What exactly is a Redox Flow Battery ?

We all know what a battery is, that same thing in almost every portable electronic device in your room has. Most batteries we use in our day to day are static batteries, i.e. once they are manufactured, their chemical composition doesn't really change. Now the chemicals used in a battery dictate everything about it, like its -

1. Cyclability 

2. Efficiency

3. Safety

4. Supply chain constraints.

Let me explain in a little more detail using examples -

The AA and AAA batteries (chemical composition is Zinc and Manganese) normally used in T.V remotes are not rechargeable, they are single use. Once the energy inside them is used them, they are pretty much dead. They are mildly toxic, but safe to use, efficiency doesn't come into picture as they can't be recharged, energy density is ( 50-150 Wh/kg )

The Lithium Ion batteries in your phones and EVs are rechargeable (Duh !). Its of many types, i am aware of only two-

a) NMC (Nickel Manganese Cobalt along with Lithium) 

b) LFP (Lithium Iron Phosphate)

Both catch fire, have very high efficiency (that's why we use them), have maximum density, life is about 7 -10 years if you maintain them really well, but lithium mining is one of the most water intensive and environmentally destructive activities humans do. And China has a monopoly on their mass manufacturing, price and distribution.

Similar to Lithium (Li-ion) is a newer and cheaper chemistry which is Sodium Ion (Na-Ion). It has all the advantages of Li-ion but no disadvantages. As of now it is being commercialized only in China, there are many startups in the rest of the world, but not as big as Chinese battery makers.

Enough about static batteries, moving on to our darlings-

Source

Flow batteries are as the name suggests, batteries through which a chemical electrolyte flows, these are not like the batteries we are used to, they don't store energy internally in their cells, instead they store energy in external tanks, the tanks store the chemicals, these are connected through the pipes to the cells or cell stack, and the liquid flow is maintained by pumps.

 

Now you may think, that's unnecessary complication, pipes, pumps, tanks, but here's the beauty of flow batteries- 

1. Cyclability -> 20,000 + cycles, if you assume 1 cycle per day, their life is 27 years, that's more than my current age.

2. Efficiency (around 80%, lower than lithium but not bad at all)

3. Safety-> the electrolyte is liquid, so no fire at all, ever

4. Supply chain constraints -> Much less Chinese control than lithium batteries, in my case none.

Unfair Advantage of Flow batteries-

Remember how the tanks stored energy, energy not power, quick physics revision-

Energy = Power x Time

Energy is stored in tank, power is transferred through the cell stacks and the time duration can be manipulated by changing the size of tanks. If this were a phone battery you would have to whole battery packs. Lets take a theoretical example -

Let us say my house working power load is 5 kW, if i need to provide a 8 hour backup, to 1000 of such homes, I need 5 MW of power and 40 MWh of stored energy. Simple arithmetic, now I have 2 options -

1. Buy two 5 MW worth of lithium ion battery packs (10 MW), this will store energy for 4 hours at max (that's 40 MWh). So my total installed power is 10 MW. But my original requirement was 5 MW, but to cover for the energy storage requirement, I had to buy 2x the power.

2. Buy a Flow battery with stack sized exactly to 5 MW, but size the tanks according to 40 MWh, i don't need to buy extra power, in case i need to add power, say another 5 MW, i just add another 5 MW cell stack. If i need to increase backup duration to 16 hours instead of 8, i just add bigger tanks.

I think the answer is pretty clear and it is becoming more so as the grid needs longer duration backup (or energy storage), this is known as "Long Duration Energy Storage" or LDES.

None of this is AI generated, so feel free to fact check me and looking forward to your views and help. 

Mayank Jately

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