With increasing global interest in transitioning from old, expensive fossil fuels to clean energies, particularly solar energy, which is abundant in most regions of the world, the challenge arises of storing solar energy generated during the day for reuse at night. Therefore, many researchers have focused their efforts on developing and enhancing thermal storage systems for solar thermal energy. Phase change materials are considered the most suitable and least expensive when used in thermal solar energy storage systems, especially in shell-and-tube heat exchangers. In this study, the issue of latent heat storage in a shell heat exchanger was addressed using different tube shapes (tube, nozzle, and reducer), surrounded by annular fins. The main objective was to simulate the solidification process for the proposed models, which represent the state during which thermal energy is released into the phase change material during the night and stored during the day. The models were designed using SolidWorks software. CFD technology, employing finite volume and enthalpy methods, was utilized with ANSYS-Fluent 19 software for numerical simulations. The results obtained from the CFD predictions indicated that the variation in liquid fraction among the proposed models was minimal. The tube model exhibited a lower average liquid fraction than other models, suggesting faster solidification of paraffin compared to the others. In the upper part of the reducer model, where there is less paraffin, the temperature of the paraffin is close to the wall surface temperature, similar to the nozzle model. The reduction in solidification time achieved by the tube model was 25.7% compared to the nozzle model, while the reducer model delayed solidification time by approximately 16.3% compared to the tube model. Designing the heat exchanger with a tube is more suitable for the solidification process, whereas the nozzle is preferred for the melting process. There was a satisfactory agreement between numerical and published experimental findings.
