(584q) On-Sun Demonstration of Hydrogen Production Via Solar Thermochemical Water Splitting

A sustainable hydrogen economy based on the low-cost production of H₂ from renewable resources has the potential to drastically transform the energy sector. While solar energy is the most abundant renewable energy resource, the capture, storage, and distribution of it remains a challenge. Solar thermochemical water splitting (STWS) provides a promising route for efficient utilization of this dispersed resource because it allows for use of the entire solar spectrum to convert water to a renewable energy carrier and fuel, H₂. Simple two-step reduction/oxidation (redox) cycles are preferred because the production of oxygen and hydrogen can be spatially or temporally separated, eliminating the need for high-temperature gas separation, while also avoiding the design complications and inefficiencies of multistep processes. In this process, oxygen is generated by the reduction of a metal oxide during a high temperature step in an inert atmosphere. In the second step, the reduced metal oxide is exposed to high temperature steam to reoxidize the material and produce hydrogen fuel. Although the STWS reaction schemes appear straightforward, their implementation in 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, and robust redox active particles. In this work we present results showing near-isothermal on-sun hydrogen production utilizing dual lab-scale fluidized beds. The use of fluidized beds that are cycled between oxidation and reduction negates the need for energy intensive moving solids. Active hercynite particles were redox cycled at the High Flux Solar Furnace at the National Renewable Energy Laboratory and production rates of H₂ exceeding 500 umol/g were measured. The effect of environmental variability on the incident solar radiation and hydrogen production is evaluated during on-sun testing. This work demonstrates the viability of solar thermal water splitting leveraging continuous operation of both reduction and oxidation at near-isothermal conditions.