Page 46 - Analysis and Design of Energy Geostructures

Energy and geotechnologies 15

circumstance and only machines that force a heat carrier fluid to flow between the
ground and the target environment are required.
Shallow geothermal systems can be used to provide heating, cooling and hot water,

using temperatures available underground of less than 25 C. These systems are
suitable for small-scale and domestic use in almost any geographical location. Deep
geothermal systems can be used to provide heating and hot water as well as electrical
power, using temperatures available underground greater than 25 C and up to 200 C

[the temperatures required for electrical power generation being generally greater than
175 C(Narsilio et al., 2014)]. These systems are suitable for medium- to large-scale

uses, but can be applied in more particular locations than shallow geothermal systems.
In addition to the previously proposed classification of geothermal systems, one
additional criterion is often employed to characterise these systems and is related to the
presence of closed- or open-loops in the heat exchanger. Based on this criterion, geo-
thermal systems can be classified in most cases as either closed-loop or open-loop systems.
Closed-loop systems use a water-based mixture circulating through sealed pipes to
transfer the heat from the ground to the superstructure or vice versa. Open-loop sys-
tems directly use groundwater extracted from or injected into aquifers through wells
in the considered heat exchange process.
The crucial difference between closed- and open-loop systems is that in the latter
systems mass exchange occurs, in contrast to the former, and heat transfer is more
favourable. However, although providing the highest energy yield, open-loop sys-
tems require a greater financial input and pose technical risks related to, for example
underground pollution (Boënnec, 2008). Because there is no mass exchange with
the ground, closed-loop systems minimise environmental risks and mineral precipita-
tion issues, and do not require the need to obtain extractive licensing (Narsilio et al.,
2014). Shallow geothermal systems can be either open- or closed-loop. Deep geo-
thermal systems are open-loop.
Fig. 1.11 presents typical examples of shallow and deep geothermal systems.
Typical shallow geothermal systems are horizontal geothermal boreholes, geothermal

baskets, groundwater capture systems, vertical geothermal boreholes and energy geostruc-
tures. Typical deep geothermal systems are thermal springs, hydrothermal systems and
petrothermal systems.
Horizontal geothermal boreholes are the shallowest type of geothermal systems. These
systems typically consist of closed polyethylene pipes ploughed or dug down horizon-
tally in the ground next to buildings at a few metres of depth (from a depth of
z 5 2 5 m). In the pipes a circulating heat carrier fluid allows the exchange of the
heat present in the ground (predominantly as a consequence of solar radiation), espe-
cially for heating purposes in residential, agricultural, or aquaculture applications.
However, storage purposes can also be achieved in those situations in which the bore-
holes are placed under buildings.