Geothermal Cooling - Low Energy Cooling for Sustainable Buildings - page 106

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Standard

-5

-10

Half

-15

-20

-25

Quarter

-30

Distance between heat exchangers

Figure 4.29 Influence of heat exchanger distance on energy output

thermal relaxation the temperature levels are still about 1 K higher than from the
undisturbed start.
Regarding the mutual distance of vertical ground heat exchangers, Zhang and
Murphy (2003) state that the spacing of the boreholes is of major significance re-
garding the thermal effectiveness and that the single heat exchangers can be regarded
as isolated from each other if the distance is greater than 6m.
This finding is confirmed by the current numerical parameter study (see Figure 4.29).
Starting with the standard distance of 6m, the distances are subsequently decreased
to 3m and 1 . 5m, keeping the total soil volume of the geothermal system constant.
This results in a two times, respectively four times, greater number of heat exchangers
with decreasing distance. Although the overall energy output rises, that of each heat
exchanger is reduced due to the mutual influence.
An increase of mass flow inside the tubes improves the energy withdrawal by up to
9% for high soil heat conductivities. At lower soil heat conductivities the improvement
due to mass flow reduces to zero.

Influence of Climate on the Cooling Performance of Geothermal
Heat Exchangers

A field of five vertical ground heat exchangers with a length of 80 m each and with a
total volume flow of 2 . 4m 3 h − 1 is simulated under different climatic boundary condi-
tions in order to investigate the influence of climate on cooling performance. The aver-
age ambient temperatures of several cities in warm climates represent the undisturbed
soil temperatures around the heat exchangers. Each system is operated for 4 months,