Page 45 - Analysis and Design of Energy Geostructures

14 Analysis and Design of Energy Geostructures

1.4 Geothermal systems
1.4.1 Classification of geothermal systems
Geothermal systems are technological systems that harvest geothermal energy. Various
classifications of geothermal systems can be made, but one classification that is often
employed relies on the exploitation depth of the thermal energy present in the subsur-
face. Depending on whether a lower or greater depth than 400 m (by governmental
definition in several countries) is considered, geothermal systems can be classified as
shallow geothermal systems or deep geothermal systems, respectively. Shallow geothermal
systems deal with low temperature and enthalpy. Deep geothermal systems deal with
medium to high temperature and enthalpy.

1.4.2 Features and use of geothermal systems

Geothermal systems are made up of three main components: a heat source,a heat sink
and a heat exchanger. Typically the heat source is the ground while the heat sink is a
built environment (in general, a structure). However, the opposite can also occur,
that is the heat source is a built environment while the heat sink is the ground. The
heat exchanger is generally constituted by one or more elements containing a fluid
that transfers the heat between the heat source and the heat sink.
Employing the ground as a heat reservoir, that is a heat source or sink, is a result of
long historical developments that can often be addressed only on the basis of indirect
considerations (Cataldi, 1999). Some early development examples of their type are
listed hereafter. In early Sumerian and Akkadian times, the ground was employed as a
heat sink to store ice (and food) in so-called “ice houses” constructed partly or
completely underground (e.g. Dalley, 2002), based on the limited affection of these
environments to the surface thermal conditions. In palaeolithic and ancient Roman
times, the ground was used as a heat source for bathing and space heating (e.g.
Armstead, 1973), based on the presence of sources of hot water in the subsurface (e.g.
thermal springs). In 1904 the ground was used as a heat source for electrical power
production via the construction of the first geothermal power plant by Prince Piero
Ginori Conti in Italy (e.g. Lungonelli, 2003).
One key feature of geothermal systems is the way the thermal energy that is har-
vested from the ground is used via such systems. In shallow geothermal systems, an
indirect use of geothermal energy is typically made. Machines or devices that modify
(enhance or lower) the energy input transferred between the ground and the target
environment, in addition to machines or devices that force a heat carrier fluid to flow
(exchanging heat) between them, are employed in such cases. In deep geothermal sys-
tems a direct use of geothermal energy can be made when an indirect use is not tar-
geted. In contrast to the previous case, machines that modify the energy input
transferred between the ground and the target environment can be avoided in this