Geothermal Cooling - Low Energy Cooling for Sustainable Buildings - page 85

Figure 4.1 Variation of ground temperature with depth for German climatic conditions
with similar systems exposed to cooling tower temperatures or hot ambient air
(Karagiorgas et al ., 2004).
By cooling the soil below its undisturbed temperature prior to circulating fluid
through the heat exchanger. When horizontal heat exchangers at shallow depths are
used, soil temperature may be influenced by surface treatment. At greater depths, the
groundmay be cooled in the winter season as heat pumps in the heatingmode extract
energy from the soil. Li et al . (2006) showed that unbalanced heat extraction from
the ground reduces temperature levels by 6 ◦ Cwithin 5 years for heat pump operation
only. If only heat is rejected to the ground from a cooling machine, after 13 years the
soil temperature will be over 35 ◦ C and no longer suitable for air-conditioning. Only
balanced heat fluxes kept ground temperatures constant over a 30-year period. Ekl of
and Gehlin (1996) carried out thermal response test measurements at a borehole
cooling installation for telephone switch stations in Sweden located at Drevikstrand
and Bromma. In Drevikstrand, four boreholes were placed in a line with about 5m
spacing on average. The systems were designed to cool the circulating fluid from
22 to 16 ◦ C. With a measured high thermal conductivity of about 4Wm − 1 K − 1 and a
rather low borehole resistance of 0 . 09 K mW − 1 the mean capacity was determined
to be 30Wm − 1 . In Bromma, 13 boreholes at 130-160 m depth were placed in an
irregular pattern. Similar soil conductivities and even lower borehole resistances
were measured. The required temperature levels were lower at 20 ◦ C return and
14 ◦ C supply. The thermal capacity is significantly lower at 17Wm − 1 .
Environmental and geothermal energy obtained via horizontal or vertical ground
heat exchangers is usually used for heating (mostly via ground-coupled heat pumps),
but such heat pump systems can also be used for space cooling. However, although
there are over 500 000 ground source heat pumps installed worldwide (IEA, 2002),
there is very little experience with such systems coupled to cooling machines.