My adventures building a DIY Zn/I flow battery

[danielfp248]

After all the adventures trying to build the Mn-Fe flow battery, I have now shifted to a Zn-I flow battery. Since I now have a full setup to actually test flow batteries, I have arrived at this chemistry after testing several other alternatives. You can see some of my experimental results on my blog (https://chemisting.com/2023/09/14/testing-a-zinc-iodide-flow-battery-with-a-microporous-membrane/). At its current state, this chemistry already seems viable for DIY flow battery system.

 

[danielfp248]

We have also started a non-profit for the development of open source flow batteries (https://opencollective.com/fbrc). It is called FBRC (Flow Battery Research Collective). I started it with my colleague Kirk Smith and professor Sanli Faez (https://scholar.google.com/citations?user=X0lp5rQAAAAJ&hl=en) from Utrecht University in the Netherlands.

Our first objective is to build a kit for less than 1000 EUR that anyone could buy which will include everything to build and test your own small scale flow batteries. The kit will include all hardware (including the potentiostat) and all the hardware (pumps, cell, separators, electrodes, etc) needed to carry out the experiments.

The non-profit accepts donations through the link above. All developments are going to be fully open source.

 

[DIYrich]

What is the usable energy of 30,000 litres?
What is the cost of 30,000 litres?
I'm wondering if it can be used for shifting summer production to winter usage.

 

[danielfp248]

What is the usable energy of 30,000 litres?
What is the cost of 30,000 litres?
I'm wondering if it can be used for shifting summer production to winter usage.

15,000 catholye + 15,000 anolyte at 35Ah/L would give you 525kAh which at a mean discharge voltage of 1.23V would give you 645 kWh, this is 0.645MWh, so very massive system. At 1mL per cm2 of electrode area you would also need to have 1500 m2 of electrode area, which at a standard 25cmx25cm per cell would imply having at least 24,000 cells. This is a massive system. Probably a couple of containers filled with stacks of cells to process what is literally a pool of electrolyte. Since the energy efficiency is 70-75%, you will need to put at least 0.86MWh in to get that 0.645MWh out.

At bulk prices of:

ZnCl2 - 1700 USD/ton
KI - 2900 USD/ton
NH4Cl - 450 USD/ton

For 30,000L you would need 8.17 tons of ZnCl2, 3.20 tons of NH4Cl and 19.9 tons of KI. The total cost of the salts would be 32.1K USD.

The above doesn't include pumps, tank costs or cell costs. Note that since no ion selective membranes are used, this is going to be significantly lower cost compared with a Vanadium based system. Big systems have significant additional issues - for example pumping efficiency becomes a huge concern - so I'll have clearer costs for you once we implement the first 25x25cm cells.

We are however FAR from anything at this scale. Right now we are focusing on the small scale. Once everything is optimized the costs for larger scales might also drop further. Hopefully significant improvements in the energy density are still possible since the solubility does allow for much higher densities.

 

[DIYrich]

That's about $50/kWh to store summer production for winter usage. Not cost effective. I think it would need to be under $5/kWh to make it cost effective.

For long-term storage (summer to winter), the size/cost of the reactants is the major issue. You can have a "small" 2kW reactor vessel to convert the reactants to electricity. 2kW running continuously is 48kWh per day. Constant trickle to use immediately, or charge batteries for higher demand times. I suppose a larger reactor would be more efficient to use the electricity immediately vs conversion inefficiency to store in battery for higher demand times.

For daily use, 75% efficiency is not very attractive compared to 90% for LiFePO4. That conversion loss adds up pretty quickly for daily use.

 

[danielfp248]

That's about $50/kWh to store summer production for winter usage. Not cost effective. I think it would need to be under $5/kWh to make it cost effective.

For long-term storage (summer to winter), the size/cost of the reactants is the major issue. You can have a "small" 2kW reactor vessel to convert the reactants to electricity. 2kW running continuously is 48kWh per day. Constant trickle to use immediately, or charge batteries for higher demand times. I suppose a larger reactor would be more efficient to use the electricity immediately vs conversion inefficiency to store in battery for higher demand times.

For daily use, 75% efficiency is not very attractive compared to 90% for LiFePO4. That conversion loss adds up pretty quickly for daily use.

For LiFePO4 the cost per kWh is still above 130 USD. It is definitely still too early to be talking about anything at large scale, but flow batteries - even at these lower energy efficiencies - can often make better sense than Lithium batteries. The 75% EE is only my first practical result, I'm sure we will improve things with time as we gain more experience with the chemistry.

 

[DIYrich]

, but flow batteries - even at these lower energy efficiencies - can often make better sense than Lithium batteries.

under what use case?

 

[danielfp248]

under what use case?

I think this would make the most economic sense for off loading peak renewable production for same-day use, so for short term grid regulation operations.

 

[Vorg]

This guy as a video about making a zinc/iodine battery and mentions flow batteries: