Experimental studies and modeling activities on pressurized SOFC in the hybrid power plant project

Abstract

The increasing demand of electrical energy worldwide requires power plants with high electrical efficiency and low emissions. One way to reach this goal is the hybrid power plant, consisting of a pressurized solid oxide fuel cell (SOFC) coupled with a gas turbine. DLR has started a project which aims to build a real hybrid power plant in the 50 kW class. To design the system and its control the knowledge of the characteristics of pressurized SOFC is one essential part. In the context of the hybrid power plant the fuel cell will be operated in a pressure range from 1 to 8 bar. Our aim is to examine the SOFC at elevated pressures with regard to two different aspects. On the one hand the effect of the elevated pressure on the electrochemical behavior has to be understood. On the other hand, on account of our system focus, the steady state and transient operational behavior is an important research area. A combined theoretical and experimental approach is very valuable to gain a thorough knowledge of pressurized SOFC. Therefore, experiments and modeling activities are combined at DLR. Due to complex and interdependent mechanisms inside a SOFC the behavior at elevated pressures cannot be derived from measurements at ambient pressure. In addition, no sufficient literature data is available. Hence, a test rig for characterization of SOFC at pressures up to 8 bar has been planned and built. The presentation will focus on the effect of pressure variations on SOFC performance in the interesting range from 1 to 3 bar. V(i)-characteristics demonstrate the influence of varying pressure. The experiments are performed on planar, anode-supported 5-cell short stacks provided by ElringKlinger AG. For comparison, reference to ambient pressure measurements and simulation results will be presented. Modeling activities include a detailed examination of electrochemistry on cell level and in-depth simulation on system level. At the reference voltage of 0.9 V the performance is 207 mW/cm² at 1.4 bar, 266 mW/cm² at 2 bar and 279 mW/cm² at 3 bar with hydrogen/nitrogen 1/1 on the anode and air on the cathode at 750 °C. As expected in modeling [2] and seen in literature [6,7] the performance gain from 1 to 2 bar is comparatively higher than from 1 to 3 bar. Here, 23.5 % performance increase from 1.4 to 2 bar and 34.8 % from 1.4 to 3 bar are obtained. Furthermore, it can be seen that a rise in temperature has a higher benefit with rising pressure. Moreover, the effect of gas variation decreases with rising pressure.