Thermochemical oxygen pumping for improved hydrogen production in solar redox cycles

Kurzfassung

Solar thermochemical cycles are promising processes for the efficient production ofrenewable hydrogen at large scale. One area for process optimization is the high temper-ature reduction step. The oxygen released during this step has to be removed from thereactor in order to increase the reduction extent of the redox material. If low partialpressures of oxygen are required, the removal of oxygen can result in a significant energypenalty for the process. Two options for oxygen removal are mainly considered so far: theuse of sweep gas and vacuum pumping. Here, a third promising option is discussed -thermochemical oxygen pumping. This approach shows large energy saving potentialsespecially at low partial pressures of oxygen. In this study, the interaction between split-ting material and pumping material is theoretically analyzed for the conditions of ademonstration campaign previously published. The presented model approach is able tocapture the main mechanisms of the interaction between the two materials and the gasphase and provides predictions of the thermochemical oxygen pumping effect on thereduction extent of the splitting material. A parametric study shows the importance of theoptimization of the relative material amounts. Furthermore, the influence of usingdifferent perovskite materials on the energy consumption of such a process is addressed ina more generic thermodynamic analysis. The results indicate, that by using perovskite-based redox materials, the lower limit of oxygen partial pressures for solar thermochem-ical cycles from an energy demand perspective might be pushed well below 10�10bar. Atlow oxygen partial pressures, thermochemical pumps seem to be far more efficient thanmechanical pumps, and their efficiency can be further improved by recovering the heatreleased during the oxidation of the pumping material.