Performance assessment and evaluation of SOC stacks designed for application in a reversible operated 150 kW rSOC power plant

The coupling of renewable energy sources with reversible operated high temperature solid oxide cells (rSOC) seems to be a promising option to store and supply clean energy in the future. This work provides the results of experimental investigations on the performance of a rSOC stack as a main unit of a large-scale module installed at a real industrial power plant. In order to determine the requirements as well as to propose strategies for safe and stable operation of the large-scale module the stack was tested under system relevant gas mixtures and operational conditions. Hereby, the focus was on steam electrolysis (EC) and fuel cell (FC) operation utilizing a H₂/H₂O/CO/CO₂/N₂ gas mixture, predefined by the reformer installed within the module at the power plant. Furthermore, the alternating operation between EC and FC mode was analyzed. The comprehensive analysis of the performance of the stack includes polarization curves, electrochemical impedance spectroscopy (EIS), distribution of relaxation times (DRT), gas analysis and temperature measurements. Thus, numerous combinations of operational parameters are linked to the individual processes within the stack. During EC operation higher hydrogen partial pressures presented significantly lower diffusion losses especially at low current densities, whereby in FC mode low fuel flows presented increased concentration losses. During alternating operation, prolonged voltage stabilization periods with increasing operation time were observed. Additionally, the processes represented by the peaks calculated from the DRT showed enhanced unstable behavior and presented a significant shift towards higher frequencies compared to constant mode operation.