Carnot power plant performance

The second law of thermodynamics may be used to show that a cyclic heat power plant (or cyclic heat engine) achieves maximum efficiency by operating on a reversible cycle called the Carnot cycle for a given (maximum) temperature of supply (T ax) and given (minimum) temperature of heat rejection (T jn). Such a Carnot power plant receives all its heat (Qq) at the maximum temperature (i.e. Tq = and rejects all its heat (Q ) at the minimum temperature (i.e. 7 = 7, in) the other processes are reversible and adiabatic and therefore isentropic (see the temperature-entropy diagram of Fig. 1.8). Its thermal efficiency is 

Clearly raising 7 ,ax and lowering 7 ,in will lead to higher Carnot efficiency. 

The Carnot engine (or cyclic power plant) is a useful hypothetical device in the study of the thermodynamics of gas turbine cycles, for it provides a measure of the best performance that can be achieved under the given boundary conditions of temperature. 

It has three features whieh give it maximum thermal effieieney  

In his search for high efficiency, the designer of a gas turbine power plant will attempt to emulate these features of the Carnot cycle. 

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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. 

In 1894, the German physical chemist Wilhelm Ostwald formulated the idea that an electrochemical mechanism can be used instead of combustion (chemical oxidation) of natural kinds of fuel, such as those used in thermal power plants. Because in this case the reaction will bypass the intermediate stage of heat generation, this would be cold combustion, the direct conversion of chemical energy of a fuel to electrical energy not being subject to Carnot-cycle limitations. A device to perform this direct energy conversion was named fuel cell. 

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