Efficient Solar Thermochemical Hydrogen Production in a Reactor Train System With Thermochemical Oxygen Removal

Solar Thermochemical Hydrogen Production (STCH) is a promising technology that uses high-temperature heat directly to split water. The authors have previously proposed a Reactor Train System (RTS) that addresses the largest source of inefficiency in state-of-the-art STCH systems — solid heat recovery — by using multiple moving reactors that exchange heat radiatively between STCH steps. In this work, another major source of inefficiency — oxygen removal during metal reduction — is addressed. Two oxygen pumping schemes are considered — vacuum pumping (VP) and thermochemical oxygen pumping (TcOP). For vacuum pumping, the modularity of RTS enables a ‘Pressure Cascade’ which reduces pumping work by a factor of four and the capex by a factor of five as compared to a single-step VP scheme. The optimized RTS + VP system achieves 31% heat-to-hydrogen conversion efficiency with ceria despite the low efficiency of vacuum pumps at low pressures. Thermochemical Oxygen Pumping (TcOP) uses a second redox material — SrFeO3 — to pump oxygen. This material is transported in reactors moving in the opposite direction to the main RTS train. The optimized RTS + TcOP achieves morethan 40% heat-to-hydrogen efficiency, while producing twice as much hydrogen per kilogram of ceria as the RTS + VP system.