Effect of Porosity Gradient on the Solidification of Paraffin in a Thermal Energy Storage Tank Filled With Metal Foam

Thermal energy storage (TES) systems are a promising solution for reutilizing industrial waste heat (IWH) for distributed thermal users. These systems have tremendous potential to increase energy efficiency and decrease carbon emissions in both industrial and building sectors. To further enhance the utilization rate of industrial waste heat, optimizing TES systems has attracted significant attention. This study explores the solidification process of a vertical shell-and-tube TES unit with the annulus filled with a composite phase-change material (PCM) comprising paraffin and copper foam. Numerical simulations are employed, which are verified by visualization experiments of the TES unit. To improve the thermal performance of the unit, porous media with nonuniform parameters is implemented. Nonuniform pore structures, featuring radially varying gradients (positive, i.e., porosity increasing in the positive radial direction, and negative, i.e., porosity decreasing in the positive radial direction) that are oriented perpendicular to the flow direction of the inner tube, are compared to uniformly dispersed pore structures. Results indicate that, compared to the uniform structure, the utilization of the positive gradient shortens the time to complete solidification by 15.9% while simultaneously increasing temperature uniformity by 14.6%. In contrast, the negative gradient results in a 5.7% increase in complete solidification time and a 31.0% decrease in temperature uniformity. The optimum gradient porosity combination (0.87-0.94-0.97) is obtained by the response surface method to optimize the structural parameters of the radial gradient porosity.