Second Law Efficiency

The Second Law efficiency, n2nd, of an energy conversion device indicates its degree of reversibility, comparing the actual work against the maximum work potential. For example, the performance of an actual heat engine would be divided by the work produced by a Carnot cycle engine, as in Eq. (3.40).

n2nd,heat engine -^W

The expression could also be written in terms of thermal efficiencies, n, comparing the actual to the maximum. For fuel cells, using the thermal efficiency expressions of Eqs. (3.37) and (3.38), the Second Law efficiency becomes a voltage efficiency.

n 2nd,heat engine nFE nFE°

Equation (3.41) is therefore a comparison of the actual voltage to the maximum voltage, which is 1.23 V for a hydrogen-oxygen cell at 25°C and 1 atm. If the voltage were 0.7 V, the Second Law efficiency would be n2nd = 0.57, indicating that 43% of the available energy was not converted to work. This exergy (work potential) is lost, dissipated as heat, because of the inefficiencies or polarizations within the fuel cell (see Section 3.5.3 for Overpotential).

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