(258e) On-Sun Demonstration of Continuous Redox Driven Solar Thermochemical Hydrogen Production

A sustainable hydrogen (H2) economy based on the low-cost production of H2 from renewable resources has the potential to drastically transform the energy sector. A simple two-step reduction/oxidation (redox) cycle allowing for the production of oxygen (O2) and H2 by solar thermal water splitting (STWS) has been demonstrated on-sun. The H2 and O2 are produced in two separate steps, eliminating the need for high-temperature H2/O2 gas separation. In this redox process, oxygen is generated by the high-temperature reduction of an active metal oxide spinel, forming O-vacancies; followed by a second step in which the vacancies are filled during a high-temperature steam exposure, producing H2 fuel. Although the STWS reaction scheme appears straightforward, its implementation as an economically viable process is extremely demanding. In order to commercially realize this technology, several challenges must be overcome including an energy efficient reactor design, stable reactor and heat exchanger materials, robust redox active particles, and an efficient process for recycling inert gas. Here, we present results showing near-isothermal on-sun H2 production utilizing dual lab-scale fluidized beds operated in which redox cycling produces H₂ continuously. Active hercynite particles were redox cycled at the High Flux Solar Furnace at the National Renewable Energy Laboratory. Production rates of H₂ exceeding DOE project targets and a final deliverable of H2 production were demonstrated. The impact of incident solar radiation and hydrogen production is evaluated during on-sun testing. Results of a techno-economic analysis will be discussed as will the path forward. This work demonstrates the viability of STWS leveraging continuous operation of both reduction and oxidation at near-isothermal conditions within a multi-tube solar thermal cavity reactor. This is the first demonstration of continuous STWS to be reported.