Geology Reference
In-Depth Information
Chapter11
The Exergy of the Earth and its Mineral
Resources
11.1 Introduction
This chapter is devoted to the calculation of the standard thermodynamic properties
of the Earth (enthalpy, Gibbs free energy and chemical exergy) with a focus on each
of its outer layers: the atmosphere, hydrosphere and upper continental crust. Thus,
in order to do this, the geochemistry of the planet described in Chap. 5 is required,
together with the thermodynamic tools provided in Chap. 9.
Finally, the exergy of the mineral resources (both of fuel and non-fuel in origin)
is calculated, added to the energy sources described in Chap. 6 and compared with
the global chemical exergy of the crust.
11.2 The properties of the Earth
As explained in previous chapters, the thermodynamic properties of the Earth relate
to the compounds contained within it and not to their elements. Assuming that each
of the three
sp
heres are composed of a
n
ideal solution of substan
c
es, their average
enthalpy (
H
f
), Gibbs free energy (G
f
) and chemical exergy (b
ch
), expressed as
kJ/mol of atmosphere, hydrosphere or upper crust, is calculated as:
X
H
f
=
(x
i
H
f i
)
(11.1)
i=1
X
RT
0
lnx
i
)
G
f
=
x
i
(G
f i
+
(11.2)
i=1
X
RT
0
lnx
i
)
b
ch
=
x
i
(b
chi
+
(11.3)
i=1
whereby x
i
, is the molar fraction of the compounds included in each sphere of the
Earth, and H
f i
, G
f i
and b
chi
, their enthalpy, Gibbs free energy and chemical
exergy in kJ/mol, respectively.
317
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