Abstract: To enhance the energy utilization efficiency of concentrated solar power (CSP) systems, an integrated solar collector and thermal energy storage system based on Al-12%Si alloy is proposed to enable efficient heat transfer of phase change materials (PCMs) under simultaneous charging and discharging (SCD) conditions. To address limitations in previous studies, such as neglecting axial heat transfer and the low thermal conductivity of PCMs, a tubular latent heat storage unit with a large aspect ratio (H/D≈2.22) is designed, employing external radiation as the heat source and supercritical CO₂ as the cold source. Numerical simulations investigate the SCD phase change process of Al-12%Si starting from a fully solidified state. Analysis of solid-liquid distribution, temperature fields, and velocity fields reveals dynamic response mechanisms under varying heat fluxes. The melting process exhibits three distinct stages: an initial conduction-dominated phase driven by the high thermal conductivity of the solid PCM; a transitional phase characterized by strengthening natural convection and a shift from conduction to convection heat transfer; and a final phase where a stable, counterclockwise natural convection loop forms, significantly enhancing heat transfer efficiency. Heat flux critically influences the development of this large-scale convection: high heat flux generates sufficient buoyancy to overcome flow resistance in the high-aspect-ratio channel, promoting fully developed convection; low heat flux (20 kW/m²) results in insufficient buoyancy, stabilizing the system in a solid-liquid coexistence state. Field synergy analysis indicates that under high heat flux, the synergy angle distribution displays a symmetric "right-storage, left-discharge" pattern, achieving optimal synergy efficiency. The large aspect ratio structure effectively extends the natural convection path, improving alignment between velocity and temperature gradient fields and significantly enhancing overall thermal performance. The findings provide theoretical support and engineering guidance for integrating high-temperature PCMs in CSP systems and optimizing thermal storage unit designs.