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Due to the non-uniform heat transfer process of phase change materials, a gradient metal foam structure is designed with varying porosities from inner to outer regions to enhance heat transfer in horizontal phase change energy storage units under rotational conditions. Numerical simulations use the enthalpy-porosity method to verify a numerical model of solid-liquid phase variation under rotation. The solidification characteristics of different gradient metal foam structures are compared and analyzed through an orthogonal test. Results indicate that a metal foam structure with a positive porosity gradient from inner to outer regions significantly improves heat transfer efficiency and uniformity compared to structures with uniform or negative porosity gradients. Specifically, the solidification time of Case 1 with a 0.97–0.98-0.99 porosity gradient foam combination is 12.22 % and 43.60 % lower than that of Case 3 with a uniform foam structure and Case 5 with a negative gradient foam combination, respectively. Furthermore, the mean heat release rate and temperature response are increased by 15.07 % and 18.20 % compared to Case 3, and by 81.82 % and 92.90 % compared to Case 5. The orthogonal experiment demonstrates that the porosity combination has a greater impact on double optimization objectives than PPI, with no interaction between the two factors. The optimal structure identified is a porosity combination of 0.97–0.98-0.99 with PPI = 50, showing the highest mean heat release rate and the shortest solidification time.