This work aims to contribute towards enhancing the melting process of phase change materials (PCMs) inside an innovative thermal energy storage (TES) unit, which is established based on two concepts of triplex-tube and helical-coil. To appraise the performance of the proposed TES unit, named the triplex-tube helical-coil TES unit, a three-dimensional numerical simulation is developed. After validation and verification of the numerical model with available experimental and numerical data in the literature, a comparative qualitative and quantitative study is made between the proposed TES unit and conventional vertical and horizontal straight triplex-tube units as well as double-tube helical-coil one. Afterward, the effects of various geometrical parameters such as coil pitch, diameter, and inclination on the melting process are analyzed. Considering an identical volume of the storage zone, the results show a significant reduction in melting time by using the new design, where this unit shortens the charging time by nearly 80.4% (52.5 min), 44.7% (10.35 min), and 16% (2.24 min), respectively, compared to the double-tube helical-coil, vertical, and horizontal straight triplex-tube TES units. Furthermore, the triplex-tube helical-coil unit with the tilt angle of 0◦ reflects the best performance among other studied inclinations, and the variations of coil pitch and diameter have negligible effects on melting time. Finally, to accelerate the melting rate, the influence of adding three different nanoparticles, including TiO2, Al2O3, and CuO to pure PCM with various volume fractions are studied. The results show that the maximum melting time reduction belongs to the Al2O3-PCM with a volume fraction of 4% by 13.3% compared to the pure PCM.

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