Designing the flow frames for 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
