468 | Thermodynamics
In thermodynamics,the first-law efficiency(or thermal efficiency) of a
heat engine is defined as the ratio of net work output to total heat input.
That is, it is the fraction of the heat supplied that is converted to net work.
In general, the first-law efficiency can be viewed as the ratio of the desired
output to the required input. The first-law efficiency makes no reference to
the best possible performance,and thus the first-law efficiency alone is not
a realistic measure of performance. To overcome this deficiency, we defined
the second-law efficiency, which is a measure of actual performance relative
to the best possible performance under the same conditions. For heat
engines, the second-law efficiency is defined as the ratio of the actual ther-
mal efficiency to the maximum possible (reversible) thermal efficiency
under the same conditions.
In daily life, the first-law efficiencyor performanceof a person can be
viewed as the accomplishment of that person relative to the effort he or she
puts in. The second-law efficiencyof a person, on the other hand, can be
viewed as the performance of that person relative to the best possible perfor-
mance under the circumstances.
Happinessis closely related to the second-law efficiency.Small children
are probably the happiest human beings because there is so little they can do,
but they do it so well, considering their limited capabilities. That is, children
have very high second-law efficiencies in their daily lives. The term “full
life” also refers to second-law efficiency. A person is considered to have a
full life, and thus a very high second-law efficiency, if he or she has utilized
all of his or her abilities to the limit during a lifetime.
Even a person with some disabilities has to put in considerably more effort
to accomplish what a physically fit person accomplishes. Yet, despite accom-
plishing less with more effort, the person with disabilities who gives an
impressive performance often gets more praise. Thus we can say that this
person with disabilities had a low first-law efficiency (accomplishing little
with a lot of effort) but a very high second-law efficiency (accomplishing as
much as possible under the circumstances).
In daily life, exergy can also be viewed as the opportunities that we have
and the exergy destruction as the opportunities wasted.Time is the biggest
asset that we have, and the time wasted is the wasted opportunity to do
something useful (Fig. 8–50).
The examples above show that several parallelscan be drawn between the
supposedly abstract concepts of thermodynamics related to the second law
and daily life, and that the second-law concepts can be used in daily life as
frequently and authoritatively as the first-law concepts. Relating the abstract
conceptsof thermodynamics to ordinary eventsof life benefits both engi-
neers and social scientists: it helps engineersto have a clearer picture of
those concepts and to understand them better, and it enables social scientists
to use these concepts to describe and formulate some social or psychological
phenomena better and with more precision. This is like mathematics and sci-
ences being used in support of each other: abstract mathematical concepts
are best understood using examples from sciences, and scientific phenomena
are best described and formulated with the help of mathematics.(anonymous)I have only just a minute,
Only 60 seconds in it,
Forced upon me—can’t refuse it
Didn’t seek it, didn’t choose it.
But it is up to me to use it.
I must suffer if I lose it.
Give account if I abuse it,
Just a tiny little minute—
But eternity is in it.FIGURE 8–50
A poetic expression of exergy and
exergy destruction.