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

Abstract

A two-step solar thermochemical cycle based on Co 3 O 4 /CoO redox reactions integrated into an Air Brayton cycle is considered for thermochemical heat storage. The two-step cycle encompasses (1) the thermolysis of Co 3 O 4 to CoO and O 2 driven by concentrated solar irradiation and (2) the re-oxidation of CoO with O 2 to Co 3 O 4 , 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 Co 3 O 4 and the re-oxidation of CoO with O 2 were performed using a combination of isothermal and non-isothermal thermogravimetry. The Co 3 O 4 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 À1 . 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 mol À1 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.

Isothermal Redox Kinetics of Co3O4-Fe2O3 Nano-Composite as a Thermochemical Heat Storage Material

“…Cobalt oxide has a better redox kinetics and reversibility than other metal oxides and temperature required for its redox reactions of cobalt oxides is in the range of 885-905 °C that is close to the practical temperature (900-1000 °C) of CSP plants. Redox reaction of cobalt oxides is as follows [5][6][7][8][9][10][11][12]:…”

“…Kinetics study is useful for a deep understanding of the reaction mechanism and their practical applications. Various researches [5,7,[17][18][19][20][21][22][23][24] have been carried out to study the kinetics of reduction and re-oxidation of cobalt oxide powder and single crystals during 1971 to 2015. They reported various kinetics models and mechanisms along with redox activation energies.…”

“…In fact, the information presented in Table 1 only shows the energy activation range of reduction and re-oxidation of cobalt oxide and probable reaction mechanisms. [7] Recently, model-free (isoconversional) methods for kinetic analysis have been used extensively due to their ability to calculate the energy of activation without the need to determine the kinetics model governing a process. Isoconversional methods are model-free methods that evaluate the activation energy (Ea) at progressive reacted fraction (α) values [25,26].…”

Reversible formation of alcohol solvates and their potential use for heat storage

“…The focus was laid not only on the detailed comparative study of the calorimetric behavior, but also on the mapping of the heat-induced changes in these solvates. Besides coupled thermogravimetric-mass spectrometric (TG-MS) [30,31], thermogravimetric analysis/differential scanning calorimetric (TGA/DSC) [32] techniques are generally applied in heat storage studies. We believed that Raman spectroscopy [33,34] and X-ray powder diffraction (XRD) [35] methods are informative in sample characterization.…”

Progress in thermochemical energy storage for concentrated solar power: A review

“…Some single metal redox pairs such as Co 3 O 4 /CoO, BaO 2 /BaO, Mn 2 O 3 /Mn 3 O 4 , CuO/Cu 2 O, and Fe 2 O 3 /Fe 3 O 4 , and polymetallic oxide such as Fe–Co– and Ca–Mn–, can absorb and release O 2 (g) within a temperature range of 350 to 1100°C. Those reactants are considered to be potential TCES material, since their redox reaction is reversible.…”