Two different routes of solar thermochemical hydrogen production are reviewed. One is two-step water splitting cycle by using a metal-oxide redox pair. The first step is based on the thermal reduction of metal oxide, which is a highly endothermic process driven by concentrated solar thermal energy. The second step involves water decomposition with the thermally-reduced metal oxide. The first thermal reduction process requires very-high temperatures, which may be realized in sun-belt regions. Another hydrogen production route is solar reforming of natural gas (methane), which can convert methane to hydrogen via calorie-upgrading by using concentrated solar thermal energy. Solar reforming is currently the most advanced solar thermochemical process in sun belt. There is also possibility for the solar reforming to be applied for worldwide solar concentrating facilities where direct insolation is weaker than that in sun belt. Our experimental studies to improve the relevant catalytic technologies are shown and discussed.

The solar reforming of methane with CO2 is investigated using a direct irradiated absorber subjected to solar flux levels in the range 180-250 kWm−2. This solar thermochemical process can upgrade the calorific value of the methane feed by 17% to produce hydrogen via the water-gas shift reaction. The volumetric receiver-reactor is best suited for this application because of its compactness and low thermal capacity. The new type of catalytically-activated “metallic foam” absorber–an Ni-Cr-Al-foam absorber applied with Ru/Al2O3–was found to have a superior thermal performance at relatively low solar fluxes when compared to conventional ceramic foam absorbers.

A nickel−magnesia solid solution Ni−Mg−O was examined as a catalyst for solar CO2 reforming of methane. The activity was tested in a laboratory-scale transparent (quartz) reactor under direct irradiation of the catalyst by high-flux visible light from a sun-simulator. The 8−11 wt % Ni−Mg−O catalyst gave the high reforming activity or about 100% of chemical conversion, with little coking, under a high-flux irradiation of 890 kW m-2 and at a short residence time of about 0.15 s while passing a 1:1 CH4−CO2 gas mixture at 1 atm. The comparison of the activity data with those obtained in a light-irradiated, nontransparent (steel) reactor showed that the intensification of heat supply by the direct light irradiation of the Ni−Mg−O catalyst leads to considerable reaction rate enhancement. Applying this Ni−Mg−O catalyst, a new type of “catalytically activated” ceramic (alumina) foam absorber was prepared and tested on activity in a laboratory-scale volumetric receiver-reactor using the sun-simulator. This new absorber may be applied in solar receiver-reactor systems for converting concentrated solar high fluxes to chemical fuels via endothermic natural-gas reforming.

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