Abstract
Ocean thermal energy conversion (OTEC) thermal engine have the advantage of providing continuous power and are therefore regarded as an attractive energy source for underwater vehicles. However, their operating frequency was constrained by the inherently low melting rate of the internal phase change material (PCM). Conventional enhancement using metal foam could accelerate PCM melting, but it markedly increased the structural weight of the device and could even lead to higher parasitic energy consumption. To address this challenge, the present study investigated a radially layered porosity strategy for copper foam. Through combined experiments and numerical simulations, the temporal and spatial evolution of liquid fraction and temperature under different gradient directions and gradient magnitudes was examined, and the temperature uniformity, power output, and power-to-weight performance were evaluated. The results showed that a lower porosity led to a faster melting rate; specifically, the copper-foam configuration with a porosity of 0.8 reduced the complete melting time by 48.7% compared with pure PCM. Both positive and negative graded configurations improved the temperature uniformity by 15.4–42.4%. Finally, although the low-porosity layer provided a higher effective thermal conductivity, it weakened natural convection and thereby suppressed the melting process, whereas a moderate positive porosity gradient maintained a balance between self-weight and thermal performance. This work thus offered useful insights for the development of high–power-density OTEC thermal engine.