Modeling and Simulation of Power Yield in Chemical and Electrochemical Systems

Fuel cells are treated as flow engines driven by fluxes of chemical reagents and electrochemical mechanism of electric current generation. Analyzed are performance curves of a SOFC system, power limits and the effect of typical design and operating parameters on the cell performance. The theory combines a recent formalism worked out for chemical machines with the Faraday’s law which determines the intensity of the electric current generation. Steady-state model of a hightemperature SOFC is considered, which refers to constant chemical potentials of incoming hydrogen fuel and oxidant. Lowering of the cell voltage below its reversible value is attributed to polarizations and imperfect conversions of reactions. A power formula summarizes effect of transport laws, irreversible polarizations and efficiency of power yield. Reversible electrochemical theory is extended to the case with dissipative chemical reactions; this case includes systems with incomplete conversions, characterized by “reduced affinities” and an idle run voltage. Effect of incomplete conversions is modeled by assuming that substrates can be remained after the reaction and that side reactions may occur. Optimum and feasibility conditions are discussed for some important process parameters such as the efficiency, power output, and electric current density. Calculations of maximum power show that the data differ for power generated and consumed. These data provide bounds for SOFC energy generators, which are more exact and informative than classical reversible bounds for electrochemical transformation.