The present study proposes the phase change material (PCM) as a thermal energy storage unit to ensure the stability and flexibility of solar-energy-based heating and cooling systems. A mathematical model is developed to evaluate the PCM melting process, considering the effect of nanoparticles on heat transfer. We evaluate the role of nanoparticles (Al2O3-, copper- and graphene-based nanofluids) in enhancing the performance of the melting process of phase change materials. The results show that natural convection due to the buoyancy effect dominates the flow behaviour even in the initial stage of the PCM melting process. High natural convection at the bottom of the annular tube moves the melted PCM upward from the lateral, which pushes the liquid-solid interface downward. The addition of 3% vol Al2O3 nanoparticles boosts PCM melting performance by decreasing the melting time of PCM by approximately 15%. The comparison of Al2O3, copper and graphene nanoparticles demonstrates that higher thermal conductivity, ranging from 36 to 5000 W m-1 K-1, does not contribute to a significant improvement in the melting performance of PCMs.
Keywords: PCM; energy storage; heat transfer; liquid-solid interface; melting process; nanofluid; nanoparticles; natural convection; phase change material.