Performance of solid particles as thermal storage media in thermal power flexibility retrofits: Effects of charging and discharging flow rates on single piece stacking bed

An innovative, efficient, and cost-effective energy storage method was introduced for retrofitting thermal power plants, which can be connected in series to scale up quickly (like a battery) and aimed to replace expensive, hazardous molten salts. Charging/discharging processes among steam and solid particles were investigated using energy storage devices with capacities in the tens of kilowatts. Results of the study confirm the excellent performance of steel waste as a thermal energy storage medium. Key findings include that the time required for complete charging decreases from 75 to 300 kg/h, taking 173, 128, 106, and 105 min, respectively. The most efficient charging occurs when the bed temperature is most inhomogeneous, taking 68, 42, 34, and 28 min for 75–300 kg/h. The highest average charging power and heat transfer coefficient are 5 kW and 210 W/(m2·K) at 300 kg/h. In addition, 145 kg/h achieves optimal speed and quantity coupling, resulting in the highest charging efficiency of approximately 84 %. Time-optimized and efficiency-optimized charging flows are given. Real-time efficiency shows that the poorest temperature uniformity corresponds to the highest charging efficiency, with 145 kg/h exhibiting the strongest thermal inertia and 75 kg/h the weakest. Higher flow rates improve internal heat transfer and reduce stratification, facilitating multiple charge-discharge cycles. At discharge rates of 50, 75, and 100 kg/h, steam production is sustained at approximately 30, 40, and 50 kg/h for 11, 7, and 7 min, respectively. Discharge efficiency increases with flow rate, rising from 30 % to 95 %. Phase transition region migrates at approximately 10∼20 mm/s across different discharge rates, with 75 kg/h showing the fastest discharge state. As the discharge rate increases, the phase transition region's total area proportion rises, reaching about 70 % at 100 kg/h. Research demonstrates the potential of this new energy storage medium for efficient and safe retrofitting of thermal power plants.