Generalized Simulation Models of Transport Processes within Sodium-Sulfur Batteries

With the ever growing demand for energy and drastic issues related to global warming, energy storage and production are becoming even more important issues.  The development of large capacity energy storage devices to be used in conjunction with renewable energy resources can help meet these demands.  Due to the cost of materials and time consuming testing procedures, development of new battery and fuel cell technology is a slow and expensive practice.  Recently, progress has been made regarding solid electrolyte materials for Sodium-Sulfur cells which will allow for significantly lower operating temperatures.  The purpose of this report is to present the capabilities of two models designed to aid in the technical design process of Sodium-Sulfur cells.  A zero-dimensional, transient, lumped model derived from an integral analysis of the transport of mass, energy and charge throughout the battery has been created. Additionally a 3-dimensional, unsteady, multiphase continuum simulation model for Sodium-Sulfur batteries using general electrochemical models has been developed. Together these tools aim to decrease the time and expense required to complete the design procedure.  Transport processes, including energy, specie and charge transfer, are simulated numerically by discretizing their respective equations.   The processes are coupled with the use of Faraday’s law of electrolysis and solutions for the species concentrations, electrical potential and current density are produced in a time marching fashion.  For the continuum model, a generalized grid generation strategy allows for meshing of square planar, circular planar and cylindrical cells with electrodes and electrolyte of varying thickness and mesh densities.  The model is also capable of performing simulations with unique geometries through the use of immersed boundary method.  Properties required for solving the transport equations, such as fluid density and ionic conductivity, are calculated and updated at each grid location as a function of time based on the volumetric content and phase of each species within the control volume surrounding each node.  Specie boundary conditions consist of both open and closed electrodes, which closely model certain fuel cell and battery designs respectively.  Electro-chemical boundary conditions can be as simple as a prescribed voltage or current density or more practical such applying circuitry including resistance, power consuming or power producing loads (for the purpose of charging).  The present model was developed for Sodium-Sulfur cells but could be adapted to study the behavior of seemingly any theoretical battery under given operating conditions with the use of small scale experimental results for model calibration and verification.  Such a tool could be used to expedite and reduce costs of the iterative design process of new energy storage technologies.