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Thermal energy storage has attracted more and more attentions due mainly to its ability of peak load shifting. Shell-and-tube configuration is a typical heat exchanger for thermal energy storage. To enhance phase change heat transfer, open-cell metal foam has been involved in various kinds of shell-and-tube heat exchangers. To further improve the overall thermal performance of a shell-and-tube heat exchanger, metal foams with gradient porosities were inserted into the shell side. Positive and negative gradients in porosity were studied for comparison. Numerical model was developed based on the finite volume method and three sets of numerical simulations were performed. Transient melting front and melting fraction were illustrated for comparison. Results demonstrated that the positive gradient in porosity outperformed the other two kinds of configurations, resulting in a 17.5% reduction in full melting time.