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

A major action to reduce CO 2 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, Mn 2 O 3 /Mn 3 O 4 and Co 3 O 4 /CoO are between 1115 K and 1179 K. The present research studies both redox reactions as examples. Commercial Mn 2 O 3 and Co 3 O 4 were previously investigated in detail, and suffer from incomplete reversibility. The present study investigates the use of self-made Mn 2 O 3 and Co 3 O 4 mesoporous particles, of micrometer or nanometer scale, respectively. The average particle size of self-made Mn 2 O 3 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 Co 3 O 4 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 Al 2 O 3 or Fe 2 O 3 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. Image 1 • Reducing the CO 2 emissions calls upon the use of renewable energy and its grid integration will involve energy storage. • Redox pairs of Mn 2 O 3 /Mn 3 O 4 and Co 3 O 4 /CoO were studied as potential TCES materials. • Mechanisms and kinetics of the reversible reactions were analyzed and assessed. • Both oxide pairs can operate as TCES materials in multiple redox cycles. • Their use in a concentrated solar pilot plant is illustrated. [ABSTRACT FROM AUTHOR]