Fuel cell Carnot heat engine

Data presented in Table 4.3 shows variations in enthalpy of reaction and entropy of reaction with increase in temperature. The change in Gibbs function is estimated based on Equation 4.30a, and results show a decrease in negative of Gibbs energy of formation. Thus, the reversible work of a fuel cell also decreases with increase in temperature. This is in contrast to the reversible work of a Carnot heat engine where the net reversible work and, hence, the thermal efficiency increase with increase in high-temperature heat somce. 

It is commonly expressed that a fuel cell is more efficient than a heat engine because it is not subject to Carnot Cycle limitations, or a fuel cell is more efficient because it is not subject to the second law of thermodynamics. These statements are misleading. A more suitable statement for... 

The portion AQ = AH - AG = TAS of AH is transformed into heat. Ideal theoretical efficiencies % determined by the types and amounts of reactants and by the operating temperature. Fuel cells have an efficiency advantage over combustion engines because the latter are subdued to the Carnot limitation. High thermodynamic efficiencies are possible for typical fuel cell reactions (e.g., e,h = 0.83 (at 25°C) for H2 + I/2O2 -> H20(i)). The electrical potential difference between anode and cathode, = -AG/W(f, which is also called the electromotive force or open-circuit voltage, drives electrons through the external... 

The combination of a SOFC with a heat engine allows highest electric efficiencies. This is caused by the comparable low entropy production within high temperature fuel cells. Generally, the combination of a reversible fuel cell and a reversible heat engine, as represented by the Carnot cycle, results in a reversible process at any operating temperature of the fuel cell. This combination can be used as refer-... 

Note that the enthalpy of combustion is 283.0kjmol an unrelated fact. The product of a Carnot efficiency of, say, 50% and the combustion enthalpy is 143.5 kW smol (cf 534.5). The potential superiority of the fuel cell is evident. Referring the efficiency of heat engines to the fuel... 

---