Melting and solidification of phase change materials in metal foam filled thermal energy storage tank: Evaluation on gradient in pore structure

Solar energy as a renewable energy has sufficient development potential in energy supply applications, with the help of heat storage equipment that deals with its intermittence problem. To further improve melting/solidification efficiency, a novel energy storage tank filled by phase change materials with graded metal foams is proposed. Three gradient structures (positive gradient, non-gradient, and negative gradient) in porosity or pore density are designed. Three pieces of metal foam with the fixed porosity of 0.94 but varying pore densities of 15, 45, and 75 pore per inch is packed to form gradient in pore density design. For gradient in porosity, three selected porosities of 0.90, 0.94, and 0.98 are employed. A test bench for the phase interface visualization is set up and experiment on melting/solidification evolution and temperature are carried out. Achieving satisfactory agreement with experiments, numerical models are employed to explore the thermal features for phase change materials embedded in various kinds of graded metal foams during melting and solidification procedures. Results demonstrate that gradient design in pore density does not affect the melting and solidification procedures, while graded porosity helps notably the melting and solidification processes. Design on graded porosity reduces the complete melting time by 21.1% compared with the non-gradient structure. The global phase change process can be strengthened only if the melting is strengthened. The optimal gradient in porosity reduces the overall cycle of melting and solidification time by 10.9%, compared to the uniform filling pattern.