Conventional Power Generation—Heat Engine

A major requirement for a heat engine is that it operates between two temperature limits a high-temperature heat source from which heat is added and a low-temperature heat sink at which a fraction of the heat is rejected. This requirement limits the maximinn efficiency of a heat engine to less than 100%. 

The maximum possible performance of a heat engine set by that given by a reversible heat engine operating on a Carnot cycle, which involves four reversible processes (i) reversible isothermal heat addition, Qh, (ii) reversible adiabatic expansion (work), W, (iii) reversible isothermal heat rejection, Ql, and (iv) reversible adiabatic compression. Thermal efficiency of the heat engine is given by 

Rankine vapor cycle external combustion heat engine. 

The thermal efficiency of a heat engine is defined as the ratio of net work output to the total heat energy added at the high-temperature source and expressed as 

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A conventional power plant fired by fossil fuels converts the chemical energy of combustion of the fuel first to heat, which is used to raise steam, which in turn is used to drive the turbines that turn the electrical generators. Quite apart from the mechanical and thermal energy losses in this sequence, the maximum thermodynamic efficiency e for any heat engine is limited by the relative temperatures of the heat source (That) and heat sink (Tcoid) ... 

The fission reaction has been successfully applied to industrial power production here the reaction is conducted not explosively, but in such a way as to provide a steady source of energy for power generation by conventional heat engines. This method is suitable -also for large vehicles such as atomic energy submarines and atomic energy aircraft (See also Atomic Weapons and Ammunition)... 

The conventional generation of electrical energy from a fuel requires the use of a heat engine which converts thermal energy to mechanical energy. All heat engines operate by the Carnot cycle, and their maximum efficiency is about 40-50% (for the modern gas-fired power stations, the efficiency is about 55%). 

Properly designed fuel cells may be as much as 70 percent efficient, about twice as efficient as an internal combustion engine. In addition, fuel-cell generators are free of the noise, vibration, heat transfer, thermal pollution, and other problems normally associated with conventional power plants. Nevertheless, fuel cells are not yet in widespread use. A major problem lies in the lack of cheap electrocatalysts able to function efficiently for long periods of time without contamination. The most successful application of fnel cells to date has been in space vehicles (Figure 19.12). 

Thermodynamic considerations lead to the first heat engines that were used to generate mechanical or electrical energy. Since the eighteenth centuiy, conventional reciprocating steam engines have served as mechanical power sources, with notable improvements being made by lames Watt. The first conunercial central electrical... 

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