Aluminium redox cycles: modeling and preliminary experimental assessment of an innovative process for energy storage

The increasing share of Renewable Energy Sources (RES) in the energy mix brings the necessity to find innovative and efficient methods for long-term energy storage. Recyclable electrofuels are a promising solution, allowing to store electrical energy in a compact and safe way through energy-dense materials thanks to Power-to-Power cycles. Among them, aluminium shows many advantages such as safety, efficiency, and sustainability. In charging phase, pure electricity is exploited to produce aluminium thanks to electrochemical processes in centralized smelters; in the discharging phase the metal is oxidized in air or water, returning part of the energy previously stored. In this work, a comprehensive assessment of aluminium as energy vector is performed, starting from a theoretical thermodynamic analysis of the chemical redox reactions involved in the production and utilization of the metal, leading to a theoretical energy density of 23.3 kWh/l. The attention is focused on medium/high temperature aluminium-water reactions allowing the cogeneration of hydrogen and heat, that can be then converted efficiently to electricity. A case study on a district/regional level is proposed: the calculated round-trip efficiency of 30% is competitive with compressed hydrogen Power-to-Power systems, especially if the hydrogen consumed for transportation is taken into account. A model of a highly efficient Aluminium-to-Power plant was developed in Aspen Plus, including 3 generation units (steam turbine, gas turbine, and solid oxide fuel cell), that was optimized to reach an electrical efficiency up to 78.8%. Finally, an experimental section is included, in which the oxidation of Al is studied in TGA (thermo-gravimetric analysis) and in a microreactor. TGA, performed both in temperature ramp and in isothermal mode, confirmed the possibility of reaction of pure aluminium powders in air without activation methods, with full oxidation reached at 1500°C. Tests in microreactor show the possibility of hydrogen production during temperature ramps, while in isothermal mode (e.g., 900°C, above melting point) the oxidation of the sample appears slower, suggesting that the microstructural changes of the material at different temperatures could play a crucial role to overcome the passive external oxide layer and activate the reaction.