ABSTRACT
This chapter focuses on several thermodynamic concepts that are important in the study of fuel cells. Since in fuel cells reactants (fuel and oxidant) react to produce the useful electric energy output, we first review several thermodynamic concepts related to reacting systems that are specifically useful for fuel cell analysis: absolute enthalpy, enthalpy of reaction, heating values, Gibbs function, Gibbs function of formation, and Gibbs function of reaction. Next we examine the maximum possible performance for fuel cells, namely, the reversible cell potential by using the first and second laws of thermodynamics, and its variations with the operating conditions such as temperature, pressure, and reactant concentrations in the reactant streams. Finally, the issue of energy conversion efficiency is presented with the help of the first and second laws of thermodynamics. The maximum possible efficiency for fuel cells is investigated; a comparison is made with Carnot efficiency which is the maximum possible efficiency for heat engines against which fuel cells are competing for commercial success. Then the possibility of more than 100% efficiency for fuel cells is examined. The energy conversion efficiency for a fuel cell system comprised of fuel cells and auxiliary equipment is considered, and efficiency loss mechanism for operating fuel cells is also discussed. Examples are presented, where necessary, to illustrate the concepts and principles involved.
