Scientific Approach Within the last years, the development work on reversible solid oxide cell (rSOC) systems has been intensified. This is mainly because this technology can deliver a valuable contribution to carbon-neutral energy supply by storing surplus electrical power into hydrogen and converting it again if necessary. Motivated by this application, in 2018 research by Frank et al. [1] suggests that a round trip efficiency of 50% is possible for a pressurized storage at 70 bars. Based on these results, Forschungszentrum Jülich developed an rSOC demonstration system whose design point is 10 kWAC in fuel cell mode and 40 kWAC in electrolysis mode. The system layout and the evaluation of balance of plant components (BoP) are described by Peters et al. [2]. Figure below shows on the left side the core components of the demonstrator system including the Integrated Module (IM). This IM consists of four 20-layer sub-stacks in the mark H20-design by Forschungszentrum Jülich. The fuel and air heaters are located at the top and bottom of the module. The system can be heated up by in total five heating plates which are arranged on top and below each sub-stack. These plates are also used to maintain the temperature management of the stacks during the endothermic electrolysis operation. In order to ensure the most compact system design, the BoP components are suitably arranged in the vicinity of the IM. The right side of Figure shows the core system installed in the laboratory test environment including the full thermal insulation. This environment provides the necessary media supply and disposal as well as the supporting safety equipment and control units. The system was set into operation on the first of June in 2021. The operation started with stationary operating points in fuel cell and electrolysis mode, after passing the commissioning phase. The performance and efficiency data achieved during this operating phase are shown below. An overall power range from 1.7 kWAC to 13 kWAC was achieved in fuel cell mode. At an output power of 10.4 kWAC and a fuel utilization of 98 %, a system efficiency of 63.3 % could be achieved. During the electrolysis mode, an efficiency of 71.1 % could be achieved with an input power of -49.6 kWel and a steam utilization of 80 %. An analysis of the loss mechanisms showed that in the fuel cell mode about 75 % of the losses are caused by the heat production of the stack itself. In the electrolysis mode, the largest share of about 65 % is caused by the power consumption of the steam generator. The system efficiency can be further increased by a skillful heat recovery from the fuel side off-gas into the steam generation process. Outlook In future work new methods of stack temperature control based on artificial neural networks will be investigated on basis of the presented system. Afterwards it is planned to apply realistic load profiles to the system while investigating the performance, temperature and degradation behavior. Furthermore, electricity and gas storage as well as heat decoupling for district heating application will be studied. Acknowledgement The authors would like to thank their colleagues at Forschungszentrum Jülich GmbH for their great support and the Helmholtz Society, the German Federal Ministry of Education and Research as well as the Ministry of Culture and Science of the Federal State of North Rhine-Westphalia for financing these activities as part of the Living Lab Energy Campus. References [1] Frank M, Deja R, Peters R, Blum L, Stolten D. Bypassing renewable variability with a reversible solid oxide cell plant. Applied Energy. 2018;217:101-12. [2] Peters R, Tiedemann W, Hoven I, Deja R, Kruse N, Fang Q, et al. Development of a 10/40kW-Class Reversible Solid Oxide Cell System at Forschungszentrum Jülich. ECS Transactions. 2021;103:289-97. Figure 1