Carnot limited

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... 

For ZT 1, Carnot efficiency is obtained for power generation and the Carnot limit to the COP is obtained for refrigeration. Materials currently used in thermoelectric devices have ZT values between 0.4 and 1.0. 

If we try to assess the relative economy of a heat pump versus burning fossil fuel directly, we must bear in mind that, if the electrical energy to run the heat pump comes from fossil fuel, the power plant is subject to the Carnot limitation. The overall efficiency of a modern steam power plant is about 35 percent. Thus, just to break even on fossil fuel consumption, the heat pump coefficient of performance must be at least 1/0.35 = 2.9. 

A schematic of the principles behind integrating an OTM-based hydrocarbon fuel reformer with a SOFC for the non-Carnot limited conversion of this feedstock into electricity is shown in Fig. 7.7. 

A fuel cell is an electrochemical device for converting the chentical energy of a fuel directly into electricity, without the efficiency losses of conventional combustion processes which are constrained by the Carnot limit. In principle, the conversion efficiency is 100% for FCs, while it is around 35-40% for the Carnot corrversion at the temperatures itsed. In real cases, efficiencies of FCs are lowered by the requirements of other plant componerrts, such as reforming arrd preheating phases as well as irreversibilities affecting the process. 

Electrochemical reactions can give energy conversion to electricity in a completely different way which is free of this Carnot limitation and thus of the energy loss referred to. In fact, theoretical maximum efficiencies in electrochemical reactions are in the 90% region and practical ones above 50%. Such energy conversion arises by means of a fuel cell, the principle of which is shown in Figure 4. 

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