Geothermal Cooling - Low Energy Cooling for Sustainable Buildings - page 102

Table 4.2 Measured energy delivered by the five vertical ground heat exchangers to the SIC building
in Freiburg, Germany

Hours Hours

Cooling Heating COP COP cooling cooling Hours

Year / kWh / kWh cool heat seminar floor heating

2005 2759 2846 20.1 12.8 233 2289 856

2006 4873 246 13.5 5.2 327 2911 289

Due to the significantly longer operating hours of both the ventilation system of
the seminar room and the floor cooling system in 2006, the cooling energy delivered
by the geothermal heat exchangers nearly doubled from 2.76 to 4.87 MWh. This
corresponds to 7-12 kWh per metre of heat exchanger and summer. The low COP
for heating in 2006 is due to the fact that the geothermal system was often operated
at a low temperature difference between the ambient air and ground heat exchanger.
Table 4.2 summarizes the measured results.

4.1.4 Modelling of Geothermal Heat Exchangers

Numerical heat transfer models for geothermal heat exchangers were developed and
implemented in the simulation environment INSEL. They are all based on the heat
conduction equation with explicit finite difference solutions. The simplest vertical
heat exchanger model uses a rectangular discretization geometry. Horizontally, nine
zones have been defined and, vertically, 30 elements are calculated for each time step
with only one-dimensional heat transfer considered (see Figure 4.23). To determine

Near soil Filling material

Far soil

Brine inlet Brine outlet

1 2 3 4 5 6 7 8 9

Undisturbed soil

Figure 4.23 Geometry of horizontal discretization of the two-dimensional model for a vertical earth
heat exchanger