Cyclic performance of high-temperature magnetite rock bed thermal energy storage: An experimental study

The growing need for reliable, affordable energy storage underscores the importance of rock-based thermal energy storage (RTES) systems for high-temperature applications. Despite their potential, experimental studies systematically examining the cyclic thermal performance of magnetite-based RTES systems remain limited. Among natural rocks, magnetite is advantageous due to its high density, specific heat capacity, and thermal conductivity. These properties allow for faster heat transfer and greater energy storage than in other rocks. Additionally, magnetite maintains exceptional stability during repeated thermal cycles, ensuring long-term durability. This work includes several laboratory-scale charging and discharging tests to examine the cyclic thermal performance of a magnetite RTES system. The experiments were performed for 8 h, with equal charging and discharging time intervals in each cycle. Three charging and discharging times were considered: 4 h, 2 h, and 1 h. The inlet temperatures of charging and discharging modes were 500 ℃ and 50 ℃, respectively. Several cycles of temperature evolution, heat transfer rate, thermal efficiency, thermal energy index, and normalized thermal energy were examined to assess storage capacity and system stability. Additionally, the study provides new insight into the transient evolution of the dimensionless transport parameters, showing that high-temperature changes in fluid and solid transport properties progressively shift the flow from an inertia-dominated regime to a more viscosity-dominated regime. Although more energy is always stored during charging than is released during discharge, this difference gradually decreases as discharge recovery improves. Although further tuning is required to reduce early-cycle losses, the system exhibits stable cyclic behavior overall, with robust charging capacity and steadily increasing discharge efficiency. The thermal energy released during discharging in the 1-cycle charging/discharging mode is approximately 89.87% of the thermal energy received during charging (heat recovery). The second charging cycle in the 2-cycle charging/discharging mode stores around 21.04% less heat than the first. Furthermore, during the second cycle discharge of this mode, the released thermal energy is approximately 13.98% lower than in the first cycle. Additionally, the first cycle's heat recovery percentage is 84.84%, while the second cycle's is 92.43%. The charging operations of the second, third, and fourth cycles store about 7.69%, 13.36%, and 14.98% less heat than the first cycle in the 4-cycle charging/discharging mode. The discharge state in the fourth cycle releases approximately 0.98%, 7.29%, and 15.73% more heat than the discharge states of the third, second, and first cycles. It can be concluded that, over time, the RTES system reaches a steady state and the heat recovery percentage increases.