Maximum efficiency of direct energy conversion systems. Application to fuel cells

This work develops a methodology to calculate the maximum First Law efficiency of any power device. To do so external irreversibilities due to heat transfer are suppressed by means of Carnot engines which mechanical work is added up to the one resulting from the device under hypothetical internally reversible mode of operation. By applying the developed methodology comparisons between Direct Energy Conversion Systems (DECS) and power cycles can be properly set on balanced assumptions in each type of device. In this paper, the main equations of the methodology are presented and the specific equations for fuel cells are derived. The potential of this methodology is illustrated by comparing fuel cells and power cycles First Law efficiency as a function of temperature.