Solarthermal cycles

Diffusion of gaseous reactant (e.g. CO2) through the film surrounding the particle to the surface of the solid 

Diffusion of the reactant through the blanket of ash (e.g. MOx) to the surface of the unreacted core (e.g. MOx y representing the reaction surface) 

Diffusion of the gaseous product through the gas film into the main body of the fluid 

Solid-state diffusion, which is involved in the release of oxygen, proceeds generally through the movement of point defects. The vacancy mechanism, the interstitial mechanism, and the interstitialcy mechanism can occur depending on the distortion of the solid lattice and the nature of the diffusing species. When one of the steps 1-5 is the slowest step representing the major resistance, that step is the rate-controlling one, which is not necessarily the chemical reaction (step 3). 

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In this way, the unwanted back reaction and the separation of the reaction products are avoided. Before presenting planned industrial applications of solarthermal cycles for the C02 splitting, the fundamental aspects are discussed to understand the challenges and advantages of this process. 

For the production of chemicals, the rate of the reaction is a key parameter for the productivity defined in Equation (5) as the number of molecules produced per time. In homogeneous systems, the reaction rate depends on temperature, pressure, and composition [1]. In the case of solarthermal cycles, a metal oxide is used for the C02-splitting reaction rendering the reaction medium a heterogeneous two-phase system consisting of a solid (metal, metal oxide) and a fluid (CO2, CO, or carrier gas with O2). Therefore, the reaction kinetics becomes much more complex. Whereas microscopic kinetics only deals with time-dependent progress of the reaction, macroscopic kinetics additionally takes the heat- and mass-transport phenomena in heterogeneous systems into account. The transfer of species from one phase to the other must be considered in the overall mass balance [1]. The reaction of a gas with a porous solid consists of seven steps ... 

An important parameter to evaluate solarthermal cycles is the solar-to-fuel energy efficiency q, which is defined as the ratio of stored chemical energy and the applied energy according to Equation (7), with CO being the produced fuel [2] ... 

Finally, specific material properties are required for efficient solarthermal cycles for CO2 activation. For solarthermal applications the metal oxides must effectively absorb solar radiation and convert it into heat without substantial reradiation. Therefore, bright materials are unfavorable because they always cause reradiation. 

The CeO CejOs System The redox properties ofceria and ceria-based materials have been studied in great detail because of their oxygen storage and release capacities, which are highly relevant for their application in modem three-way automotive catalysts [6, 7]. Unfortunately, the temperatures used in these studies are usually far lower than those necessary for thermal reduction in inert atmospheres. Therefore, the applicability of these data must be regarded as limited. Some case studies of ceria-based systems in solarthermal cycles are published in the literature [2,4,14] ... 

Figure 5.2.1 Model redox cycle for solarthermal carbon-dioxide splitting.

The ZnO/Zn System The solarthermal activation of ZnO has attracted considerable attention for energy storage. In addition to thermochemical cycles the production of elemental Zn for battery applications has been investigated within the EU-funded project SolZinc. Thermochemical cycles for solar H2, CO, and combined syngas production have been proposed ... 

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