Thermochemical hydrogen processes

The focus of this chapter is on thermochemical water-splitting cycles, with an emphasis on solar energy as the input. However, other sources of thermal and electrical energy (e.g., nuclear) can drive these cycles. Further emphasis will be placed on comparing those with no electrochemical step to those that take advantage of one. In general, water splitting that employes an electrochemical step is less complex but often less efficient. The simplest example is water electrolysis, where water is split directly into oxygen and hydrogen using electricity. However, since there are thermodynamic inefficiencies in generating electricity from thermal energy, thermal water splitting would be attractive from an efficiency standpoint. Unfortunately, thermal dissociation of water occurs at temperatures in excess of 2500°C. Therefore, hundreds of different thermochemical cycles involving chemical species in addition to water are under investigation. These various thermochemical cycles split water at lower temperatures (∼500–1000°C), with the other chemical species recycled in the system. Thermochemical cycles can be divided into two broad categories: (1) direct processes (i.e., all chemical steps); or (2) hybrid processes (i.e., a combination of chemical and electrochemical steps).