Page 58 - Analysis and Design of Energy Geostructures

28 Analysis and Design of Energy Geostructures

geostructures for providing heat to prevent the icing of pavements and decks of infra-
structures such as roads, bridges, station platforms and airport runways can be typically
achieved via energy piles supporting bridge piers, energy slabs and energy pavements.
The use of energy geostructures for storing heat in the subsurface for subsequent use
can also be achieved via energy piles, energy walls, energy slabs and energy tunnels.

2.2.2 Materials and technology
Energy geostructures are typically made of reinforced concrete. From a technological
perspective, they differ from conventional geostructures only because pipes are fixed
along their reinforcing cage or are placed within the filling material (cf. Fig. 2.2).
Placing the pipes along the reinforcing cage on the groundside is common when deal-
ing with energy walls or tunnels and there is the need to avoid potential issues due to
maintenance of the geostructure or the adjacent environment (e.g. fixing supports to
the geostructure that may pierce the pipes embedded within the reinforced concrete).
Embedding the pipes within the concrete is otherwise preferable to ensure adequate
concrete cover on the reinforcing cage.
Fixing the pipes to the reinforcing cage of energy geostructures can be performed
either in a plant or on site. The latter is more common (Brandl, 2006), whereby the
piping is delivered to site on reels and a special working area is used. At the inflow
and outflow of the pipework of each energy geostructure a locking valve and a
manometer are fixed (Brandl, 2006). These instruments allow the pipe circuit to be
pressurised within a range of 5 8 bar for integrity check. In most applications the
locking valves and manometers are also used upon concreting to resist the head of the
wet concrete without collapsing. Pressure testing for 24 hours after concreting is good
practice. The pressure in the pipes is again applied before the working phase involving
the construction of the superstructure starts (Brandl, 2006).
Inside the pipes a fluid is pumped via electrically driven machines and is used as a
thermal energy carrier for the operation of the energy geostructures in most shallow,
closed-loop geothermal systems. The pipes of energy geostructures are usually made of
high-density polyethylene and are characterised by a diameter of 10 40 mm with a
wall thickness of 2 4 mm (cf. Fig. 2.3). Thermal insulation of the pipes can be con-
sidered for the first meters of the inlet and outlet to limit the influence of the climatic
condition on the heat exchange process, aiming at optimising the energy efficiency
(Gao et al., 2008; Batini et al., 2015).
The heat carrier fluid (i.e. the heat transfer medium) circulating in the pipes usually
consists of water, water with antifreeze or a saline solution. Water antifreeze mixtures
containing additives to prevent corrosion are also a well-performing and durable solu-
tion. Antifreezes are employed to lower the freezing point of water-based mixtures
that may be characterised by freezing otherwise. The usual antifreeze additives