(636d) Fundamentals for Designing Flow Batteries

Selected flow battery systems will be used to present the fundamentals of and advances in electrochemical engineering design.  Applications of these design principles will allow assessment of performance, degradation, and opportunities for various load cycles.  The interaction of economics with the principals of thermodynamics, kinetics, mass transfer, and potential distributions will be discussed.  In the spirit of a tutorial, analogies will be drawn between common chemical engineering concepts and electrochemical engineering principles.  The goal is to provide the basis for an understanding of the limitations and the advantages of flow batteries.  Examples of the use of experimental designs to isolate limiting behavior and opportunities for improvement will be discussed.   Opportunities for improvements using commercial computational fluid dynamics packages will also be discussed.

The fundamentals are introduced with a simple algebraic model for a hybrid flow battery and stack that uses zinc-bromine chemistry. Performance is shown to be lost though subtractions from the thermodynamic voltage by ohmic losses which can be a function of geometry, by columbic efficiency losses which can be a function of separator design, and by voltage efficiency losses which are related to kinetic, mass transfer in the cell, and shunt currents in the stack.  A review of more sophisticated models  and the opportunities for cell and stack improvement will be discussed. 

Then the paper will discuss the necessary concepts for extension to a complete flow battery with an example of replacing the zinc negative electrode with hydrogen.  Novel designs and limitations are discussed for this system with a brief review of porous electrode theory.  The economics of the system are discussed components as well as the interactions of the effects of geometry, component selection, and auxiliary power and energy requirements.