Thermochemical Oxygen Pumping for Improved Hydrogen Production in Solar Redox Cycles

Kurzfassung

Solar thermochemical cycles are promising processes for the efficient production of renewable hydrogen at large scale. 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. Alternatively, also carbon dioxide can be processed to open the path for syngas production and thereby the production of liquid hydrocarbons as renewable fuels. A 50 kW pre-commercial plant is currently build at Mostoles, Spain, within the EU project SUN-to-LIQUID to demonstrate the full process chain to solar liquid fuels. Along with the experimental validation of the complete solar fuel production plant, the project aims at further improving the efficiency of the cycle. One area of optimization is the high temperature reduction step (typically at 1500 °C). The oxygen released during this step has to be removed from the process 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. Often, two means for oxygen removal are considered so far: sweep gas and vacuum pumping. Here, a third option will be presented - thermochemical oxygen pumping. For thermochemical oxygen pumping, an additional redox material is used in the process to absorb the released oxygen during the high temperature step. This approach shows large energy saving potentials especially at low partial pressures of oxygen. In the presentation the concept of thermochemical oxygen pumping in the context of solar thermochemical cycles will be introduced and first experimental results of a lab scale demonstration campaign will be shown using SrFeO3-δ as an oxygen absorbing material. The influence of using different perovskite materials on the energy consumption of such a process is addressed in the following. The results indicate, that by using perovskite-based redox materials, the lower limit of oxygen partial pressures for solar thermochemical cycles which can be reached at reasonable energetic costs might be pushed well below 10−10 bar.