Redox cycle of calcium manganite for high temperature solar thermochemical storage systems

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Thermochemical energy storage via metal oxide redox cycling is a potential cost-effective approach to store solar energy both chemically and thermally at high temperatures, enabling efficient solar thermal power production round-the-clock and on-demand. Perovskite manganites are a low-cost redox material that undergoes reversible reaction without side reactions up to 1000 °C and offers high storage capacities, which makes them promising energy storage materials in next-generation solar thermal power plants. In this study, the thermal properties of a typical perovskite manganite, CaMnO₃, has been studied. CaMnO₃ shows 893.2 kJ/kg energy storage capacity from 200 °C to 1000 °C, and 0.133 non-stoichiometry in N₂. The material exhibits excellent stability with 150 redox cycles between 500 °C and 1000 °C. The thermodynamic efficiencies of two solar-driven combined cycle power systems with CaMnO₃ based thermochemical energy storage system are also investigated. The steady-state mass and energy conservation equations are solved for all system components to calculate the thermodynamic properties and mass flow rates at all state points in the system. Preliminary results for a system with 120 MW nominal solar power input at a solar concentration ratio of 1000 are presented, designed for constant round-the-clock operation with 9 h of on-sun and 15 h of off-sun operation. The CaMnO₃ particles store and release energy via the sensible heat and reaction heat, yield a net power block efficiency of approximately 60% and an overall energy conversion efficiency of up to 48.7%. Required storage tank sizes for the solids are estimated to be up to 2–3 smaller than those of state-of-the-art molten salt systems.