Geothermal Cooling - Low Energy Cooling for Sustainable Buildings - page 84

heat transfer between the air and the ground, and the resulting temperature of the air
supplied to the building.
Direct cooling of the building is only possible if the temperature of the soil is lower
than the desired temperature of the indoor air. With alternated signs, this is equally true
for direct heating (Rafferty, 2004). This restriction cannot be met in practice in many
locations, especially in the warmer regions most in need of cooling, such as North
Africa, the Middle East, parts of China, India and most tropical countries. As a result,
although commercial installations of ground source cooling systems are gradually
becoming more common, the application of this technology is still restricted mainly
to temperate climates. Indicative of this situation is the fact that while the Association
of German Engineers (VDI) has already published a series of engineering guidelines
on the planning and installation of ground-coupled heat pumps (VDI, 2000), other
countries do not have a single system installed to date.
Water- or brine-filled ground heat exchangers are either designed in a horizontal
configuration with a shallow depth of about 2 m or installed as vertical loops. Ver-
tical ground heat exchangers are constructed by inserting one or two high-density
polyethylene U-tubes into vertical boreholes of 75 to 150 mm diameter. Vertical loops
are usually connected in parallel to reduce the pressure drop. In Europe, double
U-tubes are common, whereas in the USA single tubes prevail. Zeng et al . (2003)
showed that double U-tubes reduce the borehole resistance by 30-90% and thus in-
crease heat transfer.
In Germany, vertical pipes up to a depth of 100 m are increasingly used for closed-
loop direct cooling of buildings, with water as the heat transfer fluid. The higher heat
capacity of water is advantageous, as the electrical energy needed for circulating the
fluid through the earth heat exchanger is considerably lower than if air is used. The
water cooled through contact with the earth is then distributed in the building using
either activated concrete slabs with buried pipes or an air-based ventilation system, in
which the air is cooled by the water (in an additional heat exchanger). Several large
office buildings in Germany now use this low-energy cooling system (the passive
office building Energon in Ulm, Solar Info Centre in Freiburg, Post Tower in Bonn,
with heat exchangers to air-based distribution systems; Technikum Biberach, the
office building DS Plan in Stuttgart with direct water circulation in activated con-
crete ceilings). The main restriction of the system is the available ground temper-
ature level, which at 15m depth corresponds approximately to the annual average
air temperature above ground and then increases again by about 3 K per 100 m (see
Figure 4.1).
If this temperature level is too close to the desired room temperature, the cooling
power of such a system is too low to be cost effective. This problem may be overcome
by one of two methods:
By employing indirect cooling systems, such as reversible heat pumps (chillers),
which use the ground as a heat sink and are therefore more efficient compared