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My adventures building a DIY Zn/I flow battery

danielfp248

Battery researcher
Joined
Sep 7, 2020
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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.
 
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.
 
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.
 
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.
 
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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.
 
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.
 
This guy as a video about making a zinc/iodine battery and mentions flow batteries:
 
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.
Not with 75% efficiency. That is a lot of power loss for 1 day. You can live with that much loss for shifting from summer to winter because the options are few.
 
Not with 75% efficiency. That is a lot of power loss for 1 day. You can live with that much loss for shifting from summer to winter because the options are few.

It depends on the levelized cost of storage. If you can store 75% for a very low lifetime cost, it definitely beats storing 0%. If you're losing the energy anyway, anything you store for a low cost is a big win. You're not storing energy you would have used, but energy you wouldn't have done anything with if no storing was available. Storing 90% at a much higher cost per kWh might not be worth it.
 
This guy as a video about making a zinc/iodine battery and mentions flow batteries:

Robert is very good at making people interested in science and DYI things, but his numbers about efficiencies - given that he doesn't seem to measure actual cycling - are often plain wrong or poorly explained.

A density of ~400 Wh/kg for these batteries is not true. The numbers in Li-Ion batteries are calculated based on the limiting power density side for the batteries - the weight of the cathode material - so Li-ion batteries actually exhibit these densities. The 400 Wh/kg he is referring to is based on the weight of the Zn anode alone, but the iodine side of the battery is much more limiting.

Using a 1% iodine solution - a lugol solution (1% I2 + 2% KI) - your power density is going to be extremely poor. At 0.07M, you are going to be getting 1-2 Ah/L, while the low end of LiFePO4 is ~150 Ah/L. The Zn-I chemistry isn't viable at all until you reach Iodine concentrations in the 4-6M range. Orders of magnitude more concentrated than a lugol solution.

Also bear in mind that the energy efficiency in a setup like he is showing is likely in the 10-15% region. The Zn is reacting with the iodine immediately after you put it in and the huge separation between electrodes and massive separator size and omhic resistance would imply huge losses. Besides just for demonstrating purposes, such a setup would never be viable for home storage, implying otherwise is very misleading.

I wish Robert would just measure a single charge/discharge cycle for the devices he proposes, so that people could see how far from anything actually viable they are. When you watch his videos it gives you the impression that we could all be building viable DIY rechargeable batteries in our houses with off-the-shelf chemicals. As you can see in all my threads, the devil is certainly in all the details.
 
It depends on the levelized cost of storage. If you can store 75% for a very low lifetime cost, it definitely beats storing 0%. If you're losing the energy anyway, anything you store for a low cost is a big win. You're not storing energy you would have used, but energy you wouldn't have done anything with if no storing was available. Storing 90% at a much higher cost per kWh might not be worth it.
Assume a $0.10/kWh daily cost differential between production time and use time. At 75% vs 95% conversion efficiency, that is a $0.02 differential per day between flow vs LiFePO4. $7.30/yr, or $109 over 15 years. DIY LiFePO4 is about $200/kWh to bus bar (wire, fuse, bms, etc). Flow would have to be 1/2 the cost to break even, and LiFePO4 prices are still dropping, while electric prices are increasing (making the lost power even more valuable).
 
Assume a $0.10/kWh daily cost differential between production time and use time. At 75% vs 95% conversion efficiency, that is a $0.02 differential per day between flow vs LiFePO4. $7.30/yr, or $109 over 15 years. DIY LiFePO4 is about $200/kWh to bus bar (wire, fuse, bms, etc). Flow would have to be 1/2 the cost to break even, and LiFePO4 prices are still dropping, while electric prices are increasing (making the lost power even more valuable).

An important advantage of flow batteries over normal batteries in general is that all parts of a flow battery are serviceable and repairable, batteries always need to be scraped and disposed of when their life cycle is over. A flow battery might only require the replacement of specific parts but you'll never need to just scrap all of it and start from scratch. This is why the levelized cost of storage of the flow batteries won't be just half of LiFePO4 but likely 10-20x lower.

Another important point is the current density. For LiFePO4 to sustain high efficiencies, it needs to be charged at low currents per cm2 (<10mA/cm2) larger currents will heavily lower the lifetime of the battery. Flow batteries can sustain really high currents, even at 80-120mA/cm2, so you can put a lot of power in or get a lot of power out of them quickly.
 
Not to hijack this thread, but I wonder how far one could be able to push the damp paper battery as a viable solution. If anything, that tech would be super DIY friendly and not too toxic.
 
Not to hijack this thread, but I wonder how far one could be able to push the damp paper battery as a viable solution. If anything, that tech would be super DIY friendly and not too toxic.
Not seen it, but I'll wager that he doesn't mention whether it's remotely viable.
He (quite skillfully) avoids letting users know if his suggestions are remotely viable.
I'm sure he's fully conversant with kW, kWh etc but I've yet to hear it across any of his projects. His unit of power is "wow those LEDs are really bright"
Most of his projects wouldn't charge a phone
 
A low cost solution to two typical energy storage problems are required:
1. Excess solar production during the day - needed to be stored for evening/night early next day use. Repeat daily.
2. Excess solar production during summer - needed to be stored for winter use. Repeat annually.

Most of the flow batteries I have seen demonstrated are best suited to 1. above, none seem suitable (yet) for 2.
With efficiencies of 70%, it would seem better to build transmission lines. Solar farms in the south can supply loads in the north - likely with better efficiencies/lower losses.
What is the round-trip efficiency of pumping Lake Ontario back into Lake Erie with solar during the day, and then using the existing hydro electric power plants at Niagara Falls/Buffalo to generate power after the sun goes down?
 
A low cost solution to two typical energy storage problems are required:
1. Excess solar production during the day - needed to be stored for evening/night early next day use. Repeat daily.
2. Excess solar production during summer - needed to be stored for winter use. Repeat annually.

Most of the flow batteries I have seen demonstrated are best suited to 1. above, none seem suitable (yet) for 2.
With efficiencies of 70%, it would seem better to build transmission lines. Solar farms in the south can supply loads in the north - likely with better efficiencies/lower losses.
What is the round-trip efficiency of pumping Lake Ontario back into Lake Erie with solar during the day, and then using the existing hydro electric power plants at Niagara Falls/Buffalo to generate power after the sun goes down?

Flow batteries are definitely better suited to 1. Flow batteries can have high CE and EE values (>95% and >90%) but those are much more costly to achieve. Currently such installations are at around 300 USD per 100 kWh, the lower EE ones using microporous membranes and none vanadium chemistries will likely be in the 50-75 USD per 100 kWh range.

In the end it's also a matter of whether the arbitrage is economical or not.

About pumping, the roundtrip efficiency of water pumping is usually around 70-80%.
 
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