SOLAR THERMOCHEMICAL HYDROGEN PRODUCTION FROM WATER ON A SOLAR TOWER

Solar-powered thermo-chemical cycles are capable of emission-free hydrogen production from water in centralised plants. A two-step cycle based on a metal oxide redox pair system, which can split water molecules by abstracting oxygen atoms and reversibly incorporating them into their lattice, coated on ceramic honeycombs that can absorb concentrated solar radiation has been developed. An easy way of operation is gained by the combination of a ceramic substrate as absorber structure, which can be heated to high temperatures with concentrated solar radiation, and of a metal oxide coating which is capable of splitting water. This offers advantages over comparable processes, because in this case the entire process can be conducted in a single solar heated converter. In the first step, the steam flowing past the metal oxide is split by binding the oxygen to the excited metal oxide lattice, and H2 is produced. In the second step, at temperatures of 1150-1200 °C, the oxygen, which has previously been incorporated into the lattice, is released again, and the metal oxide is regenerated. Because of the immobilization of the redox pair material on the substrate, not only no solids need to be circulated, but hydrogen production and oxygen release take place at different steps, eliminating thus the need for high-temperature gas separation processes. A slightly modified process can also been applied for the thermo-chemical reduction of CO2 enabling to provide the precursors for the production of liquid fuels and other organic chemicals. The two steps of the process are carried out in a cyclic mode two identical chambers of the receiver-reactor. The multi-chamber arrangement of the reactor enables a quasi-continuous solar hydrogen production. The present work describes the realisation and successful test operation of a 100kW pilot plant on a solar tower, which aimed at the demonstration of the feasibility of the process on a solar tower platform under real conditions. The pilot plant has been installed at the Small-Solar-Power-System (SSPS), the small solar central receiver system at the Plataforma Solar de Almeria in Spain. The first part of the testing of the plant was dedicated to an exhaustive thermal qualification of the pilot plant took place, using uncoated ceramic honeycombs as absorbers. Some main aspects of these tests were the development and validation of operational and measurement strategy, the gaining of knowledge on the dynamics of the system, in particular during thermal cycling, the determination of the controllability of the whole system, and the validation of practicability of control concept. The thermal tests enabled to improve, to refine and finally to prove the process strategy and showed the feasibility of the control concept implemented. It could be shown that rapid changeover between the modules is a central benefit for the performance of the process. This reduces a time period with lower hydrogen production or ineffective regeneration of the modules. After that the absorber was replaced and honeycombs coated with redox material were inserted. With that arrangement several experimental campaigns were carried out. Hydrogen was produced in a quasi-continuous manner by running several production cycles. Potential operation ranges and the influence of key operation parameters was investigated.