Oriented gradient design of fluid-coupled graphite-based phase change composites for rapid and uniform latent heat storage

Extensive research efforts have been devoted to developing graphite-based phase change composites (PCCs) with high thermal conductivity for high-power-density latent heat storage. However, when integrated with heat transfer fluid, uniformly designed graphite-based PCC blocks always suffer from spatial non-uniformity of melting, which results in a sluggish melting region (known as “dead zone”), suppresses the heat storage rate and deteriorates temperature uniformity. We herein propose a novel design strategy for graphite-based PCCs with gradient thermal conductivity, realized by a facile infiltration of molten PCMs into prefabricated expanded graphite (EG) frameworks with gradient porosity distribution. Experimental melting tests are conducted to validate the numerical model, which is subsequently employed to comprehensively investigate the melting characteristics of gradient EG-PCCs. Comparative analysis reveals that the linear gradient design outperforms both step-like and uniform designs, and the optimal linear gradient demonstrates a 41.7% enhancement in final stage heat storage rate and a 24.6% improvement in temperature uniformity. Furthermore, a systematic parametric optimization is conducted to evaluate the effects of four critical parameters (thermal conductivity, aspect ratio, inlet temperature difference, and flow velocity) on the optimal gradient distribution. Based on this, a dimensionless design correlation is formulated via regression analysis, serving as a quantitative tool for predicting the optimal gradient distribution under given parameters. This work offers a new route and methodology in design fluid-coupled graphite-based PCC systems for accelerating latent heat storage.