Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions

The production of hydrogen from water using solar energy via a two-step thermochemical cycle is considered. The first, endothermic step is the thermal dissociation of ZnO(s) into Zn(g) and O2 at 2300 K using concentrated solar energy as the source of process heat. The second, non-solar, exothermic step is the hydrolysis of Zn(l) at 700 K to form H2 and ZnO(s); the latter separates naturally and is recycled to the first step. Hydrogen and oxygen are derived in different steps, thereby eliminating the need for high-temperature gas separation. A 2nd-law analysis performed on the closed cyclic process indicates a maximum exergy conversion efficiency of 29% (ratio of ΔG298 K°|H2+0.5O2→H2O for the H2 produced to the solar power input), when using a solar cavity-receiver operated at 2300 K and subjected to a solar flux concentration ratio of 5000. The major sources of irreversibility are associated with the re-radiation losses from the solar reactor and the quenching of Zn(g) and O2 to avoid their recombination. An economic assessment for a large-scale chemical plant, having a solar thermal power input into the solar reactor of 90 MW and a hydrogen production output from the hydrolyser of 61 million-kWh/yr, indicates that the cost of solar hydrogen ranges between 0.13 and 0.15$/kWh (based on its low heating value and a heliostat field cost at 100–150$/m2) and, thus, might be competitive vis-à-vis other renewables-based routes such as electrolysis of water using solar-generated electricity. The economic feasibility of the proposed solar process is strongly dependent on the development of an effective Zn/O2 separation technique (either by quench or by in situ electrolytic separation) that eliminates the need for an inert gas. The chemical aspects of the reactions involved and the present status of the pertinent chemical reactor technology are summarized.