C. Corgnale, J. Monnier, D. Ginosar, M. Gorensek, Z. Ma
Keywords: hydrogen production, solar plant, low cost, high efficiency, thermochemical cycle, eletrochemical system, direct solar receiver reactor, process modeling, catalyst material development
Summary:A hydrogen economy requires primary sources to provide the power required to produce hydrogen from water, fossil fuels, biomass or similar compounds. Water-splitting processes are particularly attractive, since water is abundant around the planet, the unique products from water splitting are hydrogen and oxygen and a variety of primary energy sources can be used to produce the hydrogen. External electric power can be used to produce hydrogen by conventional water electrolysis, but it firstly requires the production of electricity to realize the electrochemical splitting of the water molecule. A more efficient and potentially more cost-effective approach is to use thermal power to drive a thermochemical water splitting process. The thermo-electrochemical Hybrid Sulfur (HyS) is one of the most appealing cycles. It has only fluid reactants and is comprised of only two global reaction steps: a low temperature electrochemical exothermic section, operating at temperatures on the order of 100 °C, and a high temperature thermal section operating at max temperatures of about 800 °C. In the electrochemical section SO2 and H2O are combined together to produce electrochemically H2 at the electrolyzer cathode and H2SO4 at the electrolyzer anode. Sulfuric acid is recycled inside the plant, concentrated and decomposed to produce oxygen. Solar driven HyS process is being studied by the Greenway Energy and the University of South Carolina within the U.S. Department of Energy HydroGEN program (part of the DOE Energy Material Network initiative), partnering with the Idaho National Laboratory, the Savannah River National Laboratory and the National Renewable Energy Laboratory. The project mostly focuses on the high temperature sulfuric acid decomposition section, developing new optimized catalyst formulations, to be integrated in novel solar reactor systems. A novel bimetallic catalyst formulation is being developed and synthesized achieving high conversion yields and very limited sintering effects resulting in reduced degradation. Test results, under sulfuric acid environment and operating temperatures on the order of 800 °C, will be shown and discussed. The new catalyst will be placed in a novel direct solar receiver reactor. The proposed concept is based on a solar cavity receiver system, realizing the sulfuric acid decomposition and internal heat recovery in a single unit, directly heated by the incident solar radiation. This system, along with the actual H2SO4 decomposition into SO2, allows: (1) efficient internal heat recovery from the decomposition products and (2) connections with the metallic interfaced HyS equipment (e.g. tubing, valves etc) at low temperatures. Preliminary results from detailed transport model analysis will be shown, demonstrating effective decomposition and internal heat recovery. The integration of the high temperature decomposition section with the low temperature electrochemical section of the HyS process has also been studied from a process modeling point of view. A novel SO2 electrolysis concept is proposed and discussed showing the potential to achieve high efficiencies, with actual voltage values lower than 600 mV and current densities higher than 500 mA/cm2 . A solar plant process analysis will be presented and discussed, demonstrating thermochemical efficiencies ≥ 37% and hydrogen costs < 2.50 $/kg.