Optimization study of multiple-stage PCMs thermal energy storage system employing continuous topology optimization model

Using cascaded multiple-stage phase change material (mPCM) in latent thermal energy storage (LTES) system can increase the overall temperature difference. The spatial arrangement of high-conductivity fins and PCM in LTES units will strongly affect the heat transfer rate, and adopting the same fin scheme for independent each-stage unit has obvious limitations. This study establishes a heat transfer enhancement scheme by combining mPCM with optimized fins, and proposes to separately optimize each-stage fins in mPCM LTES system. A continuous topology optimization (TO) model framework is innovatively proposed to achieve the free evolution of fin structure embedded in PCM, thereby maximizing the charging/discharging efficiency. The enthalpy-porosity model is used to describe the solid-liquid phase transition. The implementation of continuous optimization for multi-stage fins in the TO model is based on the Parametric Sweep model of independent variables. The optimized design variable of the TO problem is formulated using the variable-density method. Firstly, the TO model for the 3-stage LTES system is established, debugged and verified. Secondly, specific optimal TO-fins for each stage PCM unit are obtained, and their evolution mechanisms, the inherent trade-off relationship between objective performance and structural differences are explored and compared. Thirdly, the excellent performance advantages achieved by TO fins are verified and the physical essence of performance improvement under the TO method is explored by comparing with traditional fin structure. The structural features of TO-fin can significantly shorten the heat conduction path and increase the area and intensity of local convection flow. Finally, the cumulative gain effect of 3PCM coupling specific TO-fin on improving the overall temperature difference and heat transfer rate is quantitatively analyzed. The heat storage process time for 3PCM LTES with specific TO-fin is reduced by 57.3 % compared to sPCM LTES with rectangular fins.