Numerical investigation of a phase change material including natural convection effects

Nowadays, Organic Rankine Cycle (ORC) is one of the most promising technologies analyzed for electrical power generation from low-temperature heat such as renewable energy sources (RES), especially solar energy. Because of the solar source variation throughout the day, additional Thermal Energy Storage (TES) systems can be employed to store the energy surplus saved during the daytime, in order to use it at nighttime or when meteorological conditions are adverse. In this context, latent heat stored in phase-change transition by Phase Change Materials (PCM) allows them to stock larger amounts of energy because of the larger latent energy values as compared to the specific heat capacity. In this study, a thermal analysis of a square PCM for a solar ORC is carried out, considering four different boundary conditions that refer to different situations. Furthermore, differences in including or not natural convection effects in the model are shown. Governing equations for the PCM are written with references to the heat capacity method and solved with a finite element scheme. Experimental data from literature are employed to simulate the solar source using a time-variable temperature boundary condition. Results are presented in terms of temperature profiles, stored energy, velocity fields and melting fraction, showing that natural convection effects are remarkable on the temperature values and consequently on the stored energy achieved.