Thermal performance, mechanism analysis, and engineering applications of a Sandwich-structured single-tank molten salt thermal energy storage system

Conventional double-tank molten salt heat storage systems suffer from high cost and poor adaptability to small-scale intermittent waste heat recovery. To address these issues, this paper proposes a single-tank shell-and-tube system with a sandwich structure and cross-tree fractal fins based on the Fibonacci sequence. Both experimental and numerical methods are employed to investigate the thermal performance and heat transfer mechanism. Compared with traditional fins, the bionic fins increase the average molten salt temperature by 8 °C and the liquid fraction by 10 percentage points at 3000 s during melting. Based on the experimental and numerical results, a two-scale dimensionless parameter analysis is conducted to reveal a decoupled heat transfer mechanism: before the knee point, radial heat transfer has crossed the molten salt layer multiple times, while axial natural convection dominates temperature homogenization, enhancing axial heat transfer by approximately 150 times compared with pure conduction. New empirical correlations for the knee point time and temperature difference are established, providing a simple yet accurate tool for predicting the conduction-to-convection transition. The proposed system is optimized through response surface methodology and applied to a mobile storage vehicle whose discharging performance is studied under the effects of cooling water temperature and flow rate. Finally, 2560 storage units are coupled with an Organic Rankine Cycle, achieving an average thermal efficiency exceeding 12.9% under optimal conditions. This study provides a reference for compact molten salt storage in small-to-medium waste heat recovery.