Geothermal Cooling - Low Energy Cooling for Sustainable Buildings - page 107

Table 4.3 Simulation results for different climatic boundary conditions

T amb T max P mean P min P max Energy Cost

Location / ◦ C/ ◦ C /Wm − 1 /Wm − 1 /Wm − 1 / MWh / D per kWh

Madrid 13.9 19.4 20.0 17.5 54.0 11.6 0.31

Seville 18.8 21.0 8.0 7.0 21.3 4.6 0.78

Bangkok 28.1 — — — — — —

Crete 19.1 21.1 7.3 6.3 19.3 4.1 0.86

Athens 18.3 20.8 9.0 8.0 24.7 5.3 0.68

12 hours every day. An average sand-type soil was assumed as the surrounding
soil. The inlet temperature to the ground heat exchangers is set to a constant 22 ◦ C,
representing the output of a thermally activated concrete slab from the building.
Table 4.3 shows the investigated locations, the average ambient temperature ( T amb ), the
maximum outlet temperature of the ground heat exchangers ( T max ) as well as the mean
( P mean ), minimum ( P min ) and maximum ( P max ) available cooling power. The costs are
derived from an annuity of D 3570, which corresponds to the typical cost of vertical
ground heat exchangers in Germany.
Obviously, the climate in the area of Bangkok is not suitable for direct geothermal
cooling, as the inlet temperature from the building is lower than the ground tem-
perature. Active chillers are needed here in order to reach the desired temperatures.
However, even in the warm climate of Crete, the outlet temperature of the ground heat
exchangers does not exceed 21 ◦ C at the end of the summer period. The direct use of
geothermal energy via thermally activated concrete slabs is thus possible, although the
achievable cooling power is relatively low bearing in mind the high investment cost.
At all locations, the ground temperature regenerates to between 0.5 and 1 . 5 ◦ C above
the undisturbed temperature at the end of each 12 hours of operating cycle, rising only
slightly during the 4-months period. This aspect is important regarding the long-term
performance of any geothermal system (Pahud et al ., 2002).
In order to determine the influence of the temperature level of the direct cool-
ing system on the performance of the geothermal system, the same geothermal
simulation as above was executed with several different inlet temperatures. Crete,
with a mean ambient air temperature of 19 . 1 ◦ C, was chosen as the location. In-
let temperatures between 20 and 24 ◦ C were chosen to represent the combined
operation with a floor cooling system, whereas the inlet temperatures of 35 and
40 ◦ C represent the replacement of a standard cooling tower by the borehole heat
exchangers.
From Table 4.4 it can be seen that the maximum outlet temperature T max , mean
thermal power P mean , minimum thermal power P min as well as maximum specific
thermal power P max rise approximately proportional to the inlet temperature. This leads
to the conclusion that the ground temperature around the heat exchanger regenerates