Designing the large-format cell
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For the "large-format" cell, we'd like to target as large of a geometric area as possible, and ideally have the flow frame be useable both for single-cell and stack testing. This means it needs to possess adequate internal fluid manifolds and flow diffuser/spreading geometry. We must also consider shunt currents once we progress to stack testing, so we'd like to design the cell with that issue in mind upfront.
We plan to start with a flow-through design, as it is much, much simpler to design and manufacture than flow field-based approaches.
This thesis has some helpful figures - I haven't read it yet myself, but it looks quite useful. The author is now a professor at University of Padua.
Basically we want to make our own version of this. Ideally we could prototype it with polypropylene FDM printing... but in any real application it would be injection molded.
A good image showing the path of a shunt current, which leads to a drop in energy efficiency as well as uneven current distribution (and possibly plating, for hybrid RFBs):
Image from University of Padua researchers: https://iopscience.iop.org/article/10.1149/MA2024-0217mtgabs
Right now, we need a flow frame that:
- Has the correct geometry for flow-through graphite felt electrodes (and possibly polymer felts/spacers, for hybrid chemistries)
- Doesn't leak
- Distributes flow evenly through the felts
- Offers adequate pressure drop and shunt currents when implemented in a stack
References
PhD thesis: https://www.research.unipd.it/handle/11577/3422708
Optimization paper: https://doi.org/10.1016/j.electacta.2021.139667
Flow field optimization paper (we don't have flow fields, but the simulation framework/procedure is interesting): https://linkinghub.elsevier.com/retrieve/pii/S0378775321009563
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We would like to design our flow frame using an open-source toolchain like we have the kit. I can make a rough design of the flow frame in FreeCAD.
We will need to do some FEM and fluid mechanics simulations on this design, in addition to shunt current calcultations.
For fluid FEM and pressure drop/flow distribution, we could probably use OpenFOAM. There is a plug-in workbench to run this directly in FreeCAD, I have messed around with it and gotten it working on my laptop. I can run simulations in less than a minute for the small cell barbed flow frame:
FreeCAD + OpenFOAM (CfdOF)
This is something I did up quickly for the dev kit flow frame (2 sq cm).
Minimum working example for CFD that produced the above simulation results: https://codeberg.org/FBRC/RFB-dev-kit/media/commit/215285a9ef93c7eaaf68583418111a83b9d7b0e7/CAD/CFD.FCStd
Mesh:
Flow distribution:
Pressure drop:
For shunt currents (and pressure drop too), we are probably best off manually calculating them first. They will depend on:
- Electrolyte conductivity
- Electrolyte viscosity
- Flow rate
- Manifold cross-section area
- Flow frame geometry
- and more...
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K kirk referenced this topic on
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Probably too far involved but this sort of approach could be cool, if it was constrained to manufacturable shapes:
We introduce Fireshape, an open-source and automated shape optimization toolbox for the finite element software Firedrake. Fireshape is based on the moving mesh method and allows users with minimal shape optimization knowledge to tackle with ease challenging shape optimization problems constrained to partial differential equations (PDEs).
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K kirk@social.coop shared this topic on
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I've just finished the CAD assembly of the first iteration of the large-format cell, which we will manufacture and test for leaks and other issues.
The repository is here.
Active area is currently 175 cm², and flow frames are hopefully printable on a 200 mm x 200 mm FDM printer. The flow frame is sized to fit on the printbed and still leave room for a healthy brim.
Manufacturing files in
CAD/exports.Filename Process/Material Quantity endplate-EndplateBolt Holes.step12 mm (1/2 in) plywood/MDF 2 outer-current-collector-Outer Current Collector.step1 mm brass 2 flow-frame-Flow Frame - Barbed, For Single Cell, Northeast.stlFDM / PP or ABS 1 flow-frame-back-Flow Frame (Back).stlFDM / PP or ABS 1 Gasket.svgTBD - thin <0.5 mm gasket material 3 Outer Gasket.svgTBD - thin <0.5 mm gasket material 1 Inner Current Collector.svg1 mm graphite foil 2 M10x50 bolts/nuts/washers 12 All flow connections are on one flow frame, which passes the other electrolyte through to the next half cell through internal manifolds.
There is some shunt current flow paths and flow diffuser geometry in place, but these are not optimized and mostly representative. We will see if this type of geometry is even printable/sealable before spending much effort on CFD/FEM/shunt current modeling.
The "back" flow frame doesn't have barbs, and connects through the barbed flow frames internal manifolds to receive/output flow. It's flow fields are mirrored versions of the other flow frame, and the backside of this flow frame is completely blocked.
For endplates, the design is a simple flat sheet. We will see how 12mm plywood holds up with a 12-bolt pattern. Eight of these bolts straddle evenly the four barbed flow ports. There is a tab cutout for the current collector connection. Instead of making the current collector tab longer, I decided to cut out the endplate slightly like this, so that the current collector is cheaper to make on services like SendCutSend (which calculates largely on rectangular part area, and brass >> plywood for these thicknesses). Technically, only need the large 20 mm port holes on one side of this cell, since all the barbs are on the same side, but to keep unique part count/costs down when ordering and simplicity for now, I haven't created separate port-hole-less parts yet for the endplate and inner/outer current collectors.
Gaskets and inner current collectors all largely follow the shape of the flow frames, with the
Outer Gasket.svghaving larger port holes to allow the barbs through.
I've also modeled the pump we're using:
The code to control the pumps using an Arduino Uno and thyristors is here, and a corresponding GUI based on the MYSTAT software is here.
I haven't written any documentation yet because things are still changing quite a lot right now - I'll keep posting updates in this thread as we progress!
