<p>The partially filled metal foam (MF) strategy improves the low heat transfer efficiency of phase change material (PCM) while remaining cost-efficient compared to the full-filled MF. To explore the optimal distribution of the partially filled MF, this study investigates a rectangle latent heat thermal energy storage system with different filling parameters. The melting performance, including the liquid fraction variation, temperature field, and the thermal energy storage efficiency, is investigated. The results reveal that the addition of MF suppresses natural convection while greatly enhances the heat conduction, leading to a more uniform distribution of temperature, velocity, and liquid fraction in MF-PCM region. Then, the effects of the displacement and filling ratio of MF are analyzed. The results show that the displacement mainly affects the intensity of natural convection which tends to be more pronounced in the upper region. The decrease in melting time in MF-PCM region and full region can be up to 36.3% and 69.2%, respectively. Compared to the full-filled case, 75% reduction in the mass of MF increases the full melting time by 47% in MF-PCM region and 263% in full region, respectively, but the uniformity of temperature and the amount of total thermal energy storage is better. Besides, the thermal energy storage rate is more sensitive to the displacement than the mass of MF in partially filled cases, and the maximum deviation between different cases can be up to 228.4%. Through comprehensive consideration, the optimal displacement and filling ratio of partially filled metal foam is determined.</p>