Accelerating phase change in latent heat thermal storage with flat-tube geometry

Latent heat thermal energy storage systems can enhance energy flexibility and efficiency in applications such as renewable energy integration and industrial waste heat recovery. However, their performance is constrained by the low thermal conductivity of phase change materials, which limits heat transfer rates and slows thermal response. To address this limitation, this study investigates the influence of inner tube flatness on local thermal behavior and system-level performance in a shell-and-tube latent heat thermal energy storage system. A three-dimensional transient conjugated heat transfer model was developed and validated using experimental measurements from a laboratory-scale system filled with paraffin-based phase change material. To examine the influence of inner tube shape, three configurations were tested under the same phase change material volume. The configurations were a circular tube and two flattened tubes with long axes of 10 mm and 22 mm, called Flat-10 and Flat-22. The Flat-22 configuration enhanced melting and solidification by increasing the heat transfer area and rapidly growing the melted phase change material layer thickness, which intensified buoyancy-driven convection and improved conductive heat transfer. Despite a higher pressure drop, the Flat-22 design reduced pumping energy by approximately 55% in charging and 39% in discharging and shortened the phase-change duration by 55% and 44%, respectively, resulting in coefficient of performance improvements of 117% and 61% relative to the circular baseline. These results highlight tube flatness as a critical factor for enhancing phase-change rates and system efficiency, offering design insights for compact, scalable latent heat thermal energy storage systems.