THEORETICAL AND EXPERIMENTAL ANALYSIS AND OPTIMIZATION OF THERMAL CONDUCTION IN ELECTROCHEMICAL AND LATENT HEAT ENERGY STORAGE SYSTEMS

Abstract

Phase change materials (PCMs) are used commonly for energy storage purposes. Thermal energy can be stored in a PCM with a variety of configurations such as PCM slabs, cartesian and cylindrical finned systems. Heat transfer in PCM storage systems can be from a heat transfer fluid (HTF) or a heat source at constant temperature to adjacent PCM. The rate of heat transfer is limited by thermal properties. Fin insertion can improve the heat transfer inside the PCM. Chapters 2 and 3 derive and solve the governing energy equations to determine the transient temperature distribution in the PCM due to the presence of a fin in both Cartesian and cylindrical finned PCM systems. Results show existence of an optimal fin size which maximizes heat stored in the system. In addition, the analytical method is extended to determine heat transfer and melting front propagation in PCM slab systems due to convective heat transfer of flow past the slabs, which is a nonlinear and transient problem. It is important to treat the PCM-fluid flow heat transfer problem as a conjugate problem with convective heat transfer in the fluid and phase change heat transfer in the PCM. Heat transfer coefficient at the PCM-flow interface is a function of space and time where the heat transfer from a plate to flow past over a plate is due to either heat flux or temperature on the plate. Chapters 4 and 5 present an approach based on integral method to determine convective heat transfer from a plate into a fluid flow, where the boundary condition of heat flux or temperature on the plate is a known general function of time and space. Chapter 6 solves the conjugate problem of PCM-flow problem by implementing an iterative approach to determine transient convective heat transfer and temperature distribution on the fluid-PCM interface. The integral method showed in chapter 5 is used to solve the flow sub-problem. An approximate solution based on perturbation method solves the PCM sub-problem. An iterative approach integrates and solves these two subproblems. The effect of thermal properties of heat transfer fluid and PCM is investigated. This study improves the fundamental understanding of heat transfer in PCM storage systems and provides practical guidelines for design of such systems. Electrochemical energy storage in Li-ion cells is another key technology in energy storage purposes with multiple applications such as electric vehicles and consumer electronics. Li-ion cells are very sensitive to temperature. Chapter 7 investigates dual-purpose thermal management of a Li-ion cell using solid-state thermoelectric elements. Both cooling and heating of a Li-ion cell are demonstrated, with good agreement between experimental data and numerical simulation results which makes thermoelectric coolers an effective approach for thermal management of Li-ion cells at aggressive operating conditions. It is expected that the theoretical and experimental works presented in this dissertation will help in understanding heat transfer, design optimization and improving the performance of phase change energy and electrochemical storage systems.