Solid Oxide Steam Electrolysis with Integration of Solar Heat

Kurzfassung

Water electrolysis is considered as a suitable pathway for the production of large amounts of hydrogen to be used as energy carrier for electricity storage. Among the existing water electrolysis technologies solid oxide steam electrolysis exhibits the highest electrical efficiency. Moreover, from thermodynamic considerations the efficiency can be further increased when part of the energy demand is provided by the integration of external high temperature heat, such as solar thermally generated heat, to be used for water evaporation and heating up of steam. This reduces the needed electrical energy significantly and enhances overall efficiency. The successful experimental coupling of a commercial high-temperature steam electrolyzer and a solar thermal steam generator has been realized for the first time at the German Aerospace Center (DLR). A solar steam receiver has been developed at DLR to generate superheated steam by using a high flux solar simulator with xenon short-arc lamps. The solar thermally generated steam was fed to a commercial 12-cell electrolyzer stack from an industrial supplier (SolidPower, Mezzolombardo, Italy) which was operated at 770 °C. A high-temperature steam storage device was integrated in the system to bridge short-term interruption of irradiation which may occur during cloud passages. The main challenge of the coupling experiment was to provide a continuous hot steam flow without major fluctuations by applying an appropriate control system. After optimization of the operating parameters of the different system components, e.g. water supply, gas volume flows, pressure and temperature, the system could be operated properly. The electrolyzer stack was operated at 770 °C operating temperature and a steam flow of 12.0 slpm (standard litres per minute), 0.58 kg/h respectively. The open circuit voltage (OCV) of the stack was 10.3 V. Approximately 8.4 slpm hydrogen was produced at a current density of -1.25 A/cm2 and a voltage of approximately 17 V corresponding to a steam conversion rate of 70%. The electrical efficiency of the electrolyzer was 93% related to the lower heating value. The experimental setup of the prototype system consisting of a solar simulator, a solar steam generator, a steam accumulator and a solid oxide electrolyzer is presented. Results with regard to solar steam generation and electrochemical performance of the electrolyzer are reported.