Asymmetric melting–solidification in horizontal latent thermal energy storage: Impact of fins location and of natural convection

This paper presents a theoretical-numerical investigation of melting-solidification cycles in a horizontal latent thermal energy storage (LTES) system enhanced by metallic fins and natural convection. The storage is made of a double rectangular cavity filled with octadecane, separated by a central heat transfer fluid channel. Four fin arrangements are analyzed: no fins, fins in the upper cavity, fins in the lower cavity, fins in both cavities. The finite element numerical model is based on the apparent heat capacity method for phase change, accounting for conduction-convection interactions. The results highlight that melting is dominated by natural convection in the upper cavity and conduction in the lower one, while solidification remains largely conduction-controlled, emphasizing the strong asymmetry between melting and solidification driven by buoyancy effects. Adding metallic fins significantly enhances the phase change processes; yet their effectiveness strongly depends on their location. Fins in the upper cavity accelerate local melting but do not reduce the total cycle time, because it is governed by the lower cavity. Conversely, fins in the lower cavity substantially improve overall performance by accelerating the limiting phase change process. The configuration with fins in both cavities yields the shortest cycle time. A comparison between full-cycle and partial-cycle strategies shows that early switching between melting and solidification significantly reduces the cycle duration but comes with a reduction in total heat exchanged. The presented outcomes provide physical insight into the impact of fins placement and operating strategies, offering guidelines to design more efficient and cost-effective LTES systems.