Geology Reference
In-Depth Information
Exergy is the potential for change of a system whilst not in thermodynamic
equilibrium with the reference environment (R.E.) that is to say at “dead state”.
At dead state a system is at the temperature, pressure and concentration of its
surroundings; it has no kinetic or potential energy and no reactions occur. The
R.E. must be characterised by a set of intensive properties
4
such as temperature
(T
0
), pressure (p
0
), concentration (x
0
), chemical potential of the substances in the
environment (
0i
) or extensive ones such as specific volume (v
0
), entropy (s
0
), and
the number of moles of substance i in the environment (N
0i
) plus height (z
0
) and
velocity (C
0
).
With respect to the R.E., all materials have a definable and calculable exergy
content which is itself subject to the given material's properties. Once the R.E.
has been specified, a value can be assigned to exergy allowing it to become a pro-
perty of the system. As an absolute R.E. does not exist, it is selected in function
of the problem needed to be solved. Therefore, exergy is in no way an absolute
property of matter, even if the system is in equilibrium with its intensive proper-
ties already specified. It is an extensive property, with the same units as energy.
Moreover, exergy cannot be negative
5
and unlike mass or energy, it is not con-
served but destroyed by irreversibilities. Thus and worth reiterating, in all physical
transformations of matter or energy, it is always exergy that is lost.
The specific exergy of a system per unit of mass (b) is frequently defined as:
+
1
2
(C
2
C
0
)
b = (
uu
0
) + p
0
(v
v
0
) T
0
(ss
0
)
|
+ g
(zz
0
)
|
{z }
potentialexergy
(9.27)
{z
}
| {z }
kineticexergy
thermo-mechanicalexergy
where subindex 0 denotes the dead state. Accordingly, the specific exergy of a sys-
tem is defined by a set of measurable parameters characterising its thermodynamic
properties. In Eq. (9.27) these are: internal energy (u) temperature (T), pressure
(p), specific volume (v), entropy (s), velocity C and altitude z. The exergy method
associates each parameter with its corresponding exergy component, which in this
case includes thermo-mechanical, kinetic and potential. Note that Eq. (9.27) only
takes into account a few exergy components which although relevant in the ana-
lysis of industrial systems, have limited application in the assessment of mineral
resources. Indeed, other properties are far more appropriate, as explained in the
next section.
4
Properties are considered to be intensive if they are independent of the size of the system
(temperature, pressure, density :::) or extensive if their values depend on the size or extent of the
system (mass, volume, total energy, etc.).
5
As will be seen in Chap. 11 the chemical exergy of certain substances (especially anions dis-
solved in seawater) is negative. This issue can be avoided by choosing an appropriate reference
environment but such a task is not free of di
culties.
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