Theoretical Practical tank size limit
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I don't really get your point. Afaik what counts is the ratio between "charged" and "uncharged" ionic species, eg. I-/I3- or Zn/Zn2+. If that ratio is too low, the kinetics will be bad and voltage goes down. Since it's all about the ratio, this scales linearly with liquid volume.
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Oh, I think I understand your concern now. The question can be reformulate like this. Is the extractable energy (integral(iu) dt) the same for a volume V of fully charged and volume V of depleted electrolyte vs 2V of mixture of "half charged" electolyte. For that, you need to do some detailed modeling but maybe some simple modeling could give you a rough answer. My uneducated guess is that a very simple model will give you the answer that you get the same energy. A realistic model might give some discrepancies due to complicated losses etc. You would probably even need to model the pumps and the hydraulic circuit and shunt losses to get a good answer.
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The inability to use some volume of electrolyte at some given current density is irrespective of volume and will always be a fixed percentage of the available capacity at some given current. There is no fundamental limit of tank size due to this.
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The inability to use some volume of electrolyte at some given current density is irrespective of volume and will always be a fixed percentage of the available capacity at some given current. There is no fundamental limit of tank size due to this.
@danielfp248 So 2 small tanks with switching VS 1 large tank of the same volume wouldn't mater? Just use one big tank for each side?
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@danielfp248 So 2 small tanks with switching VS 1 large tank of the same volume wouldn't mater? Just use one big tank for each side?
@Vorg Exactly it doesn't matter. In both tanks you wouldn't be able to access some % capacity at some given current density, but that % would be the same for both tanks, no matter their size.
However, for the Zn/I system there is a coupling of the electrode area with the capacity, so you do need to have at least 1cm^2 per around 120mWh of capacity. This means that for a 1kWh system you would need to have at least 8333cm^2 of area. Our current large scale model has 169cm^2 per cell, so a Zn/I 1 kWh system requires a stack with around 50 cells.
A 10 cell stack, which would give ~12V and would therefore be a good match for solar power electronics, would have a capacity limit of around 0.25kWh.
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But a .25kWh battery is hardly worth messing with. Need at least in the 10's of kWh. Last month we used 1271.84kWh (Summer time with record temps 110+) which comes to ~43kW/day. On 7/10/25 we used 47.36kWh.
And you can only collect during the day. Assuming an off the wall number of 30% of useable daylight, that leaves you needing to store ~25-28kWh for night.I am confused, The big advantage of flow batteries is that the capacity is decoupled from the cells putting it in the size of the tanks. Need more kWh, use bigger/more tanks. Need more amp's, then you need more/bigger cells.
Oh, and solar with battery systems tend to use 48v because you need less amperage for a given amount of wattage.
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The Zn/I system is a hybrid flow battery meaning that on one side there is metal (Zn) deposition while loading similar to a Pb battery while on the other side all redox species (Iodine ions) are in solutions. The Zn deposition leads to capacity being couple to electrode area. This is however only a thing in hybrid flow batteries. True flow batteries don't have this issue. I guess the Zn/I chemistry is not necessarily meant to be the final chemistry being developed by fbrc. It has many positive points to be used as an educational tool thoug, amongst others it being relatively non-toxic, not so dangerous and easy to get chemicals.
Afaik, the quest for the perfect chemistry for home RFBs is still ongoing.
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The cell in itself can be used with other chemistries as far as I know. Also don't take my word for granted. You should speak to @danielfp248 and @kirk about their plans wrt. chemistries used. I think many aspects of the cell are independent of the chemistryand can easily be transposed to different systems. Others like the material used might be unique. But PP as material in touch with the electrolyte or the graphite felt electrode should work for quite a few other chemistries.
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@Vorg You can use the cell for true complete flow battery chemistries as well. We chose Zn/I as the first demonstration chemistry because it is a reliable and reproducible, lower toxicity chemistry that doesn't have many of the problems of Vanadium or Fe/Cr chemistries. While the metal deposition based chemistries like Zn/I, Zn/Br and Fe/Fe are not completely decoupled, they offer some crucial advantages over V and Fe/Cr, such as cost of materials. The coupling limitation is really not that bad when you consider that in real life capacity and power requirements are often coupled as well.
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But a .25kWh battery is hardly worth messing with. Need at least in the 10's of kWh. Last month we used 1271.84kWh (Summer time with record temps 110+) which comes to ~43kW/day. On 7/10/25 we used 47.36kWh.
And you can only collect during the day. Assuming an off the wall number of 30% of useable daylight, that leaves you needing to store ~25-28kWh for night.I am confused, The big advantage of flow batteries is that the capacity is decoupled from the cells putting it in the size of the tanks. Need more kWh, use bigger/more tanks. Need more amp's, then you need more/bigger cells.
Oh, and solar with battery systems tend to use 48v because you need less amperage for a given amount of wattage.
@Vorg To be realistic, it will be probably years before we get to capacities in the 10kWh region. The volume of electrolyte required to get to these capacities is dangerous, so a lot of additional safety requirements have to be met to try to get to these capacities. At the 25-30Wh/L of most viable chemistries (even completely true flow batteries like V or Fe/Cr) these means you will need 400L of solution. This amount of active material can be quite dangerous when charged so I wouldn't think about a system this large lightly.
Our project is in its infancy, although our aim is to get to the kWh scale, this will require a lot of time and effort. The good is that - contrary to what happens with companies that try to go there and fail due to commercial problems - our efforts are always going to be open source and the path we walk will be available for others to build on.
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But a .25kWh battery is hardly worth messing with. Need at least in the 10's of kWh. Last month we used 1271.84kWh (Summer time with record temps 110+) which comes to ~43kW/day. On 7/10/25 we used 47.36kWh.
And you can only collect during the day. Assuming an off the wall number of 30% of useable daylight, that leaves you needing to store ~25-28kWh for night.I am confused, The big advantage of flow batteries is that the capacity is decoupled from the cells putting it in the size of the tanks. Need more kWh, use bigger/more tanks. Need more amp's, then you need more/bigger cells.
Oh, and solar with battery systems tend to use 48v because you need less amperage for a given amount of wattage.
@Vorg Also for 48V you would need basically a 40 cell stack. In the Zn/I system, this would have a max capacity of 1kWh with 50L of total electrolyte volume (assuming 20Wh/L). This would supply a current of around 7A. So it would work at a power of around 336W.
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I was hoping there was something here. Lithium batteries cost way too much tripling or more the cost of a solar system and are way to dangerous with their high fire risk. But it really takes a battery to make solar worth doing because it averages out the load to make better use of the input even when not looking for extending through the night. I Just saw a 12kW split phase (we use 240 split phase in the US) hybrid inverter for under $500 which is cheep. Solar panels are also getting down if you look. But the battery is still a project killer.
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I was hoping there was something here. Lithium batteries cost way too much tripling or more the cost of a solar system and are way to dangerous with their high fire risk. But it really takes a battery to make solar worth doing because it averages out the load to make better use of the input even when not looking for extending through the night. I Just saw a 12kW split phase (we use 240 split phase in the US) hybrid inverter for under $500 which is cheep. Solar panels are also getting down if you look. But the battery is still a project killer.
@Vorg We will slowly get there but no, right now we don't have any kWh capacity battery. As I mentioned, it will be several years before we get there. The fun is to walk this path and make everything open source on the way.
