From Thermodynamics to Economics and Ecology - Thanatia: The Destiny of the Earth's Mineral Resources - page 71

temperature maintenance; and impermeable walls to avoid the exchange of chemical
substances or to keep them at a given metastable state. Effectively, this means that
such systems are stored in reservoirs so as to uphold their intensive properties over
time.
Mechanical, thermal and chemical equilibrium inherently implies that there are
also three causes of disequilibrium and subsequently three sources of irreversibility 3 :
Mechanical irreversibilities appear in processes that involve friction due to pres-
sure changes. The greater this change, the greater the irreversibility created
within the system. Consequently, entropy generation or exergy destruction oc-
curs.
Thermal irreversibilities appear when there is a finite temperature change within
the system as in every heat exchanger, for instance. Heat spontaneously passes
from a warm body to a cold one, thereby losing exergy. Again, the irreversibility
produced increases with the temperature difference 4 .
Chemical irreversibilities appear when some chemical disequilibrium exists, such
as occurs with mixtures, solutions and chemical reactions. For example when
water and salt mix, exergy is destroyed. This exergy loss is equivalent to the
minimum work that would be needed to desalinise water. Dispersion is a typical
chemical irreversibility. Contamination (mixing) is a spontaneous process which
needs a lot of exergy to reverse the process (clean up).
Thus, minimising the mechanical, thermal and chemical gradients that appear
in processes is key to saving energy and achieving good e ciencies as will be seen
later in Chap. 16.
3.3 From Thermodynamics to Economics: Thermoeconomics
Irreversibility is the root of the physical cost and connects Thermodynamics to
Economics to such an extent that a new branch of knowledge was created: Ther-
moeconomics. As explained in Sec. 2.6.5, this discipline was intended to build a
narrow but solid bridge between the Second Law and Economics.
The next sections describe the fundamentals of Thermoeconomics and exergy
cost.
3 In reality for each pair of intensive-extensive properties, there exists a “wall” that upon being
broken an associated irreversibility appears, i.e. as in the case of magnetic force-magnetisation.
4 Surprisingly, the e cacy of a heat exchanger increases with the temperature difference, T,
between the hot and the cold streams. On the contrary, its Second Law e ciency decreases with
T. So whilst cheap and small heat exchangers are popular they are not a recommendable energy
saving strategy.