Comparison Between a Fuel Cell and an Engine -in Thermodynamic View-

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  • 熱力学的考察による燃料電池とエンジンの比較

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<p>Fuel cells are said to be more efficient than combustion engines. But, is this true? The Carnot factor indicates the maximum efficiency of a combustion engine, and ΔGrxn / ΔHrxn (Gibbs energy change divided by enthalpy change for a reaction) gives that of a fuel cell. Clearly, the expression for the maximum efficiency differs between them. Hence, this comparison is not reasonable. This study aims to provide a reasonable comparison with theoretical calculations based on a thermodynamic model. The model developed in this study assumes that a fuel cell consists of only one reactor, a so-called cell. An isothermal combustion engine comprises multiple series of combustors (high temperature heat sources), Carnot cycle engines and low temperature sinks. In addition to these components, an adiabatic combustion engine embeds a heat exchanger. The processes in the fuel cell and the combustor progress under constant atmospheric pressure. To ensure a reasonable comparison, the inlet and outlet for both fuel cell and combustion engine specify just reactants (fuel and oxygen) and products (water) at standard condition, respectively. Theoretical calculations based on these models indicate the maximum efficiencies (W/ΔHrxn(T0)) of 95% for a fuel cell, 92% for an isothermal combustion engine, and 77% for an adiabatic combustion engine, respectively. In addition to the entropy generation by mixing hydrogen and oxygen gases, temperature dependence of the enthalpy and entropy changes leads to a slightly less efficiency of an isothermal combustion engine than a fuel cell. The entropy generation with an adiabatic process results in a significant drop in the efficiency of an adiabatic combustion engine.</p>

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