In this work, hydrogen-fuelled reversible solid oxide cell (RSOC) systems are analysed on cell level, stack level and system level. In the theoretical part of this work, the basic applications and requirements for electrical energy storage systems with connection to the grid are described. Based on the requirements for different tasks along the electrical value chain, an application-technology pairing is defined to identify the possible uses for a reversible solid oxide cell system. The specific applications include power bridging tasks like load following, and energy management tasks like seasonal storage. In comparison to other electrical storage systems based on key performance indicators (KPIs) a positioning of reversible solid oxide cells is done. Competitors to RSOC systems are vanadium redox flow batteries (VRFBs) and secondary batteries like Li-ion and Ni-Cd. On cell level, the different polarization types are analysed in detail and the differences for anode supported cells (ASCs), electrolyte supported cells (ESCs), and metal supported cells (MSCs) are described. For reversibility, metal supported cells are an attractive alternative due to high tolerances versus thermal cycling and redox cycling. Though the current tested life capability in steady state is shorter compared to ASCs and ESCs. On system level, different system designs are analysed focusing on hydrogen as fuel. Considering a recirculation loop and condenser in the balance of plant (BoP), the possibility to increase the electrical efficiency of the system is given. The strategies of upscaling reversible solid oxide cell systems from the kW-range into the MW-range are demonstrated by existing plants from Sunfire Inc., “Forschungszentrum Jülich” and VTT Technical Research Centre. High power ranges are possible by combining individual modules containing 4-8 stacks with a total module voltage between 400-600 Volts.
In the practical part of this work, the suitability for reversibility of a 180-cell stack module produced by the Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) is tested. Using the softwares Matlab/Simulink and AVL CameoTM two separate models for SOEC and SOFC mode are used to perform a design of experiment (DoE) including optimisations with different target functions in stationary operating points. Corresponding to the target functions of the optimization, different operation strategies are defined and compared to each other. The results give information about the fuel input, air input, temperature and other influencing parameters to reach maximum efficiency, maximum output or maximum heat input in SOEC mode in different loading conditions of the system. In the course of evaluation, the influence of the gas input temperature, of the fuel utilization and the consideration of a condenser in the recirculation path is determined for SOFC mode. In fuel cell mode, the system reaches a maximum electrical power output of 5 kW with a maximum electrical efficiency of 51.1 %. At half load condition with an electrical power output of 2.5 kW, the maximum electrical efficiency reaches 62.4 %. Furthermore, the part load behaviour is analysed in this work. In SOFC mode, the efficiency increases at low part loads. Reason for this is the decreasing blower power consumption. In SOEC mode, the influence of the gas input temperature as well as the accessibility to waste heat is analysed. The system reaches a maximum hydrogen production rate of 6 Nm³/h with an electrical input of 18 kW and an electrical efficiency of 78.1 %. If waste heat is accessible in SOEC mode the efficiency is increased to a maximum of 95 %. Contrarily to SOFC mode, the efficiency increases at high part load conditions. Based on the efficiencies of SOFC mode and SOEC mode, the roundtrip efficiency for RSOC system is determined and compared to literature. The maximum roundtrip efficiency of the system equals 57 % at part load of 50 % and 47 % at full load of 100 %.