Accelerated solidification process in a compact helical thermal energy storage system using two-pass flow and axial fins

• Two-pass helical design boosts PCM solidification efficiency. • Configuration suits compact, fast, and reliable TES applications. • HTF velocities from 0.1–0.3 m/s assessed for discharging performance. • Finned system ensures more uniform and stable PCM solidification. • Axial fins improve radial conduction and cut solidification time. • Fins reduce solidification time by up to 15.7% compared to no-fin case. This research conducts a numerical investigation into discharging (solidification) performance of a helical two-pass coaxial thermal energy storage (TES) system using water as the heat transfer fluid (HTF) and a phase change material (PCM) for latent heat storage. The system comprises three concentric cylinders in a compact helical configuration. The HTF flows upward through the inner tube and returns downward through the outer annulus to enhance heat exchange with the PCM. Simulations were conducted for various HTF inlet velocities (0.1–0.3 m/s ), both with and without axial fins embedded in the PCM domain. Results show that the two-pass flow path and helical geometry enhance PCM solidification by increasing the HTF–PCM contact surface and inducing secondary flows. Without fins, solidification times for HTF velocities of 0.1, 0.2, and 0.3 m/s were 660 s , 498 s , and 420 s , respectively. The addition of axial fins reduced these times by up to 15.7% because of the improved radial thermal conduction. Temperature and liquid fraction distributions show faster and more uniform solidification in the finned case. The existence of fins also stabilizes HTF outlet temperatures, promoting consistent thermal output. This configuration is particularly effective for TES applications requiring compact design, fast discharging, and reliable heat delivery.