Counter flow sweep gas demand for the ceria redox cycle

Abstract

The exploitation of solar radiation as abundant energy source and its utilization in solar fuels are crucial for a large scale deployment of a sustainable energy economy. In the search for related efficient, scalable, and economic technologies several two-step solar thermochemical redox cycles are under assessment. Generic thermodynamic process assessments have identified the parasitic power consumption for maintaining low partial pressures of oxygen during the reduction of the redox material as possibly critical efficiency limiting factor. Two options are generally considered for maintaining low oxygen pressures: reduction of the total pressure and sweeping. In case of sweeping with an inert gas, the parasitic power demand is proportional to the sweep gas amount and comprises the pumping power as well as the power for heating the sweep gas to the targeted reduction temperature and the power related to the provision of clean sweep gas. Different models have been proposed in literature to predict the required sweep gas amount for the redox cycle – with highly diverging results. A proposed counter flow model leads to negligible sweep gas demands over a wide set of operational conditions while another model leads to prohibitively high sweep gas demands below 5 kPa. In this study a refined counter flow model is proposed, which considers the actual oxygen release characteristic of ceria and the oxygen uptake capability of the sweep gas stream. While the model predicts much higher values of required sweep gas than the previous counter flow model, the application of a counter flow still leads to considerable savings. In addition, a numerical model is used to analyze different operational implications for a counter flow arrangement, showing that a significant additional reduction of the sweep gas demand can be reached by keeping the reduction extent below the thermodynamic equilibrium value. The derived counter flow sweep gas model is compared to the alternative reduction of the total pressure by the use of vacuum pumps on a system level. The results show the relative performances and operational window of sweep gas for redox cycles.