THERMODYNAMIC BASIS OF FUEL CELL SYSTEMS
Power maximization approach is applied for fuel cells 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 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 high-temperature 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. The power formula summarizes the effect of transport laws, irreversible polarizations and efficiency of power yield. The 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. The efficiency decrease is linked with thermodynamic and electrochemical irreversibilities expressed in terms of polarizations (activation, concentration and ohmic). Effect of incomplete conversions is modelled in a novel way assuming that substrates can be remained after the reaction and that side reactions may occur. Optimum and feasibility conditions are obtained and discussed for some important input parameters such as the effciency, power output, and electric current density of the cell. Calculations of the maximum power show that the data differ for power generated and consumed, and depend on parameters of the system, e.g., current intensity, number of mass transfer units, polarizations, electrode surface area, average chemical rate, etc.. These data provide bounds for SOFC energy generators, which are more exact and informative than reversible bounds for electrochemical transformation.
CYBERNETICS AND PHYSICS, VOL. 1, NO. 1, 2012, 67-72.