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Energy and geotechnologies 13

Figure 1.10 Sketch of typical temperature evolution with depth in the Earth’s subsurface. Redrawn
after Boehler, R., 1996. Melting temperature of the Earth's mantle and core: Earth’s thermal structure.
Annu. Rev. Earth Planet. Sci. 24 (1), 15 40.

approximately 0.1 C per 100 m of depth (cf. Fig. 1.10). These temperature levels
can be used in geothermal applications to meet human activity needs in the built
environment.

1.3.3 Features of geothermal energy
Geothermal energy represents the second most abundant source of primary energy on
Earth, after solar energy (Lee et al., 2007). It is classified as both renewable and sus-
tainable (Lund, 2009), and represents one of the energy sources that can be used in
the construction sector for the development of low-carbon buildings (in several
countries, also resorting to the support of government grants and incentives).
Geothermal energy can be considered as a clean and environment-friendly energy
source as it generates no (or minimal) greenhouse gas emissions because the conver-
sion and utilisation processes do not involve any chemical reactions (e.g. combustion)
(Lee et al., 2007). This energy source is also available continuously, regardless of the
weather conditions and everywhere on Earth, which makes it attractive in contrast to
other renewable energy sources such as solar and wind energy. Among other various
positive attributes (Lee et al., 2007), geothermal energy reduces the current depen-
dence on nonrenewable energy sources and it can be used for various purposes from
a local to a relatively large scale. Because geothermal energy is available almost every-
where, the consequent reduction of energy imports means a reduced dependence on
external economic or political situations (Brandl, 2006). Geothermal energy ensures
the security of supply.