High temperature Mn₂O₃/Mn₃O₄ and Co₃O₄/CoO systems for thermo-chemical energy storage

A major action to reduce CO₂ emissions is replacing fossil fuels by renewable energy sources. Matching the energy supply and demand by the mostly intermittent renewable resources (wind, solar, wave) is hence a hot topic, and energy storage has become crucial. Thermo-chemical energy storage (TCES) has a higher energy density than sensible and latent heat storage, and allows energy to be stored in the reaction products for multiple reuse and even off-site application. Design parameters are the equilibrium temperature, the reaction heat and the reaction rate, as obtained from both thermodynamic and kinetic assessments. Equilibrium temperatures of the selected metal oxides, Mn₂O₃/Mn₃O₄ and Co₃O₄/CoO are between 1115 K and 1179 K. The present research studies both redox reactions as examples. Commercial Mn₂O₃ and Co₃O₄ were previously investigated in detail, and suffer from incomplete reversibility. The present study investigates the use of self-made Mn₂O₃ and Co₃O₄ mesoporous particles, of micrometer or nanometer scale, respectively. The average particle size of self-made Mn₂O₃ particles is < 5 μm, with a BET surface area of 239.7 m²/g, and T_eq of 1177 K at ambient pressure. Self-made Co₃O₄ was of nano size, with average size of about 100 nm, a BET surface area of 54.2 m²/g, and T_eq of 1109 K at ambient pressure. The redox reactions of these ultrafine particles are fast and nearly fully reversible. The effect of adding inert Al₂O₃ or Fe₂O₃ was also studied, but proven to offer no kinetic benefit, while reducing the reaction heat due to their inert additive character. The findings were used in the design of a 10 kW TCES pilot plant that is currently being tested in a concentrated solar furnace.