, cheaper (unless you have a secret source for surplus 18650s), and minimal fire risk.

As someone completely ignorant in the field, my impression is that the point of this is to create something that relative amateurs (or research professionals on a budget) can use to tinker with various chemistries to achieve those huge breakthroughs (that we keep seeing in every other kind of battery, because it turns out batteries have a lot of room for breakthroughs).

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