From Thermodynamics to Economics and Ecology - Thanatia: The Destiny of the Earth's Mineral Resources - page 68

temperatures. The maximum work reached under these conditions will then be:

W max = Q(1 T 0 =T) (3.6)

This expression is very important because it allows for the making of analogies.
Going back to the hydraulic turbine, it can be said that the maximum mechanical
work of a water flow at elevation Z with respect to Z 0 is W max = mg(Z Z 0 ).
This magnitude is known as the potential energy of a mass flow relative to Z 0 . In
the same way, one could identify the term Q(1 T 0 =T) as the potential energy for
doing mechanical work that a heat flow has at temperature T with respect to the
base temperature T 0 . This magnitude was coined Exergy by the Yugoslavian Zoran
Rant in 1956, here denoted 1 by B.
The exergy concept (formerly referred to as availability) is attributed to Gibbs
and Maxwell, for their respective work in the last quarter of the 19th century. Ne-
vertheless it was at Rant's suggesting of the word “exergy” that the property became
popular among engineering thermodynamicists and boosted the use of Second Law
analysis in the teaching, design and improvement of energy systems. A plethora
of contributors, particularly in Europe, independently developed the ideas behind
this property. Relevant authors include Baehr, Bosnjakovich, Brodiansky, Gaggioli,
Gyftopoulos, Le Goff, Szargut and Kotas.
Once a reference condition(s) 2 denoted by 0, is set, exergy is converted into an
absolute and generalisable property. The kinetic, potential, electric and magnetic
exergies are equal to their respective energies. The chemical exergy of a substance
is close to its Gibbs free energy, which is normally tabulated. The chemical exergy
of fossil fuels is close to its High Heating Value (HHV). Only exergy presents a less
usual form when heat flows are considered, since, as seen before, it depends on the
temperature T 0 towards which heat is transferred.

B Q = Q(1 T 0 =T) (3.7)

If thermal energy were not the most common and degraded energy form, chang-
ing the name energy into exergy would not make sense, as it is arguably more
di cult for the reader to understand the Second Law than the First. This can be
understood through a simple illustration: How can one accept that when a piece of
wood is burnt, its energy is not lost when in fact it can only be burnt once? Exergy
is capable of simultaneously expressing both the quantity and quality of an energy
flow so the answer is that whilst its energy is conserved and transferred, its exergy
degrades into the form of a heat flow, “unhelpfully” heating the atmosphere. Thus
it is not the quality of energy that is conserved but rather its quantity.
Eq. (3.7) states a great deal about the difference between heat flow Q and its
exergy B. In fact, if they were to be represented graphically against T, Fig. 3.1
would be obtained.
1 Note that exergy can be also denoted by the letters E or Ex (Tsatsaronis, 2007).
2 A feature which will be analysed in detail in Chap. 10.