Page 186 - Entrophy Analysis in Thermal Engineering Systems

Exergy 181

exergy analysis in the thermodynamic model, it would be at the cost of
incorporating the function F(E i ) in the model whose effect would eventually
be null. Thus, it would comparably be less cumbersome to simply use
entropy analysis which would yield a relatively less complex thermodynamic
model than incorporating exergy analysis.

11.8.2 Limitation of the second law

Thermodynamics is the science of heat and work. It was referred by its foun-
ders as the Mechanical Theory of Heat. The first law was founded on the obser-
vations that heat can be produced by spending work, and work may be
generated by consumption of heat. The second law is based on the impos-
sibility of complete conversion of heat to work in an engine, or impossibility
of spontaneous transference of heat from a cooler to a warmer body. The
formulation of the second law, however, is based on a special condition, i.e.,
reversibility, that guarantees a production of maximum work from a given
amount of heat. The condition of reversibility is characterized by zero
entropy generation.
Unlike the first law that can be applied to a variety of processes with or
without a presence of work, an application of the second law to a system
with no work interaction is not expected to produce meaningful results.
The first law is a form of the conservation of energy; a general principle
whose truth is independent of the nature or kind of energy. On the other
hand, the formulation of the second law rests on the presence of both
heat andwork. It maysimplybestated asthe equivalence ofreversibility,
zero entropy generation, and maximum work; the second law requires
all these three elements at once. If this be agreed as a simple statement
of the second law, the question is then what would reversibility or zero
entropy generation represent in a system that does not include any work
interaction externally?
Not every machine invented by mankind is a heat engine, a refrigera-
tor, or a heat pump. There are many devices that are designed to fulfill
other objectives where a production or consumption of work (power) is
of no or minor importance. An example is heat exchangers whose primary
task is to transfer heat from a hot fluid to a cold fluid. Once the hot fluid is
cooled down to a desired temperature and, at the same time, the cold fluid
is heated up to a preset temperature, it may be said that the heat exchanger
has fulfilled its objective. In addition to exchanging heat, the two fluids
may experience retardation of motion due to, for example, friction,