The Resources of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 137

Table 6.2 Estimates of bulk continental crust heat production from heat flow data
(Jaupart and Mareschal, 2004).

Age group Range of heat Range of crustal Fraction of total

production heat flow, continental

Wm 3

mWm2 surface, %

Archean 0.56-0.73 23-30 9

Proterozoic 0.73-0.90 30-37 56

Phanerozoic 0.95-1.10 37-43 35

Total continents 0.79-0.95 32-38

That said, geothermal energy represents only a tiny fraction of all geothermal
heat. As with tidal energy, geothermal energy can only ever be important locally.
According to the Renewables Global Status Report (Martinot, 2006), the 2005 world-
wide geothermal capacity was 28 GW for direct thermal use and 9.3 GW for elec-
tricity production. The latter figure increased to 10.7 GW in 2010 and is expected
to grow to 18.5 GW by 2015, according to the International Geothermal Associa-
tion. Meanwhile, the Geothermal Energy Association (Gawell et al., 1999) reports
that geothermal resources using today's technology have the potential to support
between 35,448 and 72,392 MW of electrical generation capacity. Using enhanced
technology (permeability enhancement, drilling improvements) which is currently
under development, means that geothermal resources could eventually support be-
tween 65,576 and 138,131 MW of electrical generation capacity. Assuming a 90%
availability factor, which is well within the range experienced by geothermal power
plants, as much as 1:09 10 9 MWh of electricity annually (124 GW) could be
produced (Table 6.8).

6.4.2 Nuclear energy

Nuclear energy is derived from the huge binding force of the elemental nucleus.
There are two kinds of processes that can release nuclear energy: fusion and fission.
Fusion consists in binding together light elements, such as hydrogen and lithium
and thereby forming heavier ones. This is how the sun creates its energy. It is
however yet to be achieved in the laboratory under conditions where the energy
produced exceeds the energy sacrificed to create it. Nevertheless, many scientists
believe that it could be the solution for future energy supply and security. Her-
mann (2006) estimated the exergy reservoir for the fusion cycle between deuterium
(coming from the ocean) and tritium (bred from an isotope of lithium) as around
74 Ttoe 3 . Furthermore, if deuterium, the isotope of 1 in every 5,000 hydrogen
atoms, is fused with another deuterium nucleus at higher temperatures, the result-
ing resource contained in the ocean would be in the order of magnitude of 10 million
YJ 4 .
3 1 tonne of oil equivalent (toe) is equal to 41.868 GJ.
4 A yottajoule (YJ) is equivalent to 10 24 Joules.