Solar Thermochemical Hydrogen (STCH) Processes

In order for hydrogen to play a key role in the move towards renewable energy, CO2-neutral hydrogen must be generated efficiently, inexpensively, and at a large scale. This article focuses on thermochemical water-splitting cycles, especially when coupled with solar energy. There are hundreds of possible thermochemical cycles, but only a few have been considered commercially viable. Commonalities among thermochemical cycles are a series of reactions that split water at lower temperatures (∼500-1000°C) than thermal dissociation (>2500°C), with other species recycled in the system. Thermochemical cycles can be direct (all chemical steps) or hybrid (i.e., a combination of chemical and electrochemical steps) processes. If concentrated solar energy is used for the thermal dissociation step, all thermochemical cycles can be classed as solar thermochemical hydrogen (STCH) processes. This article discusses two cycles: 1) a direct thermochemical cycle (zinc oxide cycle), and 2) a hybrid cycle (hybrid sulfur cycle). The latter combines a chemical thermal dissociation step with an electrochemical step. As two-step cycles, their appeal is in their relative simplicity. Although probably several years away, the outlook for solar thermochemical hydrogen generation is strong, especially with continued advances in materials, and system design and optimization.