(486d) Impacts of Practical Considerations On the Steady-State Behavior of a Solid Oxide Fuel Cell

Temperature control is of great importance in the operation of high temperature, solid oxide fuel cells (SOFCs). SOFCs can exhibit steady state multiplicity[1,2]. The steady state multiplicity has been attributed to the temperature dependence of the electrolyte conductivity in the cells. In many fuel cell systems, the temperature of the upper steady state may be too high to be desirable for the balance of plant components. The existence of steady state multiplicity including hot spots in SOFCs as well as ignition and extinction phenomena motivates a thorough study of the steady state behavior of the cells. The multiplicity has been observed and reported by two groups [1,2]. Several practical factors were not accounted for in the models used in these previous studies. These include: (i) the concentration polarization, which has a strong effect on the fuel cell performance at high current densities, (ii) the temperature dependence of the physical properties of the gases, (iii) the dependence of the convection heat transfer coefficients on the operating conditions, (iv) the cathode and anode ohmic resistances, and (v) the variation of gas compositions in the anode and cathode channels along the channels. Also, the operating conditions that lead to autothermal operation (ignition and extinction) were not investigated thoroughly.

Contrary to the previous steady-state-analysis studies that have been based on a simplified SOFC model, this study is based on a detailed mathematical model that represents an actual SOFC more accurately. The detailed mathematical model is first developed. Heat transfer, mass transfer and electrochemical processes are taken into account. The resistivities of electrolyte, anode and cathode materials are allowed to be functions of temperature[3]. The impact of these processes on the steady state multiplicity is studied. As expected, the existence of a unique steady state or multiple steady states in the system depends on the operating conditions, e.g., feed temperature, feed composition and the external load. The range of operating conditions for which steady state multiplicity exists is determined. The hysteresis phenomenon is shown by varying the external load resistance; when the load resistance is reduced below a critical value (ignition point), a jump in the steady state cell temperature occurs, and when the load resistance is increased beyond a critical value (extinction point), the steady state temperature drops sharply.

References

1. Mangold, M.; Krasnyk, M.; Sundmacher, K. In Theoretical investigation of steady state multiplicities in solid oxide fuel cells, 2006; Springer: 2006; pp 265-275.

2. Vayenas, C. G.; Debenedetti, P. G.; Yentekakis, I.; Hegedus, L. L., Cross-flow solid-state electrochemical reactors - a steady-state analysis. Industrial & Engineering Chemistry Fundamentals 1985, 24(3), 316-324.

3. Bessette II, N.; Wepfer, W.; Winnick, J., A mathematical model of a solid oxide fuel cell. Journal of the Electrochemical Society 1995, 142, 3792.