@sepi, FYI, conventional all-iron RFBs at scale require a system to recombine the produced hydrogen from the negative side with the excess Fe (III) cations ("ferric") on the positive side. This reaction liberates protons to restore the acidity of the system and prevent hydroxide precipitation. This is, technically, a hydrogen-ferric fuel cell. However, it is another tricky system to implement, and can require catalysts (e.g. platinum). A lot of patents deal with this issue. ESS's "proton pump" is effectively this.
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Only Fe system -
Only Fe system@danielfp248 said in Only Fe system:
take it to the Nernst limit
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3D printed barbed connectorAwesome, nice work! These fittings normally come with the pumps I think? But I don't know if we specify them in the docs directly. Definitely good to have this as a backup/short-term solution!
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New Certified Open Source Hardware! -
Designing the large-format cellHere is a rough schematic of the test bench, showing where AC, DC, information is flowing:
The idea of using a DC-DC converter to source and sink current into a larger battery came from the OwnTech community. Charging a battery at low voltage (1 V) and high current (>1 A) is easy, you can just use a power supply, but discharging that while measuring performance is hard; you have to have large power resistors and shunts, etc. in lieu of a proper battery cycler (which is thousands of EUR). I had originally wanted to go straight to using the Twist as a DC-AC system, sourcing and sinking from the grid, but at a single cell voltage, we are not ready to do that yet.
Why I did not put pressure or flowrate sensors in:
I tried to find flow measurement devices but could not find something affordable that was satisfactory that could be used with zinc-iodide or similar electrolytes. Acrylic rotameters or glass/metal ones could be used (along with pressure sensors) in a flow loop of pure water to test pressure drop vs. flow rate. However, this seemed like a distraction away from making our first tests with the large-format cell with the zinc-iodide chemistry. We can always measure the approximate flowrate of the pumps by dispensing water through the cell into a bucket and weighing it after a fixed time period. FWIW, these pumps are supposed to be 5-6 L/min.
Another note on flow control:
In any case, in a real flow battery in the field, I am not sure whether it is a good idea to rely on explicit flowrate measurement in the control system. Maybe a flow switch, like “is there flow or not-→ output true or false”, to avoid turning the stack on without electrolyte inside, but not a sensor giving readings in L/min or similar. Flowrate sensors that are chemically compatible are expensive and another thing that can break.
What we care about is having sufficient mass transfer of electrolyte through the graphite felt (lowers the overpotential from mass transfer, improves efficiency), but not too much flow (reduces efficiency from needless pressure drop, improvement of mass transfer by pumping yields diminishing returns). If you measure the power consumption of the pumps (easy, cheap, and robust: a shunt or current clamp), and the current/voltage data from the stack (which is already available), a properly designed control algorithm should be able to determine the ideal flowrate (or close to it), without having to know the precise value of the actual flow. Control systems engineers to the front on this one, please!
All the parts for this rig have been ordered and are in the process of arriving. After @danielfp248 finalizes leak testing (already completed at the single cell level) with his large-format cell, he is going to send it to me. I will run the initial tests with the zinc-iodide electrolyte we optimized with the dev kit.
The funding for developing the large-format cell and the test bench needed to characterize it comes from the NLnet grant we received, we are very grateful for their support!
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Designing the large-format cellI am in the process of building the test rig for the large-format cell. It will also be able to handle larger area cells, as well as stacks, and cover us for a long range of development. Just to be clear, it will be an R&D system, not something aimed at end-use! I am using conservative initial engineering design, so it will be overbuilt a tad.
The rig will be capable of:
- starting/stopping, speed controlling, and monitoring the individual power consumption of two mag-drive polypropylene centrifugal pumps (designed for use in chemical industry, currently using 6 L/min models).
- circulating two independent fluid loops through >= 1L reservoirs and a large-format RFB cell (the one detailed already in this thread)
- Charge/discharging single cells and stacks (of at least several cells), using a DC-DC configuration for now (we will be discharging the flow cell into another DC device, like an old lead-acid battery - I have a lot of those where I'm at now). For now we will be using the OwnTech Twist for this, because it is OSHW and I know the developers, and so they have to help me debug it
- all the power electronics will be in a proper enclosure and DIN mounted, with an emergency stop
- temp monitoring of system: thinking of monitoring ambient, cell (somewhere on the current collectors?), the outside of each reservoir, the pumps (which have thermal protection, allegedly)
- remote monitoring of the system with a Raspberry Pi + webcam. This way we can have a leak alert as well as monitor the reservoir levels independently (it is hard to find a sensor that you can put in the tank that is chemically compatible - also, want to minimize hols in the tanks!)
- I am shoving the whole thing into secondary containment and adding a fume extraction system for safety during tests
This is a non-exhaustive list - I will keep this thread updated as I build it, and add a schematic so it's clearer.
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Designing the large-format cellBack with an update! Getting caught up:
@quinnale said in Designing the large-format cell:
Had a few thoughts that I hope are helpful or probing at least.
Thank you, I will take a look at that reference! A pressure-drop rig may be in our future...
@sepi said in Designing the large-format cell:
Wow, amazing news! Especially being able to power some real world device sounds almost unreal. I guess you didn't factor in the pump consumption though.
Aha yes, these pumps are at least a few watts each! However, they are capable of powering much larger cells/stacks that what we have here. In an optimized system they should consume no more than a 1-2% of round-trip energy efficiency, or so I've been told. We are a ways off from that, but this would still be a big step up for us in development, and we will have to learn/solve a lot of things... it is exciting though that it's starting to approach tangible power levels!
@danielfp248 said in Designing the large-format cell:
I sadly don't have a space where I could have that happen and then be able to ventilate that safely, nor do I have a spare room I could dedicate entirely to the project.
I have a suitable location for this which is isolated and away from living spaces; I will be carrying out the first cycling tests of the large-format cell there. For reasons pointed out, although Zn-I is a relatively safe chemistry compared to say all-V or Fe/Cr, large quantities of corrosive liquid are always a risk, plus the peculiarities of iodine (which, of course, I'd rather have to deal with than say, bromine!). Zn-I is relatively accessible and well-behaved, which is why we're using it, but we'd really like to figure out some other alternatives, which we are documenting here: https://fbrc.nodebb.com/category/6/electrolyte-development.
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Designing the large-format cellQuick video of Daniel's leak test: https://spectra.video/w/bSYUyYpJVr34N92261cd6K
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Designing the large-format cellThe current collectors and endplates as-received from SendCutSend in the US, laser-cut and milled, respectively:
Some more pics:
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My build (very slowly progressing)@czahl ah I think @danielfp248 would know this answer, he has flashed most of the firmware but I don't remember him saying he made any changes. He is traveling right now so it may take him a bit to get back to us. I have only used the MYSTAT, I haven't dealt with the firmware myself. If he made any changes then we can push them to the repo ASAP.