Maximum efficiency, heat engine work, electrical

In traditional fnel technology, the same fnel wonld be burned in air, producing an amonnt of heat = AH, the enthalpy of combnstion. The heat would then be nsed to rnn a heat engine-generator system to prodnce electrical power. The efficiency of conversion of heat to work is limited by the laws of thermodynamics. If the heat is supplied at temperature Th and if the lower operating temperature is T , the maximum work obtainable (see Section 13.4) is... 

The heat produced on merely burning 1 mole H2, with no work obtained directly, is — AH = 68.32 kcal. This is the <7h in the expression for efficiency the heat that is put into the working substance in a heat engine when the fuel bums. If the reaction can be made to take place in a galvanic cell, the maximum electrical work available (when the cell operates reversibly) per mole Hj is —AG = 56.69 kcal. This is the w in the expression for efficiency the work done by the device. The maximum efficiency of the cell then is... 

The plot shows only the maximum possible efficiency, which will not be obtained in practice for the heat engine or fuel cell. For a fuel cell, the efficiency decreases with increasing electrical power, so that it only approaches the theoretical value at open-circuit conditions, where no useful electrical work is produced. 

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