Advancement in Solid-Liquid Heat Transfer and Latent Heat Energy Storage Applications

Solid-liquid phase change processes are encountered in a wide variety of fields including: energy, environmental, material processing and microelectronics. They are found in such applications as: latent heat energy storage systems (LHESS), temperature control and management. LHESS are used to store thermal energy through the melting of phase change materials (PCMs); this energy can be recuperated when the PCMs solidifies. This enables the temporal decoupling of thermal energy input and output useful for renewable (solar, wind) or waste heat applications for example. Also, through this phase change process significantly more thermal energy can be stored in PCMs compared to sensible heating of a substance like water. However, PCMs have very low thermal conductivities which can make storing or retrieving thermal energy from them a lengthy process unless proper design and control strategies are applied to enhance the apparent thermal conductivity or optimize the heat exchange process. Study of these systems involves research in heat transfer and fluid mechanics; and it has been shown by the applicant and others that both conduction and natural convection play a significant role in the overall phase change behaviour inside LHESS. However, most existing literature on the subject presents studies where only one heat transfer mode is considered, typically conduction, which places a complete understanding of the coupled processes still out of reach. Optimization and thermal enhancement of LHESS requires a complete understanding of the impact of natural convection and the melting/solidification heat transfer processes within the systems. To address these research deficiencies, the objectives of this research program are to: 1) study the fundamentals of solid-liquid phase change heat transfer to further the understanding of the impact of natural convection in the liquid melt on the overall phase change process in various system geometries and using various PCMs; 2) design and optimize LHESS for thermal storage for renewable energy and waste heat applications to counteract the low thermal conductivity of PCM and test under real-time solar thermal operations. Outcomes from this research will be a detailed and accurate understanding of natural convection during melting and strategies to enhance heat transfer during every mode of operation of LHESS (charging, discharging, simultaneous). Experimental results will serve to validate numerical models of phase change heat transfer prepared on commercial software used to facilitate the early adoption of the models by practicing engineers. Finally, a methodology to design the heat exchangers found inside LHESS to optimize their operation, and energy input and output rates as a function of the storage application will be developed. This is an important experimental and theoretical research area because proper understanding of these processes, and their application to energy and thermal systems, will facilitate the management of energy demand, use and production. For example, LHESS can be used to store thermal energy during off-peak electricity demand periods, reducing the amount of electricity produced to meet the demand during peak periods. Similar storage systems can be used to store solar energy in order to use it later in the day when the sun is down. As another example, small amounts of PCM can be incorporated into electronic devices to manage and delay temperature increase during intermittent operations. Our country, along with the rest of the world, will face an energy crisis in the decades to come. In that context, research in the area of thermal/energy engineering has the potential to lead to the development of new and vital energy technologies for Canada.