Rotationally staggered elliptical-fin architecture for thermal enhancement of horizontal latent thermal energy storage systems

• A systematic rotationally staggered elliptical-fin architecture enhances PCM melting. • Rotational elliptical fins provide over 62% time savings compared to circular fins. • XGBoost ML captures nonlinear thermal–geometric interactions with R 2 > 0.99 accuracy. • Multi-objective optimization identifies optimal radial–azimuthal melting at θ≈75–80°. • Optimal configuration delivers 84.6 % faster melting and a 7.1 % higher temperature. Tackling the heat transfer bottleneck in PCM-based storage systems is crucial for improving charging efficiency and overall thermal performance in latent thermal storage applications. In this context, the study presents a novel thermal enhancement strategy for horizontal double-tube latent thermal energy storage (HDLTES) systems applicable to solar thermal and HVAC buffer-load shifting. An innovative rotationally staggered elliptical fin configuration is introduced to synergistically integrate the effects of fin aspect ratio (AR) variation, systematic rotational offsets, and nano-PCM concentration. A finite element model is developed and validated against experimental data to investigate the coupled thermal–geometric interactions within the proposed configuration. An ensemble tree-based machine learning (ML) model using XGBoost regressor is developed using 490 simulated datasets to establish a global correlation describing the time evolution of the liquid fraction as a function of the rotational angle. Subsequently, a multi-objective optimization framework is formulated using response surface methodology with a composite central design (RSM-CCD) to identify the optimal HDLTES architecture in terms of rotational offset angle (θ), elliptical aspect ratio (AR), and nano-PCM concentration. The findings highlight that the proposed architecture enhances angular heat transfer, minimizes thermal shadowing, and ensures uniform radial–azimuthal melting, with θ ≈ 70°–80° yielding optimal coverage and the fastest, most uniform melting performance, whereas θ > 90° induces low-flux regions and delayed melting. It is shown that the synergistic integration of elliptical fins with the rotational configuration can reduce melting time by over 62 % compared to the circular fin case, and by 80.1 % relative to the finless scenario. Furthermore, the optimization identified optimal rotational angles of 76.9°–79.0° and aspect ratios of 1.93–1.98, depending on nanoparticle concentration, achieving an enhanced melting time of 84.6 % and a corresponding temperature increase of 7.1 % relative to the bare tube.