The Exergy of the Earth and its Mineral Resources - Thanatia: The Destiny of the Earth's Mineral Resources - page 322

provided above 2 . That said, ice sheets and ice caps are an important source of
other types of exergy (not only chemical). Valero et al. (2002a) demonstrated
that the thermal and potential exergy components of ice sheets (calculated with
Eq. (9.27) and ignoring the mechanical and kinetic exergy) are far more important
than their chemical counterpart. Particularly, the potential exergy is due to the
fact that 99% of ice sheets are found in Greenland and Antarctica at an average
altitude of around 2,000 metres. This involves an associated potential exergy of
9.8 kJ/kg (176.4 kJ/mol). Thermal exergy meanwhile, is related to the extremely
low temperatures at which most ice sheets are found. Considering an average of
-28.9Â o C, Valero et al. (2002a) obtained that the thermal exergy of ice sheets was
26.02 kJ/kg (468 kJ/mol). All in all, such figures indicate that the total exergy
contained in the Earth's ice sheets is huge, about twice as much as that contained
in fossil fuel reserves Valero et al. (2002a). Considering the current melting rate
(estimated at more than 300 billion t/yr - (Rignot and Thomas, 2002)), the planet
is losing nearly 200 Mtoe of thermal exergy contained in ice sheets every year
(equivalent to the annual fossil fuel consumption of China). Moreover, if one were
to replace all ice caps with current technology, about 9,000 times the exergy of the
world's fossil fuel reserves would be required. Even if this calculation is of minor
practical use 3 , it serves to provide an order of magnitude as to the “treasures” on
Earth hidden in the form of ice sheets and ice caps, which are slowly but surely
melting because of human intervention through climate change.
Going back to the remaining water resources, Table 11.5 shows the estimated
average standard thermodynamic properties of groundwater with the mean com-
position assumed to be equivalent to that of the average compositions for granite,
shale and serpentinite based groundwater given in Table 5.10.
According to Table 11.5, the average standard enthalpy, Gibbs free energy and
chemical exergy of groundwater (gw) are then:

(H f ) gw = 286:0 kJ/mol
(G 0

f ) gw = 237:4 kJ/mol
(b 0

ch ) gw = 0:79 kJ/mol

Summarising, Table 11.6 shows the standard thermodynamic properties of the
hydrosphere. It is assumed that the waters of each reservoir are not mixed, so
therefore the Gibbs free energy and exergy of the mixture term is not taken into
account. Additionally, the composition of lakes is thought to be equal to that of an
average river and only the chemical component of ice caps and glaciers are shown.
Since the ocean constitutes 97% of all Earth's waters, the global enthalpy, Gibbs
2 They differ in the second decimal place, but this is enough to detect differences in water con-
ductivities.
3 Ice can also be used for practical applications as thermal energy storage systems for renewable
for instance. Consequently, the exergy of icebergs could well be used as “renewable ice energy
storage systems”.