Reduction of Thermal Stress during Soec Unsteady Operation

Abstract

In recent years, natural energy sources such as solar and wind power have been introduced into the electric grid. However, the wind and solar power suffer instability with respect to changes in the weather. As the proportion of these energies increases, a technique to store a surplus electric power is required. The hydrogen generation systems using solid oxide electrolysis cells (SOECs) are expected as large-capacity power storage facilities with the ability to stabilize the electrical power supply. For the use of the SOEC, it is important to grasp the unsteady characteristics, especially temperature distribution which is the one of the biggest destruction factor of the cell. In this study, the unsteady, two dimensional, tubular numerical model of a cathode supported micro-tubular SOEC was developed to clarify the temperature distributions and its responses .The temperature of anode surface were measured directly by K-type thermocouples during the simultaneous electrochemical performance measurements. The operating condition of SOEC was controlled to reduce the unsteady temperature distribution in SOEC. Rough description of the numerical model is shown in Fig.1. The model calculates heat-mass-charge transports and electrochemical reactions, simultaneously. The construction materials of SOEC cathode electrode was mixed materials of nickel (Ni) and yttria-stabilized zirconia (YSZ). The electrolyte was YSZ, and the anode electrode was mixture of lantern strontium manganite (LSM) and YSZ. The thicknesses of cathode electrode, electrolyte and anode electrode were 200 mm, 20 mm and 20 mm, respectively. The cell length was about 50 mm, with an outer diameter of about 2 mm. The flow rate of hydrogen and steam was set to be same. The operating temperature was 1123K. Experiments of SOEC were conducted in same conditions. The calculated temperature distributions at steady states are shown in Fig.2. The temperature decreased at 1.1 V, 1.2 V and increased at 1.3 V, 1.4 V. The maximum temperature gradient was observed around z=0.042 at 1.4 V. This position was consistent with the point where a clack was observed during a measurement. The maximum temperature gradients change during potential sweep with various intermediate voltage are shown in Fig.3. This shows that the maximum temperature gradient could be reduced by controlling the sweeping conditions. Figure 1