טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentHaimovich Nir
SubjectThermal Battery Modeling and Design
DepartmentDepartment of Chemical Engineering
Supervisors Professor Simon Brandon
Professor Dario Dekel
Full Thesis textFull thesis text - English Version


Abstract

Thermal Batteries are primary, one-shoot high-power density units consisting of serial-connected and thermally insulated cell-stacks. These batteries operate at elevated temperatures (300-700°C) necessary for sustaining molten salt within their porous electrodes and separators. Heat loss significantly impacts battery performance, as solidification of the electrolyte may cut the current, thus determining battery operation time. Thermal design is therefore of utmost importance, involving optimization of the geometrical details and materials employed. Within battery operation time, the electrical performance is associated with mass transfer and electrochemical phenomena within the cells, which in turn depends on temperature and therefore on heat transfer in the entire battery.

  This thesis is concerned with the development and application of a versatile thermal simulator for design and analysis of thermal batteries. The simulator is based on a transient two-dimensional model of heat transfer, enabling detailed simulation via the finite elements method, from the level of a single cell up to that of the entire battery. The thermal model rigorously accounts for heat conduction, electrolyte phase-change, heat of reactions and Joule-heating. The model is supported by an overall mass balance, involving the current drawn from the battery and a mass transfer resistance coefficient for each cell component. Simulator portability is enabled through a graphical user interface, facilitating computer-aided design and analysis. The simulator is supported by an extensive and upgradable material property library, consisting of temperature and concentration dependent data.

  Results include model validation as well as various case-studies of thermal battery designs, demonstrating the simulator robustness and flexibility at both small (0.1 A/cm2) and large (1.0 A/cm2) current densities. The significance of the phase-change process to heat transfer is revealed, as is its impact on battery operation time. Design insights and dominant processes affecting thermal performance of the batteries are unfolded, together with system level effects.

  A complimentary simulator, based on a detailed two-dimensional model involving  mass-transfer and electrochemistry, coupled with heat transfer, was also developed for rigorous electrical analysis and design of thermal batteries. This model involves both solid and liquid phases, diffusion-migration of ions, electro-neutrality, inter-phase polarization via Butler-Volmer kinetics, and phase change conditions. Model equations are solved using a numerical platform of the same features as the thermal simulator and yield prediction of cell and battery electrical performance. Sample model results, involving a single cell battery with a low current density, reveal that voltage decay throughout discharge is mainly due to changes in cell OCV.