Flow Battery Research Collective

@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.

@danielfp248 said in Only Fe system:

take it to the Nernst limit

https://www.youtube.com/watch?v=9D-QD_HIfjA

Awesome, 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!

It's official now!

Here 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!

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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!

I 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.

Back 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.

Quick video of Daniel's leak test: https://spectra.video/w/bSYUyYpJVr34N92261cd6K

The current collectors and endplates as-received from SendCutSend in the US, laser-cut and milled, respectively:

Some more pics:

@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.

This is possible, but we haven't tried. We've been able to seal it with silicone gaskets so there's been no need. Also, if one of the gaskets gets damaged, it's simple to replace, unlike resealing the flow frame. But there's no reason you can't try!

"Ironing" the top layer of the print in your slicer may also make it smoother, or manually sanding, but again we haven't found it necessary to try this

Also actually @danielfp248 made all the code improvements to the repository for MYSTAT, he just dislikes Git and so I'm the one who uploads them

I am less familiar with the MYSTAT hardware/firmware, that repository is a fork and you can see the original developers efforts much before mine! Just to say I'm not super familiar, @danielfp248 programed mine and sent it to me in the mail

Yes, the main PSU needs to be connected for current measurents to function.

Also a warning, make sure the MYSTAT is completely powered off and unplugged before inserting/removing the measurement cable to the MYSTAT, otherwise there is a risk of damage.

Are you following the calibration procedure from the original paper with a (IIRC) 1k resistor?

Yeah, nice work @czahl ! And another +1 for a drag knife being more appropriate than milling, but, it's worth giving it a go anyway! Laser cutter also possible, for gaskets, but no guarantee on the safety of fumes. IIRC cutting grafoil with a laser is possible only with a fiber laser, not CO2.

@sepi @czahl @danielfp248 I have enabled the chat plugin just now! With min. user reputation of 2 to try to prevent spam.

Public and private chat groups/PMs now possible, though I don't think it's encrypted if that's a concern (we are hosted by NodeBB).

@muntasirms oh this looks great, I hadn't seen the paper! It's exactly what we need for the non-shunt-current related issues (which are another bridge for us anyway, down the road). An efficient manifold seems like it could be decoupled from the shunt current protection scheme also. I will try to implement it in FreeCAD.

Some preliminary CFD of the simplified flow frame (U in m/s and P in Pa if I understand OpenFOAM correctly)

Conditions

• 4 L/min volumetric flowrate through one half-cell, inlet is on the top left, outlet on the lower right.
• Ambient pressure on cell outlet
• No-slip wall
• File containing CAD and CFD simulation setup is here
  Blue is inlet, red outlet, pink is porous zone
  !

Close-up of mesh:

Flow Distribution (m/s)

Pletcher and Walsh say a range of 0.05-0.4 m/s linear velocity is a good design range for electrolyte flow, if I apply a smaller range for velocity with 0.05 m/s as the upper limit, we see which areas in red have sufficient flow and where the dead zones are (in the corners, predictably)

Pressure Drop (Pa)

Big Caveat

Still need to calculate the Darcy-Forchheimer coefficients to do the porous zone simulation in the graphite felt, right now I am using the default values, which are almost certainly not correct. If anyone feels like finding that data (https://openfoamwiki.net/index.php/DarcyForchheimer). I think Antoni Forner-Cuenca's group has measured a lot on this recently. This could change the results quite a bit as far as flow distribution and pressure drop. I've mostly so far just been getting familiar with the simulation pipeline in FreeCAD --> CfdOF --> OpenFOAM --> ParaView.

Design is far from final, and I'm probably doing the CFD incorrectly, BUT it prints and doesn't leak! We will keep optimizing the flow frame later.

@czahl said in Blog: czahl's build:

the new update from @kirk where the inner diameter is increased from 2 to 3mm.

Also good catch, @danielfp248 has tested this already and it results in smoother flow/less pressure drop in the system without introducing new leakage it appears.

Nice progress, sorry that PP printing is such a pain, but if it's any consolation, this is the norm when finding a setup that works. We really hope to find a DIY-friendly chemistry soon that could work with ABS or other easier-to-print plastics!

Hi all,

It's been very exciting to see the FBRC community grow these past months! We have come a long way from just two people tinkering in their respective apartments. If we are going to develop a practical battery technology in this way, having a strong open-source community will be crucial.

So far, all the interactions here have been positive to my knowledge. We've only had one blatant spam post so far. A few people I spoke with recommended adopting a code of conduct, such as the well-used Contributor Covenant. I have uploaded it to the FBRC website here and added the contact info for the current mod team (right now, me and @danielfp248).

I'm not adding this in response to any recent incident, rather, I think it's good to have it established and in-place as the community continues to grow.

If you have any comments or feedback on this, please feel free to discuss here! It's not a static document; we can change it as we see fit.

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TL;DR: Don't be a jerk, let the mods know if someone is causing trouble, and let's make open-source batteries happen!