In 2018, a 5/15 kW_DCreversible solid oxide cell system was developed and successfully operated by Forschungszentrum Jülich. Based on the knowledge gained with this first system, an optimized system in the power class of 10/40 kW_ACwas developed afterwards in 2019 that uses the well-established Integrated Module. This module consists of four 20-layer sub-stacks, two heat exchangers and five heating plates. It represents the main components of the system. The basic system layout was retained in general from the previous system and adjusted in accordance with the higher power level, as well as the supporting balance of plant components. The layout of the demonstrator system and its results are described. During the experimental evaluation in fuel cell mode, the system could provide an electrical output power from 1.7 to 13 kW_AC. The maximum system efficiency of 63.3% based on the lower heating value (LHV) could be reached at a system power of 10.4 kW_AC. This operating point was also analyzed regarding the temperature distribution in the stack and efficiency losses. In electrolysis mode, a maximum efficiency of 71.1% (LHV) was achieved with an electrical power input of −49.6 kW_AC. At this operating point, about 11.7 Nm³h⁻¹of hydrogen are generated. The stack temperature distribution and the efficiency losses are also analyzed for the electrolysis mode. Finally, the potential for the efficiency optimization through higher heat integration in this mode is experimentally evaluated and discussed.

A four-layer solid oxide fuel cell stack consisting of standard anode-supported cells was assembled to investigate long-term stability, but at higher current densities and/or fuel utilization compared to previous investigations. The stack was operated within a furnace temperature range of 700 °C–750 °C with hydrogen fuel at a current density of up to 1 A·cm⁻² and fuel utilization of up to 80% for more than 10,000 h. The average voltage degradation rate was approximately 0.6%kh⁻¹. Increases in the ohmic resistance and anode polarization dominated the degradation behavior. An increase in the current density and fuel utilization under current testing conditions did not fundamentally influence the degradation rate. However, the possible modification in the nickel structure by local higher fuel utilization may have had a long-term impact on the lifetime of the stack. The complexity of the degradation analysis of stacks resulting from an inhomogeneous contact inside the stack was analyzed with the support of impedance measurements and a post-mortem analysis.