To achieve recovery and utilization of low-grade waste heat, we proposed a phase change thermal storage unit incorporating “tree-branch fins + foam metal”, and co-optimized its structural and material parameters using response surface methodology and orthogonal experiments.

Under typical operating conditions (80 ℃, 0.005 m/s), tree-branch fins reduced the melting time of the phase change material (PCM) by 10.7% compared to straight fins, with optimal parameters including a length ratio of 1.143, a width ratio of 1.057, and a branching angle of 68.987°. Regarding system structure, the counterflow arrangement of dual heat exchanger tubes decreased the melting time to 558 s, and further filling with foam metal shortened it to 181 s. Although increasing porosity enhances the proportion of PCM, it weakens the structural support of the metal framework, thereby prolonging the melting time.

In conclusion, the integration of tree-branch fins and foam metal significantly improves the thermal storage performance of phase change thermal storage devices. Subsequently, staged thermal energy storage can improve the efficiency of heat storage. The optimal configuration for a three-stage cascade is put forward: Stage I: 0.9 porosity nickel foam + n-octadecane; Stage II: 0.5 porosity nickel foam + n-octadecane; Stage III: 0.5 porosity nickel foam + stearic acid PCM.

It can provide an integrated design basis for the efficient capture and reuse of low-grade waste heat at 50~60 ℃.