Solar electricity via an Air Brayton cycle with an integrated two-step thermochemical cycle for heat storage based on Co3O4/CoO redox reactions: Thermodynamic analysis

Solar electricity production is considered in an Air Brayton cycle with an integrated two-step thermochemical cycle for heat storage based on Co3O4/CoO redox reactions. The two steps of the heat storage cycle are encompassed by (1) the high-temperature thermolysis of Co3O4 to CoO and O2 under vacuum pressure utilizing concentrated solar irradiation for process heat; and (2) the highly exothermic re-oxidation of CoO with O2 at elevated pressures, resulting in Co3O4 and providing the heat input to the Air Brayton cycle. The two steps may be decoupled, enabling long-term storage of heat (i.e., thermochemical storage of sunlight). A thermodynamic analysis is applied to determine cycle efficiencies over a range of operating parameters, and an exergy analysis is used to identify the major sources of irreversibilities. A maximum cycle efficiency of 44% was determined for re-oxidizing the CoO at 30 bar, with the maximum cycle efficiency reducing to 26% for a decrease in pressure to 5 bar.