Page 8 - Entrophy Analysis in Thermal Engineering Systems

Preface xiii

Chapter 5 presents a comparative assessment of the efficiency of common
heat engines. The chief goal is to illuminate that determination of the most
efficient engine is contingent on specific assumptions. For example, the Car-
not engine along with the Stirling and Ericsson engines are said to possess the
highest efficiency among all heat engines subject to an assumption that the
highest and the lowest temperatures are the same for all the engines. If, how-
ever, the engines are constrained to experience the same degree of compres-
sion, the Carnot engine is no longer the most efficient design.
Our investigation continues by applying entropy analysis to simple and
advanced power cycles. The objective is to show that entropy production
may become equivalent to an efficiency loss under specific conditions.
We will see in Chapter 6 that in endoreversible heat engines, a class of the-
oretical heat engines which experience external irreversibility only, the
thermal efficiency happens to inversely correlate with the entropy produc-
tion. Nevertheless, in practice, engines do also experience internal irrevers-
ibilities. It will be shown in Chapter 7 that a design based on minimum
entropy production rate in irreversible engines operating in closed cycles
is not equivalent to either of maximum power and maximum efficiency de-
signs. The three designs may, however, become identical if, for instance, the
thermal energy supplied to an irreversible engine operating between a heat
source and a heat sink, or the power output is treated as a fixed parameter.
In Chapter 8, we investigate the applicability of a second law-based anal-
ysis in conventional thermal power plants such as gas turbine and combined
gas/steam cycles, which are usually driven by fuel combustion. In this chap-
ter, the concept of specific entropy generation (SEG) is introduced, a new
parameter that measures the entropy production of a power cycle per unit
of fuel burned. It will be shown that SEG unconditionally correlates with the
inverse of the cycle efficiency, and it can be viewed as a measure of efficiency
losses in combustion-driven power generating systems. An application of
the SEG concept to typical thermal power plants is explored.
An investigation on the application of entropy analysis to fuel cells is
presented in Chapter 9. The primary objective is to show that the theoretical
efficiency of a fuel cell is not bound by the efficiency of a Carnot cycle oper-
ating between the same low and high temperatures. Chapter 10 examines
possibility of any connection between entropy and chemical equilibrium.
A careful assessment of the Gibbs criterion of equilibrium reveals that the
characterization of a chemical equilibrium by minimum Gibbs function is
simply a postulation without a strong experimental evidence or theoretical
proof. The last chapter explains the exergy concept and describes how it is