Experimental and numerical study of heat transfer performance of nitrate/expanded graphite composite PCM for solar energy storage

Highlights: • Thermal conductivity of nitrate/EG composite was accurately measured by considering thermal contact resistance. • Heat storage and retrieval tests were conducted with binary nitrates and nitrates/EG composites. • A comprehensive model was built to interpret the heat transfer characteristics. - Abstract: Eutectic molten salt can be used as the latent thermal energy storage medium in solar energy applications. Nitrates and their binary mixtures are suitable phase change material (PCM) for solar energy applications in middle-temperature-range of 200–300 °C. In the present study, binary nitrate (50 wt.% NaNO₃, 50 wt.% KNO₃) with a melting temperature of about 220 °C was employed as the PCM, and expanded graphite (EG) with the mass fraction of 5%, 10% or 20% was used to enhance the thermal conductivity. The thermal conductivities of pure nitrates and nitrate/EG shape-stabilized composites were measured with a steady-state test rig firstly. Results showed that the addition of EG significantly enhanced the thermal conductivities, e.g., the thermal conductivities of sodium nitrate/20 wt.% EG composite PCM were measured to be 6.66–7.70 W/(m K) in the temperature range of 20–120 °C, indicating about seven times larger than those of pure sodium nitrate. Furthermore, pure binary nitrate and nitrate/EG composite PCM were encapsulated in a cylindrical storage unit with a diameter of 70.0 mm and a length of 280.0 mm. Heat storage and retrieval tests were conducted extensively at different heating temperatures of 250 °C, 260 °C and 270 °C, and different cooling temperatures of 30 °C, 70 °C and 110 °C. Time-durations from temperature evolutions showed that both the melting and solidification processes were accelerated by EG, and the heat transfer characteristics were interpreted by the numerical analysis based on enthalpy–porosity and volume-of-fluid models. The evolution of nitrate/air interface caused by volume expansion ascended gradually during melting, while that caused by volume shrinkage descended during freezing.