Page 48 - Analysis and Design of Energy Geostructures

Energy and geotechnologies 17

Geothermal baskets represent a more compact system than horizontal geothermal
boreholes and can be used for similar purposes. These systems are typically buried in
the ground at a few metres of depth (from a depth of z 5 2 10 m) and consist of
closed polyethylene pipes fixed in a spiral geometry in which a heat carrier fluid flows.
Applications where spiral coils are located in surface water reservoirs next to buildings
are also possible, provided that such reservoirs are located deep enough to avoid prob-
lematic conditions for the system operation, for example freezing of the reservoir
water and thus of the circulating heat carrier fluid in the pipes.
Groundwater capture systems employ open wells surrounded by groundwater reser-
voirs located at shallow depths (from a depth of z 5 5 20 m). These systems can be
applied when no hydrological, geological and environmental constraints are present.
They are typically used for heating purposes by extracting the thermal energy present
in the subsurface water. For small constructions, single wells may be used. Larger con-
structions usually require doublet wells. Extraction and injection wells may be
employed to ensure a balanced underground thermal field, which is essential for per-
formance and, in some cases, environmental concerns.
Vertical geothermal boreholes consist of closed polyethylene pipes that are embedded
vertically in the ground below or next to buildings at deeper depths than the previous
applications (from a depth of z 5 50 300 m). A filling material (e.g. bentonite) is usu-
ally placed in the borehole to enhance the heat exchange between the ground and the
pipes. A heat carrier fluid circulating in the pipes allows exchanging heat for heating,
cooling, storage and hot water production purposes in the most diverse construction
types. Single boreholes can supply small residential constructions. Borehole fields are
required when aiming to supply with thermal energy larger constructions. Higher
energy inputs than those transferred through shallower geothermal systems can be
achieved through vertical geothermal boreholes because of the higher temperature
levels characterising the ground at the considered depths.
Energy geostructures are novel geothermal systems that couple (from a depth of
z 5 10 50 m) the structural support role of any structure in contact with the ground
with the heat exchanger role of shallow geothermal systems with comparable and
even more favourable outcomes than the previously described systems.
Thermal springs may generally be classified as deep geothermal systems, although
they can also be found at depths characteristic of shallow geothermal systems. These
systems employ open wells surrounded by hot groundwater reservoirs that are located
relatively deep in the subsurface (from a depth of z 5 500 1000 m). They are typi-
cally used for bathing and medical purposes by extracting the thermal energy present
in the subsurface water.
Hydrothermal systems extract groundwater through open wells from depths that
allow the temperature and thermal energy present to be sufficiently high for realising
large-scale heating applications (from a depth of z 5 1000 4000 m). Typical uses of