Environmental Engineering Reference
In-Depth Information
Table 4.4 Simulation results for different inlet temperatures for the location of Crete
T inlet
T max
P mean
P min
P max
Energy
Cost
/ C
/ C
/Wm 1
/Wm 1
/Wm 1
/ MWh
/ D per kWh
20.0
19.7
2.3
2.0
6.0
1.3
2.78
21.0
20.4
4.8
4.0
12.7
2.7
1.32
22.0
21.1
7.3
6.3
19.3
4.1
0.86
23.0
21.7
9.8
8.5
26.0
5.6
0.64
24.0
22.4
12.3
10.5
32.7
7.0
0.51
35.0
29.8
39.7
34.5
106.0
22.8
0.16
40.0
33.2
52.2
45.4
139.4
30.0
0.12
almost completely during the downtime of the circulation pump. However, considering
the whole system of ground heat exchangers and floor cooling, it is obvious that the
total cooling power depends on a momentary equilibrium of energy extracted from
the room and energy dissipated into the ground. So the efficiency of the system cannot
be increased arbitrarily by raising the inlet temperature. Regarding the operation of the
borehole heat exchangers to replace a cooling tower, it can be stated that geothermal
systems are an interesting alternative concerning effectiveness and cost.
4.1.5 Conclusions on Geothermal Heat Exchangers for Cooling
In summary, this work presents new experimental results for geothermal heat ex-
changer performance in some of the best German office buildings today. Earth heat
exchangers for precooling and preheating of air have excellent ratios of produced cold
compared with electricity used. Even air-based systems are shown to have annual
COPs between 35 and 50.
Measured vertical ground heat exchangers also reach high COPs of about 20. Supply
air temperatures can be efficiently reduced to temperature levels between 18 and 22 C.
The power dissipation measured per metre of heat exchanger was 26W per metre for
80 m deep vertical heat exchangers. In the case of the vertical heat exchangers, the
power was limited mainly by the low cooling energy uptake within the building.
Cheap solutions are available using simple horizontal heat exchangers, which can
be placed around the building. However, the heat dissipation measured was only
about 8Wm 1 , which is mainly due to a low depth and close spacing between the
horizontal absorbers. Although temperature levels are then higher than in the case of
vertical heat exchangers or deeper horizontal systems, the cooling performance is still
good.
Numerical simulation models have been developed and validated against the ex-
perimental data and reproduce the measured values well. They can now be used in
planning for geothermal heat exchangers and for parameter studies. The simulation
results show that performance is improved if the backfill material and obviously the
soil have higher conductivity values (e.g. in regions with groundwater flow). The
 
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