I don’t understand how you can make the comparison.

A lithium battery has a limited capacity, so you look at the cost per unit energy at a 100% charge. But flow batteries never get ‘full’, they can carry on storing energy until you fill a silo up. I could see a direct comparison on a power basis, or on an levelized cost basis, but those aren’t usually the figures used to index lithium batteries.

Happy to be corrected if I’m wrong. It would be fantastic news if flow batteries became competitive.

I don’t understand the article. It says you can have a flow battery that can run for 4 hours for $150, or one that runs for 8 hours for $100. Clearly the 8 hour model is superior, since it holds a charge longer, and costs less.

Based on your comment, I guess they mean to say that you get 2x the sustained wattage (not kWh) out of the $150 model.

Is that right?

The way I read it, the difference between the $150 ("4 hour") battery and the $100 ("8 hour") battery is that the 8 hour battery will be 2x in size. This would be the same KW (~volts x amps) available but 2x the hours at that power draw. The difference between $150 per KWh and $100 per KWh is the economy of scale.

The scaling of hours is going to be proportional to the tank sizes. The power (KW) rating is going to scale proportional to the electrochemical cell size. The expensive part is going to be the electrochemical cell; tanks are simple and scale well. The "4 hour" and "8 hour" examples would have the same electrochemical cell size since they have the same power (KW) rating but 2x bigger tanks for the "8 hour" version.

Ah right, so they’re limiting the tank size and volume of the electrolyte to make the comparison. And selling it as a closed system similar to a normal battery.

Yeah.

A "Flow" Battery's got the advantage of scale. To scale a flow battery, all you gotta do is build a bigger tank. A Lithium Ion battery requires you to build complex circuitry and chemicals throughout the whole battery.

In many ways, Lithium Ion batteries are like Solid State Drives: You got complex circuitry through and through. While Flow batteries are more like a Tape Drive: The drive is way more expensive to make, but "tape" (ie: Tank) is so cheap that the economics of energy storage eventually wins out.

What do you mean? How can they not get full?

You can continue to charge more electrolyte for as long as you can supply more, & stash the charged electrolyte somewhere. It's not a unit the way a lithium ion battery is.

Oh wow, really? You can just disconnect the electrolyte and store it and it will stay charged? What's the discharge rate on that like?

It doesn't stay charged. You push the more active reactants into it, it gets charged while it reacts the reactants into the less active version.

It is like a diesel powered generator, where you push diesel and oxygen into it, it creates power while it reacts them both. The difference is that flow batteries also do the reverse reaction, so you can get the original reactants back.

About how long it will stay charged, it does not discharge like a normal battery, but active reactants have a shelf-life.

I see, thank you. With those properties, I can see why it would be ideal for home energy storage.

Currently, they don't look great for many applications. But on principle they should be the best option for any stationary application.

What we are seeing here is that real vs. possible gap getting smaller.

Not quite. Lithium Ion is cheaper at home-energy scale.

Flow batteries are more expensive and complex. So they are ideal for utility-scale projects only. When you're powering an entire neighborhood or city with a single battery, you'd probably rather make a Flow battery rather than a million Lithium Ion batteries hooked up together.

Huh, so like a power ring buffer.

The storage capacity is not a property of the battery, so the concept of "full" does not apply.

It's like saying that the switch where your NAS is connect is full. You can even use that word, but it has a different meaning.