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

A two-step solar thermochemical cycle based on Co3O4/CoO redox reactions integrated into an Air Brayton cycle is considered for thermochemical heat storage. The two-step cycle encompasses (1) the thermolysis of Co3O4 to CoO and O-2 driven by concentrated solar irradiation and (2) the re-oxidation of CoO with O-2 to Co3O4, releasing heat and completing the cycle. The cycle steps can be decoupled, allowing for thermochemical heat storage and integration into an Air Brayton cycle for continuous electricity production. Kinetic analyses to identify the rate limiting mechanisms and determine kinetic parameters for both the thermolysis of Co3O4 and the re-oxidation of CoO with O-2 were performed using a combination of isothermal and non-isothermal thermogravimetry. The Co3O4 thermolysis between 1113 and 1213 K followed an Avrami-Erofeyev nucleation model with an Avrami constant of 1.968 and apparent activation energy of 247.21 kJ mol(-1). The O-2 partial pressure dependence between 0% and 20% O-2-Ar was determined with a power rate law, resulting in a reaction order of 1.506. Ionic diffusion was the rate limiting step for CoO oxidation between 450 and 750 K with an apparent activation energy of 58.07 kJ mori and no evident dependence on O-2 concentration between 5% and 100% O-2-Ar. Solid characterization was performed using scanning electron microscopy and X-ray powder diffraction. (C) 2015 Elsevier Ltd. All rights reserved.