Page 85 - Analysis and Design of Energy Geostructures

Energy geostructures 55

council building (Frodl et al., 2010). Preliminary field tests indicated thermal powers
2
between 10 and 20 W/m (Franzius and Pralle, 2011).

2.6.5 The energy walls of the Taborstraße station
The U2/2 metro line in Vienna, Austria, represents the first worldwide full-scale
application of energy geostructures to this type of transportation system (Brandl, 2006).
An extension of the U2 metro line offered the possibility to equip four stations with
approximately 103 km of absorber pipes installed in walls, slabs and tunnel linings.
The Taborstraße station was the first station to be equipped among the four (Brandl,
2006). It was opened in 2008 and included energy geostructures to supply the
metro stations with both heating and cooling energy (cf. Fig. 2.20). The system was
primarily designed to inject in the ground a large amount of heat produced in the
underground. The metro line was surrounded by a soil deposit mostly composed of
sand and silt. The U-bahn Company is the main owner and user of the energy. In a
preliminary phase of the design project, it was necessary to prove that the geothermal
system did not affect the soil and groundwater surrounding the station.
2
The Taborstraße station was equipped with a total of 1865 m of energy dia-
2
phragm walls and 1640 m of energy slabs (Brandl, 2006). The absorber pipes were
3
characterised by a diameter of d p 5 25 mm. A total fluid volume of 10 m circulates in
the absorber system and provides a maximum cooling capacity of 81 kW. The surplus
energy not used for heating is designed to be transferred into the soil via the absorber
system, thus avoiding noisy or unsightly outdoor cooling towers (Brandl, 2006).
Absorber pipes were attached to the reinforcements of structural elements and con-
nected via manifolds to the service rooms. The U2/2 station hosts a total of two heat
pumps and one cooling machine. For safety reasons no gas was used but only an elec-
tric refrigeration system. According to Brandl et al. (2010), the design of the energy
geostructure system included a total thermal power installed for heating and cooling of
185 and 114 kW, respectively. According to Brandl (2006), the maximum power for
heating and cooling was 95 and 67 kW, respectively. The average energy extracted
and injected for heating and cooling was 175 and 437 MWh/year, respectively.
Fig. 2.21 shows the simulation diagram of monthly cooling and heating energy
characterising the year 2009 (Widerin, 2011). It is worth noting that the energy con-
sumption data of the cooling machine in winter are measurement errors. The circula-
tion pump energy consumption is constant over the year. In heating mode the
efficiency ratio is 3.34 against 1.25 in cooling mode.
Fig. 2.22 presents the cooling capacity histogram and the seasonal fluid temperature
in the absorber pipes over the same year (Brandl, 2006). According to Brandl (2006),
sufficient heat can be transferred into the ground to achieve the yearly cooling of the
entire metro station.