From Thermodynamics to Economics and Ecology - Thanatia: The Destiny of the Earth's Mineral Resources - page 70

Exergy is thus the thermodynamic expression of power and distinction. To place
that into its proper context, it is not about how valued someone or something is in
their own right but instead how far they can be distinguished from the surrounding
environment. In layman's terms a Nobel prize laureate is as much praised as fewer
other laureates are around. A rich man is more powerful when poorer men surround
him. The same newspaper has a different value depending on if it is today's or
yesterday's, found by a historian of the 30th century or by an alien. And yes, for
Snow White it is easy to look beautiful around her seven dwarfs.
3.2.4 The nature of irreversibility
Entropy generation is always positive in irreversible processes and zero in reversible
ones. The more irreversible the process is, the greater its entropy generation. The
exergy balance of a steady state process is non-zero and the result is named ir-
reversibility, denoted I. Consequently, irreversibility is the exergy (useful energy)
destroyed in the process. The Gouy-Stodola theorem: I = T 0 , links irreversibility
with entropy generation through the surrounding temperature T 0 .
As opposed to mass and energy balances that are conservative and save engineers
the effort of having to measure one term per equation, an exergy balance neither
saves measuring nor calculating efforts. Energies and entropies of all entering and
exiting flows must be determined. The outcome of the balance is shown to be the
irreversibility of the system. Thus, if entropy or even exergy are not well under-
stood, it is obvious why their analysis is not so widespread in industry. Perhaps
it is a matter of too much effort for not an altogether practical message! Process
industries use the energy balance to determine energy losses, i.e. those that cross
system boundaries. The industrial basis for this is down to the simple but in the
authors' opinion false reasoning of “we lose e ciency because we lose energy”. For
if this is indeed the case, then the contrary is also true: “if we do not lose energy,
we do not lose e ciency”. Both are incorrect. The mechanisms by which energy
degrades are not located in the system boundaries but in the loss of energy quality
when energy itself is transferred. And, irreversibility is the measure of that energy
degradation. It is in short, the exergy that is destroyed forever and the key to energy
saving. Consequently, the authors would like to stress the fundamental importance
of understanding the mechanism by which irreversibility is produced, in order to
minimise exergy losses. Any practical energy saving compendium can therefore
be summarised in a single sentence: understand irreversibilities and actively avoid
them.
But how many kinds of irreversibilities exist? The answer to this question stems
from the concept of equilibrium. Energy systems spontaneously decrease their pres-
sure, temperature and chemical potential to reach equilibrium with their surround-
ings without any positive effect. If a system is to remain at equilibrium, it needs
to be enclosed within rigid walls to prevent pressure changes; adiabatic walls for