Using entropy generation as evaluating indicator of charging completion in a latent thermal energy storage system

To further promote the melting rate of latent thermal storage (LTES) system, the gradient porosity has been proved to be a feasible method. Entropy, as a long developed characteristic parameter to evaluate the irre-versibility of a system, can be a good measurement to estimate the completion of charge process of latent thermal energy storage system. In this paper, a representative elementary volume-scale (REV-scale) solid-liquid phase change lattice Boltzmann model is established to study the melting of PCM in a gradient porosity enhanced LTES unit. The entropy generation is realized and verified, and applied in LTES unit analysis. The result shows that, a bigger porosity gap of 0.82-0.98 and more addition of porous matrix below the system can better promote the melting and entropy generation. The stepped and linear porosity distribution reduce the melting time by 17.4% and 13.3% respectively. By researching the entropy generation, it is found that, the friction-caused entropy dominates the entropy generation during charge process. By studying the melting in different Rayleigh number, it is found that, the gradient porosity can only accelerate the melting in high natural convection conditions of approximately Ra > 2 x 106.