Abstract
Solar thermochemical cycles are promising processes for the efficient production of renewable hydrogen at large scale. One area for process optimization is the high temperature reduction step. The oxygen released during this step has to be removed from the reactor in order to increase the reduction extent of the redox material. If low partial pressures of oxygen are required, the removal of oxygen can result in a significant energy penalty for the process. Two options for oxygen removal are mainly considered so far: the use of sweep gas and vacuum pumping. Here, a third promising option is discussed - thermochemical oxygen pumping. This approach shows large energy saving potentials especially at low partial pressures of oxygen. In this study, the interaction between splitting material and pumping material is theoretically analyzed for the conditions of a demonstration campaign previously published. The presented model approach is able to capture the main mechanisms of the interaction between the two materials and the gas phase and provides predictions of the thermochemical oxygen pumping effect on the reduction extent of the splitting material. A parametric study shows the importance of the optimization of the relative material amounts. Furthermore, the influence of using different perovskite materials on the energy consumption of such a process is addressed in a more generic thermodynamic analysis. The results indicate, that by using perovskite-based redox materials, the lower limit of oxygen partial pressures for solar thermochemical cycles from an energy demand perspective might be pushed well below 10−10 bar. At low oxygen partial pressures, thermochemical pumps seem to be far more efficient than mechanical pumps, and their efficiency can be further improved by recovering the heat released during the oxidation of the pumping material.
Highlights
Several paths are investigated for the renewable production of hydrogen using solar energy [1,2]
As opposed to the Co3O4/CoO redox pair, where the chemical and sensible heat input are in the same order of magnitude, the heat input of perovskite-based thermochemical pumps is mainly governed by the latent heat demand
While the practical realization of this concept remains challenging, perovskite-based thermochemical pumps may play an essential role in increasing the efficiency of two step solarthermochemical cycles for fuel production through enabling the reduction of the splitting material at lower oxygen partial pressures and lower cycle times than possible using stoichiometric oxides
Summary
Several paths are investigated for the renewable production of hydrogen using solar energy [1,2]. One path with a promising efficiency potential of large scale plants are solar thermochemical redox cycles. Concentrated solar radiation is used to drive an endothermic reduction reaction of a redox material, which in a second step splits water to obtain hydrogen. Different concepts have been theoretically analyzed [14e18,40] and reactors and even complete plants have been tested at pilot scale [19,20]. During these experimental campaigns, significant reactor efficiencies have been demonstrated in the range of 5% [12]. Several challenges have to be solved before these processes can become economically attractive
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