Two-step thermochemical cycles for splitting CO 2 with Zn/ZnO and FeO/Fe 3 O 4 redox pairs using concentrated solar energy are considered. Thermogravimetric-based kinetic analyses were performed for the reduction of CO 2 to CO with Zn and FeO. Both reactions are characterized by an initial fast interface-controlled regime followed by a slow diffusion-controlled regime, which are described using a shell-core kinetic model. In the interface-controlled regime, a power rate law is applied with apparent activation energies 113.7 and 73.4 kJ mol -1 , and corresponding reaction orders 0.339 and 0.792, for the Zn/CO 2 and FeO/CO 2 systems, respectively. In the diffusion-controlled regime, limited by the ion mobility through the oxide shells, the apparent activation energies are 162.3 kJ mol -1 for Zn/CO 2 and 106.4 kJ mol -1 for FeO/CO 2 . Additional reaction mechanisms above the Zn melting point for Zn/CO 2 reactions are postulated.

The solar production of syngas from H2O and CO2 is examined via two-step thermochemical cycles based on Zn/ZnO and FeO/Fe3O4 redox reactions. The first, endothermic step is the thermal dissociation of the metal oxide using concentrated solar radiation as the energy source of high-temperature process heat. The second, nonsolar, exothermic step is the reaction of the metal or reduced metal oxide with a mixture of H2O and CO2 yielding syngas (H2 and CO), together with the initial form of the metal oxide that is recycled to the first step. Chemical equilibrium compositions for the systems of Zn and FeO with CO2 + H2O were computed as a function of temperature and pressure for different stoichiometries. A series of dynamic thermogravimetric experimental runs in the range 673−1423 K was carried out to evaluate the reaction kinetics and syngas quality of the second step. The molar flow rate fractions of the gaseous products exhibited linear dependencies on the molar flow rate fractions of the gaseous reactants for both the FeO/Fe3O4 and Zn/ZnO systems.

Latent heat storage systems are gaining the attention of researchers as possible substitutes to conventional sensible heat storage systems due to their compactness and their ability to absorb and release heat almost isothermally. Among the Phase Change Materials (PCM) for energy storage studied so far, esters are believed to show promising properties. In particular, a broad range of melting temperatures, little to no supercooling, low corrosivity, chemical and thermal stability, and high enthalpies of fusion are reported. Many esters have the advantage of being bio-based and biodegradable, making them more sustainable in comparison to other popular PCM. Still, a clear lack of experimental data exists in regards to this class. In the present study, esters derived from saturated fatty carboxylic acids (myristic, palmitic, stearic, behenic), coupled with primary linear alcohols of different length (methanol, 1-decanol) were synthesized through Fischer esterification and their properties were investigated. Purities higher than 89% were obtained for all cases as proven by gas chromatography coupled with mass spectroscopy and nuclear magnetic resonance analysis. Additionally, the esters' formation and reaction kinetics were characterized by attenuated total reflectance infrared spectroscopy. The esters produced showed to possess relatively high enthalpies of fusion above 190 J/g and thermal stability over three repeated cycles with differential scanning calorimetry. The melting points measured ranged between 20 • C and 50 • C, therefore proving to be interesting candidates for low-medium temperature applications such as heating and cooling in buildings. A correlation could be observed between the chemical structure and melting point of the produced esters. Additionally, thermogravimetric analysis revealed a higher thermal resistance for esters with longer aliphatic chains in comparison to shorter-chained ones.

Syngas production via a two-step H 2 O/CO 2 -splitting thermochemical cycle based on Zn/ZnO and FeO/Fe 3 O 4 redox reactions is considered using highly concentrated solar process heat. The closed cycle consists of: (1) the solar-driven endothermic dissociation of ZnO to Zn or Fe 3 O 4 to FeO; (2) the nonsolar exothermic simultaneous reduction of CO 2 and H 2 O with Zn or FeO to CO and H 2 and the initial metal oxide; the latter is recycled to the first step. The second step was experimentally investigated by thermogravimetry for reactions with Zn in the range 673-748 K and CO 2 /H 2 O concentrations of 2.5-15% in Ar, and for reactions with FeO in the range 973-1273 K and CO 2 /H 2 O concentrations of 15-75% in Ar. The reaction mechanism was characterized by an initial fast interface-controlled regime followed by a slower diffusion-controlled regime. A rate law of Langmuir-Hinshelwood type was formulated to describe the competitiveness of the reactions based on atomic oxygen exchange on active sites, and the corresponding Arrhenius kinetic parameters were determined by applying a shrinking core model.

We consider the solar thermochemical production of H2 and CO (syngas) from H2O and CO2 via a two-step ZnO/Zn redox cycle. The first step, driven by concentrated solar radiation, is the endothermic thermolysis of ZnO producing a gaseous mixture of O2 and Zn-vapor, which upon quenching precipitates a solid residue comprising Zn and ZnO in variable ratios. The second, nonsolar step is the exothermic oxidation of Zn by H2O and/or CO2 to form fuel (H2 and/or CO) and the solid product ZnO which is recycled to the solar reactor. It has been recognized that the presence of inert ZnO during the second step affects both the oxidation kinetics and the final asymptotic conversion of Zn. However, while the fraction of ZnO in a mixture with Zn leaving the thermolysis step generally varies with a solar reactor/quencher design and experimental conditions, all previously reported analyses have studied the oxidation kinetics of either pure Zn or of Zn particles blended with ZnO in a specific, preset mass ratio. This work examines the effect of dilution with inert particles on the Zn oxidation by CO2. Blends of commercially available Zn with ZnO or Al2O3 particles have been tested by thermogravimetry. The setup was carefully designed to ensure the absence of heat and mass-transfer intrusion on the oxidation kinetics while using a 15% CO2–Ar mixture or pure CO2 at 350 °C < T < 400 °C and ambient pressure. The effects of inert particle type, mass fraction, and blending method are reported and used to propose a simplified multipath reaction mechanism. The results are compared to the performance of a Zn/ZnO powder produced in a solar reactor.