Emf, maximum power and efficiency of fuel cells
- Marquette Univ., Milwaukee, WI (United States)
The ideal voltage of steady-flow fuel cells is usually expressed by Emf = [minus] [Delta]G[degree]/nF where [Delta]G[degree] is the Gibbs free energy of reaction'' for the oxidation of the fuel at the supposed temperature of operation of the cell. Furthermore, the ideal power of the cell is expressed as the product of the fuel flow rate with this emf. Such viewpoints are flawed in several respects. Even if the cell operates isothermally, the use of the conventional [Delta]G[degree] of reaction (a) assumes that the products of reaction leave separately from one another ( and from any unused fuel); and (b) when [Delta]S of reaction is positive, it assumes that a free heat source exists at the operating temperature, whereas if [Delta]S is negative, it neglects the potential power which theoretically could be obtained from the heat released during oxidation. Moreover, (c) the usual cell does not operate isothermally, but (virtually) adiabatically. Comment (a) is often accounted for by employing the Nernst equation to correct for the dilution of reactants and/or products. Nevertheless, comments (b) and (c) remain pertinent. Rather than with emf, the proper starting place is with power output. The ideal power is that which would be obtained if the fuel were oxidized without irreversible entropy generation. Among other factors, this ideal power output depends upon the ratio of oxidant to fuel flow rate (e.g., air-fuel ratio) and the percentage of fuel oxidation. The ideal voltage is deduced from the ideal power. Therefore, (d) ideal power is not equal to the product of emf with current (unless the amount of fuel utilized is infinitesimal). Examples are presented which illustrate such affects and their importance for the evaluation of ideal power and of efficiency.
- OSTI ID:
- 6273230
- Journal Information:
- Journal of Energy Resources Technology; (United States), Vol. 115:2; ISSN 0195-0738
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
FUEL CELLS
ENERGY EFFICIENCY
ELECTROMOTIVE FORCE
FREE ENTHALPY
FUEL-AIR RATIO
FUELS
MATHEMATICAL MODELS
OPERATION
OXIDATION
POWER
CHEMICAL REACTIONS
DIRECT ENERGY CONVERTERS
EFFICIENCY
ELECTROCHEMICAL CELLS
ENERGY
PHYSICAL PROPERTIES
THERMODYNAMIC PROPERTIES
300505* - Fuel Cells- Electrochemistry
Mass Transfer & Thermodynamics