Geothermal Cooling - Low Energy Cooling for Sustainable Buildings - page 104

Figure 4.26 Influence of soil heat conductivity on energy output of vertical heat exchangers. The
reference value is the moraine soil as in the SIC building context
The three-dimensional validated model was used for parameter studies concern-
ing the backfill and soil heat conductivity, mass flow rates and distance between the
geothermal heat exchanger. The boundary conditions are those of the SIC building
in Freiburg. The soil heat conductivities range from 0.5 representing dry clay, to
2 . 5Wm − 1 K − 1 for water-saturated sand. As can be seen from Figure 4.26, the soil
heat conductivity has a major influence on the cooling performance. Compared with
the standard moraine soil, the energy output is reduced by 39% if the subsoil consists
of dry clay, verifying the statements of Sanner and Rybach (1997), who show that the
specific energy extraction rate increases with increasing soil heat conductivity. Zhang
andMurphy (2003) support this finding but stress that the effectivity of thermal storage
may decrease.
Borehole backfill heat conductivities of 0.8 (light concrete), 1.6 (bentonite) and
3 . 2Wm − 1 K − 1 (high-performance backfill) were investigated in the next step. Al-
though light concrete is rarely used as backfill material, it was included in this study
for comparison. In Figure 4.27 it can be seen that the effect on the energy output
is relevant. However, it can be expected that the influence will decrease if the heat
conductivity of the surrounding soil is low.
Thermal response tests have been conducted at theUniversity of Applied Sciences in
Stuttgart to determine the effective heat conductivity for different backfill materials.
Two borehole heat exchangers with lengths of 80 m were built, one with standard
backfill with a heat conductivity of 1 . 6Wm − 1 K − 1 and the other with Stuewatherm
backfill with a heat conductivity of 2 . 0Wm − 1 K − 1 . The borehole heat exchangers
are tested with a thermal power of 3 kW. The heat conductivities of the complete