Graduation Semester and Year

2020

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Ankur Jain

Abstract

Theoretical understanding of heat and mass transfer processes in energy storage and conversion devices is of much interest for a wide variety of engineering applications. Two commonly used mechanisms for energy storage are electrochemical energy storage, such as in Li-ion cells, and phase change based energy storage, such as in phase change materials (PCM). Previous studies show that heat and mass transfer in both PCMs and Li-ion cells are critical processes affecting the performance and safety of these systems. This dissertation investigates several theoretical aspects of heat and mass transfer in these energy storage systems, with the goal of improving performance and safety. In the first part, this dissertation presents a solution for a one-dimensional phase change problem with any arbitrary time-dependent heat flux boundary condition using the perturbation method. The solution presented here is shown to offer key advantages both in accuracy and stability over past papers. The theoretical result is then used for understanding the nature of phase change propagation heat transfer for a wide variety of applications. The model is used to investigate phase change heat transfer including a pre-melted or pre-solidified length between the region of interest and a time-dependent temperature boundary condition. Such a scenario can occur in multiple engineering applications when the heating or cooling process is intermittent in time. Furthermore, the perturbation-based model is used to provide a theoretical understanding of how thermal conductivity and other thermophysical properties affect rate of energy stored (W) and energy storage density (J/m3) as two critical performance parameters of a system. Finally, the method is used to study phase change cooling of Lithium-ion cells. In the second part, this dissertation presents a heat transfer model to determine the core temperature of a Li-ion cell during thermal runaway using surface temperature and chemical kinetics data. The model presented here provides key insight into the internal state of Li-ion cells during thermal runaway. Later, mathematical modeling of species diffusion in Li-ion cell is carried out for improving performance and efficiency of electrochemical energy storage in Li-ion cells. Green’s functions approach is used to solve the solution phase and solid-phase diffusion limitations in composite electrodes operating under a time-dependent flux boundary condition. The mathematical models presented in this work are validated by comparison with past studies and numerical simulations. The Green’s-function based model is then used to present an analytical Single Particle Model (SPM) based model to predict the terminal voltage and consequently estimate the state of charge (SoC) of Li-ion cells operating under realistic time-dependent current profiles. The mathematical model presented here is compared against numerical simulations and past experimental data for different operating conditions. It is expected that the theoretical models developed in this dissertation will help in designing and improving the performance of electrochemical and phase change energy storage systems.

Keywords

Li-ion cells, Phase change materials, Thermal runaway, Energy storage

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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