Page 61 - Analysis and Design of Energy Geostructures

Energy geostructures 31

employed for such a purpose are ethylene glycol and propylene glycol. Ethylene gly-
col is poisonous to humans and animals, and should be handled carefully and disposed
of properly. Propylene glycol is considerably less toxic than ethylene glycol and may
be preferred in some cases.

2.2.3 Pipe configurations

Many different pipe configurations can be foreseen for energy geostructures. This
design variable influences the installation, the operation and the operating costs of the
energy system (cost resulting from the ease and speed of installation as well as from the
installed pipe length).
Examples of pipe configurations for energy piles include the single U-shaped, bent
U-shaped, parallel double U-shaped, series double U-shaped, multi U-shaped, indirect
double, W-shaped, spiral (or helix) and coaxial pipe configurations (cf. Fig. 2.4). In
many practical circumstances, the spiral pipe configuration is not employed despite its
marked heat transfer potential because of the difficulties associated with its installation,
especially in small diameter piles. The multi U-shaped pipe configuration is often used
for large diameter piles.
When dealing with energy tunnels, configurations involving pipes oriented per-
pendicular or parallel to the axial direction of the tunnel can be foreseen (cf. Fig. 2.5).
Such design solutions need to be envisaged considering the position of the header
pipes, which often represents a more significant restraint for the pipe layout of tunnels
compared to that of piles.
Various pipe configurations can also be foreseen for energy walls and energy slabs
(cf. Fig. 2.6). In designing the pipe layout for energy tunnels, energy walls and other
energy geostructures characterised by a significant heat exchange surface, a key aspect
is to obtain the largest heat exchange surface for the selected energy geostructure por-
tion at the lowest pressure drop and investment.

2.2.4 Pipe locations
Energy tunnels, walls and slabs can be characterised by both air solid and solid solid
interfaces. Therefore these energy geostructures can exchange heat with both the air
and the ground. Such a feature can have major implications on the overall efficiency
of the energy system if not properly considered. Depending on the situation, one may
target to foster or limit the heat exchange between the air or solid solid interfaces
by choosing an appropriate pipe location, or sometimes to prevent heat exchange
even by insulating one side of the geostructure (e.g. cold tunnel equipped as a geo-
thermal heat exchanger for heating purposes). An example of these solutions is shown
in Fig. 2.7. To minimise fire risk and prevent accidental damage, the pipes should be
placed at least 200 mm from the geostructure intrados (Nicholson et al., 2014).