Surrogate-based multi-objective optimization of a rotating triplex-tube latent heat storage system with eccentric and elliptical configurations

Latent heat thermal energy storage (LHTES) using phase change materials (PCMs) offers high energy density and nearly isothermal heat storage capability; however, the inherently low thermal conductivity of PCMs often results in long charging times that limit practical applications. Recent studies have shown that eccentric tube layouts and modified tube geometries in rotation-assisted PCM-based LHTES provide a relatively cost-effective and easily manufacturable method for enhancing heat transfer performance. The present study introduces two main novelties: first, the systematic characterization of the non-additive thermo-fluid interaction among middle-tube ellipticity, inner-tube eccentricity, and rotation under a constant PCM mass; and second, the optimization of these coupled geometric and operational parameters through a two-stage surrogate-assisted multi-objective framework. Sixty-four configurations with different eccentricity ratios (e/R b ), radius ratios (R s /R b ), and rotational speeds (ω) were numerically analyzed using the enthalpy–porosity method within a rotating reference frame. The analysis was restricted to the charging process of n-eicosane using a two-dimensional formulation with constant wall temperatures of 80 °C and temperature-independent selected thermophysical properties; axial and end effects, three-dimensional secondary flows, and solidification performance were therefore outside the scope of the present study. A surrogate-based multi-objective optimization framework combining a Gaussian radial basis function (RBF) surrogate model and a two-stage bound-refinement strategy employing the standard NSGA-II algorithm was then employed to simultaneously minimize charging time and maximize energy storage capacity. The results showed that rotational motion significantly accelerated the melting process, reducing the charging time by 3.83–79.33% compared with non-rotating conditions. Pareto-optimal solutions were concentrated at low rotational speeds (0.2–0.4 rpm), elliptical middle tube geometries (R s /R b ≈ 0.36–0.39), and relatively high eccentricity ratios (e/R b ≈ 0.36–0.375). Under the optimal knee-point condition, the charging time decreased by 70.74% with only a 0.6% reduction in energy storage capacity compared with the baseline case.