Heat engines, efficiency

Two additional aspects of efficiency are of interest 1) the effects of integrating a fuel cell into a complete system that accepts readily available fuels like natural gas and produces grid quality ac power (see Section 9), and 2) issues arising when comparing fuel cell efficiency with heat engine efficiency (see below). 

According to the definition of heat engine efficiency, the efficiency of the Carnot heat engine is = output/6input = [area l-2-3-4-l]/[area... 

By taking into account the rate of heat transfer associated with the endoreversible cycle, the upper bound of the power output of the cycle can be found. This bound provides a practical basis for a real power plant design. The industrial view is that the heat engine efficiency is secondary to the power output in power plants whose worth is constrained by economic considerations. 

They possess very high efficiency (75.90%). In heat engines efficiency is around 40% or less. 

CARNOT S ANALYSIS OF OPTIMAL HEAT-ENGINE EFFICIENCY 123... 

Figure 12.2 shows the above relationships [3]. The limits are fundamental, and only certain combinations of heat engine efficiency and the H/P ratio are possible. 

Finfgeld, C., Machlup, S. 1960. Well-Informed Heat Engine Efficiency and Maximum Power, Amer. J. Phys. 28, 324. 

If the aim was the study of heat engine efficiencies then power, not energy, should have been the quantity of interest yet it is energy that enters into the famous Carnot cycle [1824] and its further development by Clapeyron, Clausius and Lord Kelvin. In fact, the power efficiency for a Carnot engine is negligible... 

Now let us take a closer look at the two most commonly used heat engines (steam and gas turbines) to see whether they achieve this efficiency in practice. To make a quantitative assessment of any combined heat and power scheme, the grand composite curve should be used and the heat engine exhaust treated like any other utility. 

Carnot s cycle A hypothetical scheme for an ideal heat machine. Shows that the maximum efficiency for the conversion of heat into work depends only on the two temperatures between which the heat engine works, and not at all on the nature of the substance employed. 

Whenever energy is transformed from one form to another, an iaefficiency of conversion occurs. Electrochemical reactions having efficiencies of 90% or greater are common. In contrast, Carnot heat engine conversions operate at about 40% efficiency. The operation of practical cells always results ia less than theoretical thermodynamic prediction for release of useful energy because of irreversible (polarization) losses of the electrode reactions. The overall electrochemical efficiency is, therefore, defined by ... 

Vapor-phase coohng reduces the cost of the cooling system, increases heat recovery, and may result in improved engine efficiency. 

As you may know, the ideal thermodynamic efficiency of a heat engine is given by... 

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... 

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