Solar electricity via an Air Brayton cycle with an integrated two-step thermochemical cycle for heat storage based on Fe2O3/Fe3O4 redox reactions: Thermodynamic and kinetic analyses

Abstract

Solar electricity production via an Air Brayton cycle is considered with integrated thermochemical energy storage. The storage is realized via a two-step solar thermochemical cycle based on Fe2O3/Fe3O4 reduction-oxidation reactions, encompassing (1) the thermal reduction of Fe2O3 to Fe3O4 and O2 driven by concentrated solar irradiation under vacuum; and (2) the exothermic oxidation of Fe3O4 with a compressed air stream back to Fe2O3. The steps may be decoupled, resulting in a high temperature, pressurized airflow that is expanded across a turbine to produce on-demand electricity. A thermodynamic analysis of the system determined a maximum cycle efficiency of 46.0% at a solar concentration ratio of 4000 suns, an oxidation pressure of 30 bar, and an approximately 5:1 molar flow rate ratio of air to solid Fe2O3 exiting the re-oxidizer. Chemical kinetics for the thermal reduction of Fe2O3 were determined between approximately 1400 and 1700 K using non-isothermal thermogravimetry with heating rates between 10 and 20 K·s−1 and O2 partial pressures between 0 and 0.05 bar. The rate-limiting reaction mechanism was determined to be nucleation, and kinetic parameters were resolved using an Avrami-Erofe’ev nucleation model with a reaction order of 1.264 ± 0.010. The rate constant followed an Arrhenius-type temperature dependency with an apparent activation energy of 487.0 ± 3.6 kJ·mol−1 and pre-exponential factor 2.768 ± 0.783·1014 s−1. A power-law dependence on O2 partial pressure of order 8.317 ± 0.233 was determined. Non-isothermal thermogravimetry to examine the oxidation of Fe3O4 to Fe2O3 revealed multiple kinetic regimes, and isothermal thermogravimetry showed the reaction proceeded rapidly, within 20 s, at temperatures greater than 673 K. Solid characterization was carried out using scanning electron microscopy and x-ray powder diffractometry up to temperatures of 1073 K to verify initial and final sample compositions and structures.